EFFECTS OF TEMPERATURE ON PHRENIC-NERVE AND DIAPHRAGMATIC FUNCTION DURING CARDIAC-SURGERY

We have studied the effects of whole body cooling on phrenic nerve and diaphragmatic function in 26 patients using magnetic stimulation of t he phrenic nerves with a pair of Magstim 200 HP stimulator coils durin g cardiopulmonary bypass. The diaphragmatic electromyogram in response to magnetic pulses was recorded with needle electrodes at two tempera tures, approximately 31 degrees C (cold) and approximately 36 degrees C (warm) during the cooling or rewarming phase of hypothermic cardiopu lmonary bypass. This 5-degrees C temperature change was associated wit h clear changes in the evoked electromyographical response of the diap hragm. Median latency between stimulus and electromyographic response was 10.1 (range 8.0-11.8) ms during cold and 8.3 (5.9-10.2) ms during warm stimulation (P < 0.001). Median duration of the muscle compound a ction potential was prolonged and its amplitude reduced in cold compar ed with warm stimulations (P < 0.01). These effects were enhanced by a pplication of ice slush to the heart. We conclude that diaphragmatic f unction may be affected by mild hypothermia after cardiac surgery.