Systemic oxygen extraction can be improved during repeated episodes of cardiac tamponade

We used a tamponade model to study the relationship between oxygen uptake (VO2) and oxygen delivery (DO2) during successive, reversible decreases in blood flow. In 7 pentobarbital-anesthetized and mechanically ventilated dogs, a catheter was introduced via a left thoracotomy into the pericardium to inject and to withdraw saline. Each experiment consisted of three steps. First, cardiac output was reduced by successive pericardial fluid injections until 4 to 6 data points were obtained in the dependent region of the VO2/DO2 plot (step 1). Second, cardiac output was restored by progressive withdrawal of pericardial fluid (step 2). Third, cardiac output was lowered again by reinjection of fluid into the pericardium until death (step 3). Expired gases were collected for determination of VO2. In each animal, critical DO2 (DO2crit), below which VO2 became DO2 dependent, was determined from a plot of VO2 versus DO2. When releasing tamponade, VO2 was restored to baseline. For the 3 steps, DO2crit were 10.5 +/- 2.2 mL/kg/min in step 1, 9.8 +/- 1.8 mL/kg/min in step 2, and 8.3 +/- 1.9 mL/kg/min in step 3 (P < .01 v step 1; P < .05 v step 2, respectively). There was no significant difference in VO2 at DO2crit for the three steps. Hence, critical oxygen extraction ratio (ERO2crit) increased from 60% +/- 12% in step 1 to 64% +/- 11% in step 2 (not significant) and to 73% +/- 12% in step 3 (P < .01). The VO2/DO2 dependency slope was also steeper in step 3 than in step 1 (0.77 +/- 0.31 v 0.54 +/- 0.20, P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)