Hypothermia is defined as a drop of core temperature (usually measured in oesophagus or rectum) below 35°C. Impact of hypothermia onto electrical conduction system of heart was first described in 1892. The first observations concerning changes in ECG of a patient in accidental hypothermia date back to 1938. Hypothermia is associated with presence of characteristic changes in ECG, depending on hypothermia stage.

Motion artefacts related to shivering and tremors are characteristic of mild hypothermia (Figure 1) together with sinus bradycardia resulting from compensatory stimulation of sympathetic nervous system.

Prolonged exposure to low temperature significantly impairs both electric as well as mechanic activity of the heart. As a result of catecholamines release, advanced stages of hypothermia are distinguished by decreased peripheral vascular resistance and noticeable drop in cardiac output. Prolonging of action potential duration and decrease in conduction velocity in conduction system of the heart are related to delay in activation/inactivation of cellular ionic (sodium, potassium and calcium) currents. Hypothermia causes prolonging of both depolarisation duration as well as repolarisation of sinus node cells. Decrease of body temperature below 32°C causes substantial reduction of conduction velocity. In such situation, serious sinus bradycardia develops, with PR and QT/QTc intervals prolongation as well as prolongations of P wave, QRS and T wave durations (Figure 2). There is a non-linear relationship between reduced heart rate and decrease in core temperature. Prolongation of QT/QTc interval is caused both by delayed ventricular repolarisation and well as presence of so-called Osborn wave (J wave). In some patients, reduction of conduction velocity in atrio-ventricular node leads to development of conduction blocks.

Osborn wave (J wave) is certainly the most evident anomaly in ECG of hypothermic patients. Occurrence of the wave in hypothermic patients was first recorded in 1938 (Tomasjewski), but the full description of of the phenomenon was published in 1951 by J. Osborn (hence eponym). Osborn wave is a positive deflection appearing at junction between QRS complex and ST segment, resembling “camel hump” (Figure 3).