Serial Relationships between Ventilation-Perfusion Inequality and Spirometry in Acute Severe Asthma Requiring Hospitalization1-3

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Abstract
Patterns of a/ distribution and their relationship to spirometric indices were studied in 10 patients with acute severe asthma requiring hospitalization (7 women and 3 men 41.0 ± 5.6 yr of age, mean ± SEM) on admission and during subsequent recovery. On admission, all patients received the standard therapeutic regimen for our hospital. Spirometry and essentially noninvasive multiple inert gas elimination measurements were obtained serially, approximately once every day, whereas conventional arterial blood gases were determined every 3 days. On admission, all patients showed severe air-flow obstruction (FEV1/FVC% = 34.1 ± 4.3%) and moderate to severe hypoxemia without CO2 retention (PaO2 = 50.5 ± 2.6 mm Hg; PaCO2 = 37.1 ± 2.4 mm Hg; AaPO2 = 53.7 ± 3.0 mm Hg). Nine of the 10 patients showed bimodal blood flow distributions (dispersion of blood flow distribution, log SD Q = 1.34 ± 0.11; normal range, 0.3 to 0.6) with only small amounts of shunt (1.09 ± 0.8%). However, no significant interindividual correlations were observed between maximal expiratory flow rates (FEV1 and FEF25–75) and log SD Q (r2 = 0.14 and 0.006, respectively). This lack of correlation persisted throughout hospitalization. Despite both clinical and spirometric improvement in all patients, there was simultaneous improvement in a/ matching in only one patient. Statistically significant negative correlations between maximal expiratory flow rates and gas exchange did develop toward the end of the study (Weeks 3 and 4 after discharge) when maximal recovery of physiologic changes associated with the acute asthma attack was achieved. Our data show that gas exchange abnormalities, both on admission and during recovery, cannot be inferred from the degree of impairment in air-flow rates in acute severe asthma. The findings of the present study are compatible with the hypothesis that, during acute severe asthma, air-flow rates are determined mostly by properties of the large airways, whereas gas exchange is more related to peripheral airways obstruction because of mucus plugging and/or bronchial wall edema.

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