Drowning is defined as the process of experiencing respiratory impairment from submersion or immersion in a liquid. Drowning can cause cardiac arrest, which is typically a secondary event. The heart stops not due to a primary heart problem, but as a result of the body’s inability to breathe. This cascade is driven by a severe, prolonged lack of oxygen, known as asphyxia.
The Physiological Pathway to Cardiac Arrest
The sequence leading to cardiac arrest begins the moment the airway is submerged. The initial, voluntary breath-holding phase gives way to an involuntary reflex when oxygen stores are depleted and carbon dioxide levels rise. This triggers an overwhelming urge to breathe, resulting in the aspiration of water or a reflex closure of the voice box, called laryngospasm.
Oxygen deprivation, or hypoxia, rapidly ensues, affecting every organ system but most critically the brain and the heart. The body attempts to compensate, but the lack of oxygen forces cells to switch to anaerobic metabolism, which produces lactic acid. This accumulation of acid leads to a severe state of metabolic and respiratory acidosis.
The heart muscle, struggling under the strain of profound hypoxia and acidosis, begins to fail. The electrical signaling within the heart becomes erratic, often resulting in severe bradycardia, or an abnormally slow heart rate. This slow heart rate is typically the last stage before the final cardiac collapse, which is often asystole.
Asystole is a state where the heart muscle is completely electrically silent and motionless. Drowning-related cardiac arrest is fundamentally a problem of oxygen starvation, unlike many other types that begin with an electrical disturbance.
Types of Drowning and Associated Injuries
Drowning is now medically understood as a single process, though the effects on the lungs vary. Historically, this led to the terms “wet” and “dry” drowning. Wet drowning occurs when the laryngospasm reflex relaxes, allowing fluid aspiration into the lungs, which damages the lung lining and impairs oxygen exchange. Dry drowning referred to cases where laryngospasm persisted, preventing water entry, but the underlying mechanism of death remains asphyxia due to lack of oxygen.
Delayed Complications
Secondary or delayed drowning refers to respiratory complications that develop hours or days after a non-fatal incident. This delayed injury is usually due to acute respiratory distress syndrome (ARDS) or chemical pneumonitis caused by aspirated fluid. Even if the person appears stable after rescue, damage to the lung tissue can progressively impair breathing, necessitating careful medical observation.
Aspiration of water, whether fresh or salt, causes inflammation and destroys surfactant, a substance that keeps the lung’s air sacs open. Current medical consensus holds that the primary injury in both is severe hypoxia and lung damage, despite traditional beliefs about vastly different electrolyte imbalances.
Critical Factors Influencing Survival
The duration of submersion and the timeliness of intervention are the most significant predictors of survival and long-term neurological outcome. Irreversible brain damage begins after only a few minutes without oxygen. Therefore, prompt initiation of rescue breathing and chest compressions (CPR) is the most important intervention.
Water Temperature and Hypothermia
Water temperature is another factor influencing survival. Submersion in cold water, defined as below 70°F (21°C), can sometimes trigger the mammalian diving reflex. This reflex shunts blood away from the extremities to the core, protecting the heart and brain, and causes a drop in heart rate.
Hypothermia, or a drop in core body temperature, dramatically slows the body’s metabolism. This reduction in metabolic rate decreases the brain’s demand for oxygen, effectively buying time for rescue and resuscitation. Intact survival has been reported in cases of prolonged submersion in very cold water, particularly in children who have a more pronounced diving reflex.
The type of water aspirated can affect the body, though this is less significant than the duration of hypoxia. Freshwater is hypotonic, meaning it is absorbed rapidly into the bloodstream, potentially leading to red blood cell rupture (hemolysis). Saltwater, being hypertonic, draws fluid from the bloodstream into the lungs, causing pulmonary edema.