Dobutamine, but not dopamine, augments the normal chronotropic response to acute hypoxemia

Infusion of exogenous dopamine has been shown to alter both the hypoxic and the hypercapnic ventilatory response, but its effects on the hypoxic and the hypercapnic cardiac response have not been reported. The purpose of this study was to determine if cardiac responses to hypoxemia and hypercapnia are altered by infusion of either dopamine (DOP) or its analog dobutamine (DBT). Baseline mean arterial blood pressure (MAP) and heart rate (HR) were measured in 7 normal male subjects while normal saline was infused intravenously (NS1) for 30 min. Each subject then underwent both a eucapnic hypoxemic challenge (EHC), and a hyperoxic hypercapnic challenge (HHC). In a double-blinded randomized fashion either DOP1 or DBT1 was selected and infused at 5 micrograms/kg/min for 30 min. EHC and HHC were again performed. This sequence was then repeated following a second NS infusion (NS2) and the alternate vasopressor. On a second study day, the dose of dopamine (DOP2) selected was that sufficient to induce a similar rise in MAP as seen with DBT1. Following NS and DOP1 there was a reproducible significant linear increase in HR with progressive hypoxemia seen in all subjects. This response was significantly augmented following infusion of DBT1 (p < 0.03), but no such augmentation was noted with DOP2. No significant change in HR was noted during HHC. The mechanisms responsible for this augmentation of hypoxemic response during dobutamine infusion are unclear, and do not appear to be related to a rise in MAP. We speculate that it is due to an effect of dobutamine on beta 1-receptor activity.