Objectives: To explore the effects of dobutamine and norepinephrine on the global cardiovascular response and on the relationship between oxygen uptake (VO2) and oxygen delivery (DO2) during an acute reduction in blood flow associated with tamponade.
Design: Prospective, randomized, controlled acute intervention study.
Setting: University intensive care unit (ICU) laboratory.
Subjects: Twenty healthy, anesthetized mongrel dogs, weighing 19 to 28 kg.
Interventions: Six dogs served as control, seven dogs were given 10 micrograms/kg/min of dobutamine and another seven dogs were given 1 microgram/kg/min of norepinephrine. Data were collected at graded incremental levels of intrapericardial pressure.
Measurements and main results: VO2 was derived from expired gas analysis and DO2 was calculated from the product of thermodilution cardiac index and arterial oxygen content. In each animal, two catheters were inserted into the pericardium to induce tamponade by saline infusion and to measure the intrapericardial pressure. The critical Do2 value, below which VO2 decreased, was found at 9.4 +/- 1.3 mL/kg/min in the control animals. When DO2 decreased to below this critical value, lactic acidosis developed. Dobutamine and norepinephrine, at the dose that was administered, significantly increased cardiac index and DO2. Critical DO2 was slightly higher in the treated than in the control animals (12.1 +/- 1.6 mL/kg/min in dobutamine and 13.2 +/- 0.9 mL/kg/min in norepinephrine, NS). VO2 at critical DO2 was significantly higher in the treated groups than in the control group (7.7 +/- 1.1 mL/kg/min in the dobutamine group and 7.9 +/- 0.9 mL/kg/min in the norepinephrine group vs. 5.4 +/- 0.4 mL/kg/min in control, both p < .01). There was no significant difference in the critical oxygen extraction ratio and the slope of the supply-dependent line between the three groups. In dobutamine-treated animals, cardiac index, DO2, and VO2 were better maintained for any intrapericardial pressure than in the other groups. Critical intrapericardial pressure, at which VO2 started to decrease, was significantly higher in the dobutamine-treated group than in the control group (13.8 +/- 2.3 vs. 9.3 +/- 1.2 mm Hg, p < .05). At critical DO2, the mean blood lactate concentration was also lower in the dobutamine-treated animals than in the other animals (2.1 +/- 0.3 vs. 4.1 +/- 0.7 mmol/L in control and 3.8 +/- 0.4 mmol/L in the norepinephrine group, both p < .05).
Conclusions: During low-flow states associated with tamponade, both dobutamine and norepinephrine at the dose used increased cardiac index, DO2, and VO2, but dobutamine delayed the onset of tissue hypoxia by further increasing blood flow and oxygen availability. In the conditions of the present study, neither agent significantly influenced the oxygen extraction capabilities of the body.