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In acute massive pulmonary embolism with hemodynamic instability, monitoring of pulmonary artery pressure can be used to assess the efficacy of thrombolytic therapy. As a noninvasive alternative to pulmonary artery catheterization, we investigated the efficacy of continuous monitoring of end-tidal CO2 tension.In 12 patients with massive pulmonary embolism who required mechanical ventilation, mean pulmonary arterial pressure (MPAP) and end-tidal carbon dioxide tension (ETco2) were registered continuously during thrombolytic therapy. Paco2, cardiac index as estimated by thermodilution catheter and respiratory ratio of arterial oxygen tension and inhaled oxygen concentration (Pao2/Fio2) were determined every 60 mins.Before thrombolysis, MPAP (34.5 ± 9.8 mm Hg) and the difference between Paco2 and ETco2 (10.1 ± 4.7 mm Hg) were markedly increased compared with normal values. Continuously monitored MPAP was related to ETco2 for both all patients (r2 = .42;p < .001) and individually (mean r2 = .92; range, .79-.98;p < .001). In ten survivors, the mean cardiac index and Pao2/Fio2 increased during therapy from 1.7 ± 0.4 to 2.8 ± 0.6 L/min·m2 and 125 ± 27 to 285 ± 50 mm Hg (p < .01, respectively). In these patients, the difference between Paco2 and ETco2 decreased from 9.8 ± 4.5 to 2.8 ± 0.9 mm Hg (p < .001). Recurrent embolism was detected in two patients by sudden reduction of ETco2.Analysis of ETco2 allows monitoring of the efficacy of thrombolysis and may reflect recurrent embolism. Thus, on the basis of this small study, analysis of ETco2 appears to be useful for noninvasive monitoring in mechanically ventilated patients with massive pulmonary embolism.