Use of continuous noninvasive measurement of oxygen consumption in patients with adult respiratory distress syndrome following shock of various etiologies

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Abstract
Objective To describe the patterns of cardiac index, oxygen delivery (Do2), oxygen consumption (Vo2), and oxygen deficit (or excess) and to compare invasive and noninvasive monitoring systems for evaluation of these oxygen transport patterns. Design Descriptive study of oxygen transport interrelationships throughout critical illness in a consecutive series of surviving and nonsurviving patients with adult respiratory distress syndrome (ARDS). Setting University-affiliated city hospital. Patients A consecutive series of 55 critically ill patients with ARDS after shock of various etiologies. Measurements and Results Noninvasive Vo2 was measured by a continuous, on-line, real-time device developed in our department. Inspired and expired oxygen concentrations were measured using a polarographic oxygen analyzer. Minute ventilation measurements were time integrated over 7-min intervals. Cardiac index, Do2, and Vo2 were simultaneously measured invasively by pulmonary artery thermodilution catheters, together with arterial and mixed venous blood gases. There was good agreement (r2 = .60) in Vo2 measured by the invasive and noninvasive methods. The estimated oxygen deficit or excess was calculated as the difference between the actual measured Vo2 standardized for body temperature pressure saturated and the normative standard Vo2 of each patient corrected for temperature and sedation (Vo2 need). A total of 317 monitoring days in 55 patients were analyzed; 25 survivors were monitored for a mean of 4.6 ± 2.9 days and 30 nonsurvivors were monitored for 6.9 ± 6.6 days. Survivors had significantly higher cardiac index, Do2, and Vo2 values. Generally, oxygen excesses were found in the survivors and oxygen deficit was observed in the nonsurvivors. Survivors did not reach a plateau in their Do2-Vo2 patterns. In the septic nonsurviving patients and both nonseptic groups by contrast, a plateau was observed in the Do2-Vo2 pattern. Surviving septic patients had a critical Do2 of 16 mL/min-kg (700 mL/min-m2) and a critical Vo2 of 3.5 mL/min-kg (145 mL/min-m2). Conclusions Monitoring of Vo2 and Do2 variables is useful for evaluation of tissue oxygenation and titration of therapy in critically ill patients. Noninvasive monitoring of Vo2 values are in good agreement with Vo2 values calculated from invasive measurements of cardiac index. The increased Do2 and Vo2 values are not attributable to mathematical coupling of erroneous cardiac index values.

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