Hypothermia is a clinical entity defined as a core body temperature less than 35°C (95°F) and is classified in four stages depending on the temperature, symptomatology, and effect on specific organ physiology (Table 81.1).

All organs are ultimately affected by hypothermia, including the kidneys. The renal response to cold is rapid and varies with the different stages of hypothermia. Initially, peripheral vasoconstriction results in relative central hypervolemia producing an increase in urine output. This response, termed “cold diuresis,” has been described even in patients with mild to moderate hypothermia. The etiology of this phenomenon is multifactorial and includes an initial increase in cardiac output and renal blood flow resulting from hypothermia-induced changes in vascular capacitance. Other important contributors are nonosmotic suppression of antidiuretic hormone (ADH) release by the hypothalamus and subsequent decreased renal tubular reabsorption. These responses usually begin as soon as the core body temperature reaches 35°C and become more pronounced until moderate hypothermia, when decreased renal blood flow and glomerular filtration rate (reduced 50% at 27°C to 30°C) may lead to renal failure.

Even in the setting of a large diuresis (the urine is usually dilute, with osmolarity <300mOsm/L and specific gravity <1.003), the kidneys are unable to handle nitrogenous waste because of tubular dysfunction. Although they are uncommon, electrolyte disturbances including hypernatremia, hyperchloremia, and hyperkalemia can occur and are more frequent as hypothermia progresses in duration and/or severity. “Cold diuresis” is exacerbated by ethanol ingestion and water submersion, which may coexist with hypothermia and can potentiate inappropriate diuresis by inhibiting ADH secretion.

It is important to understand the pathophysiology of this phenomenon when making clinical decisions regarding fluid management in hypothermic patients. The “cold diuresis” can be massive and generally creates a hypovolemic state that worsens with rewarming because of the reverse changes in vascular tone as core body temperature is raised. If it is overlooked or underappreciated, this phenomenon can exacerbate electrolyte disturbances, contribute to hypotension, and result in prerenal stress.