Cardiovascular System Changes During Pregnancy

The pregnancy-induced changes in the cardiovascular system develop primarily to meet the increased metabolic demands of the mother and foetus . Despite the increased workload of the heart during gestation and labour, the healthy woman has no impairment of cardiac reserve.

Blood Volume increases progressively from 6-8 weeks gestation (pregnancy) and reaches a maximum at approximately 32-34 weeks with little change thereafter. The increase in plasma volume (40-50%) is relatively greater than that of red cell mass (20-30%) resulting in hemodilution and a decrease in haemoglobin concentration. Intake of supplemental iron and folic acid is necessary to restore hemoglobin levels to normal (12 g/dl).

The increased blood volume serves two purposes.
First, it facilitates maternal and fetal exchanges of respiratory gases, nutrients and metabolites.
Second, it reduces the impact of maternal blood loss at delivery . Typical losses of 300-500 ml for vaginal births and 750-1000 ml for Caesarean sections are thus compensated with the so-called " autotransfusion " of blood from the contracting uterus (cf. cardiac output below).

Blood Constituents . As mentioned above, red cell mass is increased 20-30%. Leukocyte counts are variable during gestation, but usually remain within the upper limits of normal. Marked elevations, however, develop during and after delivery. Fibrinogen, as well as total body and plasma levels of factors VII, X and XII increase markedly. The number of platelets also rises, yet not above the upper limits of normal. Combined with a decrease in fibrinolytic activity, these changes tend to prevent excessive bleeding at delivery . Thus, pregnancy is a relatively hypercoagulable state, but during pregnancy neither clotting or bleeding times are abnormal.

Cardiac Output increases to a similar degree as the blood volume. During the first trimester cardiac output is 30-40% higher than in the non-pregnant state.
During labor , further increases are seen with pain in response to increased catecholamine secretion; this increase can be blunted with the institution of labour analgesia . Also during labour, there is an increase in intravascular volume by 300-500 ml of blood from the contracting uterus to the venous system. Following delivery this autotransfusion compensates for the blood losses and tends to further increase cardiac output by 50% of pre-delivery values.

Cardiac Size/Position . The heart is enlarged by both chamber dilation and hypertrophy. Upward displacement of the diaphragm by the enlarging uterus causes the heart to shift to the left and anteriorly.

Blood Pressure . Systemic arterial pressure is never increased during normal gestation. In fact, by midpregnancy, a slight decrease in diastolic pressure can be recognized. Pulmonary arterial pressure also maintains a constant level. However, vascular tone is more dependent upon sympathetic control than in the nonpregnant state, so that hypotension develops more readily and more markedly consequent to sympathetic blockade following spinal or extradural anaesthesia . Central venous and brachial venous pressures remain unchanged during pregnancy, but femoral venous pressure is progressively increased due to mechanical factors. (See also hypertension in pregnancy and pre-eclampsia )

Aortocaval Compression . From mid-pregnancy, the enlarged uterus compresses both the inferior vena cava and the lower aorta when the patient lies supine (on the back). Obstruction of the inferior vena cava reduces venous return to the heart leading to a fall in cardiac output by as much as 24% towards term.

The information in this page is presented in summarised form and has been taken from the following source(s):
1. World Anaesthesia & the World Federation of Societies of Anaesthesiologists : http://www.nda.ox.ac.uk/wfsa/index.htm