Aortocaval compression syndrome is also known as a supine hypotensive syndrome. This pathophysiologic state occurs in a pregnant female, typically after 20 weeks gestation, when the patient is placed in the percent position. The impedance of blood flow back from the lower extremities to the maternal heart and central circulation occurs from compression of the uterus on the inferior vena cava, and also the aorta. This subsequently limits blood flow out to the placenta, and may result in morbidity and mortality to the mother and fetus alike. Due to the increased morbidity and mortality to both patients involved, it is critical that the health care team be aware and recognize this entity as a cause of hypotension in the pregnant patient, and immediately initiate maneuvers to correct it if suspected.
The lordotic curvature of the lumbar spine causes an outward bowing of the vertebral column in the lower (L4 - L5) spinal area towards the abdominal-pelvic cavity. The uterus is located intra-abdominally at the same L4 – L5 vertebral level. Juxtaposed between the lumbar spine and the uterus is the inferior vena cava. While gravity works to keep the uterus and enclosed fetus outward and off of the inferior vena cava and lumbar spine when the mother is upright or sitting, that is not the case when in the supine position. Once in the supine position, gravity allows the uterus to rest posteriorly onto the lordotic spine, potentially compressing the inferior vena cava under its weight. As a result of this compression, the blood flow returning from the extremities may be impeded drastically, resulting in maternal hypotension.
The description of the supine hypotensive syndrome was originated in 1953 by Howard et al. This paper noted a decrease in blood pressure in 18 of 160 gravid females studied who were placed in a supine position. They attributed this decrease in systemic blood pressure to occlusion of the inferior vena cava by the gravid uterus, though without radiologic or angiographic confirmation. In 1966, Bieniarz et al. measured blood pressure in the upper and lower extremities in conjunction with aortic angiographic measurements to infer aortic compression by the uterus with supine positioning. Following this study, the aorta was implicated in the supine hypotensive syndrome, making the term aortocaval compression syndrome synonymous with the supine hypotensive syndrome.
Unfortunately, trauma and/or accidental injury occurs not infrequently during pregnancy, with a known complication rate of six to seven percent of all pregnancies. Aortocaval compression syndrome may occur in the setting of trauma when the patient is placed in the position of safe transport, spinal immobilization precautions and resuscitation position, however, has also been noted to occur in routine procedures and deliveries when the maternal patient is placed in the same position. It occurs in patients with gestational age greater than 20 weeks, though may occur earlier in certain conditions.
The uterus has a blood flow of approximately 60 milliliters per minute in the non-pregnant state. However, at term, the uterus commands an impressive 600 milliliters per minute, which is a ten-fold increase. Because of this increased vascular dynamic state, even a mild decrease in blood return may result in marked negative effects on maternal and fetal circulation. Aortocaval compression syndrome is characterized by initial tachycardia and late bradycardia, pallor, diaphoresis, nausea, hypotension, and dizziness. All of these symptoms are attributable to the impedance of blood flow back into maternal circulation from the lower extremities, which have increased venous pressures progressively throughout pregnancy. It occurs when a pregnant woman lies on her back and subsequently resolves when she is turned on her side, thus alleviating the compressing pressure of the gravid uterus on the vena cava.
Physical examination does not reveal specific, pathognomonic signs for this syndrome. Diagnosis of aortocaval compression is based upon clinical assessment and suspicion, however, may be accompanied by ultrasound diagnosis. Without knowledge of this pathophysiologic state, the syndrome will likely go unrecognized. Particular attention should always be paid to the vital signs, with knowledge as to normal changes in pregnancy as related to trimester. Blood pressure decreased throughout pregnancy, however, returns to normal at term. In the first and second trimesters, both the diastolic blood pressure and the systolic blood pressure decrease by 15 to 20 mmHg. Therefore, any hypotension is abnormal in a third-trimester patient. As with any hypotensive patient, multiple etiologies should be simultaneously sought after and treated expeditiously. These include, but are not limited to, traumatic hepatic, splenic or renal bleeding, pelvic fractures, ruptured uterus, placental abruption, amniotic fluid embolus, pulmonary embolus, sepsis and other.
Evaluation of aortocaval compression syndrome is done clinically alone. There may be a possible role in ultrasound evaluation, though this has yet to be studied.
Once other causes of hypotension are treated and/or ruled out, then the physical movement of the uterus off of the spine (and inferior vena cava) is necessary. This may be accomplished in one of the following ways. When trauma is not involved, simply placing the patient in the left lateral position may be done. However, when trauma is present, immobilization of the spine must simultaneously occur, and different maneuvers are indicated.
The patient will likely be on a backboard with cervical spine immobilization necessary, which presents a unique challenge. There are three ways listed to accomplish the lateral position without compromising the immobilization. First, isolated elevation of the right hip alone may alleviate the compression. According to the Advanced Trauma Life Support Guidelines, tilting the backboard 15 to 30 degrees to the left (right side upwards) is an additional option. This may be done either manually or with elevators, including premade elevators or towel rolls placed under the board. This option is sometimes difficult as the weight of gravid abdomen gravitationally pulls the patient to the left side, potentially compromising spinal immobilization. Though difficult, in-line immobilization is a priority. Finally, when either of the first two maneuvers is not optional due to moribund condition and CPR, or other, a manual displacement of the uterus to the left of the midline is the treatment. This is done by placing the provider's hand on the right side of the abdomen, lateral to the gravid uterus, and shifting the uterus to the left, and thus off of the vena cava. This maneuver is simple, requires no expertise, and is not harmful to the uterus or fetus.