Air: an effective indicator of intravenously located epidural catheters

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. Doppler changes were never heard following epidural air injection (P less than 0.001 compared with iv air microbubble injection) or the sham injection (P less than 0.001 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.