Effect of intravenous dopamine on hypercapnic ventilatory response in humans

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Abstract
The effect of low-dose i. v. dopamine (3 .mu.g/kg per min) [ a neurotransmitter in the carotid body] on the hypercapnic ventilatory response was assessed in humans. Six normal healthy subjects were studied. By manipulating the inspired CO2 concentrtion, the end-tidal CO2 tension was raised in a stepwise fashion from 41 to 49 Torr and held at this level for 4 min. The end-tidal CO2 tension was then lowered back to 41 Torr in a stepwise fashion. The end-tidal O2 tension was held constant at 106 Torr throughout the experiment. The ventilatory response to this normoxic hypercapnic stimulus was analyzed by fitting 2 exponential functions, allowing the response to be separated into slow and fast chemoreflex loops. Each loop is described by a gain, time constant and time delay. A single eupneic threshold was used for both loops. Nine control experiments and 8 experiments performed during dopamine infusion were analyzed. The dopamine infusion caused the fast loop gain to be significantly (P< 0.05) reduced from 0.64 to 0.191 per min per Torr, while the slow loop gain was unchanged. The fast loop contribution was reduced from 28 to 11% of the total ventilatory response. None of the other model parameters were significantly affected by the dopamine infusion. Exogenously administered dopamine substantially reduced the sensitivity of the fast chemoreflex loop to CO2.

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