[1] Also, the abrupt contact with very cold water may cause involuntary inhalation, which, if underwater, can result in fatal drowning.
For people with pre-existing cardiovascular disease, the additional workload can result in myocardial infarction and/or acute heart failure, which ultimately may lead to a cardiac arrest.
Although this process is a continuum, the 4 phases were initially described in the 1980s as follows:[3][4] The first stage of cold water immersion syndrome, the cold shock response, includes a group of reflexes lasting under 5 min in laboratory volunteers and initiated by thermoreceptors sensing rapid skin cooling.
The primary components of the cold shock reflex include gasping, tachypnea, reduced breath-holding time, and peripheral vasoconstriction, the latter effect highlighting the presumed physiologic principle (i.e., warmth preservation via central blood shunting).
It is an evolutionary adaptation that helps mammals, including humans, manage the challenges of being submerged in cold water.
The diving reflex is more pronounced in aquatic mammals and is thought to have originated as a way to conserve oxygen and enhance the ability to stay underwater for longer periods.
These cold water immersion induced arrhythmias appear to be accentuated by parasympathetic stimulation resulting from facial submersion or breath holding.
Even vagally dominant diving bradycardia caused by isolated cold water facial immersion frequently is interrupted by supraventricular arrhythmias or premature beats.
In theory, atrioventricular blockade or sinus arrest due to profound parasympathetic dominance might result in syncope or sudden cardiac death, but these rhythms tend to be rapidly reversed by lung stretch receptor activation associated with breathing.
Beneficial adaptations include the following: Cold shock has been described in several species and at least part of the physiology is similar, as described above in the Diving Reflex.
To constitute as a cold shock the temperature reduction needs to be both significant, for example dropping from 37 °C to 20 °C, and it needs to happen over a short period of time, traditionally in under 24 hours.