Hypothermia I

One of the patients I admitted last call was a middle aged man found down on a sidewalk for an unknown period of time. His core temperature was 24.8 degrees Celsius (76 F). That's cold. The ambient temperature these days is 15-20 degrees Celsius. A normal temperature is about 37 degrees Celsius. Hypothermia is a temperature below 35; severe hypothermia below 28. At that temperature, enzymes in the body fail, molecules crystallize, the heart is at risk for life-threatening arrhythmias.

Hypothermia itself is a fascinating illness to me. Many of the things we normally rely on are untrustworthy. For example, hypothermia causes a "bleeding diathesis"; the normal coagulation proteins stop working and people bleed easily. However, the laboratory test for coagulation is done at 37 degrees and so results are misleading. Likewise, arterial blood gases are done at 37 degrees, electrolytes may be inaccurate, and hematocrit is concentrated as temperature drops. The EKG starts showing Osborne waves, present in this patient, and very cool to see since they are so rare. You can't use insulin in patients under 30 C because it will crystallize.

You can rewarm patients in many ways. Mild hypothermia can be managed passively with blankets (it makes me think of the mantra for newborns: warm, dry, stimulate). With more severe hypothermia, active rewarming can take the form of heating pads, warm blankets, warm humidified air by endotracheal tube, and warm fluids by IV. Even more aggressive measures include irrigating body cavities (pleural space, peritoneum, bladder) with warm fluids by chest tubes or peritoneal lavage. A patient can even be warmed by extracorporeal methods, removing blood, warming it externally, and returning it to the patient.

Of note, you have to warm the trunk before the extremities. Warming the extremities first can cause a paradoxical cooling because cold acidotic blood from the arms and legs returns to the core. Warming the extremities also causes vasodilation which can drop the blood pressure. Speaking of paradoxical phenomena, there is also this strange behavior called paradoxical undressing which happens with moderate hypothermia; people will take off their clothes, worsening the hypothermia; the reason is unclear.