Hemodynamic responses to acute carboxyhemoglobinemia in the rat

The effects of carbon monoxide on the hemodynamics of the adult rat were investigated. A number of parameters were measured using an open-chest, chloralose-urethan anesthetized preparation. Our experiments showed this anesthetic agent to have several advantages over pentobarbital sodium. One group inhaled 150 ppm CO for 0.5-2 h, carboxyhemoglobin (HbCO) reaching 16%. Heart rate, cardiac output, cardiac index, dF/dtmax (aortic), and stroke volume rose significantly; mean arterial pressure, total peripheral resistance, and left ventricular systolic pressure fell, whereas stroke work, left ventricular dP/dtmax, and stroke power changed little. These effects were evident at a HbCO saturation as low as 7.5% (0.5 h). A second group inhaled 500 ppm CO for 5-48 h, HbCO reaching 35-38%. The same parameters changed in the same direction as in the first group, with mean arterial pressure and peripheral resistance remaining depressed, while heart rate, cardiac output, cardiac index, and stroke volume remained elevated. Heart rate and arterial systolic pressure were also monitored in conscious rats; rats in one group inhaled 500 ppm CO for 24 h, and rats in a second group were injected with a bubble of pure CO ip. In both cases heart rate was sharply elevated and blood pressure depressed as HbCO saturation increased. Both parameters recovered on CO washout. There was no significant difference between the response to inhaled vs. injected CO.