Cardiovascular System

Blood Volume

The most striking maternal physiological alteration occurring during pregnancy is the increase in blood volume, which progress throughout pregnancy. The average increase in blood volume at term is 45 - 50 %. This is brought about by a 45% increase in plasma volume and a 20% increase in red blood cell volume. The disproportionate increase in plasma volume over red blood cells causes the physiological anaemia of pregnancy, where the average haemoglobin concentration falls by 2g/dl. The increase in blood volume is needed to provide extra blood flow to the uterus, kidneys and to supply extra metabolic need of the fetus.

Haemodynamic Changes in Pregnancy

During pregnancy, there is an increase in the heart rate and stroke volume (the amount of blood pumped by the heart with each heart beat) producing raised cardiac output (CO). The blood pressure does not increase despite the increase in cardiac output because there is a progressive fall in systemic vascular resistance (SVR). This fall in peripheral resistance results in maximal decrease in mean arterial pressure by the end of the first trimester. The diastolic blood pressure (BP) falls between 5 and 15 mmHg, before rising to non-pregnant levels by term, while systolic BP remains unchanged throughout pregnancy. The heart rate increase from average 72 to 85 beats/minute and stroke volume increases by up to 30%.This, together with the reduction in SVR, increases the CO to a maximum of 50% above non-pregnant levels by 24 weeks gestation. The increased blood flow is distributed mainly to the uterus and kidneys. Uterine blood flow increases from 50 ml/minute at 10 weeks gestation to 850 ml/minute at term and renal blood flow increases by 80%.

Haemodynamic Changes During Labour

CO increases further during labour. Between uterine contractions, the CO increases from pre-labour values by 10-25% during the first stage of labour, and 40% during the second stage of labour. CO and SVR increases by further 15-25% durng contractions, and this increase can be reduced by effective epidural analgesia. A progressive rise in sympathetic nervous system activity, which peaks at the time of delivery, increases myocardial contractility, SVR, and venous return. An auto-transfusion of 500 ml blood, from the placenta into circulation occurs during this period.

Heamodynamic Changes During the Puerperium

There is a state of relative hypervolemia and an increase in venous return following vaginal delivery with a sustained increase in cardiac in cardiac output and central venous pressure. Mothers with significant cardiac disease are at risk during this period.

Haematological Changes

The disproportionate increase in plasma volume over red cell volume in pregnancy causes a physiological anaemia, with a 15% drop in haemoglobin concentration and a decrease in blood viscosity. There is an increased demand for iron due to the increase in red cell volume and fetal requirements, and if iron stores are low, iron deficiency anaemia occurs. The platelets tend to remain normal but the white cell count increase during labour. Plasma protein concentration falls by 15% due to the increase in plasma volume, predisposing to edema seen in pregnancy and altering the pharmacokinetics of protein bound drugs. There is a relative hypercoagulable state during pregnancy in anticipation of haemorrhage at childbirth. This is brought about by an increase in clotting factors (V, VIII, and X) and a decrease in fibrinolytic and protein C activity. Due to this hypercoagulable state, there is an increased risk of thromboembolic disease in pregnancy.

Aorto-Caval Compression

Aorto-caval compression occurs when the pregnant woman lies supine or semi-supine and the gravid uterus compresses the inferior vena cava (IVC) and aorta. This causes a decrease in venous return to the heart and a fall in CO and blood pressure, which can compromise the mother or fetus. The reduction in venous return may partly compensated for by increased distal venous pressure pushing blood through compressed IVC, and by collateral venous pathways. Maternal symptoms and signs range from asymptomatic or mild hypotension, to total cardiovascular collapse. During regional anaesthesia, symptoms of dizziness, feeling faint and nausea often indicate a decreased blood pressure. The severity of aorto-caval compression may depend on:

  • The effectiveness of collateral venous systems
  • Gestation, with maximal effect seen at 36-38 weeks
  • Large uterine size associated with multiple pregnancies or polyhydraminios
  • The presence of sympathetic block from anesthesia.

A woman who is not anesthetized may be able to compensate for a drop in BP by increasing her SVR. However, in anaesthetized patients, these compensatory mechanisms are reduced or abolished resulting in a greater fall in BP when aorto-caval compression occurs. Even in the absence of maternal symptoms, placental blood supply may be compromised in the supine position, and after the 20th week of gestation, a left lateral tilt should ALWAYS be maintained to prevent aorto-caval compression. This tilt can be achieved by placing a wedge under the mother's hip, tilting the operating table by 15 degrees to the left, or placing the mother in the full lateral position.

Last modified: Thursday, 17 November 2016, 5:37 PM