Hypoxic pulmonary vasoconstriction and the size of hypoxic compartment

Abstract

In pentobarbital sodium-anesthetized mongrel dogs, the relationship between the amount of hypoxic pulmonary vasoconstriction (HPV) and the size of the hypoxic test segment (TS) were determined. Relative to normoxic values, the amount of HPV was defined as the following: the percent decrease in the percent of the cardiac output (.ovrhdot.QT) perfusing the TS (.ovrhdot.QTS/.ovrhdot.QT); the percent increase in pulmonary arterial pressure (Ppa); the percent increase in the pulmonary vascular resistance of the TS (PVRTS); and the HPV index was defined as the sum of normalized .ovrhdot.QTS/.ovrhdot.QT plus Ppa. The size of the hypoxic compartment was defined by the normoxic .ovrhdot.QTS/.ovrhdot.QT; and to create 3 .ovrhdot.QTS/.ovrhdot.QT intervals, the left lower lobe (LLL, .ovrhdot.QTS/.ovrhdot.QT = 0.27) and the remainder of the lung (ML, .ovrhdot.QTS/.ovrhdot.QT = 0.75) were used in group I (n = 8) and the left and right single lungs (LL and RL, .ovrhdot.QTS/.ovrhdot.T = 0.50) were used in group II (n = 8). The percent decrease .ovrhdot.QTS/.ovrhdot.QT decreased only when .ovrhdot.QTS/.ovrhdot.QT was changed from 0.50 to 0.75 (P < 0.005); the percent increase Ppa increased when .ovrhdot.QTS/.ovrhdot.QT was changed from 0.27 to 0.50 (P < 0.01) and to 0.75 (P < 0.2); the percent increase PVRTS was increased when .ovrhdot.QTS/.ovrhdot.QT = 0.50 compared with both 0.27 and 0.75 (P < 0.05); the HPV index was constant (near 1.0) at all .ovrhdot.QTS/.ovrhdot.QT sizes. At any given .ovrhdot.QTS/.ovrhdot.QT and for all 4 expressions of HPV, the distribution of the magnitude of individual HPV responses was normal (bell-shaped), but very broad. Factors that could have contributed to the HPV variability among individuals include genetic and the associated mixed venous O2 partial pressures, Ppa and compartmental alveolar O2 partial pressure values. As .ovrhdot.QTS/.ovrhdot.QT increases, the expression of HPV changes from flow redistribution to perfusion pressure increase but the overall magnitude of HPV does not change. At any given .ovrhdot.QTS/.ovrhdot.QT the limits of any given individual HPV response are wide.