W. Nahm

Middle latency auditory evoked responses and electroencephalographic derived variables do not predict movement to noxious stimulation during 1 minimum alveolar anesthetic concentration isoflurane/nitrous oxide anesthesia

UNLABELLED The electroencephalogram (EEG) and middle latency auditory evoked responses (MLAER) have been proposed for assessment of the depth of anesthesia. However, a reliable monitor of the adequacy of anesthesia has not yet been defined. In a multicenter study, we tested whether changes in the EEG and MLAER after a tetanic stimulus applied to the wrist could be used to predict subsequent movement in response to skin incision in patients anesthetized with 1 minimum alveolar anesthetic concentration (MAC) isoflurane in N2O. We also investigated whether the absolute values of any of these variables before skin incision was able to predict subsequent movement. After the induction of anesthesia with propofol and facilitation of tracheal intubation with succinylcholine, 82 patients received 1 MAC isoflurane (0.6%) in N2O 50% without an opioid or muscle relaxant. Spontaneous EEG and MLAER to auditory click-stimulation were recorded from a single frontoparietal electrode pair. MLAER were severely depressed at 1 MAC isoflurane. At least 20 min before skin incision, a 5-s tetanic stimulus was applied at the wrist, and the changes in EEG and MLAER were recorded. EEG and MLAER values were evaluated before and after skin incision for patients who did not move in response to tetanic stimulation. Twenty patients (24%) moved after tetanic stimulation. The changes in the EEG or MLAER variables were unable to predict which patients would move in response to skin incision. Preincision values were not different between patients who did and did not move in response to skin incision for any of the variables. MLAER amplitude increased after skin incision. We conclude that it is unlikely that linear EEG measures or MLAER variables can be of practical use in titrating isoflurane anesthesia to prevent movement in response to noxious stimulation. IMPLICATIONS Reliable estimation of anesthetic adequacy remains a challenge. Changes in spontaneous or auditory evoked brain activity after a brief electrical stimulus at the wrist could not be used to predict whether anesthetized patients would subsequently move at the time of surgical incision.

Wavelet Analysis of Middle Latency Auditory Evoked Responses Calculation of an Index for Detection of Awareness during Propofol Administration

Background Middle latency auditory evoked responses (MLAER) as a measure of depth of sedation are critically dependent on data quality and the analysis technique used. Manual peak labeling is subject to observer bias. This study investigated whether a user-independent index based on wavelet transform can be derived to discriminate between awake and unresponsive states during propofol sedation. Methods After obtaining ethics committee approval and written informed consent, 13 volunteers and 40 patients were studied. In all subjects, propofol was titrated to loss of response to verbal command. The volunteers were allowed to recover, then propofol was titrated again to the same end point, and subjects were finally allowed to recover. From three MLAER waveforms at each stage, latencies and amplitudes of peaks Pa and Nb were measured manually. In addition, wavelet transform for analysis of MLAER was applied. Wavelet transform gives both frequency and time information by calculation of coefficients related to different frequency contents of the signal. Three coefficients of the so-called wavelet detail level 4 were transformed into a single index (Db3d4) using logistic regression analysis, which was also used for calculation of indices for Pa, Nb, and Pa/Nb latencies. Prediction probabilities for discrimination between awake and unresponsive states were calculated for all MLAER indices. Results During propofol infusion, subjects were unresponsive, and MLAER components were significantly depressed when compared with the awake states (P < 0.001). The wavelet index Db3d4 was positive for awake and negative for unresponsive subjects with a prediction probability of 0.92. Conclusion These data show that automated wavelet analysis may be used to differentiate between awake and unresponsive states. The threshold value for the wavelet index allows easy recognition of awake versus unresponsive subjects. In addition, it is independent of subjective peak identification and offers the advantage of easy implementation into monitoring devices.

Observation of spectral interferences for the determination of volume and surface effects of thin films

SummaryThe application of a rapid scanning diode array spectrometer allows the time-resolved observation of the interferences caused by multiple reflections at the interfaces of thin films. This spectral interferometry enables the observation of changes in optical pathlengths and allows to separate volume-effects like polymer swelling from surface-effects like adsorption or deposition. Polymer/solvent interactions will give an example for an application of this method.

Irrigating fluid absorption during percutaneous nephrolithotripsy

Background: The most common complication during percutaneous nephrolithotripsy (PNL) is the destruction of organ structures with extravasation of the irrigation fluid into the retroperitoneal space. Consequently, there is an increased risk of a urosepsis and a complicated therapeutic course. In this study we aimed to show that extravascular absorption could be differentiated from intravascular absorption due to their unique absorption characteristics, and that these characteristics enable a prediction of possible post‐operative complications.

Non-invasive in vivo measurement of blood spectrum by time-resolved near-infrared spectroscopy

Abstract Investigation of rapid near-infrared (NIR) spectroscopy in combination with the fibre optics for biomedical sensing is presented. Modern diode-array technology enables sensitive measurement of changes in tissue absorbance caused by blood pulsation. In order to describe the dynamics of this system a multi-layer model based on pulse wave theory is used. The evaluation of the pulsatile part of tissue absorbance at different wavelengths allows the construction of both pulsatile and static tissue spectra. The absorption monitoring of an injected bolus of indocyanine green is presented of non-invasive measurement of an arterial blood spectrum.

Irrigation fluid absorption during transurethral resection of the prostate: Spinal vs. general anaesthesia

Background: The absorption of irrigation fluid during transurethral resection of the prostate (TURP) is determined primarily by hydrostatic pressure in the bladder and prostatic venous pressure. In comparison to spontaneously breathing patients, patients undergoing mechanical ventilation with positive pressure have a raised central venous pressure and a reduced venous return, both of which can influence intravascular absorption. The purpose of the prospective study was to compare the effects of general (GA) and spinal anaesthetic (SA) techniques on the perioperative absorption of irrigating fluid in patients undergoing TURP.

A Prototype Device for Standardized Calibration of Pulse Oximeters

Objective.To develop and test a method for standardized calibrationof pulse oximeters. Methods.A novel pulse oximeter calibrationtechnique capable of simulating the behavior of real patients is discussed.It is based on an artificial finger with a variable spectral-resolved lightattenuator in conjunction with an extensive clinical database of time-resolvedoptical transmission spectra of patients fingers in the wavelength range600–1000 nm. The arterial oxygen saturation of the patients at the timeof recording was derived by analyzing a corresponding blood sample with aCO-oximeter. These spectra are used to compute the modulation of the lightattenuator which is attached to the artificial finger. This calibration methodwas tested by arbitrarily playing back recorded spectra to pulse oximeters andcomparing their display to the value they displayed when the spectra wererecorded. Results.We were able to demonstrate that the calibratorcould generate physiological signals which are accepted by a pulse oximeter.We also present some experience of playing back recorded patient spectra. Themean difference between the original reading of the pulse oximeters and thedisplay when attached to the calibrator is 1.2 saturation points (displayedoxygen saturation SpO2) with a standard deviation of 1.9 saturationpoints. Conclusions.The tests have shown the capabilities of aspectral light modulator for use as a possible calibration standard for pulseoximeters. If some improvements of the current prototype can be achieved weconclude from the experience with the device that this novel concept for thecalibration of pulse oximeters is feasible and that it could become animportant tool for assessing the performance of pulse oximeters.

Messung der Atem-Alkoholkonzentration mit einem neuen elektrochemischen Sensor Modelluntersuchung zur Querempfindlichkeit gegenüber volatilen Anästhetika und klinische Anwendung

ZusammenfassungDie Messung der Atem-Alkoholkonzentration zur Erkennung eingeschwemmter Spülflüssigkeit bei endoskopischen urologischen Eingriffen wird bei beatmeten Patienten mit Inhalationsanästhesie limitiert durch die Querempfindlichkeit üblicherweise verwendeter Infrarotsensoren gegenüber volatilen Anästhetika und Lachgas. Ziel dieser Untersuchung ist die Validierung eines neuen Atem-Alkoholmeßgeräts mit elektrochemischem Sensor zur Anwendung bei spontanatmenden Patienten und bei mit Inhalationsanästhetika beatmeten Patienten. In einem Labormodell wurde die Querempfindlichkeit des elektrochemischen Sensors gegenüber volatilen Anästhetika und Lachgas ausgeschlossen. In einer klinischen Untersuchung wurde bei 48 spontanatmenden Patienten und bei 51 beatmeten Patienten mit Inhalationsanästhesie die mit dem Testgerät gemessene Atem-Alkoholkonzentration (AAK) gegenüber der gaschromatographisch bestimmten Blut-Alkoholkonzentration (BAK) aufgetragen. Für die spontanatmenden Patienten ergibt sich eine Korrelation zwischen AAK und BAK von r=0,961 und eine Steigung der Regressionsgeraden von 0,56. Bei den beatmeten Patienten mit Inhalationsanästhesie korreliert die AAK bis zu einer BAK von 0,4‰ähnlich linear wie bei den spontanatmenden Patienten (r= 0,856), darüber hinaus besteht keine klinisch relevante Korrelation (r=0,444).AbstractAbsorption of irrigating fluid in transurethral prostatic resection (TURP) and percutaneous nephrolitholapaxy (PNL) into veins or delayed absorption due to fluid extravasation may result in a TURP syndrome. The measurement of end-tidal breath alcohol concentration (et AC) as a monitor of absorption of irrigating fluid labelled with 2% ethanol is limited by the disturbance of infrared sensors by volatile anaesthetics and nitrous oxide (N2O) (Fig. 2). An electrochemical sensor is acceptable for this method. The aim of the present study was the evaluation of breath alcohol measurements using an electrochemical sensor device (Alcomed 3010, Envitec). The stability of the sensor in the presence of volatile anaesthetics was examined using a lung model. In a clinical investigation, the device was then applied to spontaneously breathing or mechanically ventilated patients inhaling volatile anaesthetics during endoscopic urological surgery. Method. A two-chamber lung model filled with water for performing non-invasive measurements at the mouth of a patient has already been introduced by Brunner et al. (Fig. 1). With the addition of different amounts of ethanol to the temperature-controlled water, a constant ethanol concentration is achievable in the air above the water that is dependent on adjustments of the ventilator. Increasing concentrations of volatile anaesthetics (isoflurane, enflurane, halothane, and sevoflurane) were added to the fresh gas flow (2 l O2/3 l N2O) and etACs were measured using the manually triggered self-absorbent electrochemical sensor. First, regression equations were established between breath alcohol concentrations and increased volatile anaesthetic concentrations. Regression equations were then established between end-tidal anaesthetic gas concentrations and vaporizer adjustments in order to rule out an influence of ethanol on the anaesthetic gas monitor Ultima V (Datex). In the clinical investigation, 53 intubated and ventilated patients (33 undergoing PNL, 20 undergoing TURP) and 48 patients breathing spontaneously (32 with inhalation anaesthesia, 16 with spinal anaesthesia) were investigated. The etAC was measured with the Alcomed 3010 and compared with gas-chromatographically registered blood alcohol concentrations (BAC). The study had previously been approved by the Ethical Committee of the Medical University of Luebeck. Patients with liver disease and a history of toxic abuse were excluded. Only one value per patient (maximum BAC) was included in the statistics in order to avoid a cluster effect. Results. The lung model experiments demonstrated that the measurement of etAC with an electrochemical sensor is free of interference by volatile anaesthetics (Table 1). The slope of the regression between the measured alcohol concentration and increased concentrations of anaesthetics did not differ significantly from baseline values. The measurement of end-tidal anaesthetic concentrations was not significantly different from vaporizer adjustments in the presence of increased alcohol concentrations (Table 2). During the clinical investigation, a regression between etAC and BAC was determined for both groups. For the group of patients breathing spontaneously, the correlation coefficient was 0.961 and the regression equation revealed etAC= 0.5677*BAC−0.1303 (Fig. 5). However, in the group of ventilated patients a biphasic course was shown that was dependent on BAC (Fig. 6). At BAC <0.4‰, a similar correlation (r=0.856) to the spontaneously breathing group could be seen (regression equation: etAC= 0.617*BAC−0.020). Above 0.4‰ BAC there was no acceptable correlation (r=0.444, regression equation: etAC=0.202*BAC+0.104). Conclusions. The tested electrochemical sensor does not interfere with volatile anaesthetics and N2O as demonstrated by a lung model. There is a good correlation between etAC and BAC measurements in patients breathing spontaneously with special regard to the slope of the regression (s=0.57). The application to ventilated patients during the administration of inhalation anaeshtesia should be restricted, with attention paid to the non-linear course o the regression.

Messung seitlicher Hauttemperaturprofile von Frühgeborenen in Inkubatoren mittels Thermographie - Measuring Lateral Skin Temperature Profiles of Preterm Infants in Incubators by Thermography

: Thermography enables the measurement of patients skin temperature profiles without stress caused by direct contact of probes to the skin. In previous incubator studies, frontal recordings were made through a hole in the top wall of the incubator hood. Using this method it is not possible to record the lateral temperature gradient from the back to the abdomen of the infant (in supine position), which is due to very limited heat loss near the incubator mattress. In this study temperature recordings were made from a lateral position. For this purpose a new front door of the incubator (Draeger 8000) was designed, which replaced the standard front door during measurements. In a clinical study thermography was compared to temperature measurements by standard thermistors. The mean difference between thermography and thermistors was 0.16 degree C. These results verify the use of thermography for measuring skin temperature of preterm infants in incubators.

Spectroscopical examination of photopolymerization, structure and permeability of thin polymers films

Abstract Oligomers with a dimethylsiloxane backbone coated as thin films on different substrate surfaces were thermally as well as photochemically cross-linked. The structure and the degree of cross-linking were examined spectroscopically. Diffusion of different gases in the thin polymer films was measured by time resolved infrared ATR-spectroscopy. The process of diffusion is almost immediately followed by a swelling of the polymer proportional to gas concentration. Therefore diffusion may also be measured by spectral interferometry, giving a very sensitive device for optical sensing of hydrocarbons. Furthermore, diffusion in polymers may be measured very accurately by spatially resolved UV/Vis-spectroscopy. Diffusion coefficients may also be determined indirectly from the equilibrium of monomers and excimers indicated by the fluorescence intensities. This method allows the in situ observation of the cross-linking process.