Young, 1953). The decline in the hyperpnoea may be due to hypocapnia. To

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1 365 J. Physiol. (I957) I37, THE EFFECT OF CARBON DIOXIDE ON THE RESPIRATORY RESPONSE TO NORADRENALINE IN MAN BY H. BARCROFT, V. BASNAYAKE,. CELANDER, A. F. COBBOLD, D. J. C. CUNNINGHAM, M. G. M. JUKES, AND I. MAUREEN YOUNG From the Sherrington School ofphysiology, St Thomas's Hospital, London, S.E. 1, and the Laboratory of Physiology, University of Oxford (Received 1 March 1957) A continuous intravenous infusion of either adrenaline or noradrenaline in man causes only a transient stimulation of respiration which is accompanied by a reduction in the alveolar carbon dioxide pressure (PCO2) (Whelan & Young, 1953). The decline in the hyperpnoea may be due to hypocapnia. To test this hypothesis the effect of infusions of noradrenaline on respiration has now been studied during the breathing of air containing 2-5% C2 so that the alveolar pco2 would remain high and any reduction of it which accompanied the hyperpnoea would be small. Noradrenaline was used rather than adrenaline because as a rule it gives rise to no subjective symptoms: adrenaline, on the other hand, almost always causes palpitation and 'tightness' in the chest which affect respiration to an unknown extent. METHODS To minimize psychogenic effects the infusion machine was behind a screen and the recording apparatus was out of the sight of the subject. The subject allowed us to mislead him as to the aim of the experiment; he did not know that he was going to receive noradrenaline. He was told that interest was centred on the effect of different partial pressures of C in the blood upon the urinary excretion of intravenously administered sodium chloride. Emphasis was therefore laid on the urine and the respiratory apparatus was accepted on the grounds that it was necessary for the control of alveolar PCO. The experiment was done in the morning; the subject took no food or drink after 1 p.m. on the previous day. Room temperature was 19-22o C. The first sample of urine was taken as soon as he arrived. Tubing for the stethograph was tied round his chest and abdomen and he settled himself comfortably in a deck chair. The intravenous needle was inserted near the elbow and infusion of saline was begun. The mouthpiece and noseclip were fitted and the subject breathed one of the C2-air mixtures for at least 15 min. Continuous recording of respiratory minute volume and of end-tidal pco was started. The following routine was then carried out: -15 min, control period; Haldane & Priestley end-expiration alveolar air samples collected at 7 and 12 min;

2 366 H. BARCROFT AND OTHERS 15-3 min, infusion of noradrenaline at 1,ug/min; Haldane & Priestley alveolar air samples at 23 and 28 min; 3-45 min, recovery period. The mouthpiece and noseclip were then removed. The subject was casually asked how he felt; his reply was noted and a second sample of urine was obtained. After a rest of about 2 min the experiment was resumed, the subject breathing another C2-air mixture. The routine was the same as before. Afterwards a third sample of urine was obtained (all urine samples were discarded later). The subject usually returned on another day and the effect of noradrenaline on the responses to two other C2-air mixtures was tested. At the end of this experiment he was asked if he had guessed that he had been given noradrenaline. Apparatu8. The apparatus for supplying the inspired gas mixtures and for recording respiratory frequency and pulmonary ventilation was similar to that described by Cormack, Cunningham, Jukes, Lloyd & O'Riordan (1957) with a few modifications. (1) The inspired gas mixtures of 2,4,4-5 and 5% C2in air were not warmed or humidified; (2) a standard Siebe-Gorman respiratory valve casing was used in which the rubber flaps were replaced by spring-loaded 'Perspex' diaphragms to reduce resistance to flow; (3) an infra-red analyser recorded the C2 percentage in gas sucked continuously from between the subject's teeth; the alveolar pco2 could be assessed from the end-expiration plateaus recorded (Dornhorst, Semple & Young, 1953). This record was used as a guide to the progress of the experiment and as a check on the Haldane & Priestley method of sampling. A fraction of each Haldane & Priestley sample was passed through the analyser and another portion was collected in a gas sampling tube for volumetric analysis later. Exact timing of the samples was the responsibility of the subject but their adequacy was checked by the infra-red analyser as described above. The apparatus for intravenous infusion (Duff, 1952) delivered 3 ml. of saline per minute throughout the experiment. The saline contained ascorbic acid to prevent oxidation of noradrenaline (Gaddum, Peart & Vogt, 1949). During the periods in which noradrenaline was infused, L-noradrenaline (Levophed: Bayer Products, Ltd.) was added to the saline so that it entered the body at the rate of 1,lg/min. The Levophed ampoules also contained a preservative and to ensure that the preservative was not itself a respiratory stimulant two types of control experiment were carried out. In the first, the diluted preservative alone was given throughout the control and recovery periods, and during the experimental period noradrenaline itself was the only addition to the infusion. In the second, the diluted preservative was infused for one hour to see whether it had any cumulative effect on respiration while the subjects breathed 4.5% CO2. The results were negative in both these control experiments. The concentration of lactate in the venous blood was estimated in preliminary experiments by Hullin & Noble's (1953) modification of Barker & Summerson's method (1941). Three samples were taken before, five during and three after each noradrenaline infusion. No significant change in lactate occurred either during or after noradrenaline administered while the subject was breathing 2-5% CO2. RESULTS The effect of noradrenaline on pulmonary ventilation and on alveolar pco2 was examined fourteen times on five experienced subjects. Symptoms. Most subjects had mild headaches throughout the experiments, especially when the inspired air contained % CO2. In five infusions the subjects felt symptoms which they suspected were due to receiving a sympathomimetic substance. The results of these experiments were discarded. Respiratory rate. In two subjects noradrenaline did not affect the respiratory frequency. In the other three there was a small maintained increase. In them

3 C2, NORADRENALINE AND RESPIRATION 367 noradrenaline increased the rate from 1-16 to breaths/min when they breathed 2% C2 and from 18-2 to with 5% C2. The hyperpnoea started about 1 min after the beginning of the noradrenaline infusion and reached a plateau in 2-3 min. Fig. 1. Minutes The effect of CO. on the respiratory response to noradrenaline. Typical results obtained in one subject:, 2% CO2; 4%; A, 4-5%; *, 5%. Pulmonary ventilation. The results of a typical series of experiments on one subject are shown in Fig. 1. Ventilation is expressed as the average volume expired in litres/min s.t.p. for 5-min periods. Three such values were obtained for each noradrenaline infusion; during the first of them ventilation was changing rapidly so importance is attached only to the second and third. In Fig. 1 there was only a small change in ventilation while breathing 2% C2,

4 368 H. BARCROFT AND OTHERS but at all higher levels the ventilation reached a plateau of 35-5% above the control value during the second 5 mm period of the noradrenaline infusion, and remained at about this level till the end of the infusion in spite of the slight fall in alveolar pco2 produced by the hyperpnoea. In nine out of fourteen experiments on the five subjects the increase in ventilation was maintained till the end of the infusion; in the other five experiments there was a small decline in the hyperpnoea. After the end of the infusion ventilation declined slowly but had not reached the control value in 15 min in six out of fourteen experiments. Table 1 shows the averaged results on the five subjects. The increase in ventilation during the last 5 min of the noradrenaline infusion ranged from 19% on 2% C2 to 47% on 4 5 C2. Alveolar PCO2. The alveolar pco2 figures shown in Fig. 1 and Table 1 were obtained by volumetric analysis of the Haldane & Priestley samples. In the subject whose results are shown in Fig. 1 there was a small decrease in alveolar PCO2 in all four experiments. This was typical of four of the subjects. In the fifth subject there was a rise of less than 1 mm in two experiments because an additional -25 % C2 was added to the gas mixture during the noradrenaline hyperpnoea (cf. Cormack, Cunningham & Gee, 1957). Table 1 shows that the average decrease in alveolar PCO2 during the last 1 min of the noradrenaline infusion was small, ranging between -6 and 2-3 mm Hg below the control level. Relation between ventilation and alveolar pco2. In Fig. 2 ventilation is plotted against alveolar PCO2 for all subjects. Only the results obtained during the last 1 min of the control periods and during the second and third 5 min periods of the noradrenaline infusion, when a relatively steady state had been reached, are included. Linear regressions of ventilation on PCO2 during infusion of saline and noradrenaline were calculated, the regression coefficients being respectively and There is no significant difference between them. The predicted ventilations at a PCO2 of 43-2 mm are /min during saline, and /min during noradrenaline infusion, the difference being 1-1 I./min (s.e ) which is significant at the 1 % level (t test) mm was the mean PCO2 of all the noradrenaline infusion points, and was arbitrarily selected for comparing the positions of the two regression lines. Using the pooled data from five subjects was not entirely satisfactory owing to the scatter of the points, but satisfactory calculations could not be made on individual subjects since there were insufficient results on any one of them. DISCUSSION Whelan & Young (1953) found that continuous infusions of noradrenaline given to subjects breathing air caused only a transient stimulation of breathing. The present experiments have shown that the addition of C2 to the inspired air converts this transitory response into one which is sustained throughout the

5 C2, NORADRENALINE AND RESPIRATION 369 4o o o COCO1 m COec. 4 4 p4 ) * r4~ -4 C111 :4 o 4o Pw o4 L 4) 9 CO '.4 4p ' 4) bo c~ - C- 1 CB3 t :4 -o4m. p 15l '4. 14) c f P4-4 :O m.4 o.4) *> 41. O - ce CO c s b eqc. C4) C 4 a y *E 4 P ::.:4 p4~ 4)s ' r - o. 4 o. *P '4 %4) o m *++ coa (S - a

6 37 H. BARCROFT AND OTHERS infusion period. It is interesting to recall that the hyperpnoea of anoxia is likewise sustained if the alveolar pco2 is prevented from falling (cf. Haldane & Poulton, 198; Gee, 1949). When the regressions of ventilation on alveolar pco2 were calculated there was little difference between the slopes of the lines in the absence and in the presence of noradrenaline, indicating that there was no evidence that noradrenaline increased the sensitivity to C2. However, a change of slope cannot be excluded owing to the large scatter of points. 5C fi4c O._ 2 g 3.U 2._ c 3 O ~~~~ Io O / i 1l /~~~~C '48 5 Alveolar pco2 (mm Hg) Fig. 2. Relation between ventilation and alveolar PCO2 before and during the infusion of noradrenaline in fourteen experiments on five subjects breathing CO (2-5%)-air mixtures. and lower line: infusion of saline; regression coefficient 1-76 (s.e. ±.41). and upper line: during noradrenaline infusion; regression coefficient 1-57 (S.E. +-43). It is uncertain whether noradrenaline stimulates the respiratory centre directly, or reflexly in response to some peripheral action. Vertebral and intracarotid arterial infusions of noradrenaline do not influence respiration in conscious man (Coles, Duff, Shepherd & Whelan, 1956). That this is also true for the anaesthetized animal was shown by Young (1957), who has also shown that the respiratory response to intravenous noradrenaline is independent of the activity of the chemo- and baroreceptors. She considered the possibility

7 C2, NORADRENALINE AND RESPIRATION 371 that noradrenaline was converted into, or released, some other substance which acted directly on the respiratory centre, as has been suggested by Hoff, Breckenridge & Cunningham (195). As in the experiments just described, so in severe exercise, the C2 stimulus is increased (Bannister, Cunningham & Douglas, 1954) and the excretion of catechols in the urine is raised (von Euler & Hellner, 1952). It follows that adrenaline and noradrenaline are probably among the factors contributing to the hyperpnoea of severe exercise. It is not possible to assess the relative importance of their actions; though more noradrenaline is released, its action on respiration in equivalent dosage is less than that of adrenaline (Whelan & Young, 1953; Young, 1957). A B C D E 7 1. C CL 5I II~~Mnue Minutes Fig. 3. Ventilation responses in a subject who became conditioned to the information that he was to receive a noradrenaline infusion: breathing 4-5% C2 throughout experiment. A-B: subject told he was receiving noradrenaline at 3 and 6 ug/min respectively. No noradrenaline was actually administered. C: subject told he was no longer receiving noradrenaline. D-E: noradrenaline infused at 1,ug/min; subject not told, unaware of infusion. Conscious and psychological effects on respiration have been known for many years (cf. Krogh & Lindhard, 1913). Subjective symptoms are usually marked during adrenaline infusions but generally absent with noradrenaline; it was for this reason that all the present experiments were carried out with noradrenaline. To minimize possible psychogenic effects an elaborate programme of deception was followed. The importance of such precautions is illustrated in Figs. 3 and 4. The records were obtained in preliminary experiments on two subjects who knew when the infusions of noradrenaline began

8 372 H. BARCROFT AND OTHERS and ended and the form of the expected response. The subject of Fig. 3 had previously shown by far the greatest increase in ventilation at the beginning of infusions before the establishment of the steady state; he did not know the object of this particular experiment, unlike the earlier investigations, and suspected that he was to be given an unusually large dose of noradrenaline. The result (Fig. 3) shows that he had become conditioned to the information that the drug was about to be given, for there was a greater response to the information alone than to a subsequent noradrenaline infusion given without his knowledge. Fig. 4 shows that in another subject not so intimately concerned with the planning of the experiments psychogenic influences were negligible. Though the striking result shown in Fig. 3 was quite exceptional, it emphasized the fact that when planning respiratory experiments every possible precaution should be taken to avoid errors due to psychogenic effects. A B I C 111:1 ~~~~~~~~~~~I E C Minutes Fig. 4. Ventilation responses in a subject who was unaffected by the information that he was to receive a noradrenaline infusion: breathing 4.5% CO2 throughout experiment. A: subject told he was receiving noradrenaline at lo1ug/min; no noradrenaline actually given. B-C: noradrenaline infused at 1,ug/min; subject not told and unaware of infusion. SUMMARY 1. The effect of infusions of noradrenaline (1,g/min for 15 min) on pulmonary ventilation and alveolar pco2 has been examined in subjects breathing carbon dioxide-air mixtures (2-5%). 2. A sustained increase of 2-5% in pulmonary ventilation was obtained in spite of the small fall in alveolar pco2 which accompanied the hyperpnoea. 3. At an alveolar pco2 of 43 mm Hg ventilation was increased by about 1 I./min by the infusion of noradrenaline. 4. No significant change in CO2 sensitivity was demonstrated. 5. A striking illustration is given of the need for precautions to avoid psychogenic effects in respiratory experiments. The authors wish to thank J. L. H. O'Riordan for statistical assistance and also the subjects for their co-operation. Certain of the expenses were defrayed from a grant from the Medical Research Council.

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