Cardiac arrest during pregnancy (peripartum cardiac arrest) is a rare but life-threatening event requiring immediate and specialized medical attention. While the incidence is low, affecting approximately one in every 12,000 hospitalizations for childbirth, the consequences for both the mother and the fetus can be severe. Understanding the underlying causes is paramount, as the factors leading to cardiac arrest in pregnant patients are often distinct from those in the general population. The unique physiological adaptations of pregnancy can transform a manageable medical issue into a catastrophic event.
Physiological Changes That Increase Vulnerability
Pregnancy necessitates profound adjustments to the cardiovascular system, creating a state of increased vulnerability. Cardiac output increases substantially by 30% to 50% during pregnancy, primarily through increased stroke volume and heart rate. This persistent increase in workload means the heart has little reserve capacity to compensate if a complication arises.
Total blood volume increases by up to 50%. This disproportionate rise in plasma volume compared to red blood cells leads to hemodilution, which can mask the true severity of blood loss. Additionally, the growing uterus can compress the inferior vena cava and the aorta (aortocaval compression) when the patient lies flat. This compression significantly reduces blood return to the heart, causing cardiac output to drop and compromising resuscitation efforts.
Respiratory function is altered, making pregnant patients prone to rapid oxygen depletion. The enlarged uterus pushes the diaphragm upward, decreasing the functional residual capacity (the volume of air remaining in the lungs after a normal exhale). This decreased oxygen reserve, combined with increased oxygen consumption, means the patient can become dangerously hypoxic much faster. Upper airway tissues often swell, making intubation more technically challenging during an emergency.
Immediate Triggers Stemming from Pregnancy Complications
Many frequent causes of peripartum cardiac arrest relate directly to complications of pregnancy and delivery. Massive obstetric hemorrhage, particularly postpartum hemorrhage (PPH), is a leading cause. The immense blood flow to the pregnant uterus (up to 1,000 milliliters per minute at term) means conditions like placental abruption, placenta previa, or uterine rupture can lead to rapid hypovolemic shock. The massive blood loss overwhelms the body’s compensatory mechanisms, causing systemic collapse.
Amniotic Fluid Embolism (AFE) is a rare but catastrophic cause, characterized by a sudden, severe, multi-system reaction. This event occurs when amniotic fluid or debris enters the maternal bloodstream, causing rapid cardiovascular collapse, respiratory failure, and severe coagulopathy. The resulting shock and inability of the blood to clot contribute to AFE’s high mortality rate.
Hypertensive disorders of pregnancy, such as severe pre-eclampsia and eclampsia, are significant triggers. Pre-eclampsia involves new-onset hypertension and organ dysfunction, typically after 20 weeks of gestation. Progression to eclampsia, characterized by seizures, can lead to stroke, intracranial hemorrhage, or systemic organ failure (HELLP syndrome). These events place extreme stress on the already burdened cardiovascular system, culminating in cardiac arrest.
Critical Non-Obstetric Medical Events
Non-obstetric medical events also contribute to cardiac arrest, often with increased frequency or severity due to the pregnant state. Pulmonary embolism (PE) is a major contributor, as pregnancy places the patient in a hypercoagulable state to prevent excessive bleeding during delivery. This increased tendency for clot formation means a pregnant patient is at higher risk of a clot traveling to the lungs, causing a massive PE that obstructs blood flow and leads to sudden cardiac arrest.
Pre-existing or newly developed cardiac diseases are a significant factor, especially with increasing maternal age. Peripartum Cardiomyopathy (PPCM) is a form of heart failure that develops late in pregnancy or postpartum in patients with no prior heart disease. This condition involves the weakening of the heart muscle, leading to acute decompensation and potentially fatal arrhythmias.
Sepsis, a life-threatening response to infection, remains a cause of cardiac arrest, rapidly progressing to septic shock. Infections (from the urinary tract, lungs, or uterus) can overwhelm the immune system, causing widespread inflammation, low blood pressure, and organ failure. Complications related to anesthesia, such as a high spinal or epidural block, can also result in severe, sudden hypotension that may precipitate cardiac arrest.