Does ritodrine worsen maternal hypotension during epidural anesthesia in gravid ewes?

The purpose of this study was to determine whether prior administration ritodrine worsens maternal hypotension during epidural anesthesia in gravid ewes. Twenty-four experiments were performed in nine chronically instrumented animals between 0.8 and 0.9 of timed gestation. The experimental sequence included the following: 1) at time-zero, intravenous (iv) administration of ritodrine, 0.004 mg.kg-1.min-1, or normal saline (NS) for 2 h; 2) at 120 min discontinuation of ritodrine, and administration of a 500 ml iv bolus of NS over 15 min; and 3) at 135 min epidural injection of 2% lidocaine or NS. There were three groups of experiments: 1) iv ritodrine-epidural lidocaine (n = 9); 2) iv NS-epidural lidocaine (n = 8); and 3) iv ritodrine-epidural NS (n =7). Epidural injection of lidocaine resulted in a median sensory level of T9 in both the ritodrine-lidocaine and NS-lidocaine groups. At 165 min (i.e., 30 min after the epidural injection of lidocaine), maternal mean arterial pressure was 19 +/- 3% below baseline (P = 0.0001) in the ritodrine-lidocaine group and 22 +/- 7% below baseline (P = 0.0001) in the NS-lidocaine group (NS between groups). At 120 min ritodrine had increased maternal cardiac output 45 +/- 6% above baseline (P = 0.0001) in the ritodrine-lidocaine group and 39 +/- 6% above baseline (P = 0.0001) in the ritodrine-NS group. Cardiac output remained above baseline (P less than 0.01) after epidural injection of lidocaine in the ritodrine-lidocaine group. In contrast, in the NS-lidocaine group cardiac output was 13 +/- 5% below baseline (P = 0.005) at 150 min. Fetal arterial pH did not change significantly in either the ritodrine-lidocaine or ritodrine-NS group.(ABSTRACT TRUNCATED AT 250 WORDS)