The air test as a clinically useful indicator of intravenously placed epidural catheters

Abstract

The authors performed a clinical trial in 313 patients in labor to determine the safety and efficacy of an air test for unintentional intravenous placement of epidural catheters. Following routine aspiration for blood and cerebrospinal fluid, 1 ml of air was injected through each epidural catheter while heart tones were continually monitored with a Doppler ultrasound probe placed over the maternal precordium. In 281 patients, Doppler heart tones did not change following air injection (negative air test). All but eight of these patients developed an adequate level of analgesia following anesthetic administration, and no patients with negative air tests developed signs or symptoms of local anesthetic toxicity (false-negative rate, 0%; 95% confidence limits, 0.0-1.1%). Doppler heart tone changes followed air injection in 22 cases (positive air test). In 16 of these, intravenous catheter position was subsequently shown by aspiration of blood from the catheter or by the use of test doses consisting of local anesthetics with or without epinephrine. In six cases, adequate levels of analgesia developed despite a positive air test (false-positive rate, 2%; 95% confidence limit, 0.7-4.3%). None of the 303 patients receiving the air test developed any complications attributable to the injection of air (95% confidence limits, 0.0-1.0%). The authors conclude that air, with precordial Doppler detection, is a safe and effective test for identifying intravenously located epidural catheters.