Indirect evidence has accrued to suggest that the phrenic nerve may become “frostbitten” during bypass surgery. Experimentally, damage to large myelinated fibers has been shown with 8°C exposure for as little as 30 min. Radiographic evidence of diaphragmatic dysfunction has been shown in patients undergoing CAB surgery with topical cold cardioplegia. This is less common with devices used to prevent direct contact between the slush and the phrenic nerve or with use of a myocardial cooling jacket. In an animal model developed by Marco et al immediate postoperative phrenic dysfunction was found in five dogs with an exposure similar to that of CAB surgery. birth control pills
Despite these supportive data, phrenic nerve injury appears to be uncommon with the use of topical cold cardioplegia during bypass surgery in humans. An incidence of phrenic nerve paralysis of 5 of 57 and 5 of 44 has been reported using phrenic nerve conduction studies, despite the almost invariable use of topical cardioplegia in these patients. In our study, topical cold cardioplegia was used in all five patients, only one of whom had concomitant use of a cardiac insulation pad. Even though cold may be a relatively infrequent cause of phrenic nerve injury, it would be prudent either to avoid use of saline slush or at least to use an insulating device during CAB surgery. The recovery time from axonal degeneration is critically dependent on the distance which the axons must regrow.