Because most studies of epidural test dose effectiveness have evaluated single-orifice catheters, Leighton et al. set out to determine the sensitivity and specificity of the air test to reliably detect intravascular placement with the currently used three-hole catheters. In 300 laboring women who requested epidural analgesia, blunt-tip, three-hole 20-gauge epidural catheters were placed at the L2–L3 or L3–L4 interspace. If neither blood nor cerebrospinal fluid (CSF) could be aspirated through the needle, 3 ml lidocaine, 2%, or 0.25% bupivacaine was injected. After a 3-min interval, patients were questioned about changes in lower extremity sensation or motor strength. If these signs of spinal anesthesia were absent, the team performed the air test by injecting 1 ml air through the epidural catheter and listening for maternal heart sound changes 15 s after injecting the air. Tests were repeated if the women experienced contractions during the observation period. If no blood was aspirated and no heart tone changes were heard, the team injected 10 ml bupivacaine, 0.125%, with 50 μg fentanyl. Epidural analgesia was continued if a sensory band was present 20 min after epidural injection.
Catheters through which blood was initially aspirated were air-tested and then replaced. Patients whohadair-test-positivebutblood-aspiration-negative catheters received 100 mg lidocaine through the catheter and then were questioned about toxicity symptoms (perioral numbness, tinnitus, and diplopia).
The air test detected 82% of intravascular catheters, whereas aspiration detected 91%; neither test detected all cases of intravascular placement. Eight of 11 intravascular catheters were positive for both the air test and the blood aspiration. In one air-test-positive catheter, blood could not be aspirated, and the patient developed perioral numbness after lidocaine injection. In the remaining 288 catheters, bupivacaine–fentanyl injection produced epidural analgesia in 279 patients, but no effect in another 9. The difficulty of testing multiport catheters underscores the need for careful local anesthetic dose fractionation to reduce the risk of large doses in patients with unrecognized epidurovascular or epidurointrathecal catheters.