Effect of elastic loading on ventilatory response to hypoxia in conscious man

Abstract

Ventilatory responses to isocapnic hypoxia, with and without an inspiratory elastic load (12.1 cmH2O/l), were measured in seven healthy subjects using a rebreathing technique. During each experiment, the end-tidal PCO2 was held constant using a variable-speed pump to draw gas from the rebreathing bag through a CO2 absorbing bypass. Studies with and without the load were performed in a formally randomized order for each subject. Linear regressions for rise in ventilation against fall in SaO2 were calculated. The range of unloaded responses was 0.74-1.38 1/min per 1% fall in SaO2 and loaded responses 0.71-1.56 1/min per 1% fall in SaO2. Elastic loading did not significantly alter the ventilatory response to progressive hypoxia (P greater than 0.2). In all subjects there was, however, a change in breathing pattern during loading, whereby increments in ventilation were attained by smaller tidal volumes and higher frequencies than in the control experiments. These results support the hypothesis previously proposed in our studies of resistive loading during progressive hypoxia, that a similar control pathway appears to be involved in response to the application of loads to breathing, whether ventilation is stimulated by hypoxia or hypercapnia.

Influence of body position on pressure and airflow generation during hypoxia and hypercapnia in man.

“…dyspnoea) (Cherniack & Snidal, 1956;Guz, Noble, Widdicombe, Trenchard & Mushin, 1966;Phillipson, Hickey, Bainton & Nadel, 1970;Freedman, Dalton, Holland & Patton, 1972). Hence, under both hypoxic and hypercapnic conditions, the level of ventilation and pattern of breathing are shaped by the interaction of chemical drives on the one hand and mechanical sensory information arising peripherally on the other (Rebuck, Betts & Saunders, 1975;Rebuck & Juniper, 1975).…”

“…dyspnoea) (Cherniack & Snidal, 1956;Guz, Noble, Widdicombe, Trenchard & Mushin, 1966;Phillipson, Hickey, Bainton & Nadel, 1970;Freedman, Dalton, Holland & Patton, 1972). Hence, under both hypoxic and hypercapnic conditions, the level of ventilation and pattern of breathing are shaped by the interaction of chemical drives on the one hand and mechanical sensory information arising peripherally on the other (Rebuck, Betts & Saunders, 1975;Rebuck & Juniper, 1975 During both inspiratory resistive and elastic loading, peak intrathoracic and/or intra-airway pressure is one of the chief determinants of inspiratory effort sensation (Wiley & Zechman, 1966;Killian, Bucens & Campbell, 1982;Bradley, Chartrand, Fitting, Killian & Grassino, 1986;Fitting, Chartrand, Bradley, Killian & Grassino, 1987). In addition, peak pressure is a more important determinant of respiratory 484 EFFECT OF POSTURE ON VENTILATORY RESPONSES effort sensation than is duration of pressure generation as reflected by t1 or t1/total respiratory cycle time (Bellemare & Grassino, 1982).…”