A prototype device for standardized calibration of pulse oximeters

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.

A prototype device for standardized calibration of pulse oximeters II

OBJECTIVE

There is no commonly accepted in vivo calibration method for pulse oximeters available up to now. On the basis of a prototype device for the calibration of pulse oximeters which was introduced recently, a second approach based on the same concept was tackled in order to design a reliable method for standardized calibration of pulse oximeters.

METHODS

An extensive clinical database of time-resolved optical transmission spectra of patient fingers is used to simulate the behavior of patients. A device which is capable of playing back these spectroscopic data to pulse oximeters, and a database where the oxygen status measured with the reference method (Co-Oximetry) is stored, are the main parts of the concept. The playback device has an artificial finger as interface to the pulse oximeters and serves to collect light from the pulse oximeter for analysis and to playback simulated light to the pulse oximeter. The light intensity emitted by two LEDs which illuminates the pulse oximeter detector is controlled via a computer in such a way that it is the same as if the pulse oximeter light had passed the finger. The pulse oximeter display during the data playback can thus be compared to the true SaO2 of the patient. The device is tested with 4 pulse oximeters based on 100 patient spectra.

RESULTS

For the four pulse oximeters used in this investigation, an Agilent Technologies CMS monitor (formerly Hewlett-Packard), an Ivy 2000 with Masimo Set technology and Nellcor N-3000 and N-395, there is good correlation between SPO2 and SaO2, and mean and standard deviation of in vivo SpO2-SaO2 and playback SpO2-SaO2 are in good agreement. For two instruments, Nellcor N3000 and Agilent CMS Monitor, a quantitative comparison between the in vivo and in vitro SpO, results was derived. A mean of the deviation playback vs. in vivo SpO2 is less than 0.5% SpO2. The error limits are comparable with the calibration error of the conventional calibration routine. The device is also capable of data playback even in situations with rapid desaturation changes, as displayed in Figure 2. For the other tested pulse oximeters the results are comparable.

CONCLUSIONS

Compared to the first prototype the current version is simpler and less expensive in production. Many of previously existing problems are solved and the applicability to a large variety of pulse oximeters and sensors is given. The novel concept for the calibration of pulse oximeters is a tool for assessing the performance of pulse oximeters.

Design and validation of a pulse oximeter calibrator

The performance of a new calibrator for pulse oximeters is tested with five pulse oximeters from different manufacturers. The calibrator is based on time resolved transmission spectra of human fingers. Finger spectra with different arterial oxygen saturation can be selected to simulate real patients. The results obtained with this calibration device are compared with the results of conventional calibration procedures with volunteers. Beside accuracy tests the suitability for artifact simulation with the new device is discussed. The response of the five tested pulse oximeters is in good agreement with the response of the pulse oximeters connected to real patients. A test procedure for pulse oximeters similar to the conventional desaturation practice is possible; some of the typical artifacts pulse oximetry has to cope with can be simulated easily.

[Measuring breath alcohol concentration during artificial ventilation. Model studies of the effect of temperature and humidity on measurements by various sampling systems]

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.

[Measuring pulmonary CO2 elimination--studies with the lung model using various mixed gases]

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 concentration 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-related delay and distortion of the CO2 curves. In the present study, a computer-assisted comparative analysis of CO2 elimination measurement by the sidestream and mainstream methods was carried out using different 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. Measuring 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.

[Water permeability of breathing filters]

Breathing filters or heat and moisture exchangers (HME), which are placed between a tracheal tube and the Y-piece, allow reuse of breathing tubes without changing between operations. During low-flow and minimal-flow anaesthesia, condensed water accumulates in the breathing circuit. An evaluation of the volume of condensed water is given (Fig. 1). It may be possible that water flows onto the filter surface, for example, when the breathing tubes are lifted. The water permeability of such breathing filters and HME was tested. For the experiments, a commercial breathing circuit and ventilator (Dräger Sulla 808 + Ventilog) and patient model (Fig. 2) were used; 12 breathing filters/HMEs of different manufacturers were tested. Only 3 filters were not permeable to the test volume of 20 ml water. The authors suggest the water volume be checked routinely and the breathing tubes be emptied if necessary.

[The performance of respirator alarms during simulated critical events in CMV/IPPV artificial respiration]

Alarm systems of ventilators enhance detection of possible critical events during artificial ventilation. Due to their significance, in some countries the alarm detection of ventilators is regulated by federal law. Up to now, no recommendations for the adjustment of alarm limits exist and only a few detailed investigations of the accuracy of alarm detection are available. METHODS. The response of four commercially available ventilators (Servoventilator 900C, Siemens, Inc.; Bennett 7200a, Hoyer, Inc.; Veolar, Hamilton, Inc.; EVITA, Dräger, Inc.) to critical events during artificial ventilation of a test lung were evaluated. We measured the alarm time (the time between event creation and alarm response) of ten different simulated critical events including disconnection, differentisized leaks, failure of the gas supply, and obstruction at different places in the artificial airway. DISCUSSION. All respirators were able to recognise severe critical situations such as hose disconnection, failure of gas supply, and total airway obstruction within a short time (< 15 s). The recognition of small airway leaks was more difficult for the ventilators even when the alarm thresholds were close. The alarm detection of the EVITA (software 10.0 or less) under conditions of partial airway obstruction may be a source of risk for the patient as the machine continued supplying inspiration with pressure-limited ventilation even when the pressure threshold was reached.

Intelligent alarms reduce anesthesiologist's response time to critical faults

The proliferation of monitors and alarms in the operating room may lead to increased confusion and misdiagnosis unless the information provided is better organized. Intelligent alarm systems are being developed to organize these alarms, on the assumption that they will shorten the time anesthesiologists need to detect and correct faults. This study compared the human response time (the time between the sounding of an alarm and the resolution of a fault) when anesthesiologists used a conventional alarm system and when they used an intelligent alarm system. In a simulated operating room environment, we asked 20 anesthesiologists to resolve seven breathing circuit faults as quickly as possible. Human response time was 62% faster, decreasing from 45 to 17 s, when the intelligent alarm system was used. The standard deviations in response time were only half as large for the intelligent alarm system. It appears that the computer-based neural network in the intelligent alarm system diagnosed faults more rapidly and consistently than did the anesthesiologists. This study indicates that breathing circuit faults may be more rapidly corrected when the anesthesiologist is guided by intelligent alarms.

[Are humidity filters necessary in the inspired air in the breathing circuit? A new in vivo method of measuring humidity in the air breathed]

Humidification of inspiratory gases under anaesthetic conditions still is a matter of controversial discussion. Physiological humidification and heating of breathing air are preconditions for mucociliary clearance, pulmonary cleaning and defence mechanisms. These functions of the upper respiratory tract are eliminated by application of artificial airways. In general the humidification of inspiratory gases should not remain under 70% of relative air humidity at 37 degrees C. Under clinical conditions it is problematic to ensure sufficiently rapid and reproducible measurements of humidity during breathing cycles. We developed a measuring method that enables to make these measurements without big mechanical device. Aim of this investigation was to measure air humidity in typical semiclosed systems during anaesthesia and semiopen CPAP-respiration. The necessity and efficiency of a heat and moisture exchanger (HME) was to be investigated as well. After approximately 5 minutes there was an inspiratory relative air humidity not below 70% at 28 degrees C (19 mg H2O/l humid air) within the breathing circuit with CO2 double-absorber. By using an HME it is possible to increase relative air humidity within this system to 86% at 29.5 degrees C (25 mg/l). After one hour's respiration with this system without HME a relative humidity of 87% at 30 degrees C (26 mg/l) is reached after replaced HME. Initial relative humidity in a semiopen CPAP-system is about 12% at 28 degrees C (3 mg/l). This is increased to 85% at 29.5 degrees C (25 mg/l) after 15 minutes respiration with HME.(ABSTRACT TRUNCATED AT 250 WORDS)

[Design of partial pelvic replacement with calculation of stresses on the pelvic bone using the finite element method]

Although tumors of the pelvic region are relatively rare, with regard to the provision of an individual prosthesis, they make great demands both on the engineer and on the surgeon. In the case of partial pelvic replacement, the main problem is that of fixation. The direction of the preload by the screw and counternut in the anchorage should make possible the introduction of forces into the bone that closely mimic the natural pre-operative situation, in order to keep bone remodeling and stressing to a minimum. A three-dimensional finite element (FE) analysis of the stresses at work in the pelvic bone was performed using an FE model of the pelvis constructed on the basis of computed tomographic data, since with the FE method it is possible to obtain information about deformations, internal stresses and local forces acting on the bone. The results show that the main stressing of the bony pelvis occurs in the region of the acetabulum and the iliosacral joints, and that further points of loading are located in the lower region of the ischium-and this while standing on one leg, on the side of the supporting leg.

[Future perspectives for equipment maintenance and servicing]

Based on the Equipment Safety Law, the Medical Technical Regulations in force since 1 January 1986, and the German Standard DIN 13252: "Inhalational anesthetic Apparatus--requirements for safety and testing", a maintenance system for medical equipment is presented. The maintenance system comprises the check of the equipment by the user before clinical use, maintenance procedures when the equipment is in daily clinical use (maintenance system I), inspection of the equipment at fixed intervals (maintenance system II), and the repair of the equipment should it break down (maintenance system III). Possibilities and perspectives for rationalizing test procedures by means of test simulators are shown. The initial or repeated instruction on medical equipment with life-supporting functions can be performed more systematically when appropriately designed test simulators are used.

[The oxymeter, a new device for the continuous transcutaneous measurement of the arterial oxygen partial pressure]

The transcutaneous measurement of the arterial pO2 is a practicable method for routine clinical use. The covering, calibration and fixation of the electrode are simple. In normal circulatory states the correlation between arterial and transcutaneous pO2 is high. The stability of the electrode permits its use even for long term monitoring. Recalibration is necessary at intervals of 12 h. Orientating blood gas analysis for control is nevertheless essential. After a measuring period of 5--6 h a blistering under the electrode is possible. The placement of the electrode has to be changed after 4 h. The continuous measurement opens new possibilities for the therapy, diagnosis and trend analysis.