Mean baseline values (Table 1) for Rint were moderately increased, the upper limit for Rint in normal subjects in our laboratory being 2.5 cm H20/L/s. Minute ventilation and arterial blood gas values were within normal limits, although the subjects displayed a mild degree of hyperventilation. Following histamine inhalation, all of the subjects noticed a tightness in the chest and developed a mild wheeze. There were significant (p<0.05) increases in Rint and FRC; however, minute ventilation did not alter significantly, although respiratory frequency increased significantly and tidal volume decreased significantly, immediately following the histamine inhalation. There was a significant (p<0.01) decrease in Pa02 (mean decrease 21.8 mm Hg) and a significant increase in the alveolo-arterial Po2 difference P(A-a)02, associated with a significant increase in the ratio of VDphys/VT; however, alveolar ventilation fell with a rise in PaC02, although this was not statistically significant. health and care mall
The present study indicates that, even in mild asthmatic patients with a normal resting Pa02, histamine inhalation sufficient to cause a 20 percent decrease in FEVj results in a profound fall in Pa02. This is not related to significant changes in minute ventilation or alveolar ventilation, but to marked alterations in ventilation (V)/perfusion (Q) matching. It is well recognized that during spontaneous attacks of asthma, as well as in chronic asthma, marked abnormalities of V/Q occur. Thus, previous studies’ during acute asthma have documented V/Q abnormalities as the primary cause of the hypoxemia that is an invariable accompaniment. There is a marked broadening of the V/Q relationship; on the other hand, no absolute pulmonary shunt (pulmonary blood flow through areas of totally nonventilated lung) occurs, although in extreme prolonged status asthmaticus, some shunt may be present.
Table 1—Effects of Histamine-induced Bronchoconstriction on Pulmonary Function Test Values (n = 9)
|Time <||Rint,cm/H20/L/s||Vl, L||Ve, L/min||f, min||Vt, L||PkOa, mm Hg||?aCOiy mm Hg||pH||P(A-a)Oa, mm Hg||VDphys,L||VDan, L||VDphys/VT|
|Baseline||2.94±0.80||2.70±0.55||11.5 ± 3.8||12.8 ± 3.6||0.93±0.35||98.2 ± 8.6||32.8 ± 4.7||7.456±0.042||11.6 ± 6.6||0.34±0.11||0.22±0.07||0.38±0.11|
|5 min||4.67t±0.84||3.94t±0.73||10.7 ± 3.8||17.9t ± 7.6||0.65t±0.28||76.4t ± 9.9||36.3 ± 2.8||7.431±0.003||26.3t ± 9.4||0.29±0.12||0.20±0.07||0.46f±0.10|
|10 min||4.18t±1.28||3.60±0.77||9.2 ± 2.8||12.2 ± 3.6||0.83±0.35||87.8t ± 9.3||35.2 ± 3.4||7.437±0.040||16.8ft ± 7.0||0.32±0.13||0.18±0.09||0.42±0.11|
|20 min||3.72±1.00||2.95±0.75||11.1 ± 4.9||14.4 ± 5.6||0.78±0.16||90.4$±10.7||34.6 ± 4.3||7.443±0.043||15.6 ± 6.7||0.32±0.10||0.20±0.10||0.41±0.10|