OBJECTIVE: To develop and test a method for standardized calibration of pulse oximeters. METHODS: A novel pulse oximeter calibration technique capable of simulating the behavior of real patients is discussed. It is based on an artificial finger with a variable spectral-resolved light attenuator in conjunction with an extensive clinical database of time-resolved optical transmission spectra of patients fingers in the wavelength range 600-1000 nm. The arterial oxygen saturation of the patients at the time of recording was derived by analyzing a corresponding blood sample with a CO-oximeter. These spectra are used to compute the modulation of the light attenuator which is attached to the artificial finger. This calibration method was tested by arbitrarily playing back recorded spectra to pulse oximeters and comparing their display to the value they displayed when the spectra were recorded. RESULTS: We were able to demonstrate that the calibrator could generate physiological signals which are accepted by a pulse oximeter. We also present some experience of playing back recorded patient spectra. The mean difference between the original reading of the pulse oximeters and the display when attached to the calibrator is 1.2 saturation points (displayed oxygen saturation SpO2) with a standard deviation of 1.9 saturation points. CONCLUSIONS: The tests have shown the capabilities of a spectral light modulator for use as a possible calibration standard for pulse oximeters. If some improvements of the current prototype can be achieved we conclude from the experience with the device that this novel concept for the calibration of pulse oximeters is feasible and that it could become an important tool for assessing the performance of pulse oximeters.
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.
Considering the fundamental difficulties to define the term 'depth of anaesthesia', a more feasible concept for assessment of 'adequacy of anaesthesia' will be explained. The basic requirements for a monitoring index are definite response, gradual scaling and independence from the anaesthetic technique used. Additionally the index should be predictive for appearance of clinical signs of an inadequate anaesthesia. Different signal-processing methods will be discussed to extract the relevant information from both the spontaneous and the evoked brain electrical activity. In this context well established methods like spectral analysis are investigated in combination with new and more sophisticated methods like bispectral analysis or wavelet decomposition. Since no single-parameter index has been defined for monitoring depth of anaesthesia, a set of EEG parameters may be more useful to take into account intra- and interindividual variability. In parallel to the description of the monitor concept, the investigation of neural nets and fuzzy techniques, in addition to or in substitution of conventional statistical methods, will be introduced. Examples are given for data quality assessment, parameter extraction and re-classification.
R. T. Frankenberger, O. Bussmann, W. Nahm, E. Konecny, and L. Gortner. Messung seitlicher Haupttemperaturprofile von Frühgeborenen in Inkubatoren mittels Thermograhpie - Measuring lateral skin temperature profile of premature infants in incubators with thermography. In Biomedizinische Technik. Biomedical Engineering, vol. 43(6) , pp. 174-178, 1998
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.
A prerequisite for the further improvement in the quality of warming therapy is an accurate knowledge of the interactions between the microclimate in warming therapy devices and the thermal balance of the infant. For generating this knowledge, thermal manikins can be helpful. Suitable models capable of also simulating evaporative heat loss in preterm infants have, however, not been available to date. A thermal manikin representing an infant weighing 530 g and capable of simulating convective, radiative and also evaporative heat loss has now been developed. It comprises an outer shell made of porous, anatomically shaped clay, and is divided into six compartments each of which can be heated individually. Water-filled Gore-Tex bags located immediately beneath the shell are provided to simulate evaporation. In a clinical study, temperature profiles of 8 very small preterm infants were measured thermographically. Measurements in the manikin showed that highly comparable temperature profiles with only minor differences could be obtained. Total heat and water losses by the manikin were in good agreement with clinical values. Using the model described here it is possible to simulate the heat exchange of premature infants under extreme and accurately reproducible environmental conditions. This manikin may thus serve as a tool for comparative studies, for the development of warming therapy equipment, or for training purposes.
The present paper examined the question as to the extent to which the taking of gas samples for the purpose of measuring the breath alcohol concentration (BAC) in the expired air of patients on artificial respiration is influenced by temperature and humidity. For this purpose a lung model standardized at different alcohol concentrations was used, in which the temperature (T: 25, 30 and 35 degrees C) and the relative humidity (RH: 50, 75 and 95%) were varied.
H. Gehring, W. Nahm, J. Baerwald, E. Konecny, and P. Schmucker. Atem-Alkoholmeßgeräte mit elektrochemischem Sensor - Meßgenauigkeit bei Beatmung am Lungenmodell - Breath alcohol analyzers with electrochemical sensory--accuracy of measurements in lung model ventilation. In Biomedizinische Technik. Biomedical Engineering, vol. 41(3) , pp. 54-9, 1996
Absorption of irrigating fluid by blood vessels during endoscopic urological surgery may result in cardiac insufficiency, impairment of electrolyte metabolism and neurological disorders. For detection and quantification of the volume absorbed, ethanol is added to the irrigating fluid. The resulting blood alcohol concentration can be obtained by measuring the alcohol concentration in the expired air. For artificially ventilated patients receiving a general anesthetic, electrochemical sensors that remain uneffected by volatile anaesthetics are used. In the present study, the measuring accuracy of three different alcohol analyzers using electrochemical sensors was tested against an infrared reference sensor during simulated ventilation in a lung model, and the optimal trigger time point for sampling determined. All three devices tested show the same degree of accuracy as the reference. For manual endexpiratory triggering devices with short sampling times are best suitable. Portable devices powered by rechargeable batteries and usable with both spontaneously breathing and ventilated patients are recommended for clinical application.
One way of determining pulmonary CO2 elimination during anaesthesia is the breath-by-breath method. With this technique, CO2 analysis is carried out using either the mainstream method (MSM), that is, directly in the expired air flow, or in samples of expired air. A disadvantage of MSM is the lack of sensor signal correction for changes in the composition of the gas mixture and barometric pressure. Sidestream Systems (SSM) measure respiratory gas flow and gas con- centration with adequate accuracy, and also correct the measured values for gas composition and ambient parameters. Disadvantages of breath-by-breath analysis are the SSM-system-rela- ted delay and distortion of the CO2 curves. In the present study, a computer-assisted comparati- ve analysis of CO, elimination measurement by the sidestream and mainstream methods was carried out using ctiff erent mixtures of gases in a lung model. Under the selected conditions simulated in the lung model, evaluation of CO2 elimination using SSM and MSM is possible with an error of between 0 and 10 % versus reference Systems.Measu- ring accuracy of the MSM System in particular is found to depend directly on the composition of the gas mixture. Using the method described here, the measuring error of an SSM System in terms of delay and response time can be compensated with adequate accuracy.
E. Konecny, and W. Nahm. Physikalische Messtechniken in der Medizin. Zentrum für Fernstudien & universitäre Weiterbildung, UNI Kaiserslautern, 1996.
Book Chapters (1)
J. Abke, W. Nahm, and E. Konecny. Implementierung einer Bispektralanalyse in LabVIEW zur Beurteilung von Narkosetiefe über das EEG. In Virtuelle Instrumente in der Praxis - Meßtechnik, VDE-Verlag, pp. 256-264, 1998
P. Knoop, C. Hornberger, W. Nahm, and E. Konecny. Entwicklung eines optischen Kalibrators für kommerzielle Pulsoximeter: Erste Erfolge - Development of an optical calibrator for commercial pulse oximeters: initial successes. In Biomedizinische Technik. Biomedical Engineering, vol. 43 s1, pp. 128-129, 1998
M. Kelling, Z. Hussein, W. Nahm, and E. Konecny. Experimenteller Aufbau zur nicht-dispersiven Messung des Isotopenverhältnisses 13C/12C in Gasen - Experimental development of non-dispersive measurement of 13C/12C isotope relations in gases. In Biomedizinische Technik. Biomedical Engineering, vol. 43 Suppl, pp. 280-1, 1998
O. Bussmann, W. Nahm, and E. Konecny. Modell zur Simulation der Wärmeabgabe und Thermoregulation Frühgeborener - A Model for simulating heat transfer and thermoregulation of premature infants. In Biomedizinische Technik/Biomedical Engineering, vol. 43(s1) , pp. 300-301, 1998
W. Nahm, G. Stockmanns, M. Daumer, J. Abke, and E. Konecny. Automatische EEG-Datenvorverarbeitung in einer Multicenterstudie - Automatic EEG data processing in a multicenter study. In Biomedizinische Technik/Biomedical Engineering, vol. 43(s1) , pp. 146-147, 1998
M. Kelling, W. Nahm, and E. Konecny. Absorptions-spektroskopische Methode zur Messung des Istopenverhältnisses 13C/12C - Absorption spectroscopy method for measuring 13C/12C isotope relations. In Biomedizinische Technik. Biomedical Engineering, vol. 42 Suppl, pp. 315-6, 1997
H. Gehring, W. Nahm, D. Hufker, A. Schmitz, W. Mertins, and E. Konecny. Messung der Atem-Alkoholkonzentration während der Beatmung. Modelluntersuchungen zum Einfluß der Temperatur und Luftfeuchtigkeit auf die Reproduzierbarkeit der Messung verschiedener Probenentnahmesystem - Measuring breath alcohol concentration during artificial ventilation. Model studies of the effect of temperature and humidity on the reproducibility of measurements of various sampling systems].. In Biomedizinische Technik. Biomedical Engineering, vol. 42 Suppl, pp. 356-7, 1997
R. Sevcik, W. Nahm, E. Konecny, O. Petrowicz, H. Borgolte, and D. Nolte. Analyse diskontinuierlicher Lungengeräusche durch die Wavelet-Transformation und die Illustration selten auftretender kontinuierlicher Oszillationen - Analysis of discontinuous lung sounds by wavelet transformation and illustration of rare continuous oscillations. In Biomedizinische Technik. Biomedical Engineering, vol. 42 s2, pp. 297-8, 1997
C. Hornberger, W. Nahm, P. Knoop, H. Gehring, P. Wouters, and E. Konecny. Nicht-invasive Erfassung von Blutspektren durch zeitaufgelöste in vivo Spektroskopie - Noninvasive detection of blood spectra by time resolved in vivo spectroscopy. In Biomedizinische Technik. Biomedical Engineering, vol. 42 s2, pp. 217-8, 1997
C. Du, W. Nahm, H. Gehring, and E. Konecny. Quantification of NIRS data of a blood phantom by spectral multicomponent analysis. In Proceedings of the SPIE, vol. 2626, pp. 130-140, 1995
In order to quantify the chromophore components from in vivo blood NIRS, a blood- equivalent phantom has been developed which consists of properly diluted intralipid and ICG dye. The reflection and transmission near infrared spectroscopy (NIRS) of the phantoms with different scattering backgrounds and ICG concentrations are measured and analyzed by the spectral multicomponent analysis (MCA) method to extract ICG concentration. The experimental results show that the MCA method can be used to quantify absolute ICG concentrations in scattering media if the average path lengths are known. Moreover, it was found by the experiments that both the water absorption peak at 970 nm and the ICG absorption peak at 800 nm show similar behavior during the change of the scattering background. Thus the ratio of the MCA-estimated concentration factor of ICG to water is independent of the blood-phantom scattering.