Ordinary surface ECG electrodes accurately reflect cardiac electric activity at hypothermia

It has been claimed that needle electrodes can be a useful means to detect weak ECG signals in cases of accidental hypothermia.

Four pigs were cooled by immersion in ice water, followed by direct cooling of the blood through an extracorporeal circulation system until the core temperature was lowered to 12 degrees C and surface-measured ECG indicated asystole. Following cooling, the pigs were rewarmed and weaned from extracorporeal circulation if possible. ECG and interelectrode impedance were measured between surface electrodes, needle electrodes and electrodes sewn to the epicardium during the cooling and rewarming procedure.

Needle and surface electrodes showed exactly the same ECG whatever the temperature of the skin or the core was. The impedance varied only slightly with temperature and could not explain the disappearance of surface ECG. The QRS wave amplitude showed the greatest sensitivity to temperature, disappearing completely before the P-wave disappeared. The P-wave showed the least sensitivity, and was the last wave to disappear, indicating that the sinus node is the most resistant part of the heart to cooling. Between 19 and 17 degrees C, a commercial monitor indicated asystole although P-waves could be seen in the ECG and atrial contractions could be visually observed on the heart.

Surface electrodes had a similar high accuracy to indicate electric activity as needle electrodes. Higher amplification and reduction of the timebase made it possible to detect ECG in a situation where asystole was indicated by commercial monitors.