We confirmed that in stable nonhypoxemic asthmatic patients bronchoconstriction induced by both histamine and methacholine produced arterial hypoxemia and that histamine-induced bronchoconstriction was associated with an increase in the Ttot with similar increases in both Ti and Te. These changes were reversed when the bronchoconstriction was relieved by inhaling salbutamol.
Each patient’s arterial oxygen saturation fell during both challenge procedures. This hypoxemia was probably not of major clinical significance in these patients, since the average fall in oxygen saturation was only 3 percent, and the lowest oxygen saturation recorded was 86 percent. More info
Nevertheless, this complication of bronchial challenge should be recognized, because a similar degree of bronchoconstriction might induce important hypoxemia in patients who are more hypoxic before undertaking the challenge or in patients who have a greater extent of bronchoconstriction during bronchial challenge. The degree of hypoxemia as measured by the change in ear oxygen saturation that we observed was more than that recorded by Poppius and Stenius in their 55 patients. Direct comparison of the degree of bronchoconstriction induced in the two studies is difficult, since they recorded only peak flow rate, which showed an average fall of 38±15 percent, whereas we recorded FEV!, which fell by 37 ± 11 percent, but clearly these changes are broadly similar. Their average reduction in Sa02 of 1.1 ±1.8 percent (which was only a third of that observed in this study) may simply reflect their higher level of oxygen saturation (96.6 ±0.2 percent, compared with a value of 95.0 ±0.4 percent in our study). The calculated fall in arterial oxygen tension (assuming an arterial pH of 7.40 and a blood P50 of 26.6 mm Hg) was similar, being on average in our study 12 mm Hg for histamine and 10 mm Hg for methacholine but 9 mm Hg in the study of histamine challenge by Poppius and Stenius.
The mechanism of the arterial hypoxemia is not clear from these studies. Overall, hypoventilation appears to play some role, at least in the hypoxemia produced by histamine inhalation, when we found that minute ventilation fell (p<0.05). However, in contrast we found no significant change in ventilation to accompany the hypoxia produced by methacholine, with a similar degree of bronchoconstriction, nor did ventilation rise when oxygenation improved following the salbutamol inhalation, after either the histamine or methacholine challenges.