Comparison of five measures derived from in vivo pulmonary vascular pressure-flow curves

In order to facilitate evaluation of acute changes in lung vascular characteristics in vivo, pressure-flow (delta P-Q) curves of the pulmonary circulation were obtained by step-wise flow reduction. A balloon-tipped Swan-Ganz catheter was inserted via the jugular vein into the inferior vena cava of anesthetized, open chest, ventilated rabbits. Pulmonary arterial (Ppa) and left atrial (Pla) pressures were measured via catheters, and cardiac output Q by an electromagnetic flow probe on the aorta. Inflation of the balloon for 10 s reduced Q by 30-70%. delta P-Q curves were constructed by plotting a series of values of Q against corresponding delta P (= Ppa-Pla). To evaluate the feasibility and sensitivity of the method, these curves were compared under three conditions each paired with control (normoxia): hypoxia (8% O2), isoproterenol infusion, and serotonin infusion. Within our measured flow ranges, most delta P-Q plots were fairly linear, and extrapolated to a positive delta P intercept. Comparing slope, intercept, resistance, delta P at fixed Q, and Q at fixed delta P, we found that the latter two provided the more sensitive index to differentiate vasomotor changes. Since delta P-Q curves generally miss the origin, calculated pulmonary vascular resistance must depend on Q. Therefore, using the shifts in entire delta P-Q curves to select common range of Q and/or delta P is in general more quantitatively reliable for defining altered vascular characteristics.