A new Anesthesia & Analgesia section on technology, computing, and simulation.
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Anesthesiology↗
Biomedical subjects
D B Raemer
Publications and source records attributed to D B Raemer.
Arterial-line monitoring system simulation.
OBJECTIVE: We sought to improve the realism of our patient simulation environment by developing a simulation of the arterial-line monitoring system. Properties of the system we wished to depict were: electro-mechanical delay between ECG and radial artery pressure, beat to beat amplitude variability and respiratory variation, realistic looking pulse pressure in hypertensive and hypotensive states, a functional link to the stopcock and transducer flush, and filtering characteristics of the measurement system. METHODS: A standard clinical pressure transducer and stopcock were modified to provide data about their state to a personal computer. A software program was written to modify the arterial pressure waveform from a patient simulator according to the pressure transducer and stopcock state as well as user settings to produce a new waveform. RESULTS: All of the desired improvements in the realism of the arterial waveform were implemented. CONCLUSIONS: The realism of scenarios using the patient simulator is enhanced by having the arterial-line monitoring system more accurately simulated.
Blood Pressure↗
Computerized realistic simulation: a teaching module for crisis management in radiology.
OBJECTIVE: Computerized realistic simulation technology has been used as a training tool in fields such as aviation and military training and in the nuclear power industry. More recently, it has been adapted for use in anesthesia crisis resource management. We describe the effectiveness of a simulation program like that used by anesthesiology departments that we developed to teach radiologists the principles of crisis management. MATERIALS AND METHODS: A mock CT scanner and patient simulator were used to simulate the environment in which radiologists encounter crises. Twenty-four residents attended the training program, four at each half-day session. Two responded to and two observed an initial crisis, after which they attended a lecture and watched a videotape review. The second pair then participated in a different crisis scenario. The scenario order was randomized. All scenarios were videotaped and randomly reviewed by two physicians not involved with the course. The following behavioral qualities of the participating residents were evaluated using a five-point scale, ranging from poor (1) to excellent (5): global assessment, communication skills, use of support personnel, use of resources, and role clarity. Residents then rated the course on a five-point scale using the following criteria: overall course usefulness, attainment of course goals, realism of scenarios, quality of lecture, and quality of videotape review. RESULTS: The trainees who had attended the lecture and watched the videotape review before participating in a scenario consistently scored higher than those who had not in the following areas (score after training/score before training): global assessment, 4.08/2.50; communication skills, 4.09/2.67; use of support personnel, 4.17/3.00; use of resources, 4.00/2.92; and role clarity, 4.17/2.67. Moreover, the participants gave the course the following average ratings: overall usefulness, 4.93; attainment of course goals, 4.78; realism of scenarios, 4.63; quality of lecture, 4.63; and quality of videotape review, 4.85. CONCLUSION: Although the critical assessment of a teaching method is difficult and subjective by nature, the improvement in behavioral performance scores suggests that simulation technology effectively conveyed the principles of crisis management. The course ratings show that the program was well accepted by participants.
Clinical Competence↗
FIx controller: an instrument to automatically adjust inspired oxygen fraction using feedback control from a pulse oximeter.
OBJECTIVE: To develop an instrument to help prevent pulmonary O2 toxicity, a syndrome that manifests itself in adult intensive care patients. METHODS: We designed, built, and tested a device that controls FIO2 exposure using oxygen saturation measured with a pulse oximeter (SpO2) in a negative feedback control system. A target SpO2 is designated by the clinician and the system adjusts the FIO2 from a mechanical ventilator so as to minimize the difference between the measured SpO2 and the target. Important elements of the system include a conservative artifact rejection algorithm, a gainscheduled sampled-data proportional-integral-derivative (PID) controller, and a safety system to prevent inspired mixtures with undesirably low FIO2 due to device failure. RESULTS: The control system was tuned in a series of animal experiments. Acceptable clinical response of the system was obtained using a gain-scheduled controller algorithm whereby the gain of the proportional term of a PID controller was adjusted based on the error signal and measured minute ventilation. Also, the artifact rejection algorithm and safety systems were successfully tested using simulation. CONCLUSIONS: Testing the effectiveness of this instrument will require comparison with manual control of FIO2 in an appropriately designed trial.
Algorithms↗
The effects of bronchodilator-inhaler aerosol propellants on respiratory gas monitors.
Spurious readings from a mass spectrometer have been reported following the administration of aerosol bronchodilators. We quantified the response of various respiratory gas analyzers to the aerosol propellant of albuterol inhalant (Proventil). The mass spectrometer systems tested, two Advantage systems, a SARA system, and a Model 6000 Ohmeda system, all displayed artifactual readings in response to the albuterol propellant. Each metered dose of the Proventil brand of albuterol contains 4 ml of Freon 11 (trichloromonofluoromethane) and 11 ml of Freon 12 (dichlorodifluoromethane). The concentration of propellant was expressed in doses/L, where each liter of gas contains 0.4 vol % of Freon 11 and 1.1 vol % of Freon 12 per dose. In proportion to the concentration of albuterol propellant, the two Advantage systems showed substantial readings of isoflurane (%) when no isoflurane was present (13% and 16% per dose/L) and reduced readings of enflurane (-8% and -10% per dose/L) and carbon dioxide (CO2) (-3 and +5 mm Hg per dose/L). The SARA system showed substantial CO2 readings when no CO2 was present (5 mm Hg per dose/L) and displayed small enflurane readings (0.1% per dose/L) when no enflurane was present. The Model 6000 unit showed CO2 readings when no CO2 was present (5 mm Hg per dose/L). Neither the Raman spectrometer, the infrared spectrometers, nor the piezoadsorptive analyzer we tested showed an artifactual effect of albuterol propellant on any of its readings. Simulation and clinical tests demonstrated that a single dose of albuterol propellant into a breathing circuit at the onset of inspiration resulted in concentrations of 0.8 and 0.3 dose/L, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)
Aerosol Propellants↗
Accuracy of end-tidal carbon dioxide tension analyzers.
Substantial mean differences between arterial carbon dioxide tension (PaCO2) and end-tidal carbon dioxide tension (PETCO2) in anesthesia and intensive care settings have been demonstrated by a number of investigators. We have explored the technical causes of error in the measurement of PETCO2 that could contribute to the observed differences. In a clinical setting, the measurement of PETCO2 is accomplished with one of three types of instruments, infrared analyzers, mass spectrometers, and Raman spectrometers, whose specified accuracies are typically +/- 2, +/- 1.5, and +/- 0.5 mm Hg, respectively. We examined potential errors in PETCO2 measurement with respect to the analyzer, sampling system, environment, and instrument. Various analyzer error sources were measured, including stability, warm-up time, interference from nitrous oxide and oxygen, pressure, noise, and response time. Other error sources, including calibration, resistance in the sample catheter, pressure changes, water vapor, liquid water, and end-tidal detection algorithms, were considered and are discussed. On the basis of our measurements and analysis, we estimate the magnitude of the major potential errors for an uncompensated infrared analyzer as: inaccuracy, 2 mm Hg; resolution, 0.5 mm Hg; noise, 2 mm Hg; instability (12 hours), 3 mm Hg; miscalibration, 1 mm Hg; selectivity (70% nitrous oxide), 6.5 mm Hg; selectivity (100% oxygen), -2.5 mm Hg; atmospheric pressure change, less than 1 mm Hg; airway pressure at 30 cm H2O, 2 mm Hg; positive end-expiratory pressure or continuous positive airway pressure at 20 cm H2O, 1.5 mm Hg; sampling system resistance, less than 1 mm Hg; and water vapor, 2.5 mm Hg. In addition to these errors, other systematic mistakes such as an inaccurate end-tidal detection algorithm, poor calibration technique, or liquid water contamination can lead to gross inaccuracies. In a clinical setting, unless the user is confident that all of the technical error sources have been eliminated and the physiologic factors are known, depending on PETCO2 to determine PaCO2 is not advised.
Algorithms↗
How best to monitor for detection of myocardial ischemia?
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Cardiopulmonary Bypass↗
The prospective use of population pharmacokinetics in a computer-driven infusion system for alfentanil.
Maitre et al. recently evaluated the accuracy of a set of previously determined population pharmacokinetic parameters for the opioid alfentanil using data from an earlier study in which the drug had been administered using a computer-controlled infusion pump (CCIP). The present study evaluated the accuracy of these same parameters in a CCIP prospectively in two groups of clinically dissimilar patients: 29 healthy female day surgery patients and 11 relatively older and less healthy male inpatients. In addition, another set of pharmacokinetic parameters, previously determined by Scott et al. in the CCIP in 11 male inpatients was also evaluated. The bias and inaccuracy were assessed by the median performance error (MDPE) and the median absolute performance error (MDAPE) in which the performance error was determined as the difference between measured and target serum concentration as a fraction of the target serum concentration. Unlike Maitre et al., the current study found a consistent bias in both populations. The MDPE was +53% and the MDAPE was 53%, with no difference between patient groups. In the 11 patients studied using the Scott et al. pharmacokinetic parameters, the MDPE was +1% and the MDAPE was 17%. The parameters of Scott et al. were further tested by simulating the serum concentrations that would have been achieved had they been used in the CCIP in the first 40 patients; results indicated MDPE of +2% and an MDAPE of 18%. Therefore, reasonably reliable and accurate target serum concentrations of alfentanil can be achieved using the pharmacokinetic parameters of Scott et al. in a CCIP.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
The theoretical effect of carboxyhemoglobin on the pulse oximeter.
The relationship between arterial oxygen saturation as measured by the pulse oximeter (SpO2) and the fractional arterial oxygen saturation (SaO2) in the presence and absence of carboxyhemoglobin (COHb) has been derived according to the theory of absorption spectroscopy. We find that our theoretically derived correction equation is similar to that found in the technical literature of Nellcor. However, the correction equations presented by Barker and Tremper and the technical literature of Ohmeda differ substantially from our equation when sufficient quantities of reduced hemoglobin are present and the fractional COHb saturation (SaCO) is high. Our approximated equation, derived from the Lambert-Beer law, is SaO2 = SpO2 (1 - 0.932 SaCO) + 0.032 SaCO. The equation of Barker and Tremper is SaO2 = SpO2 - 0.9 SaCO. The Nellcor equation is SaO2 = SpO2 (1 - SaCO).
Carboxyhemoglobin↗
The prevalence of hypoxemia detected by pulse oximetry during recovery from anesthesia.
Pulse oximetry was used to assess the prevalence of hypoxemia (arterial oxygen saturation of 90% or less) at various times in the immediate postoperative period: five minutes after arrival, 30 minutes later, and just before discharge. Among 149 inpatients studied, one or more hypoxemic measurements were made in 21 (14%) during their postoperative course. Of 92 outpatients, 1 (1%) was found to be hypoxemic. For inpatients, the prevalence of hypoxemia preoperatively, 5 minutes after arrival in recovery, 30 minutes later, and at discharge was 2%, 4%, 6%, and 9%, respectively. Patient factors associated with a significantly higher prevalence of hypoxemia were obesity (22%), body cavity surgical procedures (24%), age over 40 years (18%), American Society of Anesthesiologists physical status (I, 7%; II, 17%; III, 18%; IV, 100%), duration of anesthesia longer than 90 minutes (18%), and intraoperative administration of greater than 1,500 ml of fluid (20%). Unrecognized hypoxemia in postsurgical inpatients with or without these risk factors is common. Therefore routine monitoring of these patients with a pulse oximeter is suggested.
Adult↗
Safe and cost-effective cleaning of pressure-monitoring transducers.
The CDC category I recommendation for high-level disinfection of pressure-monitoring transducers between uses imposes substantial costs. This practice has not been shown to be useful when disposable transducer domes are used. A prospective, randomized, double-blind study assessed the consequences of wiping transducers with alcohol between uses rather than sterilizing with ethylene oxide. We studied 5,197 transducer courses with disposable domes involving 2,202 patients in intensive care units during 15 months. There was no significant difference between the two treatment regimens in the risk of positive cultures of fluids from monitored lines, of cultures of cannula tips or of a positive blood culture. We conclude that under normal conditions, ie, in the absence of a cluster of transducer-related bacteremias, wiping transducers with alcohol between uses is sufficient when disposable domes are used.
Catheterization↗
Vaporizer fill system leak.
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Anesthesiology↗
Supplemental oxygen after ambulatory surgical procedures.
The aim of this study was to determine the need for supplemental oxygen during recovery from general anesthesia for ambulatory surgery in healthy women without obesity or respiratory disease. Arterial oxygen saturation by pulse oximetry (SpO2) was monitored throughout the first postoperative hour in 164 patients. The patients breathed room air during recovery. Supplemental oxygen was given only to those who became hypoxemic (SpO2 less than or equal to 92%). It was discontinued at the end of 15 minutes and reinstituted for another 15 minutes if hypoxemia recurred. Twelve patients (7%) became hypoxemic and required supplemental oxygen for various periods of time up to 105 minutes. The need for supplemental oxygen increased with increasing age (P less than 0.05) but was not associated with a history of cigarette smoking, tracheal intubation, amount of opioids or sedatives given intraoperatively, anesthetic duration, or level of consciousness during recovery. Hypoxemia was neither predictable nor clinically apparent. We recommend that, unless arterial oxygenation is monitored, ambulatory patients should routinely receive supplemental oxygen during recovery from general anesthesia.
Adult↗
Hypoxemia during ambulatory gynecologic surgery as evaluated by the pulse oximeter.
Pulse oximetry was used to determine the incidence of intraoperative hypoxemia in 108 patients undergoing ambulatory gynecologic operation. Eleven (10%) experienced moderate desaturation (arterial oxygen saturation less than 90%), and 5 (5%) suffered severe hypoxemic episodes (arterial oxygen saturation less than 85%). Among patient risk factors--including operation, body habitus, smoking habits, history of asthma, age, and airway characteristics--an association with moderate hypoxemia was found only with nonlaparoscopic gynecologic operation, obesity, and age over 35 years, and an association with severe hypoxemia was found only with obesity and age over 35. Among operative events--including inspired oxygen concentration, position, mode of ventilation, and anesthesia phase--an association with moderate hypoxemia was found only with the lithotomy position, manual ventilation, and arousal. The cost per patient of monitoring with a pulse oximeter is about +1.35. A cost-benefit analysis reveals that a mortality rate of 1 in 40,000 among patients who actually become moderately hypoxemic would justify the cost of monitoring arterial oxygen saturation. We conclude that pulse oximetry should be part of routine anesthetic monitoring.
Adult↗
An electronic stethoscope is judged better than conventional stethoscopes for anesthesia monitoring.
A prototype electronic monitoring stethoscope was constructed from readily available, high-quality components. It consisted of a conventional precordial or esophageal probe connected to a microphone by a rubber adapter. The microphone was connected by lightweight wire to an amplifier and headphones. Twenty-one anesthesia clinicians evaluated the stethoscope and responded to a multiple-choice preference questionnaire. The electronic stethoscope was judged to perform better than the conventional stethoscope in most categories evaluated. The electronic device was perceived to be louder, clearer in sound reproduction, more efficacious for monitoring, and easier to use continuously, and its headphones were considered more comfortable than the conventional earpiece. Based on our results, we conclude that amplified stethoscopes have the potential to improve monitoring. Further development of electronic stethoscope monitoring seems warranted and is continuing.
Anesthesia↗
Estimation of PaCO2 by two noninvasive methods in the critically ill newborn infant.
Simultaneous measurements of arterial, transcutaneous, and peak expired carbon dioxide were obtained in 24 newborn infants receiving mechanical ventilation during the first week after birth. Two calibration algorithms designed to estimate PaCO2 from the noninvasive measurements were then examined. Both approaches entailed finding a statistical relationship by which future noninvasive measurement could be used to estimate the arterial value rather then measuring it directly. The first utilized the difference between the initial paired measurements (an in vivo calibration); the second used the mean difference between all measurements in the population. Adjusted tcPCO2 measurements by either the in vivo calibration or by the population-based factor led to estimates of PaCO2 with 95% confidence limits of +/- 6 to +/- 8 torr. In contrast, this degree of precision for the peak expired CO2 measurement was only possible using the in vivo calibration method. The use of an airway adaptor for PCO2 measurement led to CO2 retention in more than half of the infants. Transcutaneous monitoring had no significant effects on the infants, but was hampered by excessive drift and erratic sensitivity of the electrodes.
Blood Gas Analysis↗
Variation in PCO2 between arterial blood and peak expired gas during anesthesia.
Arterial PCO2 (PaCO2) can be continuously and noninvasively estimated by monitoring peak expired CO2 tension (PpeCO2). The practice of calibrating the estimate by an initial measurement of PaCO2 assumes that the difference in PCO2 tension between arterial blood and expired gas P(a-pe)CO2 remains constant. We examined the stability of P(a-pe)CO2 during anesthesia in 15 patients undergoing major surgery. Mean P(a-pe)CO2 values ranged from 0.8-7.9 torr with maximum P(a-pe)CO2 values ranging from 4.5-13.0 torr. Calibration of P(a-pe)CO2 based on a single initial measurement of PaCO2 often over- or underestimated PaCO2. Mean estimated PaCO2 from calibrated P(a-pe)CO2 varied from -7.9-6.4 torr with extreme estimates of -12.8-12.3 torr. No consistent correlation was shown between P(a-pe)CO2 and duration of anesthesia, variations in ventilation, blood pressure, blood-gas tensions, PpeCO2 or temperature. We conclude that estimation of PaCO2 by monitoring PpeCO2 is not invariably reliable.
Adult↗
Instrumentation simultaneously measuring VCO2 and VO2 in humans using titration methods.
An instrument has been developed for the simultaneous measurement of carbon dioxide excretion (VCO2) and oxygen uptake (VO2). This instrument, the Nutrimeter, gives these breath-averaged measurements continuously without having to determine respiratory flow rate, perform timed spirometric gas collections, or determine absolute CO2 or O2 concentrations. It can be used on ventilated or nonventilated patients in long- and short-term studies. VO2 is determined via the replenishment technique. VCO2 is determined via a new technique, absorption-titration, described here. Bench test results of VCO2 measurements show a standard error of the estimate (SEE) +/- 0.591% of full scale (500 ml/min) and maximum single point error (MSPE) of +/- 3.54% over a 100--350 ml/min range. VO2 measurements show SEE +/- 0.518% of full scale (1,000 ml/min) and MSPE +/- 2.42% over a 100--450 ml/min range. In 31 human clinical trials the Nutrimeter was compared with the open-circuit spirometric collection and micro-Scholander analysis technique. VCO2 measurements show SEE +/- 2.208% and MSPE +/- 10.57% over 135--315 ml/min. VO2 measurements show SEE +/- 1.134% of full scale and MSPE +/- 9.54% over 170--360 ml/min. Response time is 60 s optimally for step changes in VO2 (0--90% of steady-state value), 90 s for VCO2.
Calorimetry↗