Chapter 20 Drowning
Immersion in thermoneutral water activates protective airway reflexes and aspiration does not occur until lung oxygen stores have been used up and hypoxia causes the airway to open.
It is estimated that there were more than 400 000 victims of drowning worldwide in the year 2000.1 In several countries drowning is a major cause of accidental death, particularly amongst children. Drowning is more common in low- or middle-income countries than high-income countries,1 and is around three times more common in men than women. Alcohol is a major aetiological factor.2 For each victim of death by drowning, there are estimated to be between several cases of ‘near-drowning’ that are severe enough to require hospital admission, and probably hundreds of other less severe incidents.2 Death from pulmonary complications (‘secondary drowning’) may occur a considerable time after the accident, in patients who were initially normal.
Physiology of Immersion2,3
Cold shock describes a combination of several cardiovascular and respiratory reflexes that occur in response to sudden total-body immersion in cold water.4 Sudden immersion in water below 25°C is a potent stimulant to respiration and causes an initial large gasp followed by substantial hyperventilation. The stimulus is increased with colder temperatures, reaching a maximum at 10°C.2 Functional residual capacity is acutely increased, and individuals may find themselves breathing almost at total lung capacity, giving a sensation of dyspnoea. Breath-hold time is severely reduced, often to less than 10 seconds, which impairs the ability of victims to escape from a confined space underwater or to orientate themselves before seeking safety.
Diving reflex. In response to cold water stimulation of the face and eyes, the diving reflex produces bradycardia, peripheral vasoconstriction and apnoea in most mammals. It is particularly well developed in diving mammals, to reduce oxygen consumption and facilitate long duration dives. The reflex is present in humans,5 though of small magnitude compared with other species, and is believed to be more significant in infants than adults.2
Physiological Mechanisms of Drowning
Glottic closure from inhaled water, pulmonary aspiration, cold shock and the diving response all influence the course of events following submersion in water; the relative importance of each depends, amongst many other factors, on the age of the victim and the temperature of the water. Conflicting influences on the heart from activation of both the parasympathetic (diving reflex) and sympathetic (cold shock) systems are believed to contribute to death from cardiac dysrhythmia in some victims.2,4
Drowning without Aspiration of Water
This occurs in less than 10% of drowning victims.6 In thermoneutral water, when cold-stimulated reflexes will be minimal, the larynx is firmly closed during submersion and some victims will lose consciousness before water is aspirated. The rate of decrease of alveolar, and therefore arterial, Po2 depends on the lung volume and the oxygen consumption. Oxygen stored in the alveolar gas after a maximal inspiration is unlikely to exceed 1 litre, and an oxygen consumption of 3 l.min−1 would not be unusual in a subject either swimming or struggling. Loss of consciousness from decreased alveolar Po2
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