Cardiovascular and respiratory adjustments at altitude sustain cerebral oxygen delivery -- Severinghaus revisited.

Analysis of a paper by Severinghaus et al. (see text) has already shown that sea level oxygen delivery (D(a)O(2)) is sustained 8 h after ascent to 3810 m, despite low arterial oxygen content (C(a)O(2)), largely as a result of increased cerebral blood flow (CBF). The present study extends the analysis to show that D(a)O(2) is also sustained after 3 and 5 days at altitude, despite a progressively falling CBF. It is shown that this later compensation is a result of the improvement in C(a)O(2), which accompanies acclimatisation. Since less than 3% rise in haemoglobin occurred, the rise in C(a)O(2) was predominantly respiratory. It has been shown elsewhere that as acclimatisation occurs, the fall in arterial PCO(2) (P(a)CO(2)) results in increased arterial PO(2) (P(a)O(2)) until they are related according to P(a)CO(2)=0.25 P(a)O(2)+/-15 mmHg. The results from Severinghaus et al. at 3 and 5 days fall close to this line. We also report arterialised capillary blood gases from 18 normal subjects, acclimatised at 5300 m. The values fall in a group centred on the same line. In summary, soon after arrival at altitude (8 h), cerebral oxygen delivery is largely sustained by an increase in CBF. The present study shows that, although CBF declines during the 3-5 day period, D(a)O(2) is sustained as a result of the improvement in C(a)O(2), which is mainly due to respiratory acclimatisation.