Anesthetists have long known that short periods of hypoxia, unaccompanied by significant hypotension or cardiac arrest, are innocuous. But additional documentation that hypoxia does not cause brain damage comes from the arena of bronchopulmonary and ventilatory diseases, including asthma, anaphylaxis, occlusive bronchitis and bronchiolitis, pneumonia, croup, and epiglottidis.
One of the most amazing well-documented cases was that of a two-and-a-half-year-old boy, who had a respiratory arrest due to bronchitis. Rapid clinical action led to tracheotomy and removal of pus with forceps - pus which had formed virtual casts of the bronchi. Although the boy remained in a coma for 14-16 days, subsequent recovery was without neurologic impairment, including school performance (Sadove et al. 1961). Another paper on pure hypoxia (Gray and Horner 1970) presents a collection of profoundly hypoxic patients without vascular disease, with arterial pO2 levels under 20 mmHg and ranging to a low of 8 mmHg (with survival). The outcome of these patients showed that pure hypoxia, without global ischemia, causes no permanent brain damage. The venous pO2 corresponding to the lowest recorded arterial pO2 of 8 mmHg was 2 mmHg! Noticing that hypoxia could not cause brain damage, Rie and Bernad (1980) reported three neuropatho-logic autopsies of profoundly low arterial pO2 levels which all failed to show necrotizing brain damage. All patients died subsequently of other causes after having had 28-192 h of exposure to profoundly low arterial pO2 levels in the range of 24-38 mmHg that were commonly assumed to cause necrotizing brain damage.
That such profound arterial hypoxemia is even survivable is made possible by a flattening of the staircase of the "oxygen cascade" (Fig. 13.3) where there is a progressive, step-wise decrease in pO2 levels. In the case of respiratory obstructive disease, pulmonary venous pO2 levels are greatly reduced in the blood. The principle of flattening of the staircase of the oxygen cascade also applies to high-altitude mountaineering.
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