Dwayne R. Westenskow

Improving alarm performance in the medical intensive care unit using delays and clinical context.

INTRODUCTION: In an intensive care unit, alarms are used to call attention to a patient, to alert a change in the patients physiology, or to warn of a failure in a medical device; however, up to 94% of the alarms are false. Our purpose in this study was to identify a means of reducing the number of false alarms. METHODS: An observer recorded time-stamped information of alarms and the presence of health care team members in the patient room; each alarm response was classified as effective (action taken within 5 min), ineffective (no response to the alarm), and ignored (alarm consciously ignored or actively silenced). RESULTS: During the 200-h study period, 1271 separate entries by an individual to the room being observed were recorded, 1214 alarms occurred and 2344 tasks were performed. On average, alarms occurred 6.07 times per hour and were active for 3.28 min per hour; 23% were effective, 36% were ineffective, and 41% were ignored. The median alarm duration was 17 s. A 14-s delay before alarm presentation would remove 50% of the ignored and ineffective alarms, and a 19-s delay would remove 67%. Suctioning, washing, repositioning, and oral care caused 152 ignored or ineffective ventilator alarms. DISCUSSION: Introducing a 19-s alarm delay and automatically detecting suctioning, repositioning, oral care, and washing could reduce the number of ineffective and ignored alarms from 934 to 274. More reliable alarms could elicit more timely response, reduce workload, reduce noise pollution, and potentially improve patient safety.

Theoretical analysis of non-invasive oscillometric maximum amplitude algorithm for estimating mean blood pressure

A theoretical analysis is performed to evaluate the effect of arterial mechanical and blood pressure pulse properties on the accuracy of non-invasive oscillometric maximum amplitude algorithm (MAA) estimates of the mean blood pressure obtained using air-filled occlusive cuffs. Invasively recorded blood pressure pulses, selected for their varied shapes, are scaled to simulate a wide range of blood pulse pressures (diastolic blood pressure minus systolic blood pressure). Each scaled blood pressure pulse is transformed through an exponential model of an artery to create a series of blood volume pulses from which a simulated oscillometric waveform is created and the corresponding MAA estimate of the mean blood pressure and error (mean blood pressure minus MAA estimate) are determined. The MAA estimates are found to depend on the arterial blood pressure. The errors are found to depend on the arterial mechanical properties, blood pressure pulse shape and blood pulse pressure. These results suggest that there is no direct relationship between the mean blood presure and MAA estimate, and that multiple variables may affect the accuracy of MAA estimates of the mean blood pressure obtained using air-filled occlusive cuffs.

Measured and predicted caloric expenditure in the acutely ill

Predicted energy requirements calculated from the Harris-Benedict basal energy expenditure (BEE) formulas, and caloric allowances recommended by the Food and Nutrition Board National Research Council, were compared to metabolic expenditures measured by indirect calorimetry, using a feedback-controlled gas replenishment technique with a prototype device for the continuous determination of oxygen consumption (Vo2) and carbon dioxide production (Vco2). In a group of 50 acutely ill surgical patients, predicted metabolic requirements based on ideal body weight (1.75 BEE) averaged 59% greater than metabolic expenditures measured by indirect calorimetry. Metabolic requirements based on actual body weight averaged 52% greater; recommended caloric allowances averaged 39% greater. Thus, accepted methods of predicting metabolic requirements significantly overestimated the caloric needs of these acutely ill patients. These results should encourage the development of new bedside equipment for measuring Vo2 and Vco2, so that indirect calorimetry can be used to guide nutritional support in the clinical setting.

Use of indirect calorimetry in the nutritional management of burned patients.

The use of indirect calorimetry in assessing and monitoring nutritional support in burn patients is reported. Twenty-nine patients with a mean burn size of 35% TBSA were monitored with 228 measurements of resting energy expenditure (REE), calculations of respiratory quotient (RQ), and substrate metabolism. Daily weights, nitrogen balance determinations, and routine laboratory tests were also obtained. Oxygen consumption (VO2) was 186 +/- 39 ml/min/M2, corresponding to REE of 2,506 +/- 543 kcal/day. REE varied during the course of wound healing, demonstrating a biphasic course. Metabolic rate was also significantly increased with the performance of routine procedures such as dressings and surgery. Measurements of REE were a mean 76% of predictions based on the Curreri formula, and 1.47 times basal energy expenditure (BEE) calculated by the Harris-Benedict equation. Neither formula provided for the great variations observed in daily, and individual, measurements of REE. During the study, patients consumed 2,900 +/- 811 kcal/day, which exceeded REE by 1.14. This was associated with mean weight loss of 3.2% (range, -16 to 9%). RQ was less than 0.85 in 9% of determinations, but exceeded 1.0 24% of the time. Protein accounted for 17 +/- 3% of total metabolism, corresponding to a calorie:nitrogen ratio of 128:1. Practically, however, provision of this much protein proved difficult. Routine use of indirect calorimetry permits tailoring of nutritional support for burn patients, and is valuable in the early detection of significant under- or overnutrition.

Opioid-volatile anesthetic synergy : A response surface model with remifentanil and sevoflurane as prototypes

Background:Combining a hypnotic and an analgesic to produce sedation, analgesia, and surgical immobility required for clinical anesthesia is more common than administration of a volatile anesthetic alone. The aim of this study was to apply response surface methods to characterize the interactions between remifentanil and sevoflurane. Methods:Sixteen adult volunteers received a target-controlled infusion of remifentanil (0–15 ng/ml) and inhaled sevoflurane (0–6 vol%) at various target concentration pairs. After reaching pseudo–steady state drug levels, the Observers Assessment of Alertness/Sedation score and response to a series of randomly applied experimental pain stimuli (pressure algometry, electrical tetany, and thermal stimulation) were observed for each target concentration pair. Response surface pharmacodynamic interaction models were built using the pooled data for sedation and analgesic endpoints. Using computer simulation, the pharmacodynamic interaction models were combined with previously reported pharmacokinetic models to identify the combination of remifentanil and sevoflurane that yielded the fastest recovery (Observers Assessment of Alertness/Sedation score ≥ 4) for anesthetics lasting 30–900 min. Results:Remifentanil synergistically decreased the amount of sevoflurane necessary to produce sedation and analgesia. Simulations revealed that as the duration of the procedure increased, faster recovery was produced by concentration target pairs containing higher amounts of remifentanil. This trend plateaued at a combination of 0.75 vol% sevoflurane and 6.2 ng/ml remifentanil. Conclusion:Response surface analyses demonstrate a synergistic interaction between remifentanil and sevoflurane for sedation and all analgesic endpoints.

Instrumentation for monitoring gas exchange and metabolic rate in critically ill patients

In the critically ill patient the monitoring of oxygen consumption (VO2) and carbon dioxide production (VCO2) can identify abnormalities in tissue perfusion and metabolism. A patients metabolic utilization can be calculated by indirect calorimetry, once VO2 and VCO2 are measured. This paper evaluates a compact instrument designed for monitoring VO2 and VCO2 in the critically ill adult. Accuracy was measured under controlled laboratory conditions using oxygen-enriched air, PEEP, and intermittent mandatory ventilation (IMV). Accuracy averaged 1.3% for VO2 and 0.9% for VCO2 when room air was used. Accuracy was 11.7% for VO2 and 6.8% for VCO2 when 80% oxygen was used. PEEP of 30 cm H2O had little effect on accuracy. IMV at 2 breath/min (room air) resulted in an accuracy of 4.0% and 4.1% for VO2 and VCO2, respectively.

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.

Development and Evaluation of a Graphical Anesthesia Drug Display

Background Usable real-time displays of intravenous anesthetic concentrations and effects could significantly enhance intraoperative clinical decision-making. Pharmacokinetic models are available to estimate past, present, and future drug effect-site concentrations, and pharmacodynamic models are available to predict the drugs associated physiologic effects. Methods An interdisciplinary research team (bioengineering, architecture, anesthesiology, computer engineering, and cognitive psychology) developed a graphic display that presents the real-time effect-site concentrations, normalized to the drugs’ EC95, of intravenous drugs. Graphical metaphors were created to show the drugs’ pharmacodynamics. To evaluate the effect of the display on the management of total intravenous anesthesia, 15 anesthesiologists participated in a computer-based simulation study. The participants cared for patients during two experimental conditions: with and without the drug display. Results With the drug display, clinicians administered more bolus doses of remifentanil during anesthesia maintenance. There was a significantly lower variation in the predicted effect-site concentrations for remifentanil and propofol, and effect-site concentrations were maintained closer to the drugs’ EC95. There was no significant difference in the simulated patient heart rate and blood pressure with respect to experimental condition. The perceived performance for the participants was increased with the drug display, whereas mental demand, effort, and frustration level were reduced. In a postsimulation questionnaire, participants rated the display to be a useful addition to anesthesia monitoring. Conclusions The drug display altered simulated clinical practice. These results, which will inform the next iteration of designs and evaluations, suggest promise for this approach to drug data visualization.

Comparison of venous air embolism monitoring methods in supine dogs.

The sensitivities and durations of positive responses of routinely used methods for detecting venous air embolism and physiologic variables reflecting changes accompanying air embolism were compared in ten anesthetized dogs receiving incremental venous injections of 0.1, 0.25, 0.5, 0.75, 1.0, 1.5, and 2.0 ml/kg air. These doses of air produced a spectrum of changes ranging from subtle alterations in Doppler frequency to dramatic, life-threatening cardiovascular depression. Precordial Doppler ultrasound monitoring was the most sensitive detection method, but failed to reflect embolus size. Mean pulmonary arterial pressure and end-tidal carbon dioxide concentration (FETCO2), while slightly less sensitive than the Doppler device, reliably detected 0.25-0.5 ml/kg of injected air and quantitatively reflected the size of the embolus. Changes in Doppler, and FETCO2 were predictably positive at air volumes one fourth of those producing decreases in arterial blood pressure and cardiac output. Other frequently monitored signs of air embolism, such as a millwheel murmur and cardiac arrhythmias, were late manifestations and were seen only with injection of 1.5–2.0 ml/kg air. and FETCO2 remained altered the longest after air injections (20–30 min at 2.0 ml/kg), while changes in Doppler-transmittcd signals at the same dose lasted 15 min. Positive responses with other methods of air embolus detection, although dose-related, were significantly shorter-lasting than and FETCO2 alterations. Cardiac murmurs and arrhythmias were transient after air injection, lasting less than a minute, even with emboli of 2.0 ml/kg. The data indicate that the combination of application of a Doppler device with cither end-tidal carbon dioxide measurement or a flowdirected pulmonary-artery catheter with display provides the desired combination of sustained, sensitive and reliable detection of air embolism and quantitative evaluation of embolus size.

When is a bispectral index of 60 too low?: Rational processed electroencephalographic targets are dependent on the sedative-opioid ratio.

Background: Opioids are commonly used in conjunction with sedative drugs to provide anesthesia. Previous studies have shown that opioids reduce the clinical requirements of sedatives needed to provide adequate anesthesia. Processed electroencephalographic parameters, such as the Bispectral Index (BIS; Aspect Medical Systems, Newton, MA) and Auditory Evoked Potential Index (AAI; Alaris Medical Systems, San Diego, CA), can be used intraoperatively to assess the depth of sedation. The aim of this study was to characterize how the addition of opioids sufficient to change the clinical level of sedation influenced the BIS and AAI. Methods: Twenty-four adult volunteers received a target-controlled infusion of remifentanil (0–15 ng/ml) and inhaled sevoflurane (0–6 vol%) at various target concentration pairs. After reaching pseudo–steady state drug levels, the modified Observer’s Assessment of Alertness/Sedation score, BIS, and AAI were measured at each target concentration pair. Response surface pharmacodynamic interaction models were built using the pooled data for each pharmacodynamic endpoint. Results: Response surface models adequately characterized all pharmacodynamic endpoints. Despite the fact that sevoflurane–remifentanil interactions were strongly synergistic for clinical sedation, BIS and AAI were minimally affected by the addition of remifentanil to sevoflurane anesthetics. Conclusion: Although clinical sedation increases significantly even with the addition of a small to moderate dose of remifentanil to a sevoflurane anesthetic, the BIS and AAI are insensitive to this change in clinical state. Therefore, during “opioid-heavy” sevoflurane–remifentanil anesthetics, targeting a BIS less than 60 or an AAI less than 30 may result in an unnecessarily deep anesthetic state.