Central nervous system pathophysiology

In all cases of near-drowning, the degree of central nervous system insult determines the prognosis. Hypoxemia and cardiopulmonary arrest result in generalized tissue hypoxia. Anoxic brain damage occurs within 4 to 6 min of cessation of effective cerebral blood flow. Global cerebral ischemia results in a diffuse axonal injury pattern characterized by generalized intracellular swelling. Histological changes consist of mitochondrial swelling, particularly in the cerebral cortex, hippocampus, and cerebellum. Intracranial pressure may be elevated secondary to the increase in brain volume, without mass effect, unless there is associated trauma. Aspiration of hypotonic freshwater results in a relative hypervolemia and decreased plasma oncotic pressure, further aggravating the cerebral edema initiated by hypoxia and loss of blood-brain barrier integrity. Cerebral edema may then increase intracranial pressure and further compromise cerebral blood flow during resuscitation.

Remarkably, some near-drowning victims survive submersion for periods of 1 h or more, usually in cold water. Survival in these situations appears to be the result of a specific temporal relationship between hypothermia and the diving reflex which together result in a state of protective hypometabolism ( G.oo.d.e.n 1992). Hypothermia has cerebroprotective effects only at temperatures below 30 °C. The diving response, which is profound in 15 per cent of people, prevents aspiration of water, redistributes oxygen supply to the heart and brain, and decreases cardiac oxygen consumption. The pathophysiological importance of the diving reflex is controversial.

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