Assessment of Ventilatory Response to Hypoxia Methods and Interpretation

Abstract

Study of the ventilatory response to hypoxia in man is contributing important information concerning the physiology of ventilatory control and providing new insights concerning the pathogenesis of respiratory failure. Assessment of ventilatory responses to hypoxia in man may be carried out using a variety of procedures. Most commonly, such responses are measured under conditions in which arterial carbon dioxide tension remains unchanged so that the response of ventilation to hypoxia may be examined independent of inhibiting effects which hyperventilation-induced hypoeapnia would have on such a response. Several techniques are used, but methods employed generally fit into one of four categories. Steady-state techniques involve measurement of ventilation during stepwise decrement of inspired O2 tension where each step is of several minutes' duration. 1 Cormack RS Cunningham DJC Gee JBL The effect of carbon dioxide on the respiratory response to want of oxygen in man.. Quart J Exp Physiol. 1957; 42: 303 Crossref PubMed Scopus (57) Google Scholar Second is the technique of progressive hypoxia. Because the ventilatory response to hypoxia has a relatively short time constant of approximately 18 seconds, the ventilatory response can be described as a continuous function while the oxygen tension of the gas being breathed is gradually lowered. 2 Weil JV Byrne-Quinn E Sodal IE et al. Hypoxic ventilatory drive in normal man.. J Clin Invest. 1970; 49: 1061 Crossref PubMed Scopus (296) Google Scholar This technique yields data comparable to steady-state method. 3 Kronenberg R Hamilton FN Gabel R et al. Comparison of three methods for quantitating respiratory response to hypoxia in man.. Respiration Physiol. 1972; 16: 109 Crossref PubMed Scopus (72) Google Scholar There is concern that these first two methods entail relatively persistent hypoxia which may in time result in depression of ventilation. Hence, a number of investigators have used a third technique—rapid tests involving hypoxic periods lasting no more than a few breaths. The increase in tidal volume of subsequent breaths is used to gauge the response. 4 Dejours P Labrousse Y Raynaud J et al. Etude du stimulus gaz carbonique de la ventilation chez l'homme.. J Physiol. 1958; 50: 239 Google Scholar These latter tests in general suffer from the disadvantage that the stimulus is so rapidly changing and so transient that its precise magnitude cannot be accurately assessed. In addition, the response is also fleeting and it is not certain that the response has become fully developed. Although some investigators have shown reasonably good agreement between transient and steady-state results, 5 Edelman NH Epstein PE Lahiri S et al. Ventilatory responses to transient hypoxia and hypercapnia in man.. Respir Physiol. 1973; 17: 302 Crossref PubMed Scopus (115) Google Scholar transient tests are generally considered less quantitatively rigorous. A fourth method for measuring hypoxic responses depends upon the fact that hypoxia augments the ventilatory response to hypercapnia. The ventilatory response to carbon dioxide is measured at high and low oxygen tensions 6 Nielsen M Smith H Studies on the regulation of respiration in acute hypoxia.. Acta Physiol Scand. 1951; 24: 293 Crossref Scopus (168) Google Scholar , 7 Lloyd BB Jukes MGM Cunningham DJC The relation between alveolar oxygen pressure and the respiratory response to carbon dioxide in man.. Quart J Exp Physiol. 1958; 43: 214 Crossref PubMed Scopus (134) Google Scholar and the change in slope of the hypercapnic response is used as a measure of the hypoxic drive.