Total intravenous techniques for anaesthesia

ANAESTHESIA

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Sedation and analgesia being provided to a postoperative patient using two syringe drivers

Total intravenous techniques for anaesthesia

THIERRY BETHS

THE two most commonly used techniques for total intravenous anaesthesia (TIVA) are multiple bolus injection and continuous infusion. This article describes how these techniques may be implemented and outlines some protocols for use in a range of veterinary species. It also discusses computerised infusion systems, such as the target-controlled infusion (TCI) techniques, which have been used to good effect in human medicine and are now gaining more widespread use in the veterinary arena.

INTERMITTENT BOLUS TECHNIQUES

Agent of choice Thierry Beths graduated from the University of Liège, Belgium, in 1994. He is an associate professor in the faculty of veterinary medicine at Ross University in the West Indies, prior to which he was a full-time clinician in anaesthesia and intensive care at Glasgow. He is currently completing PhD studies on total intravenous anaesthesia in dogs. He holds the RCVS certificate in veterinary anaesthesia.

Once anaesthesia is induced, it may be maintained using small intermittent boluses of a short-acting intravenous hypnotic induction agent such as propofol (see box on the right). While this technique is very simple and only requires a syringe and cannula for intravenous access, plasma drug concentrations tend to oscillate between peaks (potentially causing increased side effects such as apnoea or hypotension) and troughs (resulting in inadequate anaesthesia). These inconsistent plasma drug levels lead to a poor quality of anaesthesia and a large total drug dose being given, which consequently results in a slower recovery (Smith and White 1998).

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Intermittent boluses. This graph shows the plasma drug concentration in a dog in which anaesthesia was induced and maintained using consecutive boluses of anaesthetic. The induction dose is followed by intermittent boluses to maintain anaesthesia, which are usually given when the patient shows signs of inadequate anaesthesia. This results in peaks and troughs in the plasma drug concentration corresponding to the intermittent boluses

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Propofol is the most suitable drug currently available for TIVA. It is a rapidly acting, short duration agent, with high clearance, a minimal tendency for accumulation and few side effects. Caution, however, is required in cats (see page 413).

CONTINUOUS INFUSION TECHNIQUES

The administration of anaesthetic agents using continuous infusion techniques eliminates the peaks and troughs in plasma drug concentration that occur with the use of intermittent boluses. The result is a better quality of anaesthesia, which is achieved using a lower total drug dose (Gepts 1998). The cheapest but least accurate way of providing continuous infusion anaesthesia is to use a bag containing the hypnotic agent(s) and a giving set. The flow rate can be adjusted by varying the diameter of the infusion tubing using a ‘regulat90 105 120 ing’ clamp. The infusion rate can be calculated by knowing the volume of each drop (20 versus 60 drops per ml) and counting the drip rate. Triple drip anaesthesia, which In Practice

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These include constant rate infusion (CRI) and ratecontrolled infusion (RCI) systems. In the case of CRI, the anaesthetist chooses a particular infusion rate that is then maintained throughout the entire procedure. Note that the plane of anaesthesia achieved is very difficult to control. Initially, the plasma drug concentration will be low and might not provide an adequate level of anaesthesia but, with time, the plasma drug level will gradually increase and the agent will become more effective.

PHARMACOKINETIC-DEPENDENT INFUSION SYSTEMS Stepped infusion systems

Stepped infusion systems allow a constant rate of infusion, which thus keeps the target plasma drug concentration consistent throughout the procedure. These systems provide a series of different constant rate infusions in a precise order for a predetermined length of time. The speed of the infusions decreases from the first to the last step. This system is usually simplified to incorporate a minimum number of steps to make it more practical and allow the desired plasma drug concentration to be reached and maintained. The speed and duration of the 2000

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PHARMACOKINETIC-INDEPENDENT INFUSION SYSTEMS

However, if the procedure is prolonged, the plasma drug concentration will carry on increasing and side effects may result. The lack of effect at the start of the procedure can be avoided by administering a loading dose to allow an effective plasma drug concentration to be reached in a very short period of time. However, this will not prevent accumulation of the drug, and hence the possible development of side effects, which may be overcome by changing the drug infusion rate to correspond to the needs of the patient. This is referred to as RCI and provides a much smoother anaesthesia. The total amount of agent used is also smaller, but frequent adjustments are needed and it is difficult to titrate to effect.

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Constant rate infusion. After receiving a bolus, the patient is maintained with a constant rate infusion of 400 µg/kg/minute. The green line represents the infusion rate (right axis) and the blue line the plasma drug concentration (left axis). Compared with the intermittent bolus technique, this method provides a more stable level of anaesthesia. Note the increase in plasma drug concentration over time

Desirable features of an infusion pump or syringe driver The instrument should ideally: ■ Be versatile (eg, have a choice of bolus or infusion modes, an internal calculator for choosing different dosing schemes, and a choice of infusion rates between 0 and 1500 ml/hour) ■ Be lightweight ■ Have a clear display showing the drug administered and the infusion rate ■ Have a simple protocol for initiating or changing infusion rates ■ Have a digital interface for record keeping or external automated control ■ Have an alarm to alert the operator in the event of tubing becoming disconnected

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Infusion rate (µg/kg/minute)

is a field technique used in horses, is an example of this method. The speed of infusion depends on gravity, the diameter and length of the connecting tube, the volume of the drops and diameter of the cannula, the viscosity of the agent, the height at which the fluid is elevated and the venous pressure of the patient. As the fluid decreases in the bag, the infusion rate reduces and needs readjusting. Infusion pumps and syringe drivers can be used to achieve a more continuous, controlled, accurate and safer infusion. Large volume infusions are more accurately administered using an infusion pump, while a syringe driver is more appropriate for smaller volumes administered at slower rates. Modern infusion devices are able to detect air in the system and line blockages. Potential hazards associated with infusion pumps and syringe drivers include: ■ An excessively rapid rate of infusion (due to, for example, the administration set not being properly clamped); ■ Power failure (mains or battery); ■ Infusion of air, if a cannula becomes dislodged. The position of a syringe driver is important. If it is positioned higher than the vascular access, syphoning may occur due to the weight of the liquid, causing more than the programmed level of drugs to be administered. The device should therefore be protected to prevent the syringe plunger from moving faster than its motor drive. Conversely, if the syringe driver is positioned lower than the vascular access, less agent than programmed is infused due to the back pressure from the venous bed and the weight of the liquid. To avoid these problems, infusion sets should incorporate a one-way valve. If the syringe driver is positioned vertically, the outlet should be placed downwards to prevent bubbles formed by gas coming out of the solution being infused into the patient.

Clamp attached to a giving set to regulate the amount of anaesthetic delivered

Infusion pumps can be used to administer large volume infusions more accurately

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successive infusions are determined by simulation software, which uses pharmacokinetic data specific for the agent used. Stepped infusions using propofol were developed for use in humans. An example system is the Bristol technique, which uses steps of 10, 8 and 6 mg/kg at 10 minute intervals, and veterinary systems are based on this scheme. Unfortunately, as with CRI systems, stepped infusion systems are very inflexible and difficult to adapt to particular clinical situations. A low target drug delivery will result in inadequate anaesthesia, while a high target will result in increased side effects such as hypotension, apnoea and delayed recovery.

Target-controlled infusion systems

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Stepped infusion. The green line shows three infusion steps. The blue line indicates the plasma propofol concentration, which stays close to the required target of 2500 ng/ml (2·5 µg/ml) during the infusion period and then drops away. Note that this infusion scheme is rigid and does not take account of individual variation, health status and premedication 2900

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Target-controlled infusion programmed to achieve a plasma drug concentration of 5000 ng/ml (5 µg/ml). Note the steady plasma drug concentration obtained (blue line) and that, following bolus administration, the infusion rate decreases over time, thus compensating for agent accumulation (green line)

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Target-controlled infusion in a dog. The target is maintained at 5000 ng/ml (5 µg/ml) for the first 30 minutes (blue line). During that time, a decrease in the infusion rate compensates for propofol accumulation (green line). At 30 minutes, the target is increased to 8000 ng/ ml (8 µg/ml), and the patient is given a bolus, followed by an infusion at an exponentially decreasing rate that is higher than the previous one. At 60 minutes, the target is decreased to 6000 ng/ml (6 µg/ml). The infusion will stop until the plasma concentration reaches the new target. At that time, the infusion will restart, but at a lower rate than previously, as the target is lower

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Two syringe drivers set in parallel during anaesthesia of a dog. One infuses an analgesic agent (eg, remifentanil), while the other is a target-controlled infusion system to provide anaesthesia using propofol

TCI systems allow plasma drug concentrations to be targeted and modified depending on the individual patient’s requirements. A TCI system consists of a syringe driver coupled with a computer programmed with the pharmacokinetic parameters of the specified drug in the relevant species. In contrast to conventional pumps, where drugs are administered at a fixed rate, TCI systems use a computer pump control algorithm to calculate the infusion rate that is necessary to achieve a specific plasma drug concentration. Thereafter, the computer controls the pump to maintain the target concentration. If the target is kept constant, the infusion rate will decrease over time to match the cumulative characteristic of the agent. The anaesthetist can choose to modify the target at any point. An increase in target concentration will result in the injection of a calculated bolus dose, followed by an infusion at an exponentially decreasing rate that will be higher than the original infusion rate. Following a decrease in target concentration, the infusion will cease until the new target (as determined by the computer) is reached. Thereafter, the infusion will resume at an exponentially decreasing rate that is lower than previously. The accuracy of a TCI system depends on the pharmacokinetic variables that have been used to programme it. Consequently, it must be validated before use in clinical practice. In humans, TCI systems have been developed mainly for anaesthesia. However, they are now also used to provide either conscious sedation (using propofol) or peri- and postoperative analgesia (using fentanyl or remifentanil). In veterinary medicine, the use of computer-driven infusion systems has been described in cats using alfentanil (Ilkiw and others 1997), and in horses using alfentanil (Pascoe and others 1993) and detomidine (Daunt and others 1993). More recently, a TCI system using propofol in dogs has been developed and assessed by a team from Glasgow (Beths and others 2001). In Practice

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TIVA IN VETERINARY MEDICINE

Different TIVA protocols, including some for field anaesthesia, have been described in various species. As for inhalation anaesthesia, TIVA must be balanced and should incorporate the use of analgesic agents whether it be for premedication or peri-/postoperative use. The table on the right lists induction and maintenance protocols for TIVA in the principal veterinary species. The infusion rates provided should be used as a guide and adapted depending on the patient’s health status, age, premedication, and so on. Tracheal intubation should always be performed to secure the airways; this is particularly important in cattle and sheep to prevent the aspiration of ruminal contents. If possible, patients should be given a gas mixture containing at least 30 per cent oxygen via an appropriate breathing system or demand valve. For each species, premedication (sedation and/or analgesia) is provided as described for inhalation anaesthesia. Note that very concentrated solutions of guaifenesin (7 per cent in cattle and sheep, and 15 per cent in horses) might cause haemolysis. A total dose of 100 mg/kg (this includes the induction dose for guaifenesin, if used) or one hour of infusion should not be exceeded, as an accumulation of this agent can cause a prolonged and difficult recovery.

TIVA PROTOCOLS

Cattle

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Thiopental (7 to 13 mg/kg) iv

Guaifenesin 5%, ketamine (1 to 2 mg/ml) and xylazine (0·1 mg/ml) mixture. Average infusion rate of 2 ml/kg/hour for adults, 1·5 ml/kg/hour for calves

Guaifenesin 5%, ketamine (1 to 2 mg/ ml) and xylazine (0·1 mg/ml) mixture (triple drip) at a rate of 1 to 2 ml/kg iv Sheep

Horses

Diazepam (5 mg/ml) and ketamine (100 mg/ml) mixed together (at a ratio of 1:1) at a dose of 0·05 to 0·1 ml/kg iv

Ketamine (2·2 mg/kg) with diazepam or midazolam (0·04 to 0·1 mg/kg) iv Thiopental (4 to 6 mg/kg) alone or with diazepam or midazolam (0·04 to 0·1 mg/kg) iv

Propofol 0·3 to 0·5 mg/kg/minute Guaifenesin 5%, ketamine (1 to 2 mg/ml) and xylazine (0·1 mg/ml) mixture (triple drip). Average infusion rate of 2 ml/kg/hour 500 mg xylazine and 1 g ketamine in 500 ml of 10% guaifenesin. Average infusion rate of 1 ml/kg/hour 25 mg romifidine and 1 g ketamine in 500 ml of 10% guaifenesin. Average infusion rate of 0·8 ml/kg/hour 10 mg detomidine and 1 g ketamine in 500 ml of 10% guaifenesin. Average infusion rate of 1 ml/kg/hour 10 mg medetomidine (off-label use) and 1 g ketamine in 500 ml of 10% guaifenesin. Average infusion rate of 1 ml/kg/hour

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Premedication with medetomidine (7 µg/kg) iv Induction with propofol (12 mg/kg) iv

Propofol 10 mg/kg/minute with medetomidine 3·5 µg/kg/hour. Hypoxaemia can result, so oxygen supplementation is advised

Ketamine (5 mg/kg) iv with diazepam or midazolam (0·5 mg/kg) iv

Ketamine 2 to 4 mg/kg/hour with medetomidine 1 µg/kg/hour

Propofol (4 to 6 mg/kg) iv

Propofol 0·4 mg/kg/minute iv ± coinfusion with fentanyl 0·2 to 0·7 µg/kg/minute iv (painful procedure) or morphine 0·1 mg/ kg/hour ± ketamine 5 to 10 µg/kg/hour iv (painful procedure) or medetomidine 1 µg/ kg/hour to provide analgesia

Ketamine (5 mg/kg) iv with diazepam or midazolam (0·5 mg/kg) iv

Propofol 0·22 mg/kg/minute iv ± coinfusion with medetomidine 5 µg/kg/hour or morphine 0·03 to 0·05 mg/kg/hour or ketamine 20 to 40 µg/kg/minute to provide analgesia

Propofol (4 to 6 mg/kg) iv

iv Intravenously

TIVA being carried out in a horse using a gravity-dependent system. A bag containing a mixture of guaifenesin, ketamine and xylazine is connected to a jugular catheter via a giving set. Anaesthesia is deepened or lightened by increasing or decreasing the diameter of the tubing of the giving set using a regulating clamp

Cats and TIVA The use of propofol for TIVA in cats is controversial and might result in prolonged recovery. Ilkiw and Pascoe (2003) reported no complications following a single infusion of propofol to maintain anaesthesia. However, Andres and others (1995) reported Heinz body formation, malaise, anorexia, diarrhoea and facial oedema after 30 minutes of anaesthesia using propofol for three consecutive days. Therefore, the author advises caution if using propofol for TIVA in cats.

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References ANDRES, J. L., DAY, T. K. & DAY, D. (1995) The effects of consecutive day propofol anesthesia on feline red blood cells. Veterinary Surgery 24, 277-282 BETHS, T., GLEN, J. B., REID, J., MONTEIRO, A. M. & NOLAN, A. M. (2001) Evaluation and optimisation of a target-controlled infusion system for administering propofol to dogs as part of a total intravenous anaesthetic technique during dental surgery. Veterinary Record 148, 198-203 DAUNT, D. A., DUNLOP, C. I., CHAPMAN, P. L., SHAFER, S. L., RUSKOAHO, H., VAKKURI, O., HODGSON, D. S., TYLER, L. M. & MAZE, M. (1993) Cardiopulmonary and behavioral responses to computer-driven infusion of detomidine in standing horses. American Journal of Veterinary Research 54, 2075-2082 GEPTS, E. (1998) Pharmacokinetic concepts for TCI anaesthesia. Anaesthesia 53 (Supplement), 4-12 ILKIW, J. E. & PASCOE, P. J. (2003) Cardiovascular effects of propofol alone and in combination with ketamine for total intravenous anesthesia in cats. American Journal of Veterinary Research 64, 913-917 ILKIW, J. E., PASCOE, P. J. & FISHER, L. D. (1997) Effect of alfentanil on the minimum alveolar concentration of isoflurane in cats. American Journal of Veterinary Research 58, 1274-1279 PASCOE, P. J., STEFFEY, E. P., BLACK, W. D., CLAXTON, J. M., JACOBS, J. R. & WOLINER, M. J. (1993) Evaluation of the effect of alfentanil on the minimum alveolar concentration of halothane in horses. American Journal of Veterinary Research 54, 1327-1332 SMITH, I. & WHITE, P. F. (1998) Intravenous anaesthesia delivery and monitoring systems. Total Intravenous Anaesthesia. Principles and Practice Series. London, BMJ Books. pp 98-127 Further reading BETTSCHART-WOLFENSBERGER, R., BOWEN, M. I., FREEMAN, S. L., FELLER, R., BETTSCHART, R. W., NOLAN, A. & CLARKE, K. W. (2001) Cardiopulmonary effects of prolonged anesthesia via propofol-medetomidine infusion in ponies. American Journal of Veterinary Research 62, 1428-1435 GLASS, P. S., DOHERTY, D., JACOBS, J. R. & QUILL, T. J. (1991) Pharmacokinetic basis of intravenous drug delivery. Baillière’s Clinical Anaesthesiology 5, 735-775 GLEN, J. B. (2003) The development and future of TCI. In Advances in Modelling and Clinical Applications of Intravenous Anaesthesia. Eds J. Vuyk and S. Schraag. pp 123-134 MILLER, D. R. (1994) Intravenous infusion anaesthesia and delivery devices. Canadian Journal of Anaesthesia 41, 639-651 MORGAN, M. (1983) Total intravenous anaesthesia. Anaesthesia 38 (Supplement), 1-9 SCHUTTLER, J., KLOOS, S., SCHWILDEN, H. & STOECKEL, H. (1988) Total intravenous anaesthesia with propofol and alfentanil by computer-assisted infusion. Anaesthesia 43 (Supplement), 2-7 WHITE, P. F. (1988) Propofol: pharmacokinetics and pharmacodynamics. Seminars in Anaesthesia 7, 4-20

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COMPANION ANIMAL PR ACTICE Malcolm McKee graduated from Glasgow in 1983. He is a partner in the Willows Referral Service in Solihull, and an RCVS specialist in small animal surgery (orthopaedics). His particular interests are complex fracture management, joint replacement and spinal surgery in the dog. He is past chairman of the British Veterinary Orthopaedic Association.

In Practice (2007) 29, 434-444

Targeted investigations are required to differentiate between orthopaedic and neurological causes of lameness and weakness

Lameness and weakness in dogs: is it orthopaedic or neurological? MALCOLM MCKEE

DIAGNOSING the cause of lameness or weakness in dogs is not always straightforward. Although orthopaedic conditions are the most common cause of thoracic and pelvic limb lameness and neurological disorders the most common cause of weakness, occasionally neurological cases may be presented due to lameness and orthopaedic cases due to weakness. Diagnosing orthopaedic and neurological disorders as causes of weakness and lameness, respectively, can be challenging. A detailed history and thorough clinical examination, with emphasis on the orthopaedic and neurological components, is essential. This article describes the orthopaedic and neurological causes of lameness, and weakness and incoordination, and highlights the specific investigations that can be carried out to differentiate between the two.

LAMENESS – IS IT ORTHOPAEDIC?

Lameness is a common presentation in small animal practice and is usually due to orthopaedic conditions involving joints or bones, such as osteochondrosis, hip dysplasia, cranial cruciate ligament rupture, patellar luxation, osteoarthritis, fractures, panosteitis and neoplasia. Signalment, particularly with regard to age and breed, and history are useful aids to diagnosis. For example, osteochondrosis is primarily a condition of immature, large-breed dogs and there is often a history of stiffness after rest. Conversely, primary bone tumours are overrepresented in middle-aged to old large- and giant-breed dogs and lameness is usually insidious in onset and progressive. Some historical features are highly suggestive of an orthopaedic condition (eg, paw problems often result in excessive licking of the area and lameness that is exacerbated on hard surfaces). A thorough and complete orthopaedic examination should follow a general physical examination (see Houlton 2006). Gait should be analysed, and conformation and posture studied. All limbs should be examined systematically from the digits to the proximal aspect of each limb. Particular attention should be paid to muscle atrophy and contralateral limbs should be compared, bearing in mind that conditions may be bilateral. Individual bones should be carefully palpated for evidence of pain, thickening and deformity. All joints should be evaluated for pain, range of motion, thickening, crepitus and instability. 434

It is important to consider that many orthopaedic conditions such as elbow dysplasia, hip dysplasia, cranial cruciate ligament rupture and osteoarthritis can be subclinical and, hence, not cause lameness. Evidence of pain is thus a critical feature. However, diagnosis is further complicated by the fact that some conditions may result in chronic lameness without a focus of pain being detectable on examination. Soft tissue conditions in the region of the shoulder (eg, glenohumeral ligament rupture) and fragmented coronoid process are examples of occult causes of lameness. Radiography, synovial fluid analysis, arthroscopy, computed tomography (CT), ultrasonography, scintigraphy, fine needle aspirates and biopsy can all be used to investigate lameness.

Historical features suggestive of spinal pain ■ ■ ■ ■ ■ ■ ■ ■ ■ ■

Yelping (unprovoked or when handled) Reluctance to jump or climb Arching of the back Low head carriage Reluctance to lower the head to eat from the floor Reluctance to look upwards Reluctance to turn in tight circles Tense neck, back and/or abdomen Restlessness Panting

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LAMENESS – IS IT NEUROLOGICAL?

NEUROLOGICAL DISEASE AND THORACIC LIMB LAMENESS

Any disease that causes compression or destruction of a spinal nerve or a peripheral nerve that originates from the cervical or lumbar intumescences may cause thoracic or pelvic limb lameness. The clinician should be suspicious of this scenario when orthopaedic examination reveals no specific abnormalities or evidence of pain. In addition, historical features, such as evidence of spinal pain, weakness or incoordination, may be suggestive of neurological involvement. Thus, owners should be carefully questioned regarding evidence of spinal pain, stumbling, dragging of the digits, weakness and incoordination. Neurological examination may reveal muscle atrophy that may be neurogenic in origin or relate to disuse. Proprioception tests, such as paw position sense, may be delayed or absent. These tests may be difficult to perform in dogs that are reluctant to bear weight on the affected limb. Pedal withdrawal and stretch reflexes (eg, triceps, patellar reflex) may be reduced or absent. Traction on the limb may stretch the affected spinal nerve and exacerbate pain and lameness – referred to as a nerve root signature.

Diseases affecting the caudal cervical spinal nerves (C6 to C8), the brachial plexus or peripheral nerves may cause thoracic limb lameness. The most common conditions are degenerative disc disease and neoplasia. Less common causes include discospondylitis, brachial plexus neuritis and orthopaedic implant-related nerve injury (eg, injury to the ulnar nerve by a bone plate on the medial aspect of the distal humerus).

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Lateral (A), ventrodorsal (B), right lateroventral–left laterodorsal oblique (C) and left lateroventral–right laterodorsal oblique (D) cisternal myelograms from a 7·5-year-old cocker spaniel with neck pain and left thoracic limb lameness, showing no evidence of extradural spinal cord compression. Note the mineralised material within the C6-C7 intervertebral disc and the left C6-C7 intervertebral foramen (arrow). The latter is extruded nuclear disc material that is compressing the regional (C7) spinal nerve and causing the clinical signs

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Sagittal (left) and transverse (right) magnetic resonance images from a 10-year-old German shepherd dog with neck pain and left thoracic limb lameness showing degeneration of the C5-C6 intervertebral disc and left ventrolateral spinal cord compression (arrow). The latter is caused by extruded disc material that has migrated caudally from C5-C6. It is compressing the ipsilateral spinal nerve

Degenerative disc disease

Intervertebral discs that undergo degenerative chondroid or fibroid metaplasia may cause clinical signs in one of two ways: extrusion of the nucleus pulposus or protrusion of the annulus fibrosus. These are referred to as Hansen type I and II lesions, respectively. With annular protrusions there may be concomitant vertebral deformity (eg, cervical spondylopathy-associated disc protrusion or so-called wobbler syndrome). CERVICAL DISC EXTRUSION The nucleus pulposus may extrude dorsolaterally through the annulus fibrosus and occupy the ventrolateral vertebral canal or intervertebral foramen. Compression of the regional spinal nerve can result in pain and thoracic limb lameness. Spaniels, terriers and chondrodystrophoid breeds are most often affected. Thoracic limb lameness is often associated with marked neck pain, which is typified by spontaneous yelping and a low head carriage. Cervical muscle spasm and exacerbation of pain when the neck is manipulated are common clinical features. The affected thoracic limb is often held off the ground in a flexed position. Survey radiographs of the cervical spine may reveal

intervertebral disc space narrowing. When the nucleus pulposus is mineralised, extruded material may be visible overlying the vertebral canal on a lateral view. Oblique views may reveal mineralised disc material within the intervertebral foramen. Myelographic contrast studies may show extradural spinal cord compression. Oblique views may be necessary to detect ventrolateral extrusions. Myelography is insensitive for diagnosing foraminal extrusions since the subarachnoid space is not compressed. Magnetic resonance imaging (MRI) is the preferred diagnostic test in these cases. CERVICAL DISC PROTRUSION The annulus fibrosus may protrude dorsolaterally into the vertebral canal and compress the regional spinal nerve, which may result in chronic thoracic limb lameness. Large-breed dogs, such as dalmatians and standard poodles, are over-represented. Neck pain is variable and muscle atrophy of the affected limb is often palpable.

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A Eight-year-old dobermann with right thoracic limb lameness and paraparesis (A), and resting (B) and traction (C) cisternal myelograms from the same animal. Note the spinal cord compression due to protrusion of the C6-C7 intervertebral disc, which is relieved with traction. The split ventral contrast column (arrow) is consistent with asymmetrical dorsolateral protrusion of the disc and regional spinal nerve compression. There is vertebral malformation and malarticulation (cervical spondylopathy) and spondylosis deformans

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Survey radiographs of the cervical spine may reveal intervertebral disc space narrowing. Mineralisation of the affected disc is rare. Myelography with oblique views may show ventrolateral extradural spinal cord compression. The degree of compression is often reduced when traction is applied to the neck. It is probable that the associated spinal nerve compression is also reduced or eliminated as the intervertebral foramen is enlarged. The favourable response to vertebral distraction–stabilisation surgery in these cases supports this hypothesis. MRI is an excellent alternative imaging modality. Traction images may be obtained to assess the effect on spinal cord and spinal nerve compression.

Lateral (A), ventrodorsal (B), right lateroventral–left laterodorsal oblique (C) and left lateroventral–right laterodorsal oblique (D) cisternal myelograms from a 7·5-year-old Yorkshire terrier with left thoracic limb lameness due to a nerve sheath tumour. Note the left ventrolateral C5-C6 extradural spinal cord compression (white arrow) and the enlarged left C5-C6 intervertebral foramen (yellow arrow)

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CERVICAL SPONDYLOPATHY-ASSOCIATED DISC PROTRUSION

Caudal cervical vertebral malformation and associated protrusion of the annulus fibrosus may compress the spinal cord and regional spinal nerves. Middle-aged dobermanns and great danes are over-represented. Neck pain is variable, while muscle atrophy of the affected limb is often marked. Survey radiographs of the cervical spine may reveal vertebral malformation, abnormal vertebral alignment and intervertebral disc space narrowing. Myelography with oblique views may show ventrolateral extradural spinal cord compression. As with other cervical disc protrusions, the degree of cord compression is often reduced with traction. MRI with traction views is increasingly used in the diagnosis of this condition. Neoplasia

Tumours arising from nerve sheaths or the caudal cervical spine may cause progressive thoracic limb lameness. NERVE SHEATH TUMOURS Malignant peripheral nerve sheath tumours (previously referred to as neurofibromas, neurofibrosarcomas or schwannomas) may involve spinal nerves, the brachial plexus or peripheral nerves. Nerve sheath tumours arising from spinal nerves within the vertebral canal may cause spinal cord compression. Neck pain is a common feature and neurological deficits may be detected in other limbs, especially the ipsilateral pelvic limb. Oblique view survey radiographs may show enlargement of the intervertebral foramen containing the affected spinal nerve. Myelography or MRI may reveal a lateralised extradural or intradural– extramedullary space-occupying lesion. Nerve sheath tumours arising from the brachial plexus generally do not cause neck pain. Neurological

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deficits are often only evident when the condition is advanced. Muscle atrophy is often marked. Careful palpation of the axilla may reveal a mass. Pain is generally exacerbated when these tumours are manipulated and this may even be evident when the dog is sedated or anaesthetised. Brachial plexus tumours may invade along one or more spinal nerves and enter the vertebral canal and spinal cord. Electromyographic tests may reveal spontaneous electrical activity in affected muscles and reduced nerve conduction velocities. MRI and ultrasonography may enable detection of a mass. Some of these tumours are extremely small, even after many months of progressive thoracic limb lameness, and false negative findings are therefore possible. CERVICAL SPINAL NEOPLASIA Extradural tumours (eg, fibrosarcomas) and vertebral tumours (eg, osteosarcomas) may cause spinal nerve compression and associated thoracic limb lameness. Survey radiographs may reveal osteolysis or abnormal new bone formation. Myelography may show evidence of concomitant spinal cord compression. MRI or CT may also be useful diagnostic aids.

(left) Eleven-year-old labrador retriever with right thoracic limb lameness due to a brachial plexus nerve sheath tumour. (middle) Transverse magnetic resonance image showing right C8 spinal nerve enlargement (arrow). (right) Brachial plexus mass (arrow) on postmortem examination

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B (A) Nine-year-old labrador retriever with right thoracic limb lameness due to a vertebral osteosarcoma. Lateral (B), ventrodorsal (C) and left lateroventral–right laterodorsal oblique (D) radiographs show osteolysis and irregular new bone formation affecting the right craniolateral aspect of C6 (arrows)

and orthopaedic implant-related nerve injury (eg, injury to the sciatic nerve by an intramedullary femoral pin or ischioilial pin [DeVita pin]). Degenerative disc disease C

NEUROLOGICAL DISEASE AND PELVIC LIMB LAMENESS

Intervertebral discs of the caudal lumbar and lumbosacral spine may degenerate and cause clinical signs in a similar way to cervical discs. Caudal lumbar disc lesions tend to be nuclear extrusions in contrast to lumbosacral disc lesions, which are invariably annular protrusions.

Diseases affecting the caudal lumbar spinal nerves (L5 to L7), the lumbosacral plexus or peripheral nerves may cause pelvic limb lameness. The most common are degenerative disc disease, neoplasia and discospondylitis. Less common causes include acetabular/ischial fractures, orthopaedic surgery (eg, triple pelvic osteotomy, femoral head and neck excision), cauda equina neuritis

LUMBAR DISC EXTRUSION The nucleus pulposus may extrude dorsolaterally through the annulus fibrosus and occupy the ventrolateral vertebral canal or intervertebral foramen. Compression of the regional spinal nerve can cause pain and pelvic limb lameness. Spaniels, terriers and chondrodystrophoid

A (A) Five-and-a-half-yearold cocker spaniel with back pain and left pelvic limb lameness. Lateral (B), ventrodorsal (C), right laterodorsal–left lateroventral oblique (D) and left laterodorsal–right lateroventral oblique (E) myelograms show left ventrolateral extradural spinal cord compression at L5-L6. The intervertebral disc space is narrow and the disc is mineralised. Extruded nuclear disc material is compressing the L5 spinal nerve and causing the clinical signs

D

B

D

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C

E

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reveal intervertebral disc space narrowing. Spondylosis deformans is common. A ventrodorsal view with the spine flexed may show new bone formation in the region of the intervertebral foramen. Myelography is an extremely insensitive test for assessing lumbosacral disc disease, as are epidurography and discography. MRI is the imaging modality of choice as it enables the detection of L7 spinal nerve compression. Some compressions are dynamic in that they are influenced by the position of the spine, and flexed and extended views may be informative in these cases. Generally, extension exacerbates compression while flexion reduces compression as the intervertebral foramen is enlarged. Lumbosacral magnetic resonance image from an 8·5-year-old Border collie with back pain and left pelvic limb lameness. Dorsolateral protrusion of the L7-S1 intervertebral disc is compressing the left L7 spinal nerve as it exits the intervertebral foramen (arrow). There is left lateral and ventral spondylosis deformans

breeds are most commonly affected. Pelvic limb lameness is often associated with back pain. Spontaneous yelping and arching of the back are typical features. The affected pelvic limb is often held off the ground in a flexed position alongside the abdomen. Survey radiographs of the lumbar spine may reveal intervertebral disc space narrowing. When the nucleus pulposus is mineralised, extruded material may be visible overlying the vertebral canal on a lateral view. Myelography may show extradural spinal cord/cauda equina compression. Oblique views may be necessary to detect ventrolateral extrusions. Myelography cannot assist in the diagnosis of foraminal extrusions – MRI is preferable in these cases. LUMBOSACRAL DISC PROTRUSION The annulus fibrosus may protrude dorsolaterally into the vertebral canal and compress the seventh lumbar spinal nerve as it exits the intervertebral foramen. Chronic pelvic limb lameness may result. Concomitant pathology (eg, spondylosis deformans, articular facet new bone and hypertrophy of joint capsule and interarcuate ligament) may contribute to vertebral canal and foraminal stenosis. The condition is often referred to as degenerative lumbosacral stenosis. Large breeds such as German shepherd dogs are over-represented. Palpation of the lumbosacral spine and extending the lumbosacral spine (lordosis test) may exacerbate back pain. Survey radiographs of the lumbosacral spine may

Neoplasia

Tumours arising from nerve sheaths or the lumbosacral spine may cause progressive pelvic limb lameness. NERVE SHEATH TUMOURS Malignant peripheral nerve sheath tumours may involve spinal nerves, the lumbosacral plexus or peripheral nerves. When they involve spinal nerves within the vertebral canal they can compress the cauda equina and cause neurological deficits. These dogs frequently have back pain. Survey radiographs are often unremarkable. MRI allows detection and assessment of the extent of the tumour within the vertebral canal and also potential involvement of the intervertebral foramen and paraspinal musculature. Nerve sheath tumours involving the lumbosacral plexus and peripheral nerves do not generally cause back pain. Neurological deficits are often only evident when the condition is advanced. Palpation of the pelvic canal per rectum may reveal a mass or thickening of the lumbosacral trunk. Pain is usually exacerbated when these tumours are manipulated. Electromyographic tests may reveal spontaneous electrical activity in affected muscles and reduced nerve conduction velocity. MRI may enable the detection of a mass. LUMBOSACRAL SPINAL NEOPLASIA Extradural tumours (generally sarcomas) and vertebral tumours (eg, osteosarcomas) may cause spinal nerve compression and pelvic limb lameness. Survey radiographs may reveal osteolysis or abnormal new bone

(above) Twelve-year-old lurcher with non-weightbearing right pelvic limb lameness due to a spinal nerve sheath tumour. (right) Dorsal magnetic resonance image showing right L6 spinal nerve enlargement (arrow)

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441

Radiograph showing a vertebral osteosarcoma in an 8·5-year-old mixed breed dog with back pain and left pelvic limb lameness. Note the osteolysis affecting the L5 vertebral body and regional extradural cauda equina compression

Lateral (above) and flexed ventrodorsal (below) radiographs showing extensive vertebral end-plate osteolysis in a 4·5year-old boxer with back pain and left pelvic limb lameness due to lumbosacral discospondylitis

formation. MRI or myelography may be necessary to provide additional information such as evidence of compression of the cauda equina or spinal nerves. Lumbosacral discospondylitis

Infection of the lumbosacral disc and adjacent vertebral endplates may result in compression of the seventh lumbar spinal nerve. This scenario is most likely in advanced cases where marked destruction of the vertebral bodies results in stenosis of the intervertebral foramina. Back pain and pelvic limb lameness can be severe. Survey radiographs show osteolysis of vertebral endplates and irregular new bone formation. MRI enables detection of early discospondylitis prior to radiographic changes, but pelvic limb lameness is not generally a feature in these cases. The infected disc space may be aspirated and material submitted for culture. Blood and urine may also be cultured.

In addition to neurological and orthopaedic conditions, there are many other causes of weakness including cardiovascular and metabolic disorders, such as shock and hypoadrenocorticism.

WEAKNESS AND INCOORDINATION – IS IT NEUROLOGICAL?

Weakness is a common presentation in small animal practice and neurological conditions are some of the most common causes. Diseases of the central nervous system (brain and spinal cord), peripheral nervous system, neuromuscular junction and muscles may all cause weakness and/or incoordination. Spinal cord disorders are the most common and, of these, degenerative intervertebral disc disease (cervical and thoracolumbar) is the most frequently diagnosed. The signalment may be useful as specific conditions are over-represented in certain breeds (eg, thoracolumbar disc disease in dachshunds). Historical features may be suggestive of particular conditions (eg, exercise-induced weakness is typical of myasthenia gravis and some polymyopathies). Spontaneous yelping is characteristic of spinal pain. Neurological examination should be thorough and complete following a general physical examination (see Garosi 2004). Gait analysis, proprioception tests, assessment of segmental spinal and cranial nerve reflexes, and evidence of spinal pain may enable neurolocalisation. Neurological causes of weakness and incoordination can be investigated using radiography, MRI, CT, myelography, cerebrospinal fluid analysis, electrophysiological testing, biopsy and blood tests. 442

WEAKNESS AND INCOORDINATION – IS IT ORTHOPAEDIC?

Orthopaedic conditions may also cause weakness and hence mimic neurological conditions. Furthermore, they may occasionally cause apparent incoordination and thus further mimic neurological disorders. An abnormal increase in flexion or extension of a joint when weightbearing is the main reason orthopaedic conditions cause weakness. This is due to a lack of passive or active joint support. Passive support depends on joints being reduced and stable. It is reduced or absent when joints luxate or ligaments rupture. Active joint support depends on intact, functioning musculotendinous units. Rupture or avulsion of these units reduces or prevents active joint support. Orthopaedic conditions cause apparent incoordination as a result of abnormal limb movement during the gait cycle. Muscle contractures and limb deformities can have profound effects on limb movement. Examination of dogs with orthopaedic causes of weakness and incoordination will often reveal abnormal joint movement, muscle contracture or limb deformity. Neurological examination will generally be unremarkable.

THORACIC LIMB WEAKNESS Glenohumeral (shoulder) luxation may cause apparent weakness, especially if it is developmental in nature and bilateral. Triceps tendon rupture or avulsion, and olecranon fractures may result in an inability to extend the elbow and bear weight. Carpal hyperextension with a palmigrade stance is not uncommon, especially in collie breeds, where the aetiology tends to be degenerative. In Practice

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Traumatic carpal hyperextension can result from a fall. In immature dogs (typically 10 to 12 weeks of age), hyperextension of the carpus can develop as a result of inadequate flexor muscle tone.

THORACIC LIMB INCOORDINATION Contracture of the infraspinatus muscle and, less Degenerative bilateral commonly, the supraspinacarpal hyperextension in a seven-year-old Shetland tus muscle results in a sheepdog with thoracic limb characteristic thoracic limb weakness and lameness posture and gait abnormality. Flexion of the shoulder joint results in external rotation of the humerus and abduction of the distal limb. Circumduction of the limb during the swing phase of the stride and lateral flipping of the paw are typical features. The condition is most frequently recognised in working dogs. There is often a history of sudden onset lameness, presumably associated with injury to the muscle, which improves prior to the development of the muscle contracture.

Bilateral cranial cruciate ligament rupture in a 2·5-year-old golden retriever, which presented with arching of the back and apparent pelvic limb weakness

PELVIC LIMB WEAKNESS Hip dysplasia with (sub)luxation of the coxofemoral joints can cause apparent weakness, especially in immature dogs. Palpation of the hips may reveal instability and positive Ortolani signs. Cranial cruciate ligament rupture results in femorotibial instability and this may cause apparent weakness, especially when bilateral. The cranial drawer test enables diagnosis. Failure of the quadriceps musculotendinous unit may result in a reduction or inability to extend the stifle when weightbearing. Patellar fractures, patellar tendon ruptures, patellar luxations and tibial tuberosity avulsions are possible causes. Similarly, failure of the Achilles mechanism due to muscle or tendon avulsion/rupture, or fracture of the calcaneus, may result in an inability to extend the hock and a plantigrade stance. Degenerative and traumatic intertarsal and tarsometatarsal hyperextension may also result in a plantigrade stance.

Bilateral gastrocnemius tendon avulsion in a nine-yearold golden retriever, which resulted in severe pelvic limb weakness

stifle when the hip is flexed results in the paw being moved caudally just prior to being placed on the ground. Furthermore, the hock tends to displace laterally and the paw medially during the swing phase of the stride. The contracted gracilis muscle may be palpated as a taut band on the caudomedial aspect of the thigh. Growth deformities such as tibial varus and tibial valgus may cause significant alterations in pelvic limb gait that can mimic neurological causes of incoordination. Careful examination reveals evidence of long bone deformity (angular ± rotational). Radiography provides additional information.

PELVIC LIMB INCOORDINATION Gracilis muscle contracture is an uncommon condition primarily recognised in the German shepherd dog. Muscle fibrosis and contracture produce a characteristic alteration in gait. An inability to fully extend the

Infraspinatus contracture in a three-year-old springer spaniel with apparent left thoracic limb incoordination. This was due, in part, to the contracture causing external rotation of the humerus and abduction of the distal limb with flipping of the paw as the shoulder joint flexed

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Bilateral gracilis muscle contracture in a 4·5-yearold German shepherd dog, which presented with apparent pelvic limb incoordination. This was due, in part, to the contracture causing internal rotation of the tibia as the hip flexed and the stifle extended

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Straight patellar tendon rupture in a 4·5-year-old Border terrier with right pelvic limb weakness. Note the increased distance between the patella and the tibial tuberosity

Acknowledgements The author is grateful to Toby Gemmill for his assistance in the final preparation of this paper, Ruth Dennis for magnetic resonance imaging and the veterinary surgeons who referred the cases.

Bilateral proximal tibial valgus deformity in a 16-monthold great dane, which presented with apparent pelvic limb incoordination

SUMMARY

Neurological disease, such as foraminal disc extrusion with spinal nerve compression and nerve sheath neoplasia, can cause lameness and mimic orthopaedic disease. Conversely, orthopaedic disease, such as hip dysplasia and infraspinatus muscle contracture, can cause weakness and incoordination, respectively, and mimic neurological disease. It is important to distinguish between orthopaedic and neurological causes of lameness, weakness and incoordination as the investigative approach and prognosis can be quite different. Differentiation necessitates a detailed history and thorough clinical examination, with emphasis on the neurological and orthopaedic components. These are prerequisites to specific investigations when assessing these often challenging cases.

Further reading BAGLEY, R. S., TUCKER, R. & HARRINGTON, M. L. (1996) Lateral and foraminal disk extrusion in dogs. Compendium on Continuing Education for the Practicing Veterinarian 18, 795-804 BREHM, D. M., VITE, C. H., STEINBERG, H. S., HAVILAND, J. & VAN WINKLE, T. V. (1995) A retrospective study of 51 cases of peripheral nerve sheath tumors in the dog. Journal of the American Animal Hospital Association 31, 349-359 CHAMBERS, J. & HARDIE, E. (1986) Localization and management of sciatic nerve injury due to ischial or acetabular fracture. Journal of the American Animal Hospital Association 22, 539-544 COCKSHUTT, J. R. & SMITH-MAXIE, L. L. (1993) Delayed onset sciatic impairment following triple pelvic osteotomy. Progress in Veterinary Neurology 4, 60-63 FANTON, J., BLASS, C. & WITHROW, S. (1983) Sciatic nerve injury as a complication of intramedullary pin fixation of femoral fractures. Journal of the American Animal Hospital Association 19, 687-694 FELTS, J. F. & PRATA, R. G. (1983) Cervical disc disease in the dog: intraforaminal and lateral extrusions. Journal of the American Animal Hospital Association 19, 755-760 GAROSI, L. (2004) The neurological examination. In Canine and Feline Neurology. Eds S. R. Platt and N. J. Olby. Quedgeley, BSAVA. pp 1-23 GILMORE, D. R. (1987) Lumbosacral discospondylitis in 21 dogs. Journal of the American Animal Hospital Association 23, 57-61 HOULTON, J. E. F. (2006) An approach to the lame dog or cat. In Canine and Feline Musculoskeletal Disorders. Eds J. E. F. Houlton, J. L. Cook, J. F. Innes and S. J. Langley-Hobbs. Quedgeley, BSAVA. pp 1-7 JEFFERY, N. D. (1993) Femoral head and neck excision complicated by ischiatic nerve entrapment in two dogs. Veterinary and Comparative Orthopaedics and Traumatology 6, 215-218 LEWIS, D. D., SHELTON, G. D., PIRAS, A., DEE, J. F., ROBINS, G. M., HERRON, A. J., FRIES, C., GINN, P. E., HULSE, D. A., SIMPSON, D. L. & ALLEN, D. A. (1997) Gracilis or semitendinosus myopathy in 18 dogs. Journal of the American Animal Hospital Association 33, 177-188 McKEE, W. M. (2000) Intervertebral disc disease in the dog 1. Pathophysiology and diagnosis. In Practice 22, 355-369 REINKE, J., MUGHANNAM, A. J. & OWENS, J. M. (1993) Avulsion of the gastrocnemius tendon in 11 dogs. Journal of the American Animal Hospital Association 29, 410-418 VAUGHAN, L. C. (1979) Muscle and tendon injuries in dogs. Journal of Small Animal Practice 20, 711-736

CLARIFICATION Emergency care of the cat with multi-trauma (In Practice, July/August 2007, volume 29, pp 388-396)

The paragraph on ‘Analgesia’ on page 395 stated that meloxicam (Metacam Oral Suspension; Boehringer Ingelheim) had been recently licensed for long-term use in cats at a dose of 0·05 mg/kg orally, once daily, for up to 14 days. Boehringer Ingelheim points out that Metacam Oral Suspension for Cats has been granted a long-term licence in which there is no restriction on the duration of use, thus allowing, where indicated, the product to be continued beyond the 14-day period stated. It says that a loading dose of 0·1 mg/kg should be administered on the first day of treatment to ensure that stable, therapeutic plasma levels are reached quickly, within the 48-hour period stated on the licence. The authors add that Metacam injection is currently licensed as a single subcutaneous injection, at a dose of 0·3 mg/kg, for preoperative use in cats for pain related to minor surgical trauma. It may therefore be used off label for pain related to trauma according to the same dosing regimen; owners should obviously be appraised of the off-label use of this product before treatment. If Metacam therapy is continued orally after this time it should be used at a dose of 0·05 mg/kg. If no improvement is seen in seven to 14 days, the authors suggest that Metacam therapy should be discontinued.

CORRECTION Total intravenous techniques for anaesthesia (In Practice, July/August 2007, volume 29, pp 410-413)

In the table of ‘TIVA protocols’ on page 413, the doses for propofol induction and maintenance in horses were incorrect. The induction dose should have read 2 mg/kg and the maintenance dose 0·1 mg/kg/minute. The author regrets the error. 444

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Total intravenous techniques for anaesthesia Thierry Beths In Practice 2007 29: 410-413

doi: 10.1136/inpract.29.7.410

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