The hypothermic ECG and the cold blood gas — interpretation and prognostication

1. The ECG — the Osborne (J) wave. The strip shows a slow, broad-complex bradycardia with a characteristic positive deflection at the J point (the junction of the QRS complex and the ST segment). This is the Osborne wave, or J wave, the eponymous sign of hypothermia, appearing below about 32 degrees and growing in amplitude as the core falls — at a core of 28 degrees it is prominent. The strip also shows a prolonged PR and QT interval, and may show atrial fibrillation. The J wave is finding examiner shows on the strip, and the candidate who names it unprompted earns the mark.^[1]

2. The blood gas — interpret it UNCORRECTED. The machine offers a value "corrected for temperature" (the patient's core) and an uncorrected value (measured at 37 degrees). The candidate acts on the uncorrected (alpha-stat) values reported at 37 C, not the temperature-corrected (pH-stat) numbers. The gas typically shows a combined metabolic and respiratory acidosis from the hypoventilation and the shock.^[1][2]

3. The serum potassium — the prognostic anchor. The electrolytes carry the prognostic weight. Potassium is a HOPE covariate. Pasquier 2018: HOPE (age, sex, temperature, potassium, asphyxia mechanism, CPR duration) beat dichotomous potassium triage; traditional potassium cut-offs alone are less reliable for ECLS triage.^[7] The glucose is checked for a coexistent hypoglycaemia,^[1] and the coagulation profile is interpreted with the caveat that it reads falsely normal (the laboratory warms the sample to 37 degrees; the patient is coagulopathic in vivo).^[10]

4. Prognostication. No single factor is decisive. The outcome is read from the cluster: the depth and duration of the hypothermia, the mechanism (an asphyxial death is not salvageable; a pure hypothermic arrest can be), the downtime, the initial rhythm, and the serum potassium. The HOPE score integrates age, sex, core temperature, potassium, asphyxia versus non-asphyxia mechanism, and CPR duration (not first rhythm). The overriding rule is that no one is dead until warm and dead — do not apply a 32 C death clock; in adults use HOPE for ECLS triage (do not use HOPE in children).^[1][3][7]