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Animals native to hypoxic environments have adapted by increasing their haemoglobin oxygen affinity, but in-vitro studies of the oxyhaemoglobin dissociation curve (ODC) in humans show no changes in affinity under physiological conditions at altitudes up to 4000m. We conducted the first in-vivo measurement of the ODC; inducing progressive isocapnic hypoxia in lowlanders at sea level, acutely acclimatized lowlanders at 3600m, and native Andeans at that altitude. ODC curves were determined by administering isocapnic steps of increasing hypoxia, and measuring blood oxygen partial pressure and saturation. The ODC data were fitted using the Hill equation and extrapolated to predict the oxygen partial pressure at which haemoglobin was 50% saturated (P50). In contrast to findings from in-vitro studies, we found a pH-related reduction in P50 in subjects at altitude, compared to sea-level subjects. We conclude that a pH-mediated increase in haemoglobin oxygen affinity in-vivo may be part of the acclimatization process in humans at altitude.

As protection against low-oxygen and high-carbon-dioxide environments, the respiratory chemoreceptors reflexly increase breathing. Since CO is also frequently present in such environments, it is important to know whether CO affects the respiratory chemoreflexes responsiveness. Although the peripheral chemoreceptors fail to detect hypoxia produced by CO poisoning, whether CO affects the respiratory chemoreflex responsiveness to carbon dioxide is unknown. The responsiveness of 10 healthy male volunteers were assessed before and after inhalation of approximately 1200 ppm CO in air using two iso-oxic rebreathing tests; hypoxic, to emphasize the peripheral chemoreflex, and hyperoxic, to emphasize the central chemoreflex. Although mean (SEM) COHb values of 10.2 (0.2)% were achieved, no statistically significant effects of CO were observed. The average differences between pre- and post-CO values for ventilation response threshold and sensitivity were -0.5 (0.9) mmHg and 0.8 (0.3) L/min/mmHg, respectively, for hyperoxia, and 0.7 (1.1) mmHg and 1.2 (0.8) L/min/mmHg, respectively, for hypoxia. The 95% confidence intervals for the effect of CO were small. We conclude that environments with low levels of CO do not have a clinically significant effect acutely on either the central or the peripheral chemoreflex responsiveness to carbon dioxide.

Nosocomial transmission of droplet-borne respiratory infections such as severe acute respiratory syndrome (SARS) may be influenced by the choice of oxygen face mask. A subject inhaled saline mist and exhaled through three oxygen masks to illustrate the pattern of dispersal of pulmonary gas. In two commonly used masks, exhaled gas formed a plume emanating from the side vents, while a third mask with a valved manifold, which was modified by adding a respiratory filter, retained the droplets. Maintaining respiratory isolation during the administration of oxygen may reduce the risk of the nosocomial transmission of respiratory infections such as SARS.