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 o₂/o₂ 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 ₂. In each animal, critical o₂ (o₂crit), below which ₂ became o₂ dependent, was determined from a plot of ₂ versus o₂. When releasing tamponade, ₂ was restored to baseline. For the 3 steps, o₂crit 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 o₂2 at o₂crit 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 ₂/o₂ dependency slope was also steeper in step 3 than in step 1 (0.77 ± 0.31 v 0.54 ±- 0.20, P < .05). A progressive decrease in arterial and in mixed venous pH was observed during the experiment. We conclude that a decrease in ₂ associated with an acute reduction in blood flow can be readily reversible. When the procedure is repeated, a progressive increase in oxygen extraction capabilities is observed. This reversible tamponade model is potentially suitable to induce several hypoxic episodes in the same animal.