MATERNAL AND FETAL EFFECTS OF ISOPROTERENOL IN THE GRAVID EWE

Detection of the intravascular placement of epidural catheters is an i mportant but difficult task. In this study, we evaluated maternal and fetal hemodynamic responses to intravenous (N) and epidural injection of isoproterenol (ISO), a proposed chronotropic test dose, in gravid e wes. Near-term, chronically instrumented, gravid ewes with single fetu ses were studied at least 48 h after surgery. We continuously recorded maternal heart rate (MHR), systemic and pulmonary blood pressures, ut erine blood flow (UBF), and fetal blood pressure and heart rate. Mater nal cardiac output was measured by thermodilution. In random sequence, each ewe (n = 11) received IV injections of saline, epinephrine (EPI) 15 mu g; ISO 4, 16, and 80 mu g; or epidural (n = 9 ewes) injections of saline, ISO 4 mu g and ISO 40 mu g. All variables returned to basel ine between experiments. Sections of lumber spinal cord were harvested from five animals for later histopathological study. IV ISO caused a dose-related increase in MHR. Cardiac output also increased transientl y after all doses of ISO but not after EPI. Maternal diastolic blood p ressure decreased after ISO 16 and 80 mu g. UBF decreased significantl y for 120 s after EPI 15 mu g. Epidural ISO did not significantly chan ge maternal systemic or pulmonary blood pressure, cardiac output, or U BF. The 40-mu g dose increased MHR significantly. No histopathological changes were seen in three ISO-exposed and two control spinal cords. IV ISO reliably induces maternal tachycardia in nonstressed gravid ewe s. Unlike EPI, IV ISO lacks a statistically significant effect on UBF. However, ISO seems to be rapidly absorbed from the epidural space. Id entifying the source of maternal tachycardia after epidural injection of a large dose of ISO could be difficult. If the absence of histopath ological change is confirmed, ISO represents an alternative to EPI as a chronotropic test dose.