Effects of hypothermia and sternal retractors on median nerve somatosensory evoked potentials.

BACKGROUND

Somatosensory evoked potentials (SEPs) are altered by hypothermia, which is often used during cardiopulmonary bypass (CPB). However, the effect of hypothermia on SHP amplitudes is unclear. Also, the sternal retractors used during open heart surgery are reported to cause brachial plexus distension and SEP changes.

METHODS

Median nerve SEPs under hypothermic CPB were studied in 29 elective patients scheduled for open heart surgery. In 23 patients who underwent left internal mammary artery (IMA) dissection, the effects of sternal retractors on cortical SEP before the initiation of CPB were investigated.

RESULTS

A latency shift of all SEP components was detected when nasopharyngeal temperature decreased from 35.7(SD 0.4) degrees C to 27.8(SD 0.25) degrees C. The mean cortical N20 latency was increased by 39% (P < 0.0001), cervical N13 by 33% (P < 0.0001), and peripheral N9 by 27% (P < 0.0001). The latency changes were reversible when normothermia was restored. The effect of hypothermia on SEP amplitudes was more complex. The mean amplitude of N20 decreased from 2.7 microV to 2.2 microV (P < 0.05) and the amplitude of N13 from 2.5 microV to 2.0 microV (P < 0.0001). In contrast, the N9 component showed an increase from 1.4 microV to 2.1 microV (P < 0.0001) during hypothermia. The sternal retractors did not cause significant cortical SEP amplitude changes during IMA dissection or sternotomy. Also, the latency changes were small, although significant (P < 0.05).

CONCLUSION

Despite the moderate amplitude changes produced by hypothermia, SEPs can be successfully monitored during hypothermia. Theoretically, the different behaviour of amplitude in peripheral and cranial components of SEP during hypothermia is interesting. Hypothermia has a more profound effect on synaptic transmission, represented by the cortical N20 latency, than on the peripheral nerve conduction velocity. Intraoperative monitoring of temperature is essential whenever SEPs are recorded. The sternal retractors were not responsible for the intraoperative SEP changes.