The present study shows that bronchoconstriction in these mildly asthmatic subjects resulted in a significant increase in P(A-a)02 and the VDphys/VT ratio which, in the absence of an increase in overall ventilation, resulted in a marked fall in Pa02. An increase in the VDphys/VT relationship, as derived in the present study by the Enghoff modification of the Bohr technique, can be due to one of three mechanisms: an increase in right-to-left shunt, a decrease in 02 saturation, or an increase in ventilation to areas with low blood flow. Increased right-to-left shunt, by elevating the PaC02, may increase VDphys/VT. However, absolute shunts do not occur in asthma and have not been noted with pulmonary radioisotope perfusion scans following induced bronchoconstriction. сanadianhealthcaremallinc.com
A decrease in 02 saturation of the blood, by reducing the release of C02 from the blood (Haldane effect) into the alveoli would tend to increase VDphys/VT; however, this would not explain the fall in Pa02. The most likely mechanism for the increase in VDphys/VT and decrease in Pa02 is an increase in ventilation to regions with low blood flow, associated with little change in minute ventilation, this would result in the observed fall in Pa02 and slight rise in PaC02. This explanation would conform to the findings following exercise-induced asthma in children and in adults, in whom areas of increased Va/Q developed. These findings contrast with those of Wagner et al, who demonstrated no areas of increased Va/Q in patients with asymptomatic asthma. It is possible that these changes in Va/Q only become manifest during acute exacerbations; alternatively, histamine and exercise challenge may result in different V/Q abnormalities compared with asymptomatic asthma. A further point, not examined in the present or previous studies, to our knowledge, in man, is the effect of bronchoconstriction on pulmonary blood flow; induced bronchoconstriction in dogs results in increased cardiac output, probably due to increases in intrathoracic pressure swings. The effects of the increased pulmonary blood flow would be to attenuate the fall in Pa02 which would occur for any given V/Q mismatch.