BRITISH JOURNAL OF ANAESTHESIA in CO, PF, MA, and max.dP/dt with no evidence of myocardial depression. The serum osmolality changes were similar to ...
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BRITISH JOURNAL OF ANAESTHESIA
in CO, PF, MA, and max.dP/dt with no evidence of myocardial depression. The serum osmolality changes were similar to those occurring during the bicarbonate infusion. A rise in Paco, of the same order of magnitude as that occurring during bicarbonate infusion was produced by adding CO, to the inspired gas. A depression of myocardial function was always observed. Conversely, tris buffer, which caused a fall in Paco, with a rise in pH, resulted in improvement of function. An injection of acid in the form of NHiCl aq. to simulate the pH change occurring during CO, inhalation caused a fall in PR and LVEDP with large increases in cardiac output, max.dP/ dt, PF and MA. To avoid the myocardial depression seen with rapid intravenous infusion of bicarbonate, the same amount (30 ml) had to be given over at least 2 min Beta blockade with 1 mg/kg practolol prolonged the phase II depression and attenuated the phase III improvement. In conclusion, the haemodynamic response to rapid infusion of bicarbonate is characterized by three phases. The first phase is a Starling effect. Phase II probably results from elevation of Pa«>, and may be avoided by slowing the rate of administration. The third phase is probably due to improved myocardial function, sympathetic stimulation and osmotic effects. This work was supported by a grant from the Medical Research Council. THE ALTERATION OF PULMONARY VASCULAR RESISTANCE INDUCED BY GRADED HYPOXIC STIMULI IN THE ISOLATED PERFUSED CAT LUNG AT NORMAL AND ACID pH TOGETHER WITH ITS MODIFICATION BY HALOTHANE J. M. GlBBS AND M. K. SYKES
Department of Anaesthesia, Royal Postgraduate Medical School, Hammersmith Hospital, London Previous studies with the isolated perfused cat lung have shown that inhalational anaesthetic agents produce changes in pulmonary vascular resistance (PVR) and reduce or abolish the vasoconstriction resulting from alveolar hypoxia (Sykes et al., 1972, 1973). A similar preparation was used for the present study. Ventilation with a control gas mixture of air and 5% carbon dioxide was compared with gas mixtures containing 15, 10 and 5% oxygen with 5% carbon dioxide and nitrogen. Two groups of perfusions were studied, one maintained at pH 7.25-7.30 (the acid group) and the other at pH 7.35-7.40 (the normal group). The control PVR was significantly higher in the acid group but the rise in PVR resulting from the graded hypoxia was greater in the normal group. However, the degree of alteration of response did not reach statistical significance. The effect of halothane 0.5% was to lower the control PVR and to reduce the rise in PVR induced by alveolar hypoxia. Restoration of pH from the acid to the normal range tended to reduce PVR but did not increase the changes in PVR associated with progressive alveolar hypoxia. REFERENCES
Sykes, M. K., Loh, L., Seed, R. F., Kafer, E. R., and Chakrabarti, M. K. (1972). Br. J. Anaesth., 44, 776. Davies, D. M., Chakrabarti, M. K., and Loh, L. (1973). Br. J. Anaesth., 45, 655. PHYSIOLOGICAL STABILIZATION OF HEART RATE AND BLOOD PRESSURE DURING PERIPHERAL NERVE STIMULATION T. G. WHTTWAM, J. NORMAN, AND D. E. FRY
Department of Anaesthetics, Royal Postgraduate Medical School, Hammersmith Hospital, London Stimulation of groups III and IV afferent nerve fibres evokes responses in sympathetic nerves in the dog
(Fussey, Kidd and Whitwam, 1968), and the baroreceptors exert a continuous, fast, inhibitory modulation of th'« evoked activity (Fussey, Kidd and Whitwam, 1973). In anaesthetized or decerebrate animals, intense stimulation of peripheral nerves causes a rise in blood pressure whereas a low intensity applied at low frequencies may evoke depressor responses (e.g., Johansson, 1962; Khayutin, 1966). The present study was designed to determine whedier the cardiovascular changes produced by trains of stimuli applied to cutaneous and muscle nerves correlate with the intensity of stimulation, as indicated by the responses in sympathetic nerves to single stimuli of the same intensity, and also whether these changes are present throughout the period of stimulation. Dogs anaesthetized with thiopentone and chloralose were artificially ventilated. Activity was recorded in renal sympathetic nerves exposed retroperitoneally. Electrical stimuli were applied to the radial nerve and the nerve to flexor digitorum profundus exposed in the foreleg. Femoral arterial pressure, an e.cg. and beat-by-beat heart rate were recorded; in several preparations left ventricular pressure and dP/dt were also recorded. Fast trains of stimuli, supramaximal for activity in cutaneous fibres of groups III and IV caused an increase in heart rate within 1-2 sec. However, as the blood pressure started to rise, the heart rate decreased even to values below control. The blood pressure reached maximum within 5-10 sec and then declined, sometimes to levels below control values, despite continuing peripheral nerve stimulation. With low intensity stimulation at relatively low frequencies there was often an initial fall in blood pressure which returned to control values within 15-30 sec. Sometimes during stimulation, the heart rate and the blood pressure oscillated with a period varying between 8 and 15 sec. The lowest heart rates occurred when the blood pressure was highest Maximal stimulation of muscle nerves, which evokes only a low-level response in sympathetic nerves related to group III fibres, produced the same types of cardiovascular responses. It was concluded that in anaesthetized dogs the baroreceptor reflexes are capable of causing rapid correction of the effects of peripheral nerve stimulation on the cardiovascular system. Hence, no quantitative correlation is to be expected between cardiovascular responses and the intensity of sustained stimulation of afferent nerves. This work was supported by the Medical Research Council. REFERENCES
Fussey, I. F., Kidd, C , and Whitwam, J. G. (1968). J. Physiol. (Lond.), 200, 77. (1973). J. Physiol. (Lond.), 229, 601. Johansson, B. (1962). Acta Physiol, Scand., 57, suppL 198, 91P. Khayutin, V. M. (1966). Acta Physiol. Acad. Sci. Hung., 29, 145.
A GAS FLOW INTEGRATOR WITH CONTINUOUS DRIFT COMPENSATION G. B. DRUMMOND
AND P. G. GOODENOUGH
Departments of Anaesthetics and Respiratory Diseases, University of Edinburgh In many circumstances in respiratory measurement it is convenient to use electrical integration of a gas flow signal from a pneumotachograph to give a volume signal. However, the use of this method is not free from sources of error. In particular, breath-by-breath measurements are hampered by drifting of the volume signal.
PROCEEDINGS OF THE ANAESTHETIC RESEARCH SOCIETY Gradual changes are caused by several factors. These include physical factors such as the actual difference in inspired and expired gas volume; and the differences in water content, composition, and temperature of the inspired and expired gas. Electrical factors such as inaccurate zero of the differential pressure signal from the pneumotachograph, and internal drift of the integrator, also lead to slow changes in volume signal. These factors are in many cases the most evident. Previous workers suggested the use of a passive resistance-capacitance (RC) integrator to reduce drift (Miller and Simmonds, 1960) but the output of such an integrator suffers phase and amplitude distortion according to frequency. An active RC integrator will have an accurate response but will suffer drift. Active RC integrators with automatic balancing have been used : this causes a discontinuity in the volume signal, which may be a disadvantage in some applications. An integrator was needed to accurately integrate the cyclical flow signals of respiration (Le. frequencies above 0.1 Hz) and neglect slow changes in flow signal that would lead to gradual drift of the resultant volume signal (i.e. frequencies below 0.02 Hz). An active RC integrator was used. A five-pole active filter was added as a low pass feedback system, associated with a subtraction stage. The block diagram is shown in figure 1.
r/ >—(£ *—
f
^
i FIG. 1. Block diagram. At and above the normal range of respiratory rates this feedback system acts as an open circuit, and the integrator has been demonstrated to have a linear response at rates above 3 b.p.m. At frequencies below 0.02 Hz the filter acts as a short circuit and the integrator acts as a voltage follower. In this way, low frequency changes in flow are not integrated, and slow changes in volume are not seen. The integrator has been incorporated in a pressurevolume display system for measurement of lung compliance and resistance by a loop-flattening technique (Mead and Whittenberger, 1953; Fryer and Pignatelli, 1966).
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SEQUENTIAL CHANGES IN TETANIC TENSION FADE AFTER THE ADMINISTRATION OF TUBOCURARINE IN MAN N. SUGAI, R. HUGHES AND J. P. PAYNE
Research Department of Anaesthetics, Royal College of Surgeons of England, Lincoln's Inn Fields, London, and St Peter's Hospitals, London Tubocurarine is considered to produce a uniform type of blockade from the onset through the entire course of its action. In this study tetanic tension and its fade (expressed as the tetanic tension ratio) were measured continuously following the intravenous injection of tubocurarine in man. Tetanic tension ratio is defined as the ratio of the height of the tetanic tension at the end of the tetanus compared with its initial peak height (dejong and Freund ; 1967). Tetanic transmission was derived from the ratio of the peak tetanic response to the control peak height and expressed as a percentage. Single twitch contractions were also measured continuously throughout this period and compared with tetanic contractions. The technique for the simultaneous recording of tetanic and single twitch responses of the thumbs has been described previously (Sugai, Hughes and Payne, 1973). Five patients were studied after informed consent had been obtained on the previous day. Anaesthesia was maintained with nitrous oxide and pentazocine in four patients and with nitrous oxide and pethidine in one patient. In each patient one ulnar nerve was stimulated tetanically at 50 Hz for 1 sec every 12 sec and the other ulnar nerve was stimulated by a single pulse of 200 MS every 12 sec. After a control period of approximately 30 min tubocurarine (0.2 mg/kg) was given intravenously. The earliest sign of neuromuscular block was always seen with tetanic contraction and later with the single twitch. Similarly the maximal depression was always greater with tetanic contractions. Tetanic tension ratio was well maintained during the onset of the depression of the tetanus but less well maintained during the recovery phase. The tetanic tension ratios and the percentage of the tetanic transmission were plotted on a graph from which the tetanic tension ratios at 30% and 50% of tetanic transmission were determined. During recovery from tubocurarine, tetanic tension ratios at 30% and 50% of tetanic transmission were always smaller compared with the equivalent values during the onset of tetanic depression. These results indicate that neuromuscular blockade by tubocurarine has different characteristics during its onset and recovery. REFERENCES
dejong, R. H., and Freund, F. G. (1967). Anesthesiology, 28, 583. Sugai, N., Hughes, R., and Payne, J. P. (1973). Br. J. Anaesth., 45, 642.
METABOLISM OF »H AH8165 IN MAN C. E. BLOGG AND B. R. SIMPSON
Anaesthetics Unit, The London Hospital REFERENCES
Fryer, D. I., and Pignatelli, A. B. (1966). J. Physiol. (Lond), 187, IP. Mead, J., and Whittenberger, J. L. (1953). J. Appl. Physiol., S, 779. Miller, J. H., and Simmonds, H. (1960). J. Appl. Physiol, IS, 967.
L. E. MARTIN AND J. A. BELL
Biochemistry Dept., Allen & Hanbttrys Research Ltd AH8165 1,1'Azobis (3-mediyl-2-phenyl-lH-imidazo (1,2a)pyridinium) dibromide, is a rapidly acting, nondepolarizing muscle relaxant which has a short duration of action in small animals (Bolger et aL, 1972), but in man its duration is comparable with pancuronium (Simpson et al., 1972). In-vitro rat experiments demonstrated that
