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«RATE OF CHANGE OF ALVEOLAR CARBON DIOXIDE AND THE CONTROL OF VENTILATION DURING EXERCISE BY CHRISTOPHER J. ALLEN AND NORMAN L. JONES From the Ambrose ...»

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Further support for APA, co./Ate as a factor in the control of ventilation at the onset of exercise is its close relationship to mean inspiratory flow rate (VT/TI). As an index of inspiratory drive (Milic-Emili, 1982), VT/TI correlates with both mouth occlusion pressure and ventilation during exercise (Siafakas, Morris & Prime, 1979). During the breath following an increase in exercise load, VT/TI increased in parallel with the increase in APA, Co2/Ate, but there was no change in VT' PA C02 or Rc02 Nor was the increase in APA, co,/Ate itself due to an increase in flow rate or to a change in the pattern of breathing. Systematic studies of these variables (Allen et al. 1984) have shown that the relationship between APA, C02/Ate and TC02o VT, and frequency of breathing (r.r.) during exercise can be described by a single multiple linear regression equation: APA, C02/Ate = 3-52 + 5-23 VCO2 - 2-23 VT-0-05 r.r. This relationship demonstrates that increases in VT or r.r. will decrease rather than increase APA, Co2/Ate during exercise, an effect predicted in a theoretical model by DuBois, Fowler, Soffer & Fenn (1952).

The mechanism by which APA co./Ate is linked to respiratory centre output (VT/TI) is too rapid to be accounted for by a feed-back mechanism: Saunders (1980) proposed a feed-forward mechanism in which APA, C02/Ate drives ventilation. In a model with the controller equation PE = 6-15 APA, C02/Ate110 acting solely in the feed-forward 8 C. J. ALLEN AND N. L. JONES mode, a simulated increase in PC02 was followed by a small initial increase in ventilation, followed after 30 S by a second rise to a new steady-state ventilation; during this second phase there was a transient rise in arterial PCo, (Saunders, 1980). From the data of the present study we derived the equation: VE = 6-76 APA, C02/Ate - 356 (95 % confidence limits for slope 4-7-8-9, and for intercept - 10-31 to + 3 20), which is remarkably similar to Saunders' (1980) theoretical controller equation and which supports the concept of a feed-forward model. During the second phase (from 30 to 60 s) there was an increase in mean alveolar PCO2 (Fig. 2). This suggests that there may be an additional error signal so that the increase in ventilation may reflect both the rate of rise of Pco2 and an increase in mean Pco2 Understanding of the physiological basis of this model continues to evolve; Band, Cameron & Semple (1969) showed that there are oscillations in arterial blood pH in cats which occur in phase with respiratory oscillations in alveolar Pco,. Infusing CO2 increased not only the amplitude of the oscillation but also the rate of rise of alveolar PCo2* Band et al. (1980) have demonstrated that an immediate increase in both the rate of change of alveolar PCO2 and arterial pH occur following the onset of exercise in man, a finding consistent with our observations. The carotid chemoreceptor is able to respond to oscillations of arterial PCo2, and carotid body stimulation increases inspiratory drive (Band et al. 1970), suggesting that the carotid body may be an important link between APA, C0O2/Ate and VT/TI. It appears that it is not an essential one; Lugliani, Whipp, Seard & Wasserman (1971) were able to demonstrate an increase in ventilation with exercise in carotid body denervated patients. However, the rate of rise in ventilation was considerably slower than it was in patients whose carotid bodies were intact.

The findings of the present study are consistent with the hypothesis that ventilation during exercise is related to CO2 output by a feed-forward control system responding to APA co2/Ate. The mechanisms and pathways mediating this response remain to be elucidated.

C. J. Allen is a Research Fellow of the Medical Research Council of Canada, which also supported this project (grant number MA 4243). The authors thank Geoff Lewis and George Obminski for development of the computer programs and Marilyn Craven, M.D. Ph.D. for her expert critical appraisal of the manuscript.

REFERENCES

ALLEN, C. J., JONES, N. L. & KILLIAN, K. J. (1984). Alveolar gas exchange during exercise, a breath-by-breath analysis. Journal of Applied Physiology (in the Press).

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BAND, D. M., CAMERON, I. R. & SEMPLE, S. J. G. (1969). Effect of different methods of CO2 administration on oscillations of arterial pH in the cat. Journal of Applied Physiology 26, 268-273.

BAND, D. M., CAMERON, I. R. & SEMPLE, S. J. G. (1970). The effect on respiration of abrupt changes in carotid artery pH and Pco, in the cat. Journal of Physiology 211, 479-494.

BAND, D. M., WOLFF, C. B., WARD, J., COCHRANE, G. M. & PRIOR, J. (1980). Respiratory oscillation in arterial carbon dioxide tension as a control signal in exercise. Nature 283, 84-85.

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AL VEOLAR PC0, AND VENTILATION DURING EXERCISE 9

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COCHRANE, G. M., NEWSTEAD, C. G., NOWELL, R. V., OPENSHAW, P. & WOLFF, C. B. (1982). The rate of rise of alveolar carbon dioxide pressure during expiration in man. Journal of Physiology 333, 17-27.

DAVIES, E. E., HAHN, H. L., SPmo, S. G. & EDWARDS, R. H. T. (1974). A new technique for recording respiratory transients at the start of exercise. Respiration Physiology 20, 69-79.

DEJOURS, P. (1964). Control of respiration in muscular exercise. In Handbook of Physiology, sect.

3, vol. 2. Washington, D.C.: American Physiological Society.

DuBois, A. B., FOWLER, R. C., SOFFER, A. & FENN, W. 0. (1952). Alveolar CO2 measured by expiration into the rapid infra-red gas analyzer. Journal of Applied Physiology 4, 526-534.

FOWLER, W. S. (1948). Lung function studies. II. The respiratory dead space. American Journal of Physiology 154, 405-416.

GRIMBY, G. (1969). Respiration in exercise. Medicine and Science in Sports and Exercise 1, 9-14.

KROGH, A. & LINDHARD, J. (1913). The regulation of respiration and circulation during the initial stages of muscular work. Journal of Physiology 47, 112-136.

LAMB, T. W., ANTHONISEN, N. R. & TENNEY, S. M. (1965). Controlled frequency breathing during muscular exercise. Journal of Applied Physiology 20, 244-248.

LINNARSSON, D. (1974). Dynamics of pulmonary gas exchange at start and end of exercise. Acta physiologica 8candinavica suppl. 415, 1-68.

LUGLIANI, R., WHIPP, B. J., SEARD, C. & WASSERMAN, K. (1971). The effect of carotid body rejection on ventilatory control at rest and during exercise in man. New England Journal of Medicine 285, 1150.

MILIC-EMILI, J. (1982). Recent advances in clinical assessment of control of breathing. Lung 160, 1-17.

MITCHELL, R. R. (1979). Incorporating the gas analyzer response time in gas exchange computations.

Journal of Applied Physiology 47, 1118-1122.

MIYAMOTO, Y., HIURA, T., TAMURA, T., NAKAMURA, T., HIGUCHI, J. & MIKAMI, T. (1982). Dynamics of cardiac, respiratory and metabolic function in men in response to step workload. Journal of Applied Physiology 52, 1198-1208.

NOGUCHI, H., OGUSHI, Y., YOSHIYA, I., ITAKURA, N. & YAMABAYASHI, H. (1982). Breath-by-breath T1co2 and V1, require compensation for transport delay and dynamic response. Journal of Applied Physiology 52, 79-84.

PEARCE, D. H., MILHORN, H. T., HOLLOMAN, G. H. & REYNOLDS, W. J. (1977). Computer based system for analysis of respiratory responses to exercise. Journal of Applied Physiology 42, 968-975.

PHILLIPSON, E. A., BowEs, G., TOWNSEND, E. R., DUFFIN, J. & COOPER, J. (1981a). Carotid chemoreceptors in ventilatory response to changes in venous C02 load. Journal of Applied Physiology 51, 1398-1403.

PHILLIPSON, E. A., BOWES, G., TOWNSEND, E. R., DUFFIN, J. & COOPER, J. (1981b). Role of metabolic C02 production in ventilatory response to steady-state exercise. Journal of Clinical Investigation 68, 768-774.

SAUNDERS, K. B. (1980). Oscillation of arterial CO2 tension in a respiratory model: Some implications for the control of breathing in exercise. Journal of Theoretical Biology 84, 163-179.

SIAFAKAS, N., MORRIS, A. J. R. & PRIME, F. J. (1979). The rate of change of mouth occlusion pressure during exercise. Clinical Science 56, 455-461.

SWANSON, G. (1978). The exercise hyperpnea dilemma. Chest 73, suppl. 277-279.

WESSEL, H. U., STOUT, R. L., BATANIER, C. K. & PAUL, M. H. (1979). Breath-by-breath variations of FRC: effect on V01 and Vco, measured at the mouth. Journal of Applied Physiology 46, 1122-1126.

WHIPP, B. J. & MAHLER, M. (1980). Dynamics of pulmonary gas exchange during exercise. In Pulmonary Ga8 Exchange, vol. II, ed. WEST, J. B. New York, London: Academic Press.

YAMAMOTO, W. S. (1960). Mathematical analysis of the time course of alveolar CO2. Journal of Applied Physiology 15, 215-219.

YAMAMOTO, W. S. & EDWARDS, M. W. (1960). Homeostasis of carbon dioxide during intravenous

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