Air

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Abstract
The authors conducted a two-part study to evaluate the efficacy of 1 ml of air as a "test dose" for detection of intravenously located epidural catheters. In part 1, a Doppler fetal heart rate monitoring probe was placed over the precordium of 33 laboring patients in whom functioning epidural catheters were in place. Each patient received, more than 90 s apart, in random order: 10 ml of agitated saline (containing less than 0.5 ml of air microbubbles) via a peripheral vein; 2 ml of air via the epidural catheter; and a sham injection (i.e., nothing injected). In all 33 cases, a blinded observer identified Doppler changes 10-30 s following the injection of air (microbubbles) via peripheral vein. Dopper changes were never heard following epidural air injection (P < 0.001 compared with iv air microbubble injection) or the sham injection (P < 0.01 compared with iv air microbubble injection). In part 2, the authors listened for Doppler heart tone changes while injecting 1 ml of air via catheters that were accidentally inserted in the epidural veins of five other patients. Unequivocal Doppler changes compatible with intracardiac air always occurred within 3 s, and no signs or symptoms of air embolism developed. The results suggest that 1 ml of air may be a suitable indicator of iv epidural catheter location.