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

Abstract

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 (Sv̄ O 2 ) will strongly influence the rate of desaturation in stage 1 but not in stage 2. We therefore examined the effect of changing preapneic Sv̄ O 2 on the rate of arterial oxygen desaturation (Ṡa O 2 ) 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 Ṡa O 2 was observed when preapneic Sv̄ O 2 was lowered; Ṡa O 2 was −3.1 ± 0.4%.sec −1 when Sv̄ O 2 = 47.4 ± 2.1% increasing to −5.8 ± 0.7%.sec −1 when Sv̄ O 2 = 28.3 ± 1.2%. During stage 2, Ṡa O 2 was −1.62 ± 0.07%.sec −1 and was independent of preapneic Sv̄ O 2 , 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 Sv̄ O 2 , we repeated the experiment with CPAP applied. Under these conditions Ṡa O 2 continued to be greater at lower preapneic Sv̄ O 2 levels. In summary, lowered preapneic Sv̄ O 2 has a potent influence on Ṡa O 2 during stage 1 of the desaturation process but not during stage 2.