Scuba diving is a recreational activity that places unique physiological demands on the human body. The question of whether diving can cause a heart attack is important for safety, and the answer lies in the interaction between the underwater environment and a diver’s underlying health. While diving itself does not typically cause a heart attack in a healthy person, the combination of environmental stressors and pre-existing, often undiagnosed, cardiovascular issues significantly increases the risk of a cardiac event. This interaction between the body and the underwater world makes cardiovascular fitness a central consideration for all divers.
How Immersion Affects the Heart
The moment a person enters the water, the body begins a series of immediate and profound cardiovascular adjustments. Immersion triggers a “blood shift” due to external hydrostatic pressure, which acts like a compression stocking on the lower body and limbs. This pressure forces blood from the peripheral vessels toward the chest cavity, increasing the volume of blood returning to the heart by up to 700 milliliters.
This centralized blood volume causes the heart’s chambers to enlarge slightly, leading to a temporary increase in cardiac output, sometimes by more than 30%. Simultaneously, the autonomic nervous system initiates the mammalian dive reflex, which is enhanced by cold water. The dive reflex causes peripheral vasoconstriction, narrowing blood vessels in the extremities to prioritize blood flow to the core, and slows the heart rate (bradycardia) to conserve oxygen.
Peripheral vasoconstriction increases the resistance the heart must pump against, known as afterload. For a healthy heart, these changes are managed effectively, but they place a substantial load on the circulatory system. The overall effect is a simultaneous activation of both the parasympathetic system (slowing the heart) and the sympathetic system (increasing blood pressure), which can predispose susceptible individuals to rhythm disturbances.
Pre-existing Cardiovascular Risk Factors
Heart attacks and other cardiac events account for an estimated 20 to 30 percent of deaths during scuba diving. These events are rarely random occurrences in healthy divers; instead, they usually arise when the physiological stresses of diving combine with chronic, underlying health issues. The most significant vulnerability is often undiagnosed or undertreated Coronary Artery Disease (CAD), where the arteries supplying the heart muscle are narrowed.
When a diver has CAD, the increased workload and oxygen demand placed on the heart by cold water and central blood volume shift can exceed the limited blood supply available. This lack of supply can trigger a heart attack (myocardial infarction) or a dangerous arrhythmia. Uncontrolled hypertension (high blood pressure) is another major risk factor because it means the heart is already pumping against consistently high resistance.
The transient increase in central blood pressure caused by immersion further strains a heart dealing with chronic hypertension, increasing the risk of acute failure or stroke. Age-related decline in cardiovascular fitness decreases the heart’s capacity to adapt to stress over time. For divers over the age of 50, the presence of risk factors like high cholesterol, obesity, and smoking significantly elevates the chance of a fatal cardiac event.
Acute Diving Stressors That Increase Cardiac Load
Beyond chronic risk factors, acute stressors encountered during a dive can act as the immediate trigger for an event in a vulnerable diver. Physical exertion, such as swimming against a strong current, carrying heavy equipment, or performing an emergency task, dramatically increases the heart’s immediate oxygen demand. If a diver’s coronary arteries cannot supply enough blood to meet this sudden demand, the lack of oxygen to the heart muscle can precipitate an acute coronary event.
Thermal stress from cold water acutely increases cardiac load by intensifying peripheral vasoconstriction. This narrowing of the blood vessels forces the heart to work harder to maintain circulation, increasing both the afterload and the heart muscle’s need for oxygen. Rapid cooling, often termed “cold shock,” can cause a surge in sympathetic nervous system activity, which may conflict with the parasympathetic activity of the dive reflex, increasing the risk of arrhythmia.
Emotional and psychological stress, such as panic or anxiety in a challenging underwater situation, also contributes to cardiac strain. Stress releases catecholamines, hormones that increase heart rate and blood pressure, further escalating the heart’s oxygen requirement. This combination of heavy exertion, cold exposure, and emotional distress creates a perfect storm of high cardiac demand and limited oxygen supply for a diver with underlying cardiovascular disease.
Medical Screening and Safety Guidelines
Proactive medical screening is essential for mitigating cardiac risk. Diving organizations recommend that all divers, especially those aged 45 and older or those with known risk factors, undergo regular medical evaluations to assess fitness to dive. Screening focuses on identifying pre-existing conditions like undiagnosed CAD or uncontrolled hypertension before they become life-threatening underwater.
A comprehensive evaluation may include a detailed medical history review, a resting electrocardiogram (ECG) for older divers, and potentially an exercise stress test to assess functional capacity and look for signs of inducible ischemia. The goal is to ensure the diver’s heart can tolerate an exercise level equivalent to 13 metabolic equivalents (METs) without developing symptoms or abnormal rhythms.
Divers should also adopt lifestyle changes to manage modifiable risk factors, including maintaining a healthy weight, controlling blood pressure, and ceasing smoking. Divers with controlled hypertension should have their blood pressure stabilized for at least three months before diving clearance is given. Planning dives within personal limits, avoiding heavy exertion, and ensuring adequate thermal protection are simple, actionable steps that reduce the acute cardiac load during every dive.