CARDIOVASCULAR CHANGES IN PREGNANCY
Although cardiac output increases by 30%–40%, blood pressure is normally lower during pregnancy due to a 15% decrease in systemic vascular resistance. Furthermore, in the supine position, venous return decreases and cardiac output can be significantly affected with a drop of 25%–30% in the latter half of pregnancy (>20 weeks) when aortocaval compression is at its greatest. This compression of the aorta and the inferior vena cava against the lumbar vertebral bodies results in decreased venous return from the lower extremities, thereby decreasing preload, stroke volume and cardiac output.
SUPINE HYPOTENSIVE SYNDROME OF PREGNANCY
Hypotension in the supine position usually does not occur due to a compensatory rise in peripheral vascular resistance. However, up to 15% of women at term can demonstrate supine hypotensive syndrome of pregnancy, defined as a decrease in systolic blood pressure of at least 15–30 mmHg. The syndrome has been demonstrated in pregnant females from the middle of the second trimester onward. Cardiac output can decrease by 30%–40% in patients with this syndrome. Pregnant patients in the supine position have compression of the inferior vena cava and aorta by the gravid uterus, which leads to decreased venous return and thus hypoperfusion. Symptoms include tachycardia, diaphoresis, nausea, vomiting, pallor, weakness, lightheadedness, and dizziness. Women can lose consciousness and even maternal and/or fetal death can occur. Symptoms usually occur within 3–10 minutes after lying down. Inadequacy of paravertebral collateral blood supply is one etiology behind this hypotension.
Women who do not develop supine hypotensive syndrome of pregnancy demonstrate compensatory mechanisms including increased collateral venous flow through the paravertebral and azygous system leading to increased cardiac preload and reflex increases in systemic vascular resistance to maintain hemodynamic stability. Risk factors include size, shape, and weight of the uterus. These risk factors are more common with multiple pregnancies and obese parturients. In addition, the syndrome is exacerbated by neuraxial blocks due to the cofounding sympathectomy.
Symptoms are usually transient and resolve with change in positioning, specifically left lateral position. With minimal compression of the vena cava by the gravid uterus, hemodynamic changes resulting in hypotension are mostly avoided. Studies have shown radiographic and physiologic improvement in aortocaval compression in the lateral compared to supine position. Pregnant women at more than 20 weeks gestation should be placed in the full left lateral position when recumbent. Left lateral position is preferred, but other options include left lateral tilt and manual displacement of the gravid uterus. Left lateral tilt to 15°–30° is achieved by placing a wedge under the right hip and is used in practice for labor and delivery as well as nonobstetric surgery in pregnant patients undergoing anesthesia to prevent supine hypotensive syndrome. Left uterine displacement is performed by manually moving the uterus away from the midline to the left side when the patient is supine. ...