Low blood pressure, or hypotension, is a common event during labor and delivery that requires careful attention. Maternal hypotension is typically defined as a systolic blood pressure below 100 mmHg or a drop of over 20% from the woman’s baseline. Regular monitoring of blood pressure is standard procedure to assess cardiovascular stability. These fluctuations must be managed quickly, as they can affect both the mother’s well-being and blood flow to the baby.
Anesthesia and Pharmacological Triggers
Regional anesthesia, such as an epidural or spinal block, is the most frequent cause of a significant drop in blood pressure during labor. Local anesthetics block the sympathetic nervous system fibers, a process called sympathectomy. This prevents the nerves from sending signals that normally maintain blood vessel tone.
When sympathetic tone is lost, blood vessels relax and widen (vasodilation). This leads to venous pooling, where blood collects in the lower extremities. Consequently, less blood returns to the heart, immediately lowering the heart’s output and decreasing systemic vascular resistance and blood pressure.
The speed and severity of the drop depend on the type of anesthesia. A spinal block causes a rapid and profound sympathetic block, leading to an immediate, sharp drop in pressure. Conversely, an epidural block infuses the anesthetic into the epidural space, resulting in a more gradual and less severe onset of hypotension.
Mechanical and Positional Factors
The enlarged uterus can mechanically interfere with the circulatory system, causing low blood pressure, known as aortocaval compression syndrome. When the mother lies flat on her back (supine position), the weight of the uterus compresses the inferior vena cava (IVC). This compression significantly reduces the blood returning to the heart, decreasing the heart’s output.
The uterus can also compress the aorta, impeding circulation and diminishing blood flow to the placenta. To prevent this, changing the mother’s position is an effective intervention. Tilting the mother onto her left side (left lateral tilt) shifts the uterus away from the major blood vessels, relieving pressure and restoring proper venous return.
Systemic Maternal Health Issues
Underlying systemic maternal health issues can also trigger hypotension during labor. Acute blood loss, or hemorrhage, is a serious cause because it suddenly reduces the total circulating blood volume. Even internal loss of blood volume can impair the heart’s ability to maintain adequate pressure.
Severe dehydration is another factor that lowers circulating volume, often resulting from prolonged labor or excessive vomiting. A reduced fluid volume in the bloodstream means less fluid is available to fill the vascular system, directly translating to a drop in blood pressure.
Systemic infection, or sepsis, is a severe pathological cause of low blood pressure. The body’s overwhelming inflammatory response causes widespread vasodilation throughout the circulatory system. This extreme widening, combined with fluid leaking into tissues, dramatically lowers systemic vascular resistance and blood pressure, potentially leading to shock.
Consequences and Clinical Response
When maternal blood pressure remains low, the mother may experience dizziness, nausea, vomiting, and faintness. The impact on the fetus is more concerning, as hypotension reduces blood flow to the placenta. This diminished uteroplacental blood flow can temporarily decrease the oxygen supply to the baby, often detected as changes in the fetal heart rate pattern.
Clinicians follow a rapid, multi-step protocol to restore blood pressure. Initial actions involve correcting the mother’s position, using a left lateral tilt to alleviate aortocaval compression. Simultaneously, medical staff administer an intravenous fluid bolus, rapidly infusing fluid into the bloodstream to increase circulating volume.
If non-pharmacological methods are insufficient, vasopressor medications are administered. These drugs, such as phenylephrine or ephedrine, cause blood vessels to constrict, increasing systemic vascular resistance and raising blood pressure. The aim is to rapidly normalize maternal blood pressure, minimizing the duration of the hypotensive event and protecting the fetus from complications like fetal acidosis.