Kai-Michael Scheufler

Does tissue oxygen-tension reliably reflect cerebral oxygen delivery and consumption?

We investigated the value of brain oxygen partial pressure (PbrO2) with respect to predicting cerebral energetic failure in a rabbit model of global cerebral ischemia and hypoxia. Local cortical blood flow (lcoCBF), PbrO2, extracellular lactate, pyruvate, and glutamate concentrations, as well as microvascular hemoglobin saturation (SmvO2), cytochrome oxidase redox level (Cyt a+a3 oxidation), and brain electrical activity, were assessed during variable degrees of cerebral ischemia and hypoxia, induced by cisternal infusion of artificial cerebrospinal fluid or an admixture of nitrous oxide to inspiratory gas in 10 animals each. Arteriovenous difference in oxygen content, cerebral metabolic rate for oxygen, and oxygen extraction were derived from multimodal data. PbrO2, SmvO2, and Cyt a+a3 oxidation were closely related to cerebral blood flow and indices of oxidative metabolism. PbrO2 ≤8 mm Hg corresponded to lcoCBF ≤15 mL · 100 g−1 · min−1, SmvO2 ≤9%, Cyt a+a3 oxidation ≤20%, and progressive loss of brain electrical activity. Adequate tissue oxygenation was reflected by cerebral metabolic rate for oxygen ≥2.8 mL · 100 g−1 · min−1, arteriovenous difference in oxygen content ≤12.5 mL O2 · 100 mL−1, and oxygen extraction ≤60%. Meaningful interpretation of low PbrO2, especially with respect to definition of energetic thresholds, requires complementary information from simultaneous assessment of lcoCBF and tissue oxygen extraction.

Individual value of brain tissue oxygen pressure, microvascular oxygen saturation, cytochrome redox level, and energy metabolites in detecting critically reduced cerebral energy state during acute changes in global cerebral perfusion.

The authors assessed the diagnostic value of brain tissue oxygen tension (PbrO2), microvascular oxygen saturation (SmvO2), cytochrome oxidase redox level (Cyt a+a3 oxidation), and cerebral energy metabolite concentrations in detecting acute critical impairment of cerebral energy homeostasis. Each single parameter as well as derived multimodal indices (arteriovenous difference in oxygen content [AVDO2], cerebral metabolic rate for oxygen [CMRO2], fractional microvascular oxygen extraction [OEF]) were investigated during controlled variation of global cerebral perfusion using a cisternal infusion technique in 16 rabbits. The objective of this study was to determine whether acute changes between normal, moderately, and critically reduced cerebral perfusion as well as frank ischemia defined by local cortical blood flow (lcoBF), brain electrical activity (BEA), and brain stem vasomotor control can be reliably identified by SmvO2, PbrO2, Cyt a+a3 oxidation, or energy metabolites (glutamate, lactate/pyruvate ratio). PbrO2, SmvO2, and Cyt a+a3 oxidation, but not cerebral perfusion pressure, were closely linked to lcoBF and BEA and allowed discrimination between normal, moderately reduced, and critically reduced cerebral perfusion (P < 0.01). Glutamate concentrations and the lactate/pyruvate ratio varied significantly only between moderately reduced cerebral perfusion and frank ischemia (complete loss of BEA and brain stem vasomotor control). Therefore, PbrO2, SmvO2, and Cyt a+a3 oxidation, but not glutamate and the lactate/pyruvate ratio, reliably predict the transition from moderately to critically reduced cerebral perfusion with impending energy failure.

Influence of nitrous oxide on motor-evoked potentials.

Study Design. Rabbits were used as an experimental model in the study of motor-evoked potentials. Objectives. To evaluate the effect of nitrous oxide on motor-evoked potentials while monitoring direct muscle and spinal cord responses. Summary of Background Data. Motor-evoked potential monitoring provides a promising tool for intraoperative assessment of descending pathways function. However, to date, this technique is still at an experimental stage, since its routine use is mainly limited because of intraoperative recording difficulties caused by the influence of anesthesia. Methods. Eight male rabbits weighing between 3000 g and 3500 g were studied. Motor-evoked potentials were recorded from the extremity muscles and from the epidural space of the thoracic cord in response to electrical stimulation of the motor cortex at baseline conditions and at increasing nitrous oxide concentrations (10-70 vol%). Results. The authors found a major suppressive effect of high nitrous oxide concentrations on the electromyographic responses. With 50 vol% nitrous oxide, electromyographic amplitudes were suppressed to 46% (fore leg) and 14% (hind leg) of the baseline values, whereas latencies did not change significantly. In contrast to muscular activity, spinal evoked responses representing neural activity were not affected by any concentration of nitrous oxide. Conclusions. Intraoperative monitoring of descending pathways by means of motor-evoked potentials during anesthesia of the rabbits based on nitrous oxide is feasible when neural activity is evaluated. Higher doses of nitrous oxide, however, are not compatible with recording of muscular activity.

Influence of fentanyl, alfentanil, and sufentanil on motor evoked potentials.

The effects of the opioids fentanyl, alfentanil, and sufentanil on motor pathways were studied in a total of 30 rabbits. Compound muscle action potentials (CMAP) were recorded from the extensor muscles of the upper extremity as well as evoked spinal cord potentials (ESCP) from the thoracic epidural space in response to electrical stimulation of the motor cortex. After establishing stable baseline values, an equipotent intravenous bolus of one of the three opioids was applied that abolished reflex motor response to noxious stimulation. Motor evoked potentials (MEP) were recorded from the time of bolus administration until recovery of MEP amplitudes and latencies. Afterwards, the opioids were administered continuously with cumulative dosage up to total absence of motor evoked response. Our results show a dose-dependent suppression of the CMAP: When reflex movement to noxious stimulation was extinguished, we found a significant (P < .001) reduction of the amplitudes to 34+/-18% (mean +/- SD) in the fentanyl group, to 43+/-24% in the alfentanil group, and to 53+/-20% of baseline values in the sufentanil group. Increasing opioid plasma levels were associated with complete extinction of the CMAP. We hypothesize that the descending volleys within motor pathways are mainly inhibited at a spinal level, because ESCP, particularly the number of spinal I-waves, are not severely affected even when CMAP are completely suppressed. In conclusion, intraoperative monitoring of descending pathways by means of MEP during anesthesia with opioids is feasible at anesthetic plasma concentrations maintaining a surgical level of analgesia. Even with high opioid plasma levels, a valid MEP monitoring could be performed evaluating neural activity of spinal MEP.

Percutaneous multilevel decompressive laminectomy, foraminotomy, and instrumented fusion for cervical spondylotic radiculopathy and myelopathy: assessment of feasibility and surgical technique.

OBJECT Extensive muscle dissection associated with conventional dorsal approaches to the cervical spine frequently results in local pain, muscle wasting, and temporarily painful and restricted neck movement. The utility of a percutaneous muscle-sparing access technique and specifically modified instrumentation for multilevel posterior cervical decompression and fusion were evaluated. METHODS Eleven patients (six men, five woman; mean age 72.8 +/- 6.3 years) presenting with refractory neck pain and progressive multilevel cervical radiculopathy and/or myelopathy due to cervical spondylosis with spinal canal and neural foraminal stenosis underwent multilevel laminectomy, foraminotomy, and subsequent instrumented posterior fusion via bilateral or unilateral percutaneous muscle dilation approaches. A novel cannulated polyaxial instrumentation system was used for unilateral transpedicular/translaminar fixation. RESULTS Significant reduction of Neck Disability Index and Nurick Scale scores and partial or complete recovery of upper extremity radicular deficits was observed during follow-up (mean 14.6 months). Mean procedural blood loss was 45.5 ml, and mean length of stay in hospital was 5.7 days. Fusion was demonstrated in 10 patients between 12 and 14 months postoperatively. Operative exposure and instrumentation were significantly facilitated by specific modifications of retractor/access port systems, surgical instruments, and implants. CONCLUSIONS Muscle sparing posterior decompression and instrumented fusion constitutes a safe and effective surgical option in a selected subgroup of patients with multilevel cervical spondylotic radiculomyelopathy. Specific modifications in surgical technique, instrumentation, and implants are mandatory for effective achievement of the surgical goals. The use of refined image guidance technology and intraoperative imaging can further improve surgical safety and efficacy.

S(+)-ketamine attenuates myogenic motor-evoked potentials at or distal to the spinal α-motoneuron

UNLABELLED We investigated the effect of S(+)-ketamine on spinal cord evoked potentials (ESCPs) and myogenic motor-evoked potentials after electrical stimulation of the motor cortex in a rabbit model. This study was designed to characterize the relationship between ESCP characteristics and corresponding changes in compound muscle action potentials (CMAPs) derived from fore and hind limbs. Direct (D) and indirect (I) corticospinal volleys (ESCP) from the upper and lower thoracic spinal cord, recorded by two bipolar epidural electrodes, were assessed during IV administration of 0.02, 0.05, 0.1, and 0.2 mg. kg(-1) x min(-1) of S(+)-ketamine, each before and after neuromuscular blockade (0.4 mg/kg of cisatracurium), in 16 New Zealand White rabbits after single-pulse bipolar electrical stimulation of the motor cortex at 50 (threshold), 60, and 70 V. CMAP amplitudes at fore and hind limbs were significantly suppressed (P < 0.01) during infusion at 0.1 and 0.2 mL x kg(-1) x min(-1), whereas neither corresponding D- nor I-waves were altered. Similar findings were obtained during variation of stimulus amplitude (50-70 V). Multivariate regression analysis of CMAP amplitudes and various ESCP characteristics demonstrated no apparent interparametric association. These findings indicate that S(+)-ketamine modulates CMAP independent from corticospinal D- and I-wave-mediated facilitation at or distal to the spinal alpha-motoneuron. IMPLICATIONS S(+)-Ketamine combines several anesthetic properties suitable for total IV neuroanesthesia, including minimal effects on neurophysiological monitoring. Recording of neural and myogenic responses after electrical stimulation of the motor cortex indicates that S(+)-ketamine modulates myogenic motor-evoked potentials by a peripheral mechanism at or distal to the spinal alpha-motoneuron.