Terry Patterson

Deep hypothermic circulatory arrest: I. Effects of cooling on electroencephalogram and evoked potentials

BACKGROUND Deep hypothermia is an important cerebral protectant and is critical in procedures requiring circulatory arrest. The purpose of this study was to determine the factors that influence the neurophysiologic changes during cooling before circulatory arrest, in particular the occurrence of electrocerebral silence. METHODS In 109 patients undergoing hypothermic circulatory arrest with neurophysiologic monitoring, five electrophysiologic events were selected for detailed study. RESULTS The mean nasopharyngeal temperature when periodic complexes appeared in the electroencephalogram after cooling was 29.6 degrees C +/- 3 degrees C, electroencephalogram burst-suppression appeared at 24.4 degrees C +/- 4 degrees C, and electrocerebral silence appeared at 17.8 degrees C +/- 4 degrees C. The N20-P22 complex of the somatosensory evoked response disappeared at 21.4 degrees C +/- 4 degrees C, and the somatosensory evoked response N13 wave disappeared at 17.3 degrees C +/- 4 degrees C. The temperatures of these various events were not significantly affected by any patient-specific or surgical variables, although the time to cool to electrocerebral silence was prolonged by high hemoglobin concentrations, low arterial partial pressure of carbon dioxide, and by slow cooling rates. Only 60% of patients demonstrated electrocerebral silence by either a nasopharyngeal temperature of 18 degrees C or a cooling time of 30 minutes. CONCLUSIONS With the high degree of interpatient variability in these neurophysiologic measures, the only absolute predictors of electrocerebral silence were nasopharyngeal temperature below 12.5 degrees C and cooling longer than 50 minutes.

Deep hypothermic circulatory arrest: II. Changes in electroencephalogram and evoked potentials during rewarming.

BACKGROUND Electrophysiologic studies during rewarming after deep hypothermic circulatory arrest probe the state of the brain during this critical period and may provide insight into the neurological effects of circulatory arrest and the neurologic outcome. METHODS Electroencephalogram (EEG) and evoked potentials were monitored during rewarming in 109 patients undergoing aortic surgery with hypothermic circulatory arrest. RESULTS The sequence of neurophysiologic events during rewarming did not mirror the events during cooling. The evoked potentials recovered first followed by EEG burst-suppression and then continuous EEG. The time to recovery of the evoked potentials N20-P22 complex was significantly correlated with the time of circulatory arrest even in patients without postoperative neurologic deficits (r = 0.37, (p = 0.002). The nasopharyngeal temperatures at which continuous EEG activity and the N20-P22 complex returned were strongly correlated (r = 0.44, p = 0.0002; r = 0.41, p = 0.00003) with postoperative neurologic impairment. Specifically, the relative risk for postoperative neurologic impairment increased by a factor of 1.56 (95% CI 1.1 to 2.2) for every degree increase in temperature at which the EEG first became continuous. CONCLUSIONS No trend toward shortened recovery times or improved neurologic outcome was noted with lower temperatures at circulatory arrest, indicating that the process of cooling to electrocerebral silence produced a relatively uniform degree of cerebral protection, independent of the actual nasopharyngeal temperature.

Global Cardiac-Specific Transgene Expression Using Cardiopulmonary Bypass With Cardiac Isolation

BACKGROUND The available techniques for intravascular gene delivery to the heart are inefficient and not organ-specific. Yet, effective treatment of heart failure will likely require transgene expression by the majority of cardiac myocytes. To address this problem, we developed a novel cannulation technique that achieves efficient isolation of the heart in situ using separate cardiopulmonary bypass (CPB) circuits for the heart and body in dogs. METHODS The arterial inflow and venous effluent from the two circuits were physically isolated. The efficiency of separation was 98% to 99% in three preliminary experiments using Evans Blue dye-labeled albumin. In 6 dogs, the cardiac circuit was perfused with oxygenated crystalloid cardioplegia at 37 degrees C containing approximately 4 x 10(11) particles of an adenovirus encoding LacZ (AdCMVLacZ) with a perfusion pressure of 170 to 200 mm Hg for 15 minutes allowing virus to recirculate through the heart approximately 15 times. Cross-clamp time was 26 +/- 2 minutes and CPB time was 90 +/- 3 minutes. RESULTS Five animals survived and were euthanized at 7 days. Beta-galactosidase activities measured using a chemiluminescent assay were three orders of magnitude higher in all areas of the heart than in the liver. Histological analyses revealed heterogeneous X-Gal staining of myocytes in all areas of the myocardium. CONCLUSIONS Despite using a constitutive promoter, this technique yields relatively cardiac-specific transgene expression and is potentially translatable to clinical applications. Future studies will allow for further optimization of the conditions necessary for vector-mediated gene delivery to the heart.

Mechanisms of electrode induced injury. Part 1: Theory

ABSTRACT. Electrodes are the essential elements of clinical neurophysiology both in recording of neural activity and in functional electrical Stimulation of the nervous system. Therefore it is important to understand the potential complications of using electrodes. In this paper, the factors that influence the chance of electrode related injury are discussed from a theoretical standpoint. The mechanical factors, especially pressure related injury, are discussed first, followed by a discussion of injury that is of chemical origin such as contact dermatitis. Next, the ways in which electrical currents flowing from electrodes can cause injury including: Joule heating, electroporation, electroconformational denaturation, and excitatory neurotoxicity are discussed. The differential effects of constant current and constant voltage Stimulation on tissue heating are examined, as are the effects of the conductivity and geometric structure of the stimulated tissue. Finally, the effects of electrochemical reactions are discussed both in the context of surface and implanted electrodes.

Comparison of Endotrachial Tube and Hookwire Electrodes for Monitoring the Vagus Nerve

Monitoring the vagus nerve and the recurrent laryngeal nerve during surgical procedures may reduce the probability of significant nerve injury. As such, a number of methods to monitor these nerves have been devised including placing electrodes directly into the vocal cords or recording from surface electrodes. In direct comparison, monitoring the identical muscles, bipolar hookwire electrodes displayed approximately one order of magnitude greater amplitude, of both spontaneously occurring and evoked electrical activity than double wire endotracheal tube electrodes. The enhanced sensitivity of the hookwire electrodes, despite the technical difficulties with placement, suggests their use when maximum sensitivity is required.

Detection of stroke during cardiac operations with somatosensory evoked responses

OBJECTIVES The objectives of this study were to determine if monitoring of intraoperative somatosensory evoked potentials could be used to detect stroke during cardiac operations and to establish indicators of cerebral ischemia based on changes in these potentials. METHODS Twenty-five patients undergoing cardiac operations underwent preoperative and postoperative neurologic examinations as well as intraoperative recording of somatosensory evoked potentials. Detailed analysis of the waveforms of these potentials was performed. RESULTS Two of the 25 patients had intraoperative strokes. These patients and only these patients had changes in their somatosensory evoked potentials during the operation suggesting cerebral ischemia. The unilateral disappearance of the cortical somatosensory evoked potential waves correlated significantly with the clinical outcome of stroke (p < 0.004). Ischemic changes were detected in real time and were related to the removal of the aortic crossclamp in one patient and to the initiation of cardiopulmonary bypass in the other. CONCLUSIONS Somatosensory evoked potentials can detect intraoperative stroke during cardiac operations. Acute, unilateral decreases in amplitude of the cortical potential are more useful than changes in latency in detecting intraoperative stroke.

Sensitivity of scalp eeg, cortical eeg, and somatosensory evoked responses during surgery for intracranial aneurysms

BACKGROUND We estimated the relative sensitivity and reliability of scalp EEG, cortical EEG and somatosensory evoked potentials (SSEPs) to detect significant changes during aneurysm surgery. METHODS Two observers independently reviewed data from 18 patients who were monitored with scalp EEG, cortical EEG, and SSEPs to determine which if any modality demonstrated significant changes during 25 different episodes of temporary intracranial vascular occlusion. RESULTS Kappa scores indicating the degree of agreement between the two observers were highest for the cortical strip EEG (kappa = 0.92) and the SSEPs (kappa = 0.82) and much greater than for the scalp EEG data (kappa = -0.07). The cortical strip recordings showed changes more often than either the scalp EEG or SSEP during temporary vascular occlusion. In no case did the scalp EEG change when neither the strip nor SSEP changed. In only 4% of events did the observers feel that the SSEP changed when the strip did not, but in 16% of cases, the strip changed without changes in any of the other modalities. CONCLUSIONS Recording of EEG from strip electrodes placed on the cortical surface detects changes more frequently than either scalp EEG or SSEPs during vascular occlusion. Different observers were more likely to agree on whether the cortical strip EEG changed during vascular occlusion than the other modalities. This argues for the possible advantages of recording from strip electrodes during aneurysm surgery.

Anesthesia and temperature effects on somatosensory evoked potentials produced by train stimuli

Objective: To determine the effect of anesthesia, temperature, and stimulus characteristics on the response of upper extremity somatosensory evoked responses (SSEP) to repetitive stimulation. Methods: Pairs and trains of electrical stimuli were used to elicit the upper extremity SSEP, and the amplitudes of the N20-P22, N13, and Erb s point potentials produced by each stimulus were measured. The ratio of the amplitude of the response to each stimulus to that produced by the first stimulus in a given train was computed. Recordings were also made directly from the cortical surface in a single patient. Results: There were only minimal effects of anesthesia, temperature, or stimulus characteristics on the amplitude ratios for the N13 and Erb s point potentials. The N20-P22 amplitude ratio demonstrated facilitation with interstimulus intervals in the 20 100 ms range and was markedly reduced with hypothermia. The degree of facilitation was strongly dependent on isoflurane and nitrous oxide concentrations. In response to stimulation with trains of four stimuli, increased amplitudes of the N20-P22 complex were seen maximally with the second response while the third and fourth responses were of lower amplitude. Conclusions: There are strong effects of anesthesia and temperature on the cortical responses to repetitive stimulation of the upper extremity. Speculations on the origin of these effects were made.

Spinal dural arteriovenous malformations. Intraoperative evoked potential evidence for pathophysiology. A case report.

Study Design This case report details intraoperative evoked potential changes during surgical removal of a T8 dural arteriovenous malformation. Objectives The pattern of changes in somatosensory‐evoked responses during surgical correction of a spinal dural arteriovenous malformation can illuminate the pathophysiologic process behind the clinical symptoms. Summary of Background Data Arteriovenous malformation of the spinal dura can manifest with multiple symptoms, including progressive myelopathy and pain. The pathophysiologic process behind these symptoms could be either direct compression of the spinal cord by the arteriovenous malformation, ischemia resulting from shunting of the blood flow away from the cord, or increased venous pressure. Methods To investigate these hypotheses further, the results of posterior tibial evoked potentials obtained during surgical removal of a T8 dural arteriovenous malformation were analyzed. Results At baseline, the cortical (P40) potential was markedly prolonged bilaterally. During surgery, just after the dura was opened, a marked increase was observed in the latencies of the P40 and P60 components of the evoked response on the right, which began to resolve as soon as the arteriovenous malformation was occluded. Only minimal changes were seen on the left. Conclusions These results are most consistent with the increased venous pressure hypothesis for the pathogenesis of neurologic symptoms in dural arteriovenous malformations.

Strategies for minimizing 60 Hz pickup during evoked potential recording.

Electrical interference at mains power supply frequency can adversely affect the recording of evoked potentials and can be especially destructive in an operating room setting. We investigated 60 Hz interference in electrode cables running from subject to preamplifier and further examined methods to eliminate such interference. We conclude that braiding electrode wires is highly efficacious in such interference reduction, presumably by reducing the magnetic flux enclosed by the wires. We further indicate that the use of flexible metal hose fabricated from Permalloy 80 may effect further interference reduction.