Intraoperative Monitoring During Extracorporeal Circulation
Modern anesthesiology has allowed surgeons to extend surgical treament to areas once considered beyond approach. The technique of cardiopulmonary bypass, associated with hypothermia, makes it possible to maintain a patient in extreme conditions; i.e., the patient is kept alive for the whole duration of a surgical procedure, with the heart at a standstill, with the lungs not breathing, and with a mean arterial blood pressure of 40 mmHg or below. The conditions necessary for heart surgery are allowed by hypothermia which lowers the metabolism of the body to facilitate survival in these conditions. This is particularly important for the central nervous system, where a temperature of 20–25 °C can effectively compensate for the reduction of blood pressure and prevent neural damage. On the other hand, it has been reported2,5,10 that hypothermia may in itself cause neurologic injury. This view, however, is not shared by all authors. An adequate balance is required between hypothermia (a factor offering some protection) and hypotension (a potentially dangerous factor). Neural function should therefore be monitored to verify whether, at specific levels of body temperature and of mean arterial pressure, the ability to respond to external stimuli is preserved. Evoked potentials (EPs) have been useful and reliable for this purpose. Within the central nervous system, the structure most sensitive to ischemia is the cortex. For this reason, the study of the cortical somatosensory response appears to be the best approach to the problem. In the literature a few studies are reported 1,7,9 concerning evoked potential monitoring during cardiopulmonary bypass. These papers mainly describe subcortical and early cortical responses, the brainstem auditory evoked potential (BAEP) and the short latency somatosensory evoked potential (SSEP). It appeared that a detailed study of the entire cortical evoked potential, including late waves, might help early identification of cortical dysfunction. Cortical SEPs are made up of a primary component, the N 20–P 25 complex, and late waves. The N 20–P 25 complex (often called simply N 20) is related to the arrival of the afferent sensory volley at the primary somatosensory cortex. All the later components are of cortical origin. The sensitivity of the primary component to anesthetic drugs is less than the sensitivity of later components, as these are mediated by a multisynaptic pathway. Our experience with intraoperative SEP monitoring during carotid surgery indicated that a change affecting late waves was the first sign of cortical ischemia, seen well before either prolongation of the N 20 latency or reduction of the SEP primary complex. In this study we tried to define the limits of EP changes seen when both body temperature (T) and blood pressure (BP) were modified in parallel.
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