RESUMO
PURPOSE: Previous studies have compared the relationship between directly measured values for cardiac output, systemic oxygen consumption (VO2), and arteriovenous oxygen difference (D(A-v)O2) with those calculated by the Fick principle. However, the validity of Fick's principle in critically ill patients undergoing physiologic changes and pharmacologic interventions is unknown. The purpose of our study was to compare directly measured values for hemodynamic and oxymetric variables with those calculated by the Fick equation in patients with acute myocardial infarction, at baseline and after the hemodynamic changes produced by pharmacologic interventions. PATIENTS AND METHODS: Cardiac output, (VO2), and (D(A-v)O2) were measured in 33 patients with acute myocardial infarction, at baseline (50 +/- 30 hours after the onset of symptoms) and after pharmacologic intervention to relieve pulmonary congestion. These values were then compared with indirect values derived from the Fick equation. RESULTS: High and significant correlations were found between thermodilution and Fick-derived cardiac output at baseline (r = 0.91, p less than 0.001) and post-intervention (r = 0.92, p less than 0.001). Similarly, VO2 values measured by expired gas analysis showed a significant correlation with VO2 calculated by the Fick principle, at baseline (r = 0.85, p less than 0.001) and post-intervention (r = 0.84, p less than 0.001). Lastly, when D(A-v)O2 of arterial and mixed venous samples was measured by spectrophotometry and compared with calculated values, there was a significant correlation at baseline (r = 0.85, p less than 0.001) and after intervention (r = 0.85, p less than 0.001). Analysis of variance revealed no difference between measured and calculated values for the three variables under those two conditions. CONCLUSION: In patients with acute myocardial infarction, cardiac output, VO2, and D(A-v)O2 indirectly calculated by the Fick principle are equivalent to directly measured values, despite the various degrees of hemodynamic dysfunction and the currently used therapeutic interventions.