Assessment: intraoperative neurophysiology. Report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology

Experience to date in both humans and controlled animal research studies substantiates that several electrophysiology tests and monitoring techniques are safe and efficacious, to a variable degree, as commonly applied in the operating room for the following procedures: EEG or SEP to monitor for cerebral ischemia; ECoG and SEP sensory cortex identification to define the limits of cortical resection; SEP spinal cord monitoring; BAEP and cranial nerve EMG monitoring during posterior fossa procedures; functional localization of cortex with direct cortical stimulation in expert hands; and EMG and compound muscle and nerve action potential measurements of various peripheral nervous system structures. All these techniques need to be applied by a well-trained, knowledgeable physician-neurophysiologist or personnel directly under his or her supervision. Clinical situations need to be chosen carefully, avoiding those in which the nervous system is only at low risk.

Mixed venous oximetry during automatic implantable cardioverter-defibrillator placement

Mixed venous oxygen saturation (SvO2) monitoring was used to assess tissue and circulatory recovery following induced ventricular tachycardia or fibrillation in 17 patients undergoing surgery for automatic implantable cardioverter-defibrillator (AICD) placement. Return of systemic arterial pressure conventionally determines adequate recovery. The duration of circulatory arrest during defibrillator threshold (DFT) testing, measured from the moment of absent phasic pressure at the radial artery until its return, was 18 +/- 8 seconds (mean +/- SD, n = 118 episodes). The absolute decrease in SvO2 from baseline to nadir for these 118 episodes was 14% +/- 6% absolute, and correlated well with the duration of circulatory arrest (r = 0.757, P = 0.0001). The time from onset of phasic arterial blood pressure to the nadir of SvO2, available for 46 episodes, was 28 +/- 14 seconds, and did not correlate with the duration of arrest. The time from onset of phasic pressure to the return of SvO2 to within 1% (absolute) of baseline saturation, available for 84 episodes, was 52 +/- 32 seconds and, in the aggregate, correlated poorly (r = 0.401) with duration of arrest. Simultaneous recording of arterial pressure and SvO2 (n = 41) showed that arterial recovery (6 +/- 3 seconds) occurred long before SvO2 recovery (48 +/- 16 seconds, P = 0.0001). The authors interpret these data as showing that mixed venous oximetry, compared to arterial blood pressure, provides a more sensitive indicator of tissue recovery following periods of circulatory arrest during DFT testing of AICDs.

Value and accuracy of dual oximetry during pulmonary resections

During thoracic surgery, one-lung ventilation (1LV) is often required. The purpose of this prospective study was to examine the usefulness and accuracy of dual-oximetry during 1LV. Prior to the induction of anesthesia, 30 patients had a radial artery and a fiberoptic pulmonary artery catheter (15 Edwards, 15 Spectramed by randomization) inserted. Arterial O2 saturation (SpO2) was monitored by pulse oximetry, and mixed venous O2 saturation (SvO2) by oximetry (Edwards or Spectramed). Arterial and mixed venous blood gases were obtained and immediately analyzed by an OSM3-Hemoximeter. Measurements, including hemodynamics and blood gases, were obtained before induction, during two-lung ventilation (2LV) in the supine and lateral decubitus positions, during 1LV, and following extubation. The change from 2LV to 1LV was associated with significant increases in cardiac index (CI) and oxygen delivery index (DO2I), whereas PaO2 and arterial and mixed venous oxygen saturation decreased. The ratio of oxygen consumption to delivery remained stable. Continuous oximetry when compared with in vitro measurements yielded a correlation coefficient for arterial oxygen saturation of r = 0.794 (P less than or equal to 0.001) and a value of bias and precision of -0.5% +/- 1.7%; for mixed venous oxygen saturation of r = 0.874 (P less than or equal to 0.001) and -1.3% +/- 2.8% for the two-wavelength Edwards catheter; and, r = 0.862 (P less than or equal to 0.001) and -0.1% +/- 3.2% for the two-wavelength Spectramed catheter. These findings demonstrate that dual-oximetry is an on-line, reliable method to measure SpO2 and SvO2. SpO2 less than 95% reflects hypoxygenation and hypoxia (PaO2 less than or equal to 70 mm Hg). SvO2 is determined primarily by oxygenation (r = 0.005; P less than or equal to 0.05) rather than by CI (r = 0.001, ns). Since DO2I increased during 1LV to maintain the oxygen supply and demand balance, SvO2 monitoring might be useful as an early indicator in identifying high-risk patients with compromised DO2I resulting from decreased CI.