RATE OF OXYHEMOGLOBIN DESATURATION IN OBSTRUCTIVE VERSUS NONOBSTRUCTIVE APNEA

Preapneic thoracic gas volume (Vtg), arterial saturation (Sa(O2)), and mixed venous oxygen saturation (SvBAR(O2)), have been shown to influence the rate of Sa(O2) fall (dSa(O2)/dt) during apnea. We asked the following question: does tissue oxygen consumption (tV(O2)) affect the dSa(O2)/dt during apnea? We attempted to answer this question by comparing dSa(O2)/dt during obstructive apneas (high tVO2) with dSa(O2)/dt during nonobstructive apneas (low tVO2) in six adult baboons. Fiberoptic central venous and arterial catheters were used for continuous monitoring of SvBAR(O2), Sa(O2), and cardiac output. A sapphire-bearing turbine monitored minute ventilation and airflow cessation. A Respitrace(R) and esophageal pressures were used to assess relative differences in Vtg. Obstructive apneas (30, 45, and 60-s) were created by clamping an indwelling cuffed endotracheal tube at end-expiration. Nonobstructive apneas were created by paralyzing the animals with atracurium and interrupting ventillation for periods equivalent to those of the obstructed apneas. The ventillator was adjusted to duplicate the respiratory rate, tidal volume, and relative Vtg of the spontaneously breathing animal. Mean tVO2 during spontaneous breathing was 110 ml/min (Fick method) and decreased to 90 ml/min during paralysis (p < 0.05). The dSa(O2)/dt for the three apnea durations (mean, all animals), obstructive versus nonobstructed were: 0.85 and 0.74%/s (n = 6), 0.87 and 0.75%/s (n = 6), and 0.60 and 0.48%/s (n = 4), respectively. The dSa(O2)/dt was significantly lower during the nonobstructive apneas. We conclude that differences in VO2 during apnea may affect the dSa(O2)/dt and that for the same duration apnea, central apneas may show less desaturation than obstructive apneas where vigorous muscular efforts at overcoming obstruction are common.