Continuous pump pressures cannot be used to identify catheter tip migration into the subarachnoid space.
BACKGROUND AND OBJECTIVES: Migration of an epidural catheter into the subarachnoid space is a potentially lethal complication of continuous epidural anesthesia. We evaluated the use of pump pressure measurement during infusion in detecting such an occurrence. METHODS: Pump pressures at two flow rates (P1 and P10) and epidural space pressure (P0) were measured hourly in eight consecutive ASA Physical Status I parturients receiving continuous epidural anesthesia and compared to those of a patient whose catheter migrated into the subarachnoid space. In addition, total resistance to infusion (Rtot) was calculated as the slope of the least squares regression line linking pressure and flow. Epidural resistance (Repi) was computed as the difference between total resistance and catheter resistance; Repi = Rtot - Rcath. RESULTS: In epidural catheters, average P0 and P10 were 13 +/- 4 mmHg and 23 +/- 4 mm Hg, respectively, and did not change over time. Rtot and Repi averaged 960 +/- 180 RU and 980 +/- 120 RU, respectively. Pump pressures in the subarachnoid catheter were 10 mmHg for P0 and 20 mmHg for P10, while Rtot was 1000 RU. CONCLUSIONS: We conclude that epidural and subarachnoid space resistance is essentially zero and the pump pressure in the system is used to overcome the catheter resistance. Both spaces offer the same total resistance to infusion and, therefore, pump pressure will not change when a catheter migrates into the subarachnoid space.