selective mechanical ventilation of dependent lung ...

Br.J. Anaesth. (1987), 59, 1027-1034

SELECTIVE MECHANICAL VENTILATION OF DEPENDENT LUNG REGIONS IN THE ANAESTHETIZED HORSE IN DORSAL RECUMBENCY G. NYMAN, C. FROSTELL, G. HEDENSTIERNA, B. FUNKQUIST, C. KVART AND H. BLOMQVIST It has been known for more than 20 years that the alveolar-arterial oxygen tension difference is markedly increased in the anaesthetized recumbent horse (Hall and Clarke, 1983; Taylor, 1984). In a study on anaesthetized horses, the position of dorsal recumbency was shown to impede pulmonary gas exchange even more than the lateral position, the arterial oxygen tension (PaOt) during spontaneous breathing of 90-100% oxygen decreasing to a value of 8-10 kPa (Nyman, Funkquist and Kvart, in preparation). However, recumbency appears to explain only partially the impairment in gas exchange (Hall, 1984) and the anaesthetic itself must be held largely responsible for this change. In this paper we present a new technique of ventilating the anaesthetized, recumbent horse, based on selective ventilation, and the application of positive end-expiratory pressure (PEEP) to dependent lung regions. The rationale of using this technique, the procedure, and preliminary results concerning arterial blood-gas tensions are described. THEORY

In the lateral position, the volume of the dependent lung in the anaesthetized horse is markedly reduced (McDonell, 1974) and the ventilation of that lung is impeded and becomes

GOREL NYMAN, D.V.M.; CLAES FROSTELL, MJJ., PH.D.; GGRAN HEDENSTIERNA, M.D., PHJX; BERIT FUNKQUIST, D.V.M., PH.D.; CLARENCE KVART, D.V.M., PH.D.; HANS BLOMQVIST, M J ) . ;

Departments of Surgery and Physiology, Faculty of Veterinary Medicine, Swedish University of Agricultural Sciences, Uppsala; Department of Anaesthesiology, Danderyd Hospital, Danderyd; Department of Clinical Physiology, Huddinge University Hospital, S-141 87 Huddinge, Sweden. Accepted for Publication: January 27, 1987. Correspondence to G.H., Huddinge.

SUMMARY The effects of selective mechanical ventilation of dependent lung regions were studied in anaesthetized horses (mean weight 486 kg) in dorsal recumbency. Blood-gas measurements were performed with the horse in the lateral position during spontaneous breathing (before selective intubation) and in dorsal recumbency during spontaneous breathing, general mechanical ventilation, and spontaneous breathing+selective mechanical ventilation. Arterial oxygen tension (PaOl) was 32.3 kPa in the lateral position during spontaneous breathing with a high inspired oxygen fraction (F/ Ot > 92%). In dorsal recumbency PaOt decreased to 10.9 kPa during spontaneous breathing and was not significantly affected by general mechanical ventilation (Pa Ol 12.6 kPa). The institution of selective mechanical ventilation with a selective positive end-expiratory pressure (PEEP) of 20 cm H%0 caused a marked increase in PaOt to an average of 35.3 kPa. It is concluded that selective intubation of dependent regions in the diaphragmatic lobes is a feasible procedure and that selective mechanical ventilation with PEEP markedly improves arterial oxygenation in the anaesthetized horse in dorsal recumbency.

only one-fifth of the total ventilation (McDonell, 1974). The perfusion of the dependent lung may also be reduced (Staddon and Weaver, 1981; Stolk, 1982), but to a lesser extent than the ventilation (Hall and Clarke, 1983), thus creating a ventilation-perfusion mismatch. In radiographic studies opacities have been seen to develop in the lower lung of anaesthetized laterally recumbent horses (McDonell, Hall and Jeffcott, 1979). Histological examination of rapidly frozen lung

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BRITISH JOURNAL OF ANAESTHESIA

1028 tissue has shown a well expanded upper lung, and atelectasis and large alveolar capillaries in the lower lung (Stolk, 1982). These findings may indicate airway or alveolar collapse, or both, and vascular congestion during anaesthesia in the recumbent horse. In addition, the weight of abdominal organs on the slanting diaphragm in the horse in dorsal recumbency may cause compression of the lower lobe of both lungs (fig. 1).

Dorsal Recwtoency FIG. 1. The horse in dorsal recumbency, illustrating the relation between the lungs, diaphragm and abdominal organs. Note how the slanting diaphragm allows the abdominal contents to encroach on the diaphragmatic lobes of the lung. Redrawn on basis of x-ray studies by McDonell, Hall and Jeffcott (1979). 1 = Heart; 2 = stomach; 3 = dorsal colon; 4 = ventral colon; 5 «= caecum.

The application of general PEEP would be expected to improve pulmonary gas exchange by increasing the lung volume and opening up airways and alveoli. However, general PEEP has been shown to have no beneficial effect on the arterial oxygenation in the anaesthetized spontaneously breathing horse (Beadle, Robinson and Sorenson, 1975; Hall and Trim, 1975). This led Sorenson and Robinson (1980) to conclude that PEEP does not increase the lung volume enough to counter airway closure and alveolar collapse, and that these events are not important in limiting gas exchange. We hypothesize that alveolar collapse and airway closure in dependent lung regions are important contributors to hypoxaemia in the recumbent, anaesthetized horse. We consider this hypothesis to be supported by the severely congested and apparently collapsed lung tissue in the diaphragmatic regions that we have observed in horse lungs excised shortly after death under general anaesthesia in a position of dorsal recumbency (fig. 2). Therefore, we assume that generally applied PEEP of up to 20 cm H,O (as has been used in previous studies) is not enough to open up closed regions. On the other hand, a further increase in overall PEEP may be detrimental, since it has been demonstrated that the

FIG. 2. Photograph of freshly excised lungs from a horse which died under anaesthesia in a position of dorsal recumbency. The dorsal aspect is shown, with the trachea to the right and caudal regions of the diaphragm to the left. Note the severely congested diaphragmatic regions. The cuts through the lung were made to show the congestion in the tissue.


DIFFERENTIAL VENTILATION IN THE HORSE increases in regional volume caused by PEEP are distributed unfavourably and cause mainly a further expansion of already well-expanded lung regions (Hedenstierna et al.,1981). This increases the danger of barotrauma (Kumar et al., 1973) and forces lung blood flow to dependent lung regions, that are poorly ventilated or not ventilated at all, enhancing ventilation-perfusion (V/Q) inequality (West, Dollery and Naimark, 1964). In man PEEP also reduces the cardiac output, presumably by impeding the venous return to the thorax (Colgan and Marocco, 1972). This will further aggravate the hypoxaemia caused by a V/Q mismatch, since the mixed venous blood will be more desaturated at low cardiac output (for a given oxygen consumption). Ideally, ventilation should be distributed in proportion to regional blood flow and with least possible interference to the bloodflow.In man this can be accomplished by placing the subject in the lateral position and ventilating each lung separately by means of a double-lumen bronchial tube. In this way the ventilation of the dependent lung can be increased to match its perfusion (independent or differential ventilation) and, by applying PEEP solely to that lung, closed and collapsed regions can be opened up to improve the distribution of ventilation within the lung (selective PEEP) without any overdistension of non-dependent lung regions. This technique has proved successful both in healthy subjects during anaesthesia and in patients in the intensive care environment (Baehrendtz and Hedenstierna, 1984; Hedenstierna et al., 1984). We have adopted the basic principles of this concept for a technique suited to the horse. Since the impairment of gas exchange appears to be much more marked in a position of dorsal recumbency than in the lateral position, we have focused our interest on developing or modifying the technique for that position.

MATERIALS AND METHODS

Technique for selective ventilation and PEEP A technique for the selective administration of ventilation and PEEP was developed and tested in adult standard-bred trotter horses anaesthetized as described below. The animal ethics committee approved the study. Material and anaesthesia. Five horses were investigated: two geldings and three mares

1029 TABLE I. Details of hones studied

No. 1 2 3 4 5 Mean

SEM

Breed Standard-bred Standard-bred Standard-bred Standard-bred Standard-bred

trotter trotter trotter trotter trotter

Sex

Age (yr)

Weight (kg)

Gelding Mare Mare Gelding Mare

11 15 4 7 3

530 525 562 440 405

8.0 ±2.2

492 ±30

weighing between 405 and 532 kg. Four of these were studied during generally applied and selective mechanical ventilation (Nos. 1,2,4,5 in table I). Individual data are given in table I. The horses were considered healthy at clinical examination, including inspection of the airways with a fibreoptic bronchoscope (CF-LB 3, Olympus). The horses did not receive any food for 12 h before anaesthesia, but had free access to water and straw bedding. They were premedicated with acepromazine 0.05 mg kg"1 i.m. 1 h before induction of anaesthesia. A centrally acting muscle relaxant, 5 % glyceryl guaiacolate was given by i.v. infusion until the horse became unstable and could be pulled down onto a mattress by means of ropes around the neck and chest. Anaesthesia was then induced with 12.5% thiopentone i.v. (0.4 g/100 kg) and the trachea intubated with a cuffed tracheal tube (Rusch, i.d. 20 mm). Anaesthesia was maintained with halothane in oxygen in a large animal circle type rebreathing system with a carbon dioxide absorber and a halothane vaporizer (Fluotec Mark 3). The fresh gas flow (oxygen) was 5 litre min"1. The halothane concentration was adjusted to maintain the level of anaesthesia at stage III, plane 1-2 (Guedel, 1937; Campbell and Lawson, 1958). During part of the study the horse was mechanically ventilated using a large animal ventilator based on the bag-in-box principle (Smith, 1971). The ventilator was set to deliver a tidal volume of oxygen 7-9 litre at a rate of 6 b.p.m. The maximum inspiratory pressure during general mechanical ventilation was 30 cm H,O. Technique. (Anatomical nomenclature according to Hare (1975).) A tracheotomy was performed in the middle of the neck, just beyond the tip of the tracheal tube. Two tracheal tubes for human use (Portex No. 9 with inflatable high volume cuffs), connected to 1.0-m long catheters, were guided through the tracheostomy to a dependent region


1030 of each diaphragmatic lobe, using a fibreoptic bronchoscope (Olympus CF-LB 3). The tracheal wound was temporarily made airtight around the tubes by means of Ardent Alginate (Class B Type II, Dental Medico). The two catheters were connected to a clinical ventilator (UV 705, AGA) by a Y-piece. The expiratory port of the ventilator was connected to tubing which ended in a water trap which was adjusted to create a PEEP of 20 cm H,O. The maintenance of a stable value of PEEP was considered proof of an airtight seal between the catheters and the bronchi. The tidal volume was set at oxygen 1.5 litre at a rate of 10 b.p.m. and the maximum inspiratory pressure was approximately 40 cm HjO. The horse was breathing spontaneously through the regular tracheal tube, ventilating the remaining, major part of the lung tissue, simultaneously with the mechanical ventilation of the dependent lung regions (fig. 3). However, there was no synchronization between the spontaneous breathing and the ventilator. Inspiratory gas samples were drawn intermittently from the large animal circle during spontaneous breathing and general mechanical ventilation, and from the open circuit (UV 705, AGA) during selective ventilation. Subsequent mass spectrometer analysis showed that the inspired oxygen fractions always exceeded 92 %. Positioning of the bronchial catheters. In all horses

BRITISH JOURNAL OF ANAESTHESIA horses and through a catheter placed in the metatarsal artery during recumbency. P a ^ carbon dioxide tension (Pa^,) and pH were measured, standard techniques being used (ABL3 Radiometer, Copenhagen, Denmark). The bloodgas data were corrected to the body temperature of the horse.

PEEP

FIG. 3. Schematic drawing illustrating selective intubation of the dependent regions of the diaphragmatic lobes. The tip* of the tubes were advanced to a position approximately 10 cm distal to the carina and lay just above the branching of the diaphragmatic lower bronchus into the lateral basal and the basal segmental bronchi. Vent. = ventilator.

the positioning of the catheters was accomplished smoothly and no difficulties were encountered in reaching the dependent regions of the lower lobes. In choosing appropriate sites for the bronchial catheters, our own observations in a dissected animal which had died in a position of dorsal recumbency were taken into consideration (fig. 2). The tips of the tubes were advanced approximately 10 cm past the tracheal bifurcation into the diaphragmatic lobar bronchus. The cuffs of the tubes were inflated immediately before the branching off the lateral basal segmental bronchus. This allowed selective mechanical ventilation of the lateral basal and dorsal basal segments of the lower lobes (fig. 4). Gas exchange study

Gas exchange was studied by means of arterial blood-gas measurements in the five horses while standing and during anaesthesia as described above. Blood-gas tensions. Arterial blood samples were taken by puncture of the carotid artery in standing

FIG. 4. Bronchial anatomy of the horse, and the positioning of the endobronchial catheters in the diaphragmatic lobar bronchus. (Redrawn from Hare (1975).) 1 = Apical lobar bronchus; 2 = accessory lobar bronchus; 3 = middle segmental bronchus; 4 = ventral basal segmental bronchus; 5 •=• lateral basal segmental bronchus; 6 = dorsal basal segmental bronchus. Nos 3-6 are branches of the diaphragmatic lobar bronchus.


DIFFERENTIAL VENTILATION IN THE HORSE

1031

Procedure. After blood sampling in the stunning, 60unsedated horse, anaesthesia was induced. The trachea was intubated and connected to the 50anaesthetic breathing system. Further blood samples were drawn while the horse was in the lateral recumbent position, breathing spon40 taneously for 30-60 min. The horse was then turned to a position of dorsal recumbency, its posture being supported by wings on a specially £30 designed surgical table. After another 30-60 min of spontaneous breathing, blood was again 20collected and blood-gas analysis undertaken, and general mechanical ventilation was instituted with a moderate hyperventilation with a tidal volume 10adjusted to keep the P a ^ , at 5-6 kPa. This was followed by further blood sampling after a similar Dorsal Awake Lat. period of time. To initiate spontaneous breathing, SB SV MV the frequency of ventilation was reduced in order to increase Pacor After the return of spontaneous FIG. 5. Arterial oxygen tension (mean ± SEM) in the awake, breathing, tracheotomy was performed and the standing horse, (Fi^O.21), and during anaesthesia in the catheters were introduced, one to each dependent lateral and dorsal recumbent positions (Fi Oi > 0.92). SB = region of the diaphragmatic lobe. While the horse Spontaneous breathing; MV = general mechanical ventiwas breathing spontaneously, selective mechanical lation; SV = selective mechanical ventilation with PEEP 20 cm H,O. *Significantly different from the awake value ventilation of and PEEP to the lower lung regions (P < 0.05); tsignificantly different from the previous value was commenced and after 30-60 min blood (P < 0.05). samples were drawn. In three horses blood-gas measurements were performed in dorsal recumbency after 15-30 min of selective ventilation of reduced (table II). With the horse in dorsal one lung only, followed by selective ventilation of recumbency, still with spontaneous ventilation, both lungs as described above. Pao, decreased markedly—to an average of ECG, arterial pressure, heart rate and fre- 11 kPa. This was significantly less than the awake quency of breathing were monitored continuously value with air breathing. Pac Ol remained increased throughout the study. Three horses were sacri- and pH remained reduced. The commencement ficed by a large injection of barbiturate at the end of general mechanical ventilation in dorsal reof the study; the other two made an uneventful cumbency caused no significant change in P a ^ . Pa COt was decreased and no longer differed from recovery from anaesthesia. Statistics. The mean value and SEM were the awake value. pH was significantly increased calculated for each variable under each condition. and also reached the awake value. The Wilcoxon matched pairs signed rank test was When selective mechanical ventilation with used for testing the significance of a difference PEEP 20 cm H,O was added to spontaneous between different conditions. breathing, with the animal in dorsal recumbency, Pao t increased markedly, reaching a value 3-3.5 times greater than during spontaneous or general RESULTS mechanical ventilation in the same position (fig. In the standing horse, breathing air, Pao, w a s 5). (The inspired oxygen tension was essentially normal and exceeded 13 kPa in all animals (fig. 5, the same as during the previous measurements table II). Pa COt and pH also lay within the normal during anaesthesia.) Pac Ol and pH did not change to any significant extent (table II). range (table II). During anaesthesia in the lateral position, with Selective mechanical ventilation of dependent the horse breathing spontaneously, Pa,}, averaged regions in only one lung, with PEEP (n = 3), 32 kPa (fig. 5). This should be compared with an increased P a ^ from a mean of 11.9 kPa (sponinspired oxygen tension of approximately 90 kPa. taneous breathing) to 21.8 kPa, and bilateral Pa COt was significantly increased and pH was selective ventilation with PEEP increased P a ^ to


BRITISH JOURNAL OF ANAESTHESIA

1032

TABLE II. Arterial blood-gas tensions (kPa) (mean ± SEM) and pH in atcake and anaesthetized horses. For abbreviations, see text. Significant differences: *from the awake value; \Jrom the above value (P < 0.05) n Awake (air) Standing, spontaneous breathing

^co.

PH

5

13.6±0.2

6.4 ±0.5

7.36 ±0.03

5

32.3±8.2*

8.4±0.5*

7.25±0.01*

Dorsal recumbency Spontaneous breathing

5

10.9±0.9*f

8.0 ±0.9*

7.25 ±0.04*

Dorsal recumbency General mechanical ventilation

4

12.6±3.9

5.7±0.4t

7.31±0.02f

Dorsal recumbency Selective mechanical ventilation

4

35.3±6.6*f

7.1±0.1

7.23 ±0.05

Anaesthetized ( > 92 % oxygen) Left lateral recumbency Spontaneous breathing

an average of 36.4 kPa. Thus unilateral selective ventilation had only a partial effect on compared with bilateral selective ventilation. DISCUSSION

This study has shown that the great difficulties in oxygenating the anaesthetized horse in a dorsal recumbent position can be largely overcome by selective ventilation of, and application of PEEP to, the most dependent lung regions. Although only tested in preliminary studies this ventilatory technique appears promising and may be of clinical importance. Possible mechanisms underlying the improvement in pulmonary gas exchange, and comparisons with previous experience in man, are discussed below. Distribution of ventilation

The marked improvement in arterial oxygenation that occurred on inflation and ventilation of the dependent lung regions strongly suggests that these regions had been collapsed or that the airways had been occluded, before selective ventilation. The morphological appearance of lung regions corresponding to the diaphragmatic dependent regions, with airlessness and congestion (fig. 2), is compatible with alveolar collapse rather than airway closure. In man, prompt development of atelectasis in dependent lung regions on induction of anaesthesia has been demonstrated (Brismar et al., 1985), as well as impairment of arterial oxygenation, which does not necessarily progress with time (Nunn, Bergman and Coleman, 1965). It has been suggested that the atelectasis may be caused by loss of

supporting forces (compression atelectasis) rather than by resorption of gas from occluded lung units (Brismar et al., 1985; Strandberg et al., 1986). In a recent study in anaesthetized horses, Nyman, Funkquist and Kvart (in preparation) found that the impairment in pulmonary gas exchange could progress for more than 1 h in the dorsally recumbent horse. This time difference, as compared with man, may suggest additional or alternative explanations for the impairment in gas exchange. One possibility is resorption atelectasis; another is increasing vascular congestion. These mechanisms may aggravate a gas exchange impairment, initially caused by compression of lung tissue. The ventilator used in the present studies had been constructed for use in anaesthetized, essentially healthy humans and was not able to maintain any greater degree of ventilation. This was at most 10-15 litre min"1 and can be expected to be 25 % of the total ventilation, the remainder being produced by spontaneous breathing. This may have resulted in a ventilation per unit lung volume in dependent lung regions that was less than the average. However, inflation of dependent lung units with oxygen without any ventilatory movements will create effective oxygenation of the pulmonary capillary blood for a number of hours, the major problem being inefficient elimination of carbon dioxide (cf. the studies on apnoeic oxygenation by Holmdahl (1956)). However, since a major part of the total ventilation was created by spontaneous breathing, Paco, was determined by the sensitivity to carbon dioxide of the respiratory centre and not by the efficiency of the selective ventilation.


DIFFERENTIAL VENTILATION IN THE HORSE Distribution of perfusion

Since the impaired arterial oxygenation in the anaesthetized horse was observed during the breathing of almost 100% oxygen, contributions by diffusion limitation and ventilation-perfusion mismatching can be considered negligible. The poor oxygenation of blood must therefore have been caused by a large shunt, that is, a blood flow not coming into contact with ventilated lung tissue. This would fit with the idea of maintained blood flow in dependent, collapsed and thus non-ventilated lung regions. In studies on anaesthetized humans, shunts as large as 15-17 % of the cardiac output have been observed, together with atelectasis no greater than 6-8 % of the transverse intrathoracic areas, as measured by computerized tomography (Hedenstierna et al., 1986). The explanation for this may be that gravitational forces create a larger blood flow in the lowermost lung regions than in upper units (West, Dollery and Naimark, 1964). By this means, even small regions of occluded or collapsed dependent lung tissue may cause a considerable shunt. The vertical blood flow distribution in the horse lung may differ (Stolk, 1982), but as yet no detailed topographical analysis has been made. Another question is whether the regional application of selective ventilation with PEEP forces blood flow away from this region. In anaesthetized, healthy humans placed in a lateral position, the dependent lung received approximately 60 % of the total lung blood flow during mechanical ventilation with no end-expiratory pressure, and this decreased to 50 % with selective PEEP of 10 cm HSO to that lung (Hedenstierna et al., 1984). To what extent this result is applicable to the horse remains to be shown. However, it can be concluded that either mechanism—recruitment of collapsed lung tissue or redistribution of blood flow—should result in reduced shunting.

CONCLUSION

This study has shown that dependent regions of the diaphragmatic lobes of the lung can easily be intubated and ventilated by means of bronchial catheters introduced through a tracheostomy, and that selective ventilation of dependent lung regions, with PEEP, results in a more than three-fold increase in arterial oxygenation from a dangerously low to a high Paot- Th e ventilation technique described in this paper may prove to be

1033

one of choice for prolonged surgery in the anaesthetized, dorsally recumbent horse. ACKNOWLEDGEMENTS This study was supported by grants from the Swedish University of Agricultural Sciences, the Swedish Medical Research Council (project no. 4X-5315) and the Karolinska Institute. The skilful assistance of Eva-Maria Hedin, Ann-Marie Lofgren and Pia Funkquist during the studies is greatly appreciated.

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1034 McDonell, W. N., Hall, L. W., and Jeffcott, L. B. (1979). Radiographic evidence of impaired pulmonary function in laterally recumbent anaesthetized horses. Equine Vet.J., 11, 24. Nunn, J. F., Bergman, N. A., and Coleman, A. J. (1965). Factors influencing the arterial oxygen tension during anaesthesia with artificial ventilation. Br. J. Anatsth., 37, 898. Smith, M. (1971). A respirator for large gnimal« Nord. Vet. Med., 23, 537. Sorenson, P. R., and Robinson, N. E. (1980). Postural effects on lung volumes and asynchronous ventilation in anesthetized horses. J. Appl. Physiol., 48, 97. Staddon, G. E., and Weaver, B. M. Q. (1981). Regional pulmonary perfusion in horses: a comparison between

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