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J Thorac Cardiovasc Surg 2002;123:531-538
© 2002 The American Association for Thoracic Surgery

Cardiopulmonary Support and Physiology (CSP)

Delayed detection of motor pathway dysfunction after selective reduction of thoracic spinal cord blood flow in pigs

From the Department of Anesthesiology,^a Department of Neurosurgery,^b Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands; Department of Surgery,^c University Hospital Maastricht, University of Maastricht, Maastricht, The Netherlands; and Department of Anesthesiology,^d University Hospital Utrecht, The Netherlands.

Mr Lips is supported by the Dutch Heart Foundation (Den Haag, The Netherlands; grant 97-193). This study was funded in part by the Departments of Anesthesiology and Experimental Surgery, University of Amsterdam (The Netherlands).

Received for publication April 23, 2001; revisions requested June 11, 2001; revisions received June 18, 2001; accepted for publication June 20, 2001. Address for reprints: Jeroen Lips, Department of Anesthesiology (H1-115), Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands (E-mail: j.lips{at}amc.uva.nl).

Objective: Clinical monitoring of myogenic motor evoked potentials to transcranial stimulation provides rapid evaluation of motor-pathway function during surgical procedures in which spinal cord ischemia can occur. However, a severe reduction of spinal cord blood flow that remains confined to the thoracic spinal cord might render ischemic only the descending axons of the corticospinal pathway. In this situation lower-limb motor evoked potentials could respond relatively late compared with a similar spinal cord blood flow reduction of the lumbar spinal cord that renders predominantly motoneurons ischemic.
Methods: Selective thoracic and lumbar spinal cord ischemia was induced by sequential clamping of segmental arteries during continuous assessment of laser-Doppler spinal cord blood flow at the thoracic and lumbar spinal cord. Myogenic motor evoked potentials were recorded from the upper and lower limbs. The time to loss of motor evoked potentials was compared (n = 11) during reduction of laser-Doppler spinal cord blood flow below 25% of baseline (ischemic segment), and flow was maintained at greater than 75% of baseline in the nonischemic segment, both during thoracic and lumbar spinal cord ischemia.
Results: Average laser-Doppler spinal cord blood flow in the ischemic segment was similar during thoracic (26% ± 15% [±SD]) and lumbar (26% ± 16%) ischemia, whereas normal flow was maintained in the nonischemic segment. The time to motor evoked potentials loss was considerably longer after thoracic spinal cord ischemia (15 ± 11 minutes) than after lumbar spinal cord ischemia (3 ± 2 minutes, P < .005).
Conclusion: In this experimental model of selective spinal cord ischemia, a severe reduction of lumbar spinal cord blood flow results in rapid loss of myogenic motor evoked potentials, whereas a similar blood flow reduction in the thoracic spinal cord results in relatively slow loss of motor evoked potentials. The effectiveness of motor evoked potentials to rapidly assess spinal cord integrity might be limited when spinal cord ischemia is confined to the thoracic segments.

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