SOME RESPIRATORY EFFECTS OF THE TRENDELENBURG POSITION DURING ANAESTHESIA

Brit. J. Anaesth. (1966), 38, 174 SOME RESPIRATORY EFFECTS OF THE TRENDELENBURG POSITION1 DURING ANAESTHESIA ./' 'U1 BY

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D. B. SCOTT, M. M. LEES AND S. H. TAYLOR

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Departments of Anaesthetics, Obstetrics and Gynaecology, and Medicpi&j < Royal Infirmary, Edinburgh • :j.> . , ( , SUMMARY

Arterial blood gas tensions and pH were studied serially before, during and1 'alter tilting into a steep Trendelenburg position in six young patients under li)ght general anaesthesia. The minute volumes and respiratory rates were also recorded' dilfWfe anaesthesia. No significant changes in any of the parameters measured' 'ciduH be attributed to the Trendelenburg position. ' ' "

It is generally accepted that the Trendelenburg position causes interference with respiratory movement during anaesthesia as a result of pressure of the abdominal viscera upon the diaphragm (Inglis and Brooke, 1956; Swain, 1960). Various reports have demonstrated that in conscious patients, the lung volume is decreased in a steep head-down tilt Altschule and Zamcheck (1942) found a reduction of 17 per cent in functional residual capacity, but no consistent alteration in tidal volume, respiratory rate, minute volume or oxygen consumption. Case and Stiles (1946) demonstrated a decrease of 7 per cent in vital capacity compared with the supine position. Investigations of this problem under anaesthesia however, have tended to be scanty. Jones and Jacoby (1954) studied patients partially curarized, but breathing spontaneously, and claimed to have shown a reduction of 12 per cent tidal volume when the Trendelenburg position was assumed. However, little data is given in their paper on the number of cases studied, the values obtained from each case, or the time for which the position was maintained. Furthermore, the use of tubocurarine in this way introduces an uncontrollable factor which may seriously affect the results obtained. Wood-Smith, Home and Nunn (1961) measured the tidal volumes of anaesthetized patients who were put into several different positions, and found that a steep but transient Trendelenburg tilt caused a reduction of 14 per cent. The tilt was, however, only maintained for very brief periods, of the order of 90 seconds; the longer term effects of this posture were not studied.

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In view of the importance of'a'ssessin'j*' the effect on respiratory function of this cornrhohly used posture, a study was designed to' urvtSftfgate the effects of the Trendelenburg position during anaesthesia in patients undergoing'minor gynaecological surgery. ' ' '' METHODS

Six patients undergoing minor; gynaecological surgery were studied. They weft aged b&ween 21 and 32 years and showed no cOrtical evidence of cardio-respiratory disease. An 'arterial 'catheter (o.d. 0.8 mm) was inserted percutaneoiisly into the brachial artery by the Seldinger method.' After samples were taken for control values, premedication, consisting of papaveretum 20 mg and atropine 0.6 mg, was given intramuscularly. After 1 hour, further samples were taken and general' anaesthesia then induced. Induction with thiopentone 400 mg was followed by nitrous oxide' and oxygen (6 and 2 l./min respectively) to which halothane from a Fluotec vaporizer outside the- circuit was added in a concentration of 4 p$r cent for 2 minutes and 1 per cent thereafter. Breathing was spontaneous throughout, carbon dioxide being eliminated by the use of a circle absorber (Boyle Mk II). Endotrachcal intubation was not performed, an anaesthetic facepiece being carefully fitted to avoid any leakage of gas. At the completion of the operative procedure which usually lasted approximately 10 minutes, the patient was placed in a supine horizontal posture. By this time, the respiration had become regular and stabilized, and the ventilatory minute

SOME RESPIRATORY EFFECTS OF THE TRENDELENBURG POSITION volume and the respiratory rate were then recorded continously until the end of the anaesthetic administration. A Wright Respirometer inserted between the anaesthetic facepiece and the expiratory valve was used for this purpose. After 5 minutes in the horizontal position, the patient was tilted into a steep Trendelenburg position (35° from the horizontal), which was maintained for 10 minutes, and then followed by a further 5 minutes in the horizontal position. Anaesthesia was then discontinued. Arterial blood samples were taken after the preliminary 5 minute period in the horizontal position, at 5 and 10 minutes when head-down, and again 5 minutes after returning to horizontal. Specimens were also taken at 15, 30, 60, 120 and 180 minutes after completion of the anaesthesia. The arterial blood samples were immediately analyzed for oxygen tension and carbon dioxide tension, pH, blood oxygen saturation, and oxygen

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carrying capacity. Oxygen tension and carbon dioxide tension was measured by means of a Clark polarographic cell and a Severinghaus electrode respectively. Arterial blood pH was determined by means of a capillary electrode system (Electronic Instruments Ltd., Wynne type SHW33). The percentage blood oxygen saturation was measured by haemoreflection with a Brinkman instrument standardized against the Van Slyke manometric method. Blood oxygen capacity was measured by a standard photometric technique on a Unicam SP600 which was also calibrated against the Van Slyke method. . RESULTS

The results are shown in table I and illustrated graphically in figure 1. Premedication was accompanied by small but not statistically significant changes in arterial oxygen tension (Paoa) and pH.

TABLE I

Systemic arterial blood gas tensions, pH and oxygen saturations before, during and after general anaesthesia. Carbon Oxygen dioxide Oxygen Minute Respiratory Time tension tension saturation volume rate State (min) (mm Hg) (mm Hg) PH fl./min) (b.pjn.) (%) — Control 95 + 1.16 41.0 ±1.67 95.3 ±0.98 7.40 + 0.012 (3.0) (4.1) (0.03) (2.4) Premedication 60 100±1.14 41.2±1.51 7.37 ±0.008 96.2 ±0.86 (2.8) (3.7) (0.021 (2.1) Anaesthesia Flat 15 108 ±1.18 56.2 ±1.84 7.28+0.008 95.0 ±0.78 6.0 18.5 (0.02T (2.9) (4.5) (1.9) Tilted 20 104 ±0.90 59.9 ±1.96 7.26 ±0.020 94.5 ±0.69 5.8 18.0 (0.05J (2.2) (4.8) (1.7) 62.5 ±2.14 25 107 ±1.35 7.23 + 0.020 94.5 ±0.73 5.6 17.5 (5.251 (3.3) (0.05)" (1.8) Flat 30 109 ±0.94 62.0 ±2.90 7.29 + 0.012 94.2 ±0.90 5.8 17.5 (2.3) (7.1) (0.03)" (2.2) . Postoperative 90.0 ±0.94 15 51.2 ±3.06 7.31+0.008 93.5 ±1.26 period (2.3) (7.5) (0.02)" (3.1) 7.33+0.016 30 92.0 ±0.94 48.8 ±2.98 93.0 ±1.16 — — (0.04T (2.3) (7.3) (3.0) 60 97.0 ±0.82 — — 46.2 ±3.02 94.1 ±0.49 7.38 ±0.020 (0.05T (2.0) (7.4) (1.2) _ _ 120 95.0 ±1.16 47.5 ±2.53 94.0 ±0.90 7.36 ±0.016 (3.0) (6.2) (0.04T (2.2) 180 98.0 ±0.82 46.2 ±2.53 7.37 ±0.016 93.9 ±0.69 — — (2.0) (6.2) (0.047 (1.7) Values are expressed as the mean ± standard error of the mean, with the standard deviation of the observations in parentheses. The minute volumes and respiratory rates during anaesthesia are also given, each value being the mean of a 5-minute observation period.

BRITISH JOURNAL OF ANAESTHESIA

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POST OPERATIVE PERIOD

PbOj

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SaO2 O;

MINUTE VOLUME litres RESPIRATORY RATE/mln

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MINUTES

1 Systemic arterial blood gas tensions, pH and oxygen saturation in six healthy subjects before, during and after anaesthesia. Mean value shown with two standard deviations for all six subjects at each period. The means of the minute volumes and respiratory rates during anaesthesia are also shown. FIG.

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SOME RESPIRATORY EFFECTS OF THE TRENDELENBURG POSITION The first arterial specimens taken 15 minutes after induction of anaesthesia showed an increase in PaO2 to a mean of 108 mm Hg. This was accompanied by a rise in arterial carbon dioxide tension (Pacoj) to 55 mm Hg, and a corresponding fall in arterial blood pH. No statistically significant changes occurred in blood gases when the patient was tilted, either into the Trendelenburg position or back to the horizontal. Changes in minute volume and respiratory rate were likewise unremarkable. There was a slight fall in Pa02 after five minutes of steep head-down tilt, but this had returned to its previous figure after 10 minutes. Postoperatively, there was a sharp fall in Pa^, the mean value 15 minutes after discontinuing the anaesthesia being 90 mm Hg; this had risen to the pre-operative level 60 minutes later. At the end of the anaesthesia, the PaOo3 fell more slowly than the Pa0:1 and was slightly above control levels throughout the remainder of the period of observation. DISCUSSION

Contrary to expectation, the Trendelenburg position had little or no effect upon overall respiratory function as measured by the changes in arterial blood gas tensions or minute ventilation in young subjects under light anaesthesia. Theoretically, this posture can affect respiration in three ways. First, the weight of the abdominal viscera may impair diaphragmatic movement. Second, by raising the diaphragm the lung volume may be reduced. Third, gravity may alter the pulmonary circulation. However, the weight of the abdominal viscera acting at an angle of 35° from the horizontal would not offer serious impediment to the movement of the respiratory muscles with the patient at rest and a tidal volume below 1 litre. It seems unlikely that these muscles, which are capable of moving very much larger volumes of air during exercise, would be incapable of the small extra effort required. This may well explain why studies carried out in conscious patients show so little change in ventilation (Altschule and Zamcheck, 1942; Case and Stiles, 1946). During anaesthesia, the function of the respiratory muscles may be impaired, particularly if relaxant drugs have been given, and this may account for the reduction in ventilation observed by Jones and Jacoby (1954). In practice,

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the use of relaxants frequently requires artificial assistance to the respiration regardless of position. Our patients did not receive relaxant drugs, but all had some degree of respiratory depression during anaesthesia, as indicated by the raised PaCOa and the low minute volumes. On clinical grounds, however, none were considered to be in need of artificial ventilation, and, as the results showed, all were able to adjust to any increased demand on their respiratory mechanism consequent upon tilting into the head-down position. A reduction in lung volume that may occur by upward displacement of the diaphragm might lead to a situation similar to that studied by Nunn and colleagues (1965). These authors showed that conscious subjects with a voluntarily reduced lung volume and normal ventilation showed evidence of hypoxaemia. The patients in the present study did not apparently suffer sufficient reduction in lung volume to demonstrate this effect. Gravity plays a considerable pan in the circulation of blood through the lungs. In the erect posture, the apical zones are poorly perfused (West, 1963), and presumably die reverse may happen in a steep head-down tilt The effect of reduction in the volume of the lower lobes that may occur in the Trendelenburg position would, however, be offset by the concomitant decrease in perfusion. The moderate degree of hypoxaemia seen 15 minutes after cessation of the anaesthesia was almost certainly due to hypoventilation as shown by the elevated Paco3. As in a previous study (Taylor, Scott and Donald, 1964), hypoxaemia without hypercapnoea was not seen. It must be emphasized that the present investigation concerned healthy patients undergoing minor surgery and short periods of anaesthesia. The effects of prolonged head-down tilt have not been determined. Moreover, it is possible that in patients with severe respiratory disease, especially when the minute volume approaches the vital capacity, the Trendelenburg position may prove a serious additional burden to spontaneous respiration. ACKNOWLEDGEMENTS

The authors are indebted to Professor K. W. Donald for allowing us the facilities for the blood gas measurements and to Professor R. J. Kellar for allowing us to study his patients. We are also pleased to record our gratitude to Dr. K. B. Slawson and Miss M. Forsyth for technical assistance. The investigation