Linda S. Aglio

Hypomagnesemia is common following cardiac surgery

Hypomagnesemia is a common disorder in noncardiac surgical patients in the postoperative period, but the effect of cardiac surgery on serum magnesium concentrations remains unclear. The authors hypothesized that cardiac surgery is associated with hypomagnesemia, and prospectively studied 101 subjects (60 +/- 13.1 years of age) undergoing coronary artery revascularization (n = 70), valve replacement (n = 24), or both simultaneously (n = 7). Blood samples and clinical biochemical data were collected before induction of anesthesia, prior to cardiopulmonary bypass (CPB), immediately after CPB, and on postoperative day 1. Blood samples were analyzed for ultrafilterable magnesium, total magnesium, ionized calcium, parathyroid hormone, and free fatty acid concentrations. Outcome variables were also determined. Eighteen of 99 (18.2%) subjects had hypomagnesemia preinduction and this number increased to 71 of 100 (71.0%) following cessation of CPB (P less than 0.05). Patients with postoperative hypomagnesemia had a higher frequency of atrial dysrhythmias (22 of 71 [31.0%] v 3 of 29 [10.3%], P less than 0.05) and required prolonged mechanical ventilatory support (22 of 63 [34.9%] v 4 of 33 [12.1%], P less than 0.05). Hypomagnesemia is common following cardiac surgical procedures with CPB and is associated with clinically important postoperative morbidity.

Transcranial magnetic stimulation coregistered with MRI: a comparison of a guided versus blind stimulation technique and its effect on evoked compound muscle action potentials

INTRODUCTION AND METHODS Compound muscle action potentials (CMAPs) elicited by transcranial magnetic stimulation (TMS) are characterized by enormous variability, even when attempts are made to stimulate the same scalp location. This report describes the results of a comparison of the spatial errors in coil placement and resulting CMAP characteristics using a guided and blind TMS stimulation technique. The former uses a coregistration system, which displays the intersection of the peak TMS induced electric field with the cortical surface. The latter consists of the conventional placement of the TMS coil on the optimal scalp position for activation of the first dorsal interossei (FDI) muscle. RESULTS Guided stimulation resulted in significantly improved spatial precision for exciting the corticospinal projection to the FDI compared to blind stimulation. This improved precision of coil placement was associated with a significantly increased probability of eliciting FDI responses. Although these responses tended to have larger amplitudes and areas, the coefficient of variation between guided and blind stimulation induced CMAPs did not significantly differ. CONCLUSION The results of this study demonstrate that guided stimulation improves the ability to precisely revisit previously stimulated cortical loci as well as increasing the probability of eliciting TMS induced CMAPs. Response variability, however, is due to factors other than coil placement.

Intraoperative Somatosensory Evoked Potential Monitoring Predicts Peripheral Nerve Injury during Cardiac Surgery

BackgroundBrachial plexus injury may occur without obvious cause in patients undergoing cardiac surgery. To determine whether such peripheral nerve injury can be predicted intraoperatively, we monitored somatosensory evoked potentials (SEPs) from bilateral median and ulnar nerves in 30 patients undergoing coronary artery bypass surgery. MethodsSEPs were analyzed for changes during central venous cannulation and during use of the Favoloro and Canadian self-retaining sternal retractors, events hereto implicated in brachial plexus injury. Brachial plexus injury was evaluated during physical examination in the postoperative period by an individual blinded to results of SEP monitoring. ResultsCentral venous cannulation was associated with transient changes in SEPs in four patients (13%). These changes occurred intermittently during insertion of the cannula but completely resolved within 5 min. Postoperative neurologic deficits did not occur in these cases. Use of the Canadian and Favoloro retractors was associated with significant changes in 21 patients (70%). In 16 of these, waveforms reverted toward baseline levels intraoperatively and were not associated with postoperative neurologic deficits. Five patients demonstrated a neurologic deficit postoperatively. In each of these, SEP change associated with use of surgical retractors persisted to the end of surgery compared to the immediate pre-bypass period. ConclusionsIntraoperative upper extremity SEPs may be used to predict peripheral nerve injury occurring during cardiac surgery.

Experimentation with a transcranial magnetic stimulation system for functional brain mapping

We describe functional brain mapping experiments using a transcranial magnetic stimulation (TMS) device. This device, when placed on a subjects scalp, stimulates the underlying neurons by generating focused magnetic field pulses. A brain mapping is then generated by measuring responses of different motor and sensory functions to this stimulation. The key process in generating this mapping is the association of the 3-D positions and orientations of the TMS probe on the scalp to a 3-D brain reconstruction such as is feasible with a magnetic resonance image (MRI). We have developed a registration system which not only generates functional brain maps using such a device, but also provides real-time feedback to guide the technician in placing the probe at appropriate points on the head to achieve the desired map resolution. Functional areas we have mapped are the motor and visual cortex. Validation experiments focus on repeatability tests for mapping the same subjects several times. Applications of the technique include neuroanatomy research, surgical planning and guidance, treatment and disease monitoring, and therapeutic procedures.

Visual hemifield mapping using transcranial magnetic stimulation coregistered with cortical surfaces derived from magnetic resonance images

The perception of a visual stimulus can be inhibited by occipital transcranial magnetic stimulation. This visual suppression effect has been attributed to disruption in the cortical gray matter of primary visual cortex or in the fiber tracts leading to V1 from the thalamus. However, others have suggested that the visual suppression effect is caused by disruption in secondary visual cortex. Here the authors used a figure-eight coil, which produces a focal magnetic field, and a Quadropulse stimulator to produce visual suppression contralateral to the stimulated hemisphere in five normal volunteer subjects. The authors coregistered the stimulation sites with magnetic resonance images in these same subjects using optical digitization. The stimulation sites were mapped onto the surface of the occipital lobes in three-dimensional reconstructions of the cortical surface to show the distribution of the visual suppression effect. The results were consistent with disruption of secondary visual cortical areas.

Non-invasive functional brain mapping using registered transcranial magnetic stimulation

The authors describe a method for mapping the functional regions of the brain using a transcranial magnetic stimulation (TMS) device. This device, when placed on a subjects scalp, stimulates the underlying neurons by generating focused magnetic field pulses. A brain mapping is then generated by measuring responses of different motor and sensory functions to this stimulation. The key process in generating this mapping is the association of the 3D positions and orientations of the TMS probe on the scalp to a 3D brain reconstruction such as is feasible with a magnetic resonance image (MRI). The authors perform this matching process by (1) registering the subjects head position to an a priori MRI scan, (2) tracking the 3D position/orientation of the TMS probe, (3) transforming the TMS probe position/orientation to the MRI coordinate frame, and (4) tracking movements in the subjects head position to factor out any head motion. The resultant process generates a high resolution, accurate brain mapping which supports surgical planning, surgical guidance, neuroanatomy research, and psychiatric therapy. When compared to other functional imaging modalities, this approach exhibits much lower cost, greater portability, and more direct active control over the functional areas being studied.

QEEG and neuropsychological profiles of patients after undergoing cardiopulmonary bypass surgical procedures.

One week after surgery neuropsychological (NP) deficits were quite common, occurring in 40.6% of the patients, with QEEG abnormality developing or increasing in the majority of patients. This change in the QEEG was an accurate predictor of NP performance 1 week after surgery. Two to three months after surgery evidence of continued NP performance deficits were still present in 28.1% of the patients. Preoperative versus one week postoperative QEEG change showed higher levels of sensitivity and specificity for predicting neuropsychological performance 3 months after CPB surgery than did preoperative versus one week postoperative NP performance. The mean values of specificity plus sensitivity were 74.5% for NP performance and 89.1% for the QEEG. These high levels of sensitivity and specificity for QEEG change for predicting postoperative cognitive function may justify the utility of performing these evaluations in the general CPB surgical population. In addition, this evidence supports the need to study the role of intraoperative QEEG monitoring to determine when QEEG change occurs so that possible remediational measures can be taken as soon as possible.

Monitoring the Electroencephalogram During Bypass Procedures

Electroencephalographic monitoring has been performed since the early days of cardiopulmonary bypass. Despite this long experience, the technology has never been widely used for cardiac operations. This review examines the reasons for the limited use and describes technological advances that may alter this pattern.

Peripheral ischemia as a complicating factor during somatosensory and motor evoked potential monitoring of aortic surgery

HE incidence of paraplegia following aortic surgery is reported to vary between 2% and 25%.lm4 This reported incidence has made intraoperative spinal cord monitoring an area of interest.5-9 Two monitoring modalities, somatosensory evoked potentials (SEP) and motor evoked potentials (MEP), can detect ischemia of nervous system structures. Of these, the SEP, although controversial, is used most for both experimental and clinical monitoring of aortic aneurysm surgery.5-i0 The SEP monitors physiologic integrity of the peripheral nerve, from the site of stimulation to the cerebral cortex, via the dorsal columns of the spinal cord. The most sensitive area of the spinal cord to ischemia is the anterior horn area, as shown by the distribution of postoperative spinal cord injury.” Although some investigators have suggested that a steal of blood from the posterior to anterior spinal cord may occur during aortic cross-clamping, changes in SEPs will, at best, indirectly monitor ischemia of the motor areas1*J3 Hence, SEP monitoring has yielded low specificity and sensitivity to postoperative neurologic deficits in the absence of shunt or partial bypass use during these operations.r4J5 As suggested by McNulty et a1,r3 use of these operative adjuncts, however, does in part improve the predictive value of SEP monitoring for postoperative neurologic deficits. Recent investigations have used MEPs induced by electrical stimulation to monitor spinal cord function during aortic occlusion.16-ia Monitoring MEPs may be more sensitive than SEPs to spinal cord ischemia. Clinical examples using magnetic transcranially induced MEPs in humans during aortic aneurysm surgery have yet to be reported. An often neglected factor in spinal cord monitoring is that an insult to any part of the nervous system pathway that is monitored will result in a change in the evoked potentials.19 Ischemia to the peripheral nervous system is an important factor that must be addressed during future clinical investigations involving evoked potential monitoring of aortic aneurysm surgery. Two cases are presented, one in which SEPs alone and one in which both SEPs and MEPs were monitored during aortic surgery. Both cases demonstrate the importance of peripheral ischemia as a possible explanation for changes in SEPs and/or MEPs during aortic surgery.

QEEG Changes during Cardiopulmonary Bypass: Relationship to Postoperative Neuropsychological Function

The relationship of changes in intraoperative QEEG and postoperative cognitive function was studied in 32 patients undergoing cardiac surgical procedures requiring cardiopulmonary bypass (CPB). All patients were anesthetized with a high dose narcotic technique in which CPB was carried out using moderate hypothermia. EEG recorded continuously throughout each procedure was analyzed using the neurometric technique. Neuropsychological (NP) evaluations were administered to all patients before, 1 week and 2-3 months postoperatively. A decrement in postoperative performance of 2 standard deviations in two or more tests from preoperative testing was defined as a new cognitive deficit. Of the patients studied, 40.6% demonstrated a new postoperative cognitive deficit at 1 week. At 2-3 months postoperatively, 28.1% continued to show a cognitive deficit. Discriminant analysis of the QEEG as a function of NP performance was calculated at select times during the surgical procedure. QEEG prediction of NP performance was just above chance at the 1 week comparison but excellent for the 2-3 month comparisons. This study suggests that with appropriate monitoring protocols, intraoperative QEEG may predict cognitive dysfunction experienced by patients 2-3 months postoperatively.