Stroke Volume Recruitability during the Third Trimester of Pregnancy.

Objective: It is unknown whether the heart operates in the ascending or flat portion of the Starling curve during normal pregnancy. Pregnant women do not respond to the passive leg-raising maneuver secondary to mechanical obstruction of the inferior vena cava by the gravid uterus. Our objective was to evaluate if administration of a fluid bolus increases baseline stroke volume (SV) among healthy pregnant patients during the third trimester.

Study Design: Healthy pregnant women who underwent elective term cesarean sections were included. A noninvasive cardiac output monitor was used to measure hemodynamic variables at baseline and after administration of a 500-mL crystalloid bolus.

Results: Forty-five women were included in the study. Fluid administration was associated with a statistically significant increase in SV from a baseline value of 71 ± 11 to 90 ± 19 mL (95% confidence interval [CI]: 13.67-21.49;  < 0.01) and a significant decrease in maternal heart rate from a baseline of 87 ± 9 beats per minute to 83 ± 8 after the fluid bolus (95% CI: -6.81 to -2.78;  = 0.03). No changes in peripheral vascular resistances or any other measured hemodynamic parameters were noted with volume expansion.

Conclusion: In healthy term pregnancy, the heart operates in the ascending portion of the Starling's curve, rendering it fluid responsive.