Nitrous oxide and air-filled balloon-tipped catheters.

The differential solubilities of gases permit nitrous oxide (N2O) diffusion into air-filled body cavities, increasing their size and/or pressure. An air-filled balloon-tipped catheter represents a body cavity that may be affected by the diffusion of N2O, and could account for the authors' clinical observation that more gas was aspirated than injected during insertion of Swan-Ganz (S-G) catheters in patients receiving N2O anesthesia. An accompanying increase in balloon diameter could conceivably account for difficulties in floating the tip into the pulmonary outflow tract. To help substantiate these observations, balloon volumes of three S-G catheters were measured at 0.5, 1, 2, 3, 4, 5, 10, 20, and 30 min in various N2O and oxygen mixtures, and the diameters of the balloon were compared with published diameters of pulmonary outflow tracts. The volume changes were near maximum between 5-10 min, increasing to 30 to 150 per cent depending on the N2O concentration. The increases in balloon diameter, when compared to pediatric pulmonary outflow tract diameters, could account for difficulties in passage of the catheter tip through the pulmonary outflow tract. These findings suggest that manipulation of S-G catheters under N2O anesthesia sould be done with intermittent deflation of the balloon every few minutes.