Effect of venous oxygenation on arterial desaturation rate during repetitive apneas in lambs

While arterial oxygen desaturation during apnea is a common occurrence in adults and infants, the factors determining the rate of desaturation are poorly understood. We describe a theoretical model which suggests that arterial desaturation during an apneic episode occurs in two stages. In the initial stage (stage 1) the oxygen store in the lung is depleted, while in the second phase of the desaturation process (stage 2) tissue oxygen needs are met predominantly by depletion of the blood store. Our model predicts that preapneic venous oxygenation (SvO2) will strongly influence the rate of the desaturation in stage 1 but not in stage 2. We therefore examined the effect of changing preapneic SvO2 on the rate of arterial oxygen desaturation (SaO2) during stage 1 and stage 2 apnea in anaesthetised 10-20 day-old lambs. Preapneic arterial oxygen saturation was maintained constant. In agreement with the model's prediction there were two stages to the desaturation process and during stage 1 a significant increase in SaO2 was observed when preapneic SvO2 was lowered; SaO2 was -3.1 +/- 0.4%.sec-1 when SvO2 = 47.4 +/- 2.1% increasing to -5.8 +/- 0.7%.sec-1 when SvO2 = 28.3 +/- 1.2%. During stage 2, SaO2 was -1.62 +/- 0.07%.sec-1 and was independent of preapneic SvO2, also in accord with the model's prediction. In order to assess whether the accelerated desaturation rate we observed in stage 1 could have resulted from a decline in lung volume during apnea rather than lower levels of SvO2, we repeated the experiment with CPAP applied. Under these conditions SaO2 continued to be greater at lower preapneic SvO2 levels. In summary, lowered preapneic SvO2 has a potent influence on SaO2 during stage 1 of the desaturation process but not during stage 2.