Moderate hypercapnia: cardiovascular function and nitrogen elimination.

Elevated carbon dioxide concentrations frequently encountered in diving operations may have cardiovascular effects. If so, changes in nitrogen loading and elimination may be induced. To study this possibility, whole body nitrogen elimination rates were determined using a rebreathing apparatus and gas chromatographic measurement of N2 in expired gas in six subjects as they breathed mixtures of 3 and 5% CO2 with 21% O2 and a balance of Ar for 125 min. No significant differences were observed among mean N2 yields, which were 815 ml (95% confidence interval +/- 51 ml), 831 ml (+/- 38 ml), and 845 ml (+/- 57 ml) for 0, 3, and 5% CO2 mixtures, respectively. Simultaneous measurements of heart rate showed a significant increase while breathing 5% CO2 as compared to 3 and 0% CO2. The increases in heart rate were not accompanied by any significant change in cardiac output, mean arterial pressure, or tissue perfusion. We conclude that at these levels of hypercarbia, tissue perfusion is not influenced enough to cause any changes in whole-body N2 elimination.