Pathophysiological Impacts of Hypothermia
Neurological Effects
- Cerebral metabolism decreases by approximately 6-7% for each 1°C reduction in core temperature, which forms the basis for hypothermia's neuroprotective properties 1, 2
- Patients become confused, uncoordinated, and somnolent as temperature drops 2
- At approximately 30°C, progression to coma occurs 2
- Below 27°C, loss of deep tendon reflexes and pupillary reflexes develops 2
Cardiovascular Effects
- Mild hypothermia (<36°C) causes increased systemic vascular resistance to occur consistently 1, 4, 5
- Bradycardia develops, which may paradoxically be beneficial (similar to beta-blocker effects, reducing diastolic dysfunction) 1
- Myocardial contractility typically increases in most patients 1
- Progressive depression of cardiac activity occurs in moderate to severe hypothermia 2, 5
Metabolic and Endocrine Effects
- Hypothermia decreases insulin sensitivity and insulin secretion, causing hyperglycaemia that requires insulin treatment 1
- Decreased clearance of medications occurs, with sedative drugs and neuromuscular blockers reduced by up to 30% at 34°C 1
Renal and Electrolyte Effects
- Hypothermia causes cold-induced diuresis initially, followed by decreased glomerular filtration rate with prolonged cooling 1, 4, 5
- Hypophosphataemia, hypokalaemia, hypomagnesaemia, and hypocalcaemia can occur due to the biphasic response 1
- The diuresis can lead to dehydration and electrolyte disturbances that further compromise organ function, particularly in patients with underlying cardiac disease 5
Hematological Effects
- Hypothermia impairs platelet function between 33-37°C and affects coagulation factor activity and fibrinogen synthesis below 33°C 2, 5
- Clinical bleeding appears negligible in most studies, despite laboratory abnormalities 1
Respiratory Effects
- At 32°C, the medullary respiratory center becomes depressed, leading to decreased minute ventilation, increased secretions, and atelectasis development 2
Infectious Complications
- Hypothermia impairs immune system function and increases infection rates, particularly pneumonia 1, 3
- Prophylactic antibiotics may reduce pneumonia incidence, and early antibiotic use has been associated with improved survival 1
Rewarming Considerations
- Rewarming must be performed slowly at approximately 0.25-0.5°C per hour to avoid complications 1
- Rebound hyperthermia after therapeutic hypothermia is associated with increased mortality and worse neurological outcomes 3