What is the pathophysiology of drowning?

Updated: Jun 19, 2019
  • Author: G Patricia Cantwell, MD, FCCM; Chief Editor: Joe Alcock, MD, MS  more...
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The most important contributory factors to morbidity and mortality from drowning are hypoxemia and acidosis and the multiorgan effects of these processes. Central nervous system (CNS) damage may occur because of hypoxemia sustained during the drowning episode (primary injury) or may result from arrhythmias, ongoing pulmonary injury, reperfusion injury, or multiorgan dysfunction (secondary injury), particularly with prolonged tissue hypoxia.

After initial breath holding, when the victim's airway lies below the liquid's surface, an involuntary period of laryngospasm is triggered by the presence of liquid in the oropharynx or larynx. At this time, the victim is unable to breathe in air, causing oxygen depletion and carbon dioxide retention. As the oxygen tension in blood drops further, laryngospasm releases, and the victim gasps, hyperventilates, possibly aspirating variable amounts of liquid. This leads to further hypoxemia.

Lunetta et al reviewed the autopsies of 578 individuals who had apparently drowned and found evidence of water in the lungs of 98.6% of those studied. As they noted, active ventilation while submerged is required to aspirate water, as water does not passively flow into the lungs once the victim is dead. [49]

Depending upon the degree of hypoxemia and resultant acidotic change in acid-base balance, the person may develop myocardial dysfunction and electrical instability, cardiac arrest, and CNS ischemia. [50] Asphyxia leads to relaxation of the airway, which permits the lungs to take in water in many individuals, although most patients aspirate less than 4 mL/kg of fluid.

Fluid aspiration of at least 11 mL/kg is required for alterations in blood volume to occur, and aspiration of more than 22 mL/kg is required before significant electrolyte changes develop. Ingestion of large volumes of freshwater, rather than aspiration, is the likely cause of clinically significant electrolyte disturbances, such as hyponatremia, in children after drowning.

Approximately 10-15% of individuals maintain tight laryngospasm until cardiac arrest occurs and inspiratory efforts have ceased. These victims do not aspirate any appreciable fluid (previously referred to as "dry drowning") (see the chart below).

Mechanism of hypoxia in submersion injury. Mechanism of hypoxia in submersion injury.

In young children suddenly immersed in cold water (< 20°C), the mammalian diving reflex may occur and produce apnea, bradycardia, and vasoconstriction of nonessential vascular beds with shunting of blood to the coronary and cerebral circulation.

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