Mechanisms of Hypoxemia in Chronic Thromboembolic Pulmonary Hypertension

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Abstract
Chronic thromboembolic pulmonary hypertension is characterized by widespread central obstruction of the pulmonary arteries with organized thrombus and thereby differs substantially from other forms of pulmonary hypertension. We studied 25 patients using the multiple inert gas elimination technique to identify and quantitate the physiologic mechanisms of hypoxemia in this disorder. All patients had chronic obstruction of the central pulmonary arteries, which was demonstrated angiographically and later surgically confirmed. All patients but one were hypoxemic (PaO2 = 65 .+-. 11 mm Hg, PaCo2 = 32 .+-. 4 mm Hg, AaPO2 = 45 .+-. 14 mm Hg), and all patients had pulmonary hypertension (mean Ppa = 45 .+-. 11 mm Hg) with an elevated pulmonary vascular resistance (mean PVR = 1,000 .+-. 791 dyne/s/cm5, normal < 300). The cardiac index was reduced (1.7 .+-. 0.6 L/min/m2), as was the P.hivin.O2 (31 .+-. 5 mm Hg). Inert gas studies revealed widened unimodal .ovrhdot.VA/.ovrhdot.Q distributions of 20 of 25 subjects, with a log standard deviation of 1.01 .+-. 0.32 (upper limit of normal, 0.6; ages 20 to 40), shunt = 0.03 .+-. 0.05 of cardiac output, and dead space of 3.4 .+-. 1.1 ml/kg (upper limit of normal, 2.9). The VD/VT ratio was 0.51 .+-. 0.10. No low (.ovrhdot.VA/.ovrhdot.Q < 0.1) or high (.ovrhdot.VA/.ovrhdot.Q > 10.0) regions were present, and no evidence for diffusion limitation of O2 transfer at rest was found. The low cardiac output and resulting low P.hivin.VO2 were responsible for approximately 33% of the increased AaPO2. The magnitude of the .ovrhdot.VA/.ovrhdot.Q abnormality correlated poorly with the PVR, the mean Ppa, or the magnitude of vascular obstruction. We conclude that the hypoxemia of chronic thromboembolic pulmonary hypertension is the consequence of a moderate .ovrhdot.VA/.ovrhdot.Q abnormality with a hypoxemic effect that is considerably amplified by a lowered P.hivin.VO2.