Laverne D. Gugino

Invariant reversible QEEG effects of anesthetics

Continuous recordings of brain electrical activity were obtained from a group of 176 patients throughout surgical procedures using general anesthesia. Artifact-free data from the 19 electrodes of the International 10/20 System were subjected to quantitative analysis of the electroencephalogram (QEEG). Induction was variously accomplished with etomidate, propofol or thiopental. Anesthesia was maintained throughout the procedures by isoflurane, desflurane or sevoflurane (N = 68), total intravenous anesthesia using propofol (N = 49), or nitrous oxide plus narcotics (N = 59). A set of QEEG measures were found which reversibly displayed high heterogeneity of variance between four states as follows: (1) during induction; (2) just after loss of consciousness (LOC); (3) just before return of consciousness (ROC); (4) just after ROC. Homogeneity of variance across all agents within states was found. Topographic statistical probability images were compared between states. At LOC, power increased in all frequency bands in the power spectrum with the exception of a decrease in gamma activity, and there was a marked anteriorization of power. Additionally, a significant change occurred in hemispheric relationships, with prefrontal and frontal regions of each hemisphere becoming more closely coupled, and anterior and posterior regions on each hemisphere, as well as homologous regions between the two hemispheres, uncoupling. All of these changes reversed upon ROC. Variable resolution electromagnetic tomography (VARETA) was performed to localize salient features of power anteriorization in three dimensions. A common set of neuroanatomical regions appeared to be the locus of the most probable generators of the observed EEG changes.

Effects of fentanyl and sufentanil on peripheral mammalian nerves.

The effects of fentanyl and sufentanil on peripheral nerves were evaluated in isolated sheathed and desheathed rabbit vagus nerves. The action potential amplitudes of A and C fibers were recorded before and after a 30-min exposure to 50 and 100 μg/ml of fentanyl and sufentanil. A reversible decrease in the action potential amplitude of A fibers in desheathed nerves was observed after exposure to 100 μg/ml of each drug. The action potential amplitude of C fibers was also decreased but not to the same degree as was the A fiber action potential. Pretreatment with naloxone failed to block the reduction in action potential amplitude produced by the two opiates. No evidence of irreversible conduction blockade indicative of local neural toxicity was seen in these studies. The results suggest that high concentrations of fentanyl and sufentanil may exert a weak local anesthetic-type action on peripheral nerves.

The Role of Length of Nerve Exposed to Local Anesthetics in Impulse Blocking Action

The quantitative relation between the concentration of local anesthetic (LA), the length of nerve exposed, and severity of conduction blockade was studied with use of a chamber where exposure length was varied as the concentration of lidocaine was held constant. Recordings of the compound action potential and of single axons established that small variations in the length of nerve exposed to LA strongly modulate conduction block even at exposure lengths in excess of 2 cm. Therefore, exposure length is a significant factor in determining blocking potency, and only at very high concentrations of LA, where voltage-dependent Na conductance is almost completely blocked, is the critical exposure length less than three nodes of Ranvier. The concentration required for 50% block of impulses in single fibers (that is, where 50% of the impulses would fail to propagate through the exposed region of the nerve) diminished as the exposed length of nerve increased, approximately halving as exposure length was changed from 6 mm to 15--25 mm. Conduction latency increased with the exposure length becoming sharply more variable as the critical exposure length for conduction block was approached. The results are consistent with the hypothesis of decremental conduction, where a partial active response in nodes exposed to marginal blocking concentrations extends the decay of the action potential along the axon, and do not support the interpretation that lengths of several centimeters affect blocking concentration because such distances increase the probability that three nodes will be blocked in succession. This study contradicts the broader common assumption that beyond three nodes, the length of nerve exposed is not a factor in nerve block with local anesthetics.

Patient state index : titration of delivery and recovery from propofol, alfentanil, and nitrous oxide anesthesia

Background The Patient State Index (PSI) uses derived quantitative electroencephalogram features in a multivariate algorithm that varies as a function of hypnotic state. Data are recorded from two anterior, one midline central, and one midline posterior scalp locations. PSI has been demonstrated to have a significant relation to level of hypnosis during intravenous propofol, inhalation, and nitrous oxide–narcotic anesthesia. This multisite study evaluated the utility of PSI monitoring as an adjunct to standard anesthetic practice for guiding the delivery of propofol and alfentanil to accelerate emergence from anesthesia. Methods Three hundred six patients were enrolled in this multicenter prospective randomized clinical study. Using continuous monitoring throughout the period of propofol–alfentanil–nitrous oxide anesthesia delivery, PSI guidance was compared with use of standard practice guidelines (both before [historic controls] and after exposure to the PSA 4000 monitor [Physiometrix, Inc., N. Billerica, MA; standard practice controls]). Anesthesia was always administered with the aim of providing hemodynamic stability, with rapid recovery. Results No significant differences were found for demographic variables or for site. The PSI group received significantly less propofol than the standard practice control group (11.9 &mgr;g · kg−1 · min−1;P < 0.01) and historic control group (18.2 &mgr;g · kg−1 · min−1;P < 0.001). Verbal response time, emergence time, extubation time, and eligibility for operating room discharge time were all significantly shorter for the PSI group compared with the historic control (3.3–3.8 min;P < 0.001) and standard practice control (1.4–1.5 min;P < 0.05 or P < 0.01) groups. No significant differences in the number of unwanted somatic events or hemodynamic instability and no incidences of reported awareness were found. Conclusions Patient State Index–directed titration of propofol delivery resulted in faster emergence and recovery from propofol–alfentanil–nitrous oxide anesthesia, with modest decrease in the amount of propofol delivered, without increasing the number of unwanted events.

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.

Continuous noninvasive monitoring of cardiac output with esophageal Doppler ultrasound during cardiac surgery.

Esophageal Doppler ultrasonography offers a continuous and noninvasive alternative to standard thermodilution cardiac output monitoring. A total of 372 simultaneous measurements of Doppler and thermodilution cardiac output were compared in 16 patients undergoing cardiac surgery. In addition, echocardiographic aortic diameter measurement, necessary for Doppler calibration, was compared with direct surgical measurement in 23 patients. Echocardiographic aortic measurement was often time consuming and correlated poorly (r = 0.31) with surgical measurement. On the other hand, Doppler cardiac output was determined easily and accurately tracked thermodilution cardiac output (R2 = 0.95, common slope coefficient 1.050, by multiple linear regression). Furthermore, Doppler cardiac output was more reproducible, showing less short-term variability than thermodilution cardiac output. The esophageal Doppler technique allows cardiac output monitoring in patients for whom invasive monitoring is not warranted.

Esmolol for control of increases in heart rate and blood pressure during tracheal intubation after thiopentone and succinylcholine

Esmolol, an ultra-short-acting cardioselective betaadrenergic blocker, was investigated in a double-blind prospective protocol for its ability to control haemodynamic responses associated with tracheal intubation after thiopentone and succinylcholine. Thirty ASA physical status I patients received a 12-minute infusion of esmolol (500 µg·kg-1·min-1 for four minutes, then 300 µg·kg-1 min-1 for 8 mnutes) or saline. Five minutes after the start of the drug/placebo infusion, anaesthesia was induced with 4 mg·kg-1 thiopentone followed by succinylcholine for tracheal intubation. Prior to induction esmolol produced significant decreases in heart rate (HR) (9.3 ± 1.8 per cent) and rate-pressure product (RPP) (13.1 ± 1.8 per cent), systolic blood pressure (SAP) (4.3 ± 1.5 per cent) and mean arterial blood pressure (MAP) (1.7 ± 2.0 per cent). Increases in HR, SAP and RPP after intubation were approximately 50 per cent less in patients given esmolol compared to patients given placebo. There were highly significant differences in HR (p < 0.0001), and RPP (p < 0.0005) and significant differences in SAP (p < 0.05) when the maximal esmolol post-intubation response was compared to the maximal placebo response. Infusion of esmolol in the dose utilized in this study significantly attenuated but did not completely eliminate cardiovascular responses to intubation.RésuméL’esmolol, un bloquer bêta-adrénergique cardiosélectif de courte durée d’action a été investigué dans une étude prospective à double insu pour sa capacité de contrôler les réponses hémodynamiques associées à l’intubation trachéale après thiopentone et succinylcholine. Trente patients ASA I ont requ une perfusion de 12 minutes d’esmolol (500 µg ·kg-1· min-1 pour quatre minutes, puis 300 µg ·kg-1 pour huit minutes) ou du salin. Cinq minutes après le début de la perfusion du médicament ou du placebo, l’ anesthésie était induite avec 4 mg·kg-1 de thiopentone suivi de succinylcholine pour l’intubation trachéale. Avant l’induction l’esmolol a produit une diminution significative de la fréquence cardiaque (HR) (9.3 ± 1.8 pour cent) et du produit fréquence-pression (RPP) (13.1 ±1.8 pour cent), de la tension artérielle systolique (SAP) (4.3 ± 1.5 pour cent) et de la pression artérielle mopenne (MAP) (1.7 ± 2.0 pour cent). Après l’intubation, l’augmentation dans la fréquence cardiaque, la pression artérielle systolique et la produit fréquence-pression était approximativement 50 pour cent moindre chez les patients ayant reçu de l’esmolol que chez les patients ayant regu du placebo. Il y avait une difference hautement significative dans la frequence cardiaque (p < 0.0001) et dans le produit fréquencepression (p < 0.0005) ainsi qu’une difference significative dans la pression artérielle systolique (p < 0.05) quand la réponse maximale post-intubation à l’esmolol a été comparée à la réponse maximale au placebo. La perfusion d’esmolol aux doses utilisées dans cette étude atténue significativement mais n’élimine pas complètement les réponses cardiovasculaires à l’intubation.

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.