Drowning - Pathophysiology

Pathophysiology

A continued lack of oxygen in the brain, hypoxia, will quickly render a victim unconscious usually around a blood partial pressure of oxygen of 25-30mmHg. An unconscious victim rescued with an airway still sealed from laryngospasm stands a good chance of a full recovery. Artificial respiration is also much more effective without water in the lungs. At this point the victim stands a good chance of recovery if attended to within minutes. Latent hypoxia is a special condition leading to unconsciousness where the partial pressure of oxygen in the lungs under pressure at the bottom of a deep free-dive is adequate to support consciousness but drops below the blackout threshold as the water pressure decreases on the ascent, usually close to the surface as the pressure approaches normal atmospheric pressure. A blackout on ascent like this is called a deep water blackout.

The brain cannot survive long without oxygen and the continued lack of oxygen in the blood combined with the cardiac arrest will lead to the deterioration of brain cells causing first brain damage and eventually brain death from which recovery is generally considered impossible.

A lack of oxygen or chemical changes in the lungs may cause the heart to stop beating; this cardiac arrest stops the flow of blood and thus stops the transport of oxygen to the brain. Cardiac arrest used to be the traditional point of death but at this point there is still a chance of recovery. The brain will die after approximately six minutes without oxygen but special conditions may prolong this (see 'cold water drowning' below).

As well as the direct effect of oxygen deprivation, there are also dangerous effects on blood chemistry if water is taken into the lungs. The mechanism for this is different for fresh and seawater.

• Freshwater taken into the lungs will be pulled into the pulmonary circulation by osmosis. The dilution of blood leads to hemolysis (bursting of red blood cells). The resulting elevation of plasma K+ (potassium) level and depression of Na+ (sodium) level alter the electrical activity of the heart often causing ventricular fibrillation. In animal experiments this effect was shown to be capable of causing cardiac arrest in 2 to 3 minutes. Acute renal failure can also result from hemoglobin from the burst blood cells accumulating in the kidneys, and cardiac arrest can also result if cold freshwater taken into the bloodstream sufficiently cools the heart.

• Sea water is hypertonic to blood (more salty). It poses the opposite danger. Osmosis will instead pull water from the bloodstream into the lungs, thickening the blood. In animal experiments the thicker blood requires more work from the heart leading to cardiac arrest in 8 to 10 minutes.

Autopsies on human drowning victims show no indications of these effects and there appears to be little difference between drownings in salt water and fresh water. After death, rigor mortis will set in and remains for about two days, depending on many factors including water temperature.