Does Hypothermia Increase Diuresis?
Yes, hypothermia causes diuresis, particularly during the initial cooling phase, through decreased antidiuretic hormone (ADH) release and reduced renal tubular reabsorption of water and sodium. 1
Mechanism of Cold-Induced Diuresis
The diuretic effect of hypothermia occurs through multiple physiological mechanisms:
Decreased ADH secretion: Plasma ADH levels drop significantly during hypothermia (from 7.5 to 3.8 pg/mL in experimental models), directly reducing water reabsorption in the renal collecting ducts 2
Impaired tubular function: Even when ADH is replaced pharmacologically with desmopressin, urine osmolality and sodium excretion are only partially normalized, indicating that hypothermia also reduces renal medullary hypertonicity and tubular conservation capacity 2
Reduced nephron reabsorption: The combination of decreased urine osmolality and increased fractional sodium excretion (4- to 6-fold increases) demonstrates that tubular reabsorption is significantly impaired during cooling 3
Clinical Pattern During Therapeutic Hypothermia
The diuretic response follows a predictable temporal pattern during targeted temperature management:
Induction phase: Urine output increases most dramatically, reaching mean rates of 157 mL/hour (adjusted increase of +51 mL/hour compared to post-rewarm baseline) 4
Maintenance phase: Urine output decreases to approximately 103 mL/hour, not significantly different from baseline 4
Rewarming phase: Urine output further decreases to approximately 70 mL/hour 4
Post-rewarm phase: Urine output stabilizes at approximately 91 mL/hour 4
Electrolyte Consequences
Mild induced hypothermia causes diuresis and electrolyte abnormalities including hypophosphataemia, hypokalaemia, hypomagnesaemia, and hypocalcaemia. 1 These disturbances result from the biphasic renal response—initial diuresis followed by decreased glomerular filtration rate with prolonged cooling 5
Clinical Management Implications
For Patients Undergoing Therapeutic Hypothermia
Monitor plasma electrolyte concentrations closely, as they can change rapidly during both cooling and rewarming phases 1
Maintain effective intravascular volume, as cold-induced diuresis can lead to dehydration that further compromises organ function, particularly in patients with underlying cardiac disease 6, 5
Adjust diuretic therapy individually to account for clinical signs of dehydration during cold exposure 6
Common Pitfalls to Avoid
Do not assume stable fluid status: The initial diuresis during cooling can cause significant volume depletion that may not be clinically apparent until rewarming begins 1
Monitor beyond the cooling phase: Plasma electrolyte concentrations and effective intravascular volume can change rapidly during rewarming, requiring continued vigilance 1
Anticipate reduced GFR: Despite initial diuresis, glomerular filtration rate decreases by approximately 50% in non-acclimatized individuals at 25°C, which can affect drug clearance and fluid balance 3
Renal Function Changes
Hypothermia produces a biphasic renal response:
Initial phase: Increased urine flow (2-fold) and sodium excretion (4- to 6-fold) despite decreased GFR 3, 2
Prolonged cooling: Progressive decrease in glomerular filtration rate, with reductions of approximately 50% at core temperatures of 25°C 3
Recovery: All renal variables return to baseline levels upon rewarming 3