Peripheral vascular and cardiac effects of nitrous oxide in the bovine

Peripheral vascular and myocardial effects of increasing concentrations of nitrous oxide (0 to 70 per cent) in oxygen were determined in 15 unanaesthetized calves before and after replacement in their natural heart (NH) with a pneumatically driven artificial heart (AH). Nitrous oxide produced concentration-related decreases in arterial and mixed venous pH and increases in minute ventilation and arterial and mixed venous carbon dioxide tensions in both NH and AH calves. Nitrous oxide resulted in significant increases in cardiac output, stroke volume and mean aortic, pulmonary artery and right atrial pressures in NH and AH calves, but did not significantly change systemic vascular resistance in either group of animals. Heart rate was increased in NH calves but was fixed in AH calves. Elevations in heart rate and cardiac output at nitrous oxide concentrations greater than 30 per cent and aortic pressure at 70 per cent nitrous oxide were significantly greater in NH than AH animals (P less than 0.05). These data demonstrate that nitrous oxide stimulates the cardiovascular system in spontaneously breathing mammals and that the changes result from improved venous return and an increase in myocardial chronotropy. Our findings also suggest that cardiovascular stimulation during nitrous oxide breathing may be related to increased concentrations of arterial and/or venous carbon dioxide.