Clinical Note

Measurement of Blood Pressure

It is noteworthy that measurements of blood pressure and cardiac output made in the late stages of pregnancy are very much dependent upon maternal position when the measurements are made. The gravid uterus, which occupies a volume of 6 to 7 L, compresses the vena cava and to a lesser extent the abdominal aorta when the subject rests on her back in the supine position. The gravid uterus may thus physically decrease venous return and cardiac output and cause a fall in systemic arterial pressure.

Similarly, when the woman is standing upright, the gravid uterus compresses the femoral veins and interferes with blood flow from the lower extremities to the vena cava. The resulting decline in venous return produces a decline in cardiac output and a fall in blood pressure. Because baroreceptor responses are blunted, the fall in blood pressure may cause some women to faint after prolonged standing or a sudden change in posture. Reliable assessment of blood pressure is usually made with the woman lying on her left side.

It is possible that increased renal blood flow partially compensates for decreased hematocrit in maintaining renal interstitial PO2 so that erythropoietin-secreting cells are only mildly stimulated. Alternatively, the PO2 sensitivity of these cells may be decreased by the hormones of pregnancy, so that the set-point for regulation of red cell formation is adjusted downward.

Expansion of the blood volume also dilutes the plasma proteins. The concentration of albumin declines by about 30%. Because albumin is the most abundant plasma protein and the major colloid osmolyte, plasma oncotic pressure also decreases by about 30%. Synthesis of albumin and some other hepatic proteins is thought to be regulated by plasma oncotic pressure; a decrease in

Gestation (weeks)

FIGURE 5 Changes in plasma and red cell volume during the course of normal pregnancies. Shaded area indicates range of variation.

Gestation (weeks)

FIGURE 5 Changes in plasma and red cell volume during the course of normal pregnancies. Shaded area indicates range of variation.

oncotic pressure sensed by hepatocytes activates transcription of the albumin gene. Once again it appears that the sensitivity of a regulatory mechanism is reduced by some actions of hormones of pregnancy. Hepatic protein synthetic capacity is not compromised, as evidenced by increased production and secretion of some globulins, fibrinogen, and clotting factors.

The changes in vascular volume and in levels of red blood cells and albumin would be considered pathological in nonpregnant women, but are normal in pregnancy and appear to result from adjustments to the set-points of normally operating feedback regulatory systems. The adaptive advantages of decreased circulating levels of albumin and red blood cells are not known; however, the combined effect of the lower red cell mass and the lower protein content is a decrease in blood viscosity (see Chapter 10) which contributes in at least a minor way to the overall decrease in peripheral resistance shown in Fig. 3. It should be emphasized that, although the hematocrit declines in pregnancy, the total red cell mass increases significantly and may therefore lessen the postpartum impact of the inevitable loss of about 500 mL of blood at delivery.

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