1. Dobutamine has been used to study the relationship between oxygen consumption (VO2) and oxygen delivery (DO2) in critically ill patients, but this has led to concerns that it could consistently increase VO2 in all patients. Although a direct thermogenic effect of the catecholamine has been primarily implicated in this increase in VO2, an increase in blood flow may contribute significantly by increasing the oxygen requirements of the heart and other organs such as the kidney and the liver. If this mechanism is predominant, it should also be observed when blood flow increases during the infusion of non-adrenergic agents. To separate the two mechanisms, we compared the effects of dobutamine with those of sodium nitroprusside on VO2/DO2 relationships in healthy volunteers.
2. Eight healthy volunteers received infusions of dobutamine at doses of 2, 4 and 6 μg min−1 kg−1 and nitroprusside at doses of 0.5, 1 and 2 μg min−1 kg−1 in an alternate order.
3. VO2 was determined by indirect calorimetry and cardiac output by electrical bioimpedance. Data were analysed by analysis of variance for repeated measurements and individual VO2/DO2 slopes were determined by linear regression.
4. VO2 increased more with dobutamine than with nitroprusside (from 138 ± 14 to 149 ± 20 ml min−1 m−2, P < 0.001, and from 131 ± 14 to 138 ± 17 ml min−1 m−2, P < 0.001, respectively). However, DO2 also increased more with dobutamine than with nitroprusside (from 531 ± 186 to 702 ± 274 ml min−1 m−2, P < 0.001, and from 523 ± 107 to 610 ± 122 ml min−1 m−2, P < 0.001, respectively). Individual VO2/DO2 slopes were similar with dobutamine and nitroprusside (6.5 ± 3.5 compared with 7.1 ± 4.6%, P not significant).
5. At the doses used, DO2 and VO2 increased more with dobutamine than with nitroprusside in healthy volunteers. However, the VO2/DO2 slopes were similar for both substances. Thus, an increase in VO2 is not exclusively observed with catecholamines. Studies of the effects of therapeutic interventions on oxygen-derived variables should report not only changes in VO2 but also VO2/DO2 slopes.