The effect of vasopressor agents upon uterine artery blood flow velocity in the gravid guinea pig subjected to ritodrine infusion.

The purpose of the present study was to assess the effects of intravenously administered vasopressors upon uterine artery blood flow velocity (UBFV) in the gravid guinea pig subjected to ritodrine infusion. Fourteen experiments were performed in 14 chronically instrumented pregnant guinea pigs near term. Immediately following a 1-h intravenous infusion of ritodrine (0.05-0.20 mg.kg.min-1), each animal received an intravenous bolus of vasopressor solution: 1) epinephrine, 0.001 mg/kg; 2) phenylephrine, 0.01 mg/kg; 3) mephentermine, 1.0 mg/kg; 4) ephedrine, 1.0 mg/kg; or 5) placebo. The experimental sequence was performed five times, so that each animal received all five solutions. The vasopressor sequence was randomly altered between animals. Infusion of ritodrine increased maternal heart rate 18 +/- 1% (P less than .0001), decreased maternal mean arterial pressure (MMAP) 4 +/- 1% (P less than .01), and decreased UBFV 5 +/- 1% (P less than .001). The four active vasopressor solutions resulted in similar, though not equivalent, increases in MMAP. Further, the MMAP response to each active vasopressor differed from the response to placebo (P less than .0001). Epinephrine and phenylephrine each significantly decreased UBFV (P less than .002). Ephedrine clearly preserved UBFV, whereas mephentermine appeared to result in an intermediate response.