A comparative analysis of enflurane anesthesia on primate motor and somatosensory evoked potentials.

The effect of increasing enflurane concentration on magnetic-induced myogenic cranial (Cr) and peripheral (Pr) motor evoked potentials (MEPs), and electrically induced median (MN) and posterior tibial (PTN) somatosensory evoked potentials (SEPs) was studied in 10 monkeys. MEP, recorded from abductor pollicis brevis and abductor hallucis muscles, and SEP (short- and long-latency scalp recorded potentials) variables were examined at 0.25, 0.5, 0.75, 1.0 MAC enflurane concentrations. Cr-MEPs progressively attenuated (P less than 0.01) with 0.25 MAC and were abolished (greater than or equal to 0.75 MAC) by graded enflurane concentration. Stimulation threshold for Cr-MEP was progressively elevated (P less than 0.01), and eventually reliable responses were lost (greater than or equal to 0.75 MAC). Marked scalp zone reduction to obtain Cr-MEP responses was noted with increasing enflurane concentration. Pr-MEPs and most SEP peaks maintained good replicability but showed significant amplitude reduction (P less than 0.01). MEP and SEP latency values were not significantly delayed as long as the wave form remained identifiable. We conclude that enflurane has a differential influence on Cr-MEPs and SEPs. Administration of enflurane should be discouraged while monitoring myogenic Cr-MEPs since even a subanesthetic concentration is profoundly detrimental.

The effect of an anesthetic induction dose of midazolam on motor potentials evoked by transcranial magnetic stimulation in the monkey.

The effect of a hypnotic dose (0.5 mg/kg) of midazolam (MDZ) on motor evoked potentials (MEPs) was examined in 12 monkeys. MEPs were elicited by transcranial magnetic stimulation (TMS) and the resultant potentials recorded from abductor pollicis brevis (APB) and anterior tibialis (AT) muscles contralateral to the stimulation site. After administration of MDZ, sequential MEP recordings were obtained at postinduction, hypnosis, awakening, emergence, and recovery periods. The results were compared with control values using one-way analysis of variance and Tukey's post-hoc test. Under hypnosis, MEP reproducibility was problematic as the potentials were occasionally ill identified and questionable. MDZ resulted in marked MEP scalp field reduction, coil demography alteration, stimulation threshold elevation, and amplitude suppression (p <0.01). Latency response was unaltered. During hypnosis, awakening, and recovery periods, the mean APB and AT thresholds were elevated by 39, 23, and 0% and by 60, 34, and 4% respectively; while APB and AT amplitudes were depressed by 95, 86, and 53% and by 99, 91, and 60%, respectively. We conclude that an induction dose of MDZ can produce profound and prolonged attenuation of TMS MEPs. The drug inhibitory effect on MEPs may persist after recovery. Anesthetic doses of MDZ should cautiously be used in the settings of MEP monitoring.

The effect of etomidate on motor evoked potentials induced by transcranial magnetic stimulation in the monkey.

Etomidate (ET) is a known hypnotic agent in neuroanesthesia. This study was designed to examine the reliability of motor evoked potentials (MEPs) after transcranial magnetic stimulation in monkeys anesthetized intravenously with ET. The ET regimen was as follows: an initial dose (0.5 mg/kg) followed by 13 doses (0.2 mg/kg every 6-12 min; mean, 8.0 +/- 1.3 min). The total dose administered was 3.1 mg/kg. The magnetic coil was placed over the MEP scalp stimulation region. Evoked electromyographic responses were recorded from the contralateral abductor pollicis brevis (APB) and abductor hallucis (AH) muscles of the fore- and hindlimbs, respectively. Reproducible MEP responses were consistently recorded while the animal was under total ET anesthesia. The coil demography was altered and the MEP scalp topography was moderately reduced by ET injections. Significant threshold elevation was noted after a total dose of 1.7 mg/kg for APB responses and 0.5 mg/kg for AH responses (P less than 0.05). Marked prolongation of latency was observed after a dose of 0.5 mg/kg for APB MEPs and 2.5 mg/kg for AH MEPs (P less than 0.05). MEP amplitude responses showed marked variability. Repeated doses of ET produced a mean threshold rise of 14 to 28% for the APB and 19 to 29% for the AH. The mean latency delay was 5 to 11% for the APB and 0.5 to 8% for the AH, while the mean amplitude depression was 24 to 59% for the APB and 15 to 50% for the AH. Apparent seizure activity or abnormalities in behavior and feeding were not noted over a 1-year period.(ABSTRACT TRUNCATED AT 250 WORDS)

Transtentorial brain herniation in the monkey: analysis of brain stem auditory and somatosensory evoked potentials.

A monkey model of transtentorial brain herniation (TBH) was created to simulate the clinically encountered situation of a gradually expanding intracranial lesion. TBH was produced by extradural balloon inflation over a 4-hour period and documented by the appearance of the pupils as dilated or fixed at midposition. Intracranial pressure (ICP), brain stem auditory evoked potentials (BAEP), and short-latency somatosensory evoked potentials (SSEP) were recorded before, during, and after TBH. Statistical significance from baseline values to TBH was found for diminution of the BAEP amplitude, rise of the ICP, and diminution of the SSEP amplitude. An ICP rise to twice the baseline value and a 25% decrease in Wave V amplitude was found 1 hour before TBH. Changes in BAEP and SSEP took several minutes after deflation to return to baseline values. Analysis of Wave V of the BAEP was as sensitive as ICP in warning of TBH. Discussion centers upon previous animal studies of brain herniation and ICP elevation, and findings reported in humans deteriorating as a result of intracranial mass lesions. BAEP and SSEP monitoring may be used as noninvasive tests for brain stem compression in the setting of primate TBH, and in the future may be used to guide the effectiveness of therapy.

Intravenous fat emulsion acutely suppresses neutrophil chemiluminescence.

The immediate effect of intravenous fat emulsion on neutrophil oxidant release was studied. Opsonized nonencapsulated S. aureus was used to stimulate neutrophil activity. Luminol enhanced chemiluminescence was followed over 15 min and recorded as peak output (P; mV), integral under the curve (I; V-sec) and rate of increase (R; mV/sec). Eighteen chronically ill patients receiving glucose based total parenteral nutrition were studied before and after a 4- to 6-hr test infusion of 500 ml of 10% fat emulsion. P decreased from 719 +/- 46 to 461 +/- 42 mV (p less than 0.001), I decreased from 169 +/- 17 to 111 +/- 12 V-sec (p less than 0.001) and R decreased from 6.9 +/- 1.0 to 4.0 +/- 0.6 mV/sec (p less than 0.001). Preincubation of normal whole blood with fat emulsion in vitro did not adversely affect chemiluminescence (11 studies), nor did incubation of normal neutrophils with patient postinfusion plasma (10 studies). We conclude that fat emulsion infusion acutely suppresses neutrophil chemiluminescence. The suppression is not a direct effect of the fat emulsion per se and is not due to inhibitory substances in the plasma following infusion.

An automatic data acquisition system for ventilator patients.

Computerized patient charting and trended data acquisition are becoming important elements of respiratory and intensive care. Continuous, remote monitoring of a patient's respiratory functions is now accessible with automated data acquisition systems. The system described in this article features real-time data collection from up to eight ventilators, automated patient charting, graphic trending, and configurable modes for viewing graphic trends. These features provide practitioners with continuous graphic presentations of ventilator data, which allows for better recording and understanding of each patient's respiratory progress.

Modified auditory brainstem responses (MABR): Part I--Rationale and normative study.

Binaural, rapid rate, ABR stimulation at four intensity levels (MABR) as well as ABR at two intensity levels were carried out in 35 normal subjects. Latency and amplitude relationship with stimulus intensity of wave V of the MABR was extensively studied in these subjects. MABR recording is a rapid, sensitive, non-invasive screening method readily done at the intensive care bedside. Less than 10 minutes is required in addition to the ABR examination time. Normality of the MABR potentials implies a high degree of peripheral auditory and brainstem functional integrity, augmenting information gained with standard ABR technique. Only clinical trials can decide if MABR sensitivity will exceed that of the ABR and prove helpful to diagnosis and patient care.

Preoperative antimicrobial preparation of the colon with povidone-iodine enema.

To evaluate the efficacy of povidone-iodine enemas as a means of preoperative bowel preparation in colonic surgery, 52 mongrel dogs were randomized into two groups. Group 1 received oral neomycin-erythromycin combinations in the usual clinical doses, while Group 2 received a single 500-ml 5 percent povidone-iodine enema preoperatively. Quantitative bacterial counts obtained at the time of colonic resection revealed that povidone-iodine was equally effective in reducing the anaerobes, but was significantly superior to neomycin-erythromycin combinations in reducing the aerobic colony counts. Bursting pressures, measured three weeks later, were equal in both groups. Despite elevated blood-iodine levels, no systemic toxicity was noted in Group 2 dogs. It is concluded that half-strength povidone-iodine, given as a single enema preoperatively, is equally effective as standard preoperative antibiotic preparations and may be the ideal preparation in urgent or emergency colonic operations. Due to reported toxicity of povidone-iodine in burn wounds, we suggest that initial trials be limited to emergency cases where the potential benefits possibly will be greater than the theoretic risk of iodine toxicity.

The role of internal sphincter in chronic anal fissures.

Changes in anal sphincteric manometric pressures in response to rectal distention were measured in eight patients with chronic anal fissures and were compared with those of ten controls. No statistically different resting pressures were noted between the two groups. Overshoot phenomenon was more commonly seen in patients with fissure. There were no differences in the anal sphincteric pressures after lateral internal sphincterotomy (LIS) or fissurectomy midline sphincterotomy (FMS). All fissures healed postoperatively, irrespective of the surgical technique (LIS or FMS) or the pressure readings. It can be concluded that the therapeutic effect of sphincterotomies might at least in part be due to anatomic widening of the anal canal rather than to decreased resting pressures of the internal sphincter.

Intravascular aggregation after acute intracranial hypertension by epidural balloon compression in cats.

The authors have studied the effect of acute intracranial hypertension produced by placement of an epidural balloon (control group) in cats, on cerebral perfusion, evoked responses, and hematological parameters. These elements were measured in similarly injured animals which underwent isovolemic hemodilution with dextran 75, after relief of intracranial hypertension. Four hours after balloon deflation, perfusion was markedly impaired in 30% of the control group, and was reduced to 11% in the dextran-infused group. The suppressed N1 amplitude of somatosensory evoked responses on the compression side, the reduced platelet aggregability, and the erythrocyte-deformability by intracranial hypertension were all significantly more restored in the dextran-infused group after decompression. The percentage of platelets with volumes between 21.75 and 48.75 cu mu (normal 9.75 to 12.75 cu mu) significantly increased after decompression. Activation of platelets during intracranial hypertension leads to an increase in platelet volume from platelet aggregation, and correlates with a decrease in platelet aggregability. It was also suggested that reduction of erythrocyte deformability was not caused by erythrocyte aggregation. The authors emphasize the role of intravascular factors such as vascular obstruction by platelet aggregates, and difficulty in passage of erythrocytes through capillaries due to reduced deformability, in the disturbance of the microcirculation following acute intracranial hypertension. The protective effect of dextran 75 by inhibition of platelets as well as hemodilution is stressed.

Acute intracranial hypertension and auditory brain-stem responses. Part 3: The effects of posterior fossa mass lesions on brain-stem function.

The auditory brain-stem responses (BER's), infratentorial intracranial pressure (ICP), systemic blood pressure (BP), and heart rate were recorded before, during and after expansion of an infratentorial epidural mass in anesthetized cats. Two types of BER's to increasing posterior fossa pressure were noted. In Type 1, there was predominantly suppression of the electrical activity of the auditory nuclei of the upper brain stem (Waves V and IV) and upward transtentorial herniation of the midbrain. In Type 2, the neural activity of the lower brain-stem nuclei (Waves III and II) was affected as well as that of the upper brain stem. There was upward and foraminal impaction of the brain stem and cerebellum which was confirmed by the postmortem brain sections. The change in the amplitudes of BER Waves V and III proved useful in detecting upward transtentorial herniation of the midbrain and foraminal herniation of the cerebellum in acute expanding lesions of the posterior fossa. Medullary paralysis was also detected by observing Wave III.

Acute intracranial hypertension and auditory brain-stem responses. Part 2: The effects of brain-stem movement on the auditory brain-stem responses due to transtentorial herniation.

Movement of the upper brain stem (inferior colliculus) was correlated with the alterations in the amplitude of wave V of the auditory brain-stem responses (BER's) during supratentorial brain compression in cats. In vivo observation of the brain stem and postmortem inspection show that suppression of the amplitude of BER wave V reflects the extent of caudal displacement of the inferior colliculus. Marked suppression of the amplitude of BER wave V (approximately 30% of control) correlates with the beginning of transtentorial herniation, and complete suppression of the wave V indicates complete transtentorial herniation of the brain-stem and supratentorial structures. The BER wave V is thought to be a sensitive index of caudal movement of the upper brain stem due to transtentorial herniation.

Acute intracranial hypertension and auditory brain-stem responses. Part 1: Changes in the aduitory brain-stem and somatosensory evoked responses in intracranial hypertension in cats.

Changes in auditory brain-stem responses (BER's) and somatosensory evoked responses (SER's) were investigated to correlate mass volume, intracranial pressure, and neurological dysfunction in mass-induced intracranial hypertension in cats. As the intracranial pressure was raised by expansion of a supratentorial balloon, the late components of the SER's were suppressed first, followed by the early components of the SER's, then Wave V and Wave IV of the BER's, in that order. This suggests that the nonspecific reticular projections are most vulnerable to compression ischemia, and the specific somatosensory pathways are the next most vulnerable. Neural activity of the auditory pathways in the upper brain stem was also gradually suppressed, but less so than that of the somatosensory pathways. After complete transtentorial herniation, in spite of immediate mass evacuation, the function of the somatosensory pathways was greatly impaired, often irreversibly. The neural activity of the auditory pathways in the upper brain stem revealed progressive recovery during a 3-hour period. The measurements of BER Wave V is thought to be useful in predicting transtentorial herniation.