TABLE 4-4. Central Hemodynamic Changes in 10 Normal Nulliparous Women Near Term and 6.2 ± 1.0 4.3 ± 0.9 +43% 18.0 ± 1.5 20.8 ± 1.0 -14% 10.5 ± 2.7 14.5 ± 2.5 -28% 48 ± 6 41 ± 8 NSC aMeasured in lateral recumbent position.
bChanges significant unless NSC = no significant change.
COP = colloid osmotic pressure; PCWP = pulmonary capillary wedge pressure.
Data from Clark, 1989.
lessened. Speciically, cardiac MR imaging shows that when a woman rolls from her back onto her left side, cardiac output at 26 to 30 weeks' gestation rises by approximately 20 percent and at 32 to 34 weeks by 10 percent (Nelson, 2015). Consistent with this, Simpson and James (2005) found that fetal oxygen saturation is approximately 10 percent higher ifa laboring woman lies in a lateral recumbent position compared with supine. Upon standing, cardiac output falls to the same degree as in the nonpregnant woman (Easterling, 1988).
In multifetal pregnancies, compared with singletons, maternal cardiac output is augmented further by almost another 20 percent. Ghi and coworkers (2015) used transthoracic echocardiography to show that irst-trimester cardiac output with twins (mean 5.50 Umin) was more than 20 percent greater than postpartum values. Cardiac output values in the second (6.31 L/min) and third (6.29 Umin) trimesters were increased an additional 15 percent compared with irst-trimester output. Left atrial and left ventricular end-diastolic diameters are also longer with twins due to augmented preload (Kametas, 2003). The greater heart rate and inotropic contractility imply that cardiovascular reserve is reduced in multifetal gestations.
During first-stage labor, cardiac output rises moderately.
During the second stage, with vigorous expulsive eforts, it is appreciably greater. The pregnancy-induced increase is lost ater delivery, at times dependent on blood loss.
Clark and associates (1989) conducted invasive studies to measure hemodynamic function late in pregnancy (Table 4-4). Right heart catheterization was performed in 10 healthy nulliparas at 35 to 38 weeks' gestation, and again at 11 to 13 weeks postpartum. Late pregnancy was associated with the expected increases in heart rate, stroke volume, and cardiac output. Systemic vascular and pulmonary vascular resistance both dropped signiicantly, as did colloid osmotic pressure. Pulmonary capillary wedge pressure and central venous pressure did not change appreciably. Thus, although cardiac output rises, left ventricular function as measured by stroke work index remains similar to the nonpregnant normal range (see Fig. 4-9). Put another way, normal pregnancy is not a continuous "high-output" state.
Changes in posture afect arterial blood pressure (Fig. 4-10). Brachial artery pressure when sitting is lower than that when in the lateral recumbent supine position (Bamber, 2003). Additionally, systolic blood pressure is lower in the lateral positions compared with either the flexed sitting or supine positions (Armstrong, 2011). Arterial pressure usually declines to a nadir at 24 to 26 weeks' gestation and rises thereafter. Diastolic pressure decreases more than systolic.
FIGURE 4-10 Sequential changes (±SEM) in blood pressure throughout pregnancy in 69 women in supine (blue lines) and left lateral recumbent positions (red lines). PP = postpartum. (Adapted from Wilson, 1980.)