Teaching sedation and analgesia with simulation

OBJECTIVE

This study reports on the efficacy of using the anesthesia simulator to teach sedation and analgesia to nurses. This provision of sedation and analgesia to a patient is accomplished with the goal of maintaining the ability of the patient to respond purposefully to auditory or tactile stimuli.

METHODS

Nurses working in areas of the hospital where conscious sedation is performed were the participants in this sedation and analgesia training course. Prior to the training session, the participants read the American Society of Anesthesiology Practice guidelines for sedation and analgesia by non-anesthesiologists. At the time of the training session, each participant completed a written pretest, had an introduction to sedation and analgesia with four clinical crisis teaching scenarios using the anesthesia simulator, a practical exam using the simulator, and a written post-test. Each participant was also given the opportunity to complete an evaluation of the session.

RESULTS

Twenty nurses completed the training session. The written tests had a maximum possible score of 30. Mean score on the written pretest was 22.9 +/- 3.54, and mean score on the written post-test was 26.0 +/- 4.24 (p < 0.001). Seventeen of the twenty subjects scored higher on the post-test. Mean practical exam score was 5.5 of a possible 6.0. Mean participant rating of the education session was 3.75 (1 = poor, 4 = excellent). All but one participant rated the length of the training session as "about right."

CONCLUSIONS

The anesthesia simulator provides an excellent tool for teaching conscious sedation skills to hospital nurses. The participants' test performance improved following the session, and they also rated the educational experience as excellent.

Semicontinuous cardiac output monitoring using a neural network

OBJECTIVES

This study compared 2-mL bolus thermodilution cardiac output measurements with standard 10-mL bolus measurements.

DESIGN

Cardiac output was measured with the new 2-mL bolus technique and the 10-mL standard thermodilution technique in a perspective series. We describe a system that automatically cools and injects 2-mL boluses of saline into a standard pulmonary artery catheter. It uses a Peltier effect solid-state cooler and pneumatically driven syringe injector to measure cardiac output once per minute.

SETTING

Animal laboratory.

ANIMALS

Eight adult Duroc swine weighing between 38.0 and 57.5 kg.

INTERVENTIONS

Once each minute, 2 mL of cooled 5% dextrose was injected through the pulmonary catheter. Once every 8 mins, four sequential measurements of cardiac output were made using 10-mL injections.

MEASUREMENTS AND MAIN RESULTS

A total of 1249 paired waveforms were processed with both a conventional algorithm and with a neural network. For the conventional algorithm, the correlation coefficient was r2 = .92 and the SD of the difference was 1.30 L/min. For the neural network, the correlation coefficient was r2 = .94 and the SD of the difference was 0.88 L/min. Output filtering improved the results in both cases.

CONCLUSION

Neural networks accurately derive cardiac output from 2-mL bolus thermodilution injections, allowing cardiac output to be monitored automatically once per minute in many patients. The technique is convenient and uses standard low-cost catheters.

A closed rebreathing system for dose maintenance during Partial Liquid Ventilation

Partial Liquid Ventilation (PLV), a treatment for acute respiratory failure in which the lungs are filled, either partially or to functional residual capacity (FRC), with perfluorochemical (PFC) liquid while the patient is on mechanical gas ventilation, has progressed to clinical trials using the PFC perflubron (PFB). Because gas expired during PLV is laden with PFB vapor, PFB is lost via evaporation, which increases dose consumption and necessitates periodic redosing. A device has been developed to minimize evaporative loss by confining PFC vapor to a gas volume breathed by the patient, which is isolated from the ventilator. This closed rebreathing system works with the ventilator such that after the lung is filled with PFB, the patient is connected to the rebreathing system, with breathing still "driven" by the ventilator. The rebreathing system consists of two gas circuits, or compartments, separated by a flexible bag (in a box) partition. One compartment is in gas communication with the lung, while the second communicates with the ventilator. The O2 level on the patient side is matched to that on the ventilator side by sensing gas concentrations and by feedback control of O2 introduction. Similarly, air is introduced into the patient side under pressure-based feedback control to maintain a constant gas volume. On inspiration, the ventilator delivers the tidal volume (breath) into the box surrounding the bag, which, in turn, is transmitted through the bag to the lung. On expiration, the process is reversed. Unidirectional circulation of gas in the rebreathing circuit is achieved via check valves, and expired CO2 is removed by a barium hydroxide lime cartridge. Airway humidification is maintained by captive water vapor in the system and water vapor from the CO2 absorber. It is recommended that flow, pressure, O2, and CO2 levels be monitored at the patient "Y," i.e., the proximal end of the endotracheal tube. Performance data from both in-vitro experiments and in-vivo PLV experiments in pigs are presented. The authors conclude that with the closed rebreathing system, the dose can be safely maintained with fewer redosing procedures, and an approximately 90% savings in dose is achieved.

Clinical evaluation of tracheal pressure estimation from the endotracheal tube cuff pressure

OBJECTIVE

Air flow through an endotracheal tube causes a pressure drop across the tube. This pressure drop creates a difference between air pressure measured in the trachea and the pressure measured in the breathing circuit, which can lead to errors when calculating pulmonary mechanics and when setting ventilators. We have developed a method of estimating tracheal pressure from the pressure in the endotracheal tube cuff and tested this system in clinical trials.

METHODS

Pressure measurement ports were placed between the Y piece of the ventilator circuit and the ETT connector, in the trachea at the carinal end of the ETT, and in the ETT cuff inflation line. Tracheal pressures and cuff pressures were found at end-inspiration and end-expiration (no flow states) and used to define a linear relationship between cuff pressure and tracheal pressure. Using the estimated tracheal pressure (Ptrach) and the measured pressure at the Y piece of the breathing circuit (PY), the pressure drop across the ETT was found as a function of flow through the tube. Tracheal pressure was then calculated from the flow-dependent pressure drop and PY. Tests of this system were performed in six patients in the operating room and six patients in the intensive care unit.

RESULTS

The flow-based tracheal pressure estimates were within 0.7 +/- 0.4 cm H2O of actual tracheal pressure (mean +/- SD). At peak inspiratory pressure the difference averaged 0.5 +/- 0.3 cm H2O. The difference between our estimate of tracheal pressure and actual tracheal pressure was always less than 1 cm H2O.

CONCLUSION

The flow-based tracheal pressure estimates were accurate during intermittent spontaneous breathing, but not during spontaneous breathing or with a poorly inflated cuff. The estimates were more immune to noise than the cuff-based estimates of tracheal pressure. The estimates of tracheal pressure measured from the ETT cuff should be accurate enough for clinical use in the operating room.

Evaluation in animals of a system to estimate tracheal pressure from the endotracheal tube cuff

OBJECTIVE

Flow through an endotracheal tube (ETT) causes a pressure loss across the tube. This loss results in a difference between pressure measured at the airway and pressure measured in the trachea. This difference can lead to errors when calculating pulmonary mechanics and when setting ventilators. We have tested a method of estimating tracheal pressure from the pressure in the ETT cuff.

METHODS

Pressure transducers were placed in the proximal ETT connector, in the trachea, and in the ETT cuff (through the inflation port). Instantaneous periods of zero flow, detected with a flow meter, were used to calculate the slope and offset of the line relating cuff pressure to tracheal pressure. The system was tested on the bench using a ventilator and lung simulator and in 2 dogs and 5 pigs. Tests were performed at various cuff pressures, trachea diameters, ETT sizes, respiratory rates, tidal volumes, and airway obstructions.

RESULTS

In bench tests, our estimate of tracheal pressure was within -4.0 +/- 2.6% of the actual tracheal pressure (mean +/- standard deviation [SD]). In animal tests, our estimation of tracheal pressure was within -0.6 +/- 5%. In all bench test measurements and in 40 of 42 animal measurements, the error was less than 1 cm H2O.

CONCLUSIONS

The cuff estimation technique gives real-time, continuous, noninvasive tracheal pressure measurements in intubated animals with cuffed ETTs.

Noninvasive blood pressure monitoring from the supraorbital artery using an artificial neural network oscillometric algorithm

OBJECTIVE

Our objective was to overcome the limitations of linear models of oscillometric blood pressure determination by using a nonlinear technique to model the relationship between the oscillometric envelope and systolic and diastolic blood pressures, and then to use that technique for near-continuous arterial pressure monitoring at the supraorbital artery.

METHODS

An adhesive pressure pad and transducer were used to collect oscillometric data from the supraorbital artery of 85 subjects. These data were then used to train an artificial neural network (ANN) to report diastolic or systolic pressure. Arterial pressure measurements defined by brachial artery auscultation were used as a reference. ANN results were compared with those obtained using a standard oscillometric algorithm that determined pressures based on fixed percentages of the maximum oscillometric amplitude.

RESULTS

The ANN produced better estimates of reference blood pressures than the standard oscillometric algorithm. Mean difference between target and actual output for the ANN was 0.50 +/- 5.73 mm Hg for systolic pressures, compared to the mean difference of the standard algorithm of 2.78 +/- 19.38 mm Hg. For diastolic pressures, the ANN had a mean difference of 0.04 +/- 4.70 mm Hg, while the mean difference of the standard algorithm was -0.34 +/- 9.75 mm Hg.

CONCLUSIONS

The ANN produced a better model of the relationship between the oscillometric envelope and reference systolic and diastolic pressures than did the standard oscillometric algorithm. Noninvasive blood pressure measured from the supraorbital artery agreed with pressure measured by auscultation in the brachial artery, and may sometimes be more clinically useful than an arm cuff device.

Refractive indices for volatile anesthetic gases: equipment and method for calibrating vaporizers and monitors

OBJECTIVE

The objective of our study was to establish the refractive indices and the virial coefficients of the volatile anesthetic vapors. These indices and coefficients will allow refractometry to be used by manufacturers to produce accurate calibration, without requiring expensive high-precision calibration gases.

METHODS

We used a precision refractometer to measure the refractive indices for five volatile anesthetic vapors. We prepared our calibration gases by mixing a gravimetrically calibrated amount of liquid agent with a constant gas flow.

RESULTS

The refractive indices for the volatile anesthetic vapors are 1,603.2 for halothane, 1,540.4 for enflurane, 1,563.3 for isoflurane, 1,538.3 for sevoflurane, and 1,211.7 for desflurane. The maximum theoretical error in our measurements, due to all sensors and all uncertainty in our measurement of apparatus and physical constants, is +/- 0.56% of the reading (+/- 0.70% for desflurane).

CONCLUSIONS

If refractometry replaced calibration gases in cylinders, as a calibration standard, manufacturers might avoid errors that now occur because calibration gases manufactured by numerous companies seem to differ. We propose that our values serve as an interim database.

A laboratory investigation of two new portable gas analysers

Two new portable infrared gas analysers, the Irina (Drägerwerke, Lübeck, Germany) and the Normac (Datex Instrumentarium Corporation, Helsinki, Finland), were tested and the results compared to those from such established methods as gaschromatography and the Beckman LB-2 (Beckman, USA) infrared gas analyser using exactly defined gas mixtures from a vaporizer developed in our laboratory. The analysers were evaluated for their accuracy and precision, noise, zero stability, gain stability and the impact of flow, over-pressure, carrier gas and humidity. All three analysers showed good accuracy and precision. The noise level was acceptable except in the older version of the Normac, but this error has now been corrected by the manufacturer. Zero and gain stability are very good; gas flow and over-pressure do not affect the measurement. The influence of carrier gases and humidity is negligible except for the strong N2O effect on the Beckman. We conclude that this new generation of highly sophisticated and reliable but handy analysers will meet the clinical demands.

Determination of the partial pressure of halothane (or isoflurane) in blood

A gas chromatographic method is described for the direct quantitative determination of the partial pressure of halothane (or isoflurane) in blood as well as the blood-gas partition coefficient. A head space technique and a flame ionization detector were used. Standard blood was obtained by equilibrating patients' blood with known gas concentrations in a tonometer. Using an infra-red analyser to measure the halothane gas concentration in the tonometer and within the anaesthetic system allowed for the direct comparison of the partial pressure in blood to the partial pressure in the inspired gas. Technical problems associated with this procedure, and with comparable methods, are discussed.

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.