EVALUATION OF THREE TRANSPORTABLE MULTIGAS ANESTHETIC MONITORS: THE BRUEL & KJAER ANESTHETIC GAS MONITOR 1304, THE DATEX CAPNOMAC ULTIMA, AND THE NELLCOR N-2500 Jacob Nielsen, MD, Torben Kann, MScEE, and Jakob Trier Motler, MD

Nielsen J, Kann T, Moiler JT. Evaluation of three transportable multigas anesthetic monitors: the Bruel & Kjaer Anesthetic Gas Monitor 1304, the Datex Capnomac Ultima, and the Nellcor N-2500.

J Clin Monit 1993;9:91-98

ABSTRACT. We compared the performance of three newly de- vdoped anesthetic agent (AA) monitors: the Bruet & Kjaer Anesthetic Agent Monitor 1304 (BK 1304), the Datex Capno- mac Ultima (ULTIMA), and the Nellcor N-2500 (N-2500). The following were investigated: the linearity and accuracy in measuring AAs, oxygen, carbon dioxide, and nitrous oxide; the linearity and accuracy during warm-up time; the effect of increasing respiratory rate on the accuracy; the consequences of a difference between monitored and delivered AA and of delivering a mixture of AAs; and, finally, the effect of water vapor and alcohol. For all three monitors we found that the accuracy in determining the respiratory and anesthetic gases was sufficient for clinical use (the N-2500 does not measure oxygen). Because of the calibration mixture supplied with the device, however, the ULTIMA recorded values that were 10 to 12% (relative) less than the AA that was present. The BK 1304 had greater accuracy at higher respiratory rates than did the other two monitors, probably favoring its use in pediatric anesthesia. The N-2500 will detect which agent (isoflurane, enflurane, or halothane) is being used, alone or in a mixture. With the two other monitors the user must define which agent is given. In some situations a difference between this and the one actually delivered can theoretically lead to an overdose of AA, with the ULTIMA up to a 14.9 minimal alveolar concentration (MAC) overdose. No interference from alcohol or water vapor in the expired air was found. When the units millimeters of mercury (mm Hg) and kilopascal (kPa) were chosen, the ULTIMA displayed the values in standard tem- perature pressure dry (STPD) instead of body temperature pressure saturated (BTPS) conditions. Following power-up, some time lapsed before the monitors accurately displayed all variables, shortest with the BK 1304 and ULTIMA (15 min) and longest with the N-.2500 (40 min).

KEY WORDS. Anesthetic techniques: inhalation. Equipment: monitoring.

From the Department of Anesthesia and Intensive Care, Herlev Hos- pital, University of Copenhagen Medical School, Herlev. Denmark.

Received Aug 26, 1991, and in revised form May 15, 1992. Accepted for publication July 27, 1992.

Address correspondence to Dr Nielsen, Department of Anesthesia and Intensive Care, Herlev Hospital, University of Copenhagen, DK- 2730 Herlev, Denmark.

The appropriate level o f patient and equipment mon i - toring during anesthesia is presently undergo ing exten- sive debate. The ability to moni to r anesthetic and respi- ratory gases is no t new, the cont inuous measurement o f oxygen and carbon dioxide having been available for decades [1,2]. Reliable mon i to r ing o f nitrous oxide and the anesthetic vapors became possible with the mass spectrometer [3,4] but, due to the large capital outlay, the maintenance costs, and the clinical shor tcomings [5], this apparatus has no t become widely used. W h e n a new drug is introduced, it is normal ly fo l lowed by a series o f investigations f r o m which users can judge to what extent it can be implemented in their clinical work. In contrast, new electrical and moni to r ing equip- ment has yielded m u c h less interest. We consider it impor tant that such equipment also be submit ted to

Copyright © 1993 by Little, Brown and Company 91

92 Journal of Clinical Monitoring Vol 9 No 2 April 1993

critical evaluation in terms of performance and patient safety. Over the past few years several transportable multigas anesthetic monitors have been developed, and some have previously undergone laboratory evaluation [6-101.

The Bruel & Kjaer Anesthetic Agent Monitor 1304 (BK 1304; Naerum, Denmark), the Datex Capnomac Ultima (ULTIMA; Helsinki, Finland), and the Nellcor N-2500 (N-2500; Hayward, CA) are three newly de- veloped microprocessor-controlled multigas anesthetic monitors designed for use in the operating room. When choosing between different monitors, one must con- sider several points: the cost of purchase and mainte- nance, reliability (accuracy of the displayed values and the effect of long-term use), flexibility (i.e., adult vs pediatric anesthesia, normal vs low-flow anesthesia), in- ter-apparatus communication (i.e., data sampling and transfer), display control, and adaptability to future de- velopment. This report describes a laboratory evalua- tion of the accuracy and some of the features of these devices, with special emphasis on their clinical use.

BRIEF DESCRIPTION OF THE INSTRUMENTS

All three monitors are transportable and weigh approxi- mately 15 kg. The gas is sampled through a thin flexi- ble tube (side stream), with the sampling site close to the endotracheal tube/face mask. The BK 1304 and ULTIMA permit breath-by-breath analysis of the in- spired and expired fractions of 02, CO2, N20, and the anesthetic agents (AAs) isoflurane (ISO), enflurane (ENF), and halothane (HAL). The N-2500 does not an- alyze 0 2. Testing of the built-in pulse oximeters was not included in this study.

The BK 1304 and the ULTIMA have a video out- put and a serial output for computer (RS-232). In addi- tion, each monitor has analog outputs for all variables.

Neither the BK 1304 nor the ULTIMA is able to detect which AA was used. This information has to be keyed in before monitoring, whereas the N-2500 has an agent-detection system, making it able to detect which AA is being delivered.

The sampling flow rates for the N-2500, BK 1304, and ULTIMA are 50, 90, and 200 mt/min, respectively.

Principles of Measurement

The measurement of CO> N20, and AAs is based on the absorption of infrared light. The ULTIMA and N-2500 use the long-established principle of measuring the amount of light transmitted through the measuring chamber. The BK 1304 introduces a relatively new

principle: photoacoustic spectroscopy. This principle is based on the fact that when absorbed in a gas, infrared radiation applies energy and causes the gas to expand, resulting in an increase in pressure. When the applied energy is delivered in pulses by chopping off the radia- tion beam with a spinning wheel, the pressure will change in a sinusoidal fashion; the resulting sound wave can be detected by an optimized condenser microphone.

Since 0 2 does not absorb infrared radiation, another measurement technique must be used. The most com- mon technique is based on the paramagnetic response of 02 molecules, when these molecules are subjected to a pulsating magnetic field. This is the technique used with the BK 1304 and ULTIMA. The force acting on the 02 molecules creates an alternating pressure, which is measured in the BK 1304 by the same microphone as the one measuring the other gases, and in the ULTIMA by a pressure transducer. The N-2500 does not mea- sure 0 2 .

METHODS

Forty-five minutes' warm-up time was used unless oth- erwise stated. Calibration of the BK 1304 and ULTIMA was done daily using the calibration gases supplied by each manufacturer. The N-2500 uses fixed calibration with automatic zero-calibration whenever turned on and at intervals during use. The testing took place in a well-ventilated laboratory, with stable temperature and humidity.

The Linearity and Precision in Determination of Anesthetic Agents, 02, C02, and N20

We constructed a measurement set-up based on the physical and chemical properties of the gases involved. Using a precision scale (Mettler PE 3600, 0 to 999.99 -+ 0.01 g; Mettler Instrumente AG, Zfirich, Switzer- land), we evaporated known amounts of liquid AAs into a glass bottle with known volume (11,361 ± 5 ml). Using a precision syringe (100.2 -+ 0.1 ml), we made known concentrations of O2, CO2, and N20, mixing pure gases in glass bottles of two volumes (5,611 ± 3 ml or 1,168 -+ 1 ml).

The glass bottles, previously filled with pure 02 or pure N20, were closed with an airtight cap mounted with a 3-way stopcock before the transfer of the AA or other gases. Underwater testing of this system showed it to be airtight. The bottles contained irregular pieces of 0.1 mm aluminum foil to facilitate mixing by shak- ing. A detailed description of the theory, as well as the preparation and accuracy of this procedure is published separately [11]. Measurement of the reproducibility in

Nielsen et al: Evaluation of Multigas Anesthetic Monitors 93

9 consecutive preparations showed a variation o f + 0.03 vol% (2 SD).

Samples o f the gas mixtures were drawn from these bottles via the 3-way stopcock with a 100-ml glass sy- ringe and supplied to the monitors via the normal sam- pling tubes for 10 seconds. Each gas mixture was ana- lyzed 3 times. No materials containing silicone were used, as the AAs dissolve in this substance.

Effect on Accuracy of Increasing Respiratory Rates

We used a small solenoid valve (type 1-17-900, General Valve Corp, Fairfield, NJ) with correspondingly small gas flow tubes to minimize the dead space and, hence, to be able to achieve a square wave shift in gas composi- tion with each valve shift. Constant concentrations o f ISO (1.0 vol%), O2 (50 vol%), C O 2 (5.4 vol%), and N20 (50 vol%) alternating with air were sampled suc- cessively. The sampling tubes were connected directly to the valve outlet, the total system dead space being approximately 1 ml. The valve shift frequency (full cy- cle) was varied between 0 and 60 shifts/min (in steps of 6), the shifting ratio between the inlets being 1:1, ascending from 0 to 60/rain, then descending to 0/rain. The gas tubes leading to the valve were supplied with air vessels permitting gas to escape during the "valve- closed" part of the cycle to achieve constant flow and no variation in pressure at the sampling site. A scaven- ger tube was connected close to the sampling site, this tube ending 10 cm under water. The gas flow rate, sup- plied via flowmeters, was adjusted to the point where gas emerged from this tube, ensuring a surplus o f gas at the sampling site.

To more closely simulate the clinical situation, we used a larger solenoid valve (type 25003, Herion, Stutt- gart, Germany), allowing a larger, constant gas flow of 3 L/rain (in both the inspiratory and expiratory phases) and connected an endotracheal tube size 5.0 (Mallinc- krodt Laboratories, Athlone, Ireland), with a length of 16 cm at the valve outlet (distal end of tube). The usual monitor sampling device was placed at the proximal end o f the tube. The flow was supplied via flowmeters and was measured at the tube connection with a Dr~iger Volumeter (Driiger, Germany).

Consequences of Monitoring An Anesthetic Agent Other Than the One Delivered

Because the BK 1304 and ULTIMA cannot detect which AA is being used, it is possible to monitor the wrong agent. These monitors will display a value for

the keyed-in AA, even though another AA is being delivered. Depending on the wave length of the infrared radiation and the gain factors the monitors use to mea- sure the different AAs, this discrepancy will have more or less impact on the displayed value. To evaluate this potential problem we made known concentrations o f ISO, ENF, and HAL and supplied them to the monitors successively in each of the AA modes. We supplied the same mixtures o f AAs to the N-2500, noting the selec- tion o f displayed AA. Each of the gas mixtures was randomly supplied three times.

Consequences of Delivering a Mixture of Anesthetic Agents

Problems similar to those described above will arise if a mixture o f AA is being delivered. The BK 1304 and ULTIMA cannot detect this situation and will only dis- play a value for the keyed-in AA. We used a gas mixture with known concentrations of ISO and HAL simulating the situation o f an almost empty ISO vaporizer refilled with HAL. To assess the ability o f the N-2500 to detect mixtures of AAs, a mixture with known concentrations of two and three AAs was produced and measured.

Effect of Water Vapor and Alcohol

To assess the effect of water vapor on measured gas concentrations in the expiratory phase, we shifted be- tween sampling a dry gas mixture of 5.4 vol% CO2 in air and the same gas mixture fully saturated with water vapor at 37°C, using an electrically heated humidifier (Ohio, N C 02 B, Ohmeda, Madison, WI). The mix- tures were sampled for 2 minutes to fully expose the desaturating material used in the sampling system of the monitors. To investigate whether the monitors have a built-in correction factor to compensate for the dif- ference between body temperature pressure saturated (BTPS) and standard temperature pressure dry (STPD), the units of the displayed values were changed from volume percent to kilopascals and millimeters o f mer- cury and the differences were noted.

To assess the effect o f alcohol in the expiratory phase, we made a mixture of ethanol in 0 2. The blood to breath ratio of ethanol is approximately 2,000:1 [12]. Therefore, concentrations o f 2 ppm and 10 ppm alcohol (corresponding to blood concentrations of 0.1% and 0.5%) were produced by weighing pure alcohol fol- lowed by evaporation in a known volume of pure 0 2. Because higher concentrations o f alcohol in air can oc- cur i f alcohol is used as a cleaning agent, we made con- centrations of 100 ppm and 1,000 ppm. These mixtures

94 Journal of Clinical Monitoring Vol 9 No 2 April 1993

were sampled and the result on the displayed values in each of the AA modes was noted.

The Warm-up Time

To evaluate the accuracy o f the monitors shortly after power-up, we simulated the typical first case of the day or emergency in clinical use. Thus, we used the follow- ing set-up. After a warm-up period of 2 hours, gas mixtures of known concentration were measured with the 3 monitors, which were then turned off. Eight hours later the monitors were turned on and the same gas mixtures were measured after 2 minutes and then every 5 minutes until readings similar to control were achieved.

Presentation of Data and Statistical Methods

Results obtained with the BK 1304 are displayed with two decimals, which is the resolution o f this monitor. The U L T I M A and N-2500 displayed only one decimal.

To evaluate the linearity and accuracy in the measure- ment o fAA, 0 2, C O 2, and N 2 0 , the calculated concen- trations were plotted against the measured concentra- tions, and (using the linear model Y = a + bX) the regression coefficient (r), the slope (a), and the intercept (b) were calculated. Because the variation in concentra- tion o f the produced gas mixtures was known to be -- 0.03 vol% (2 SD), and because the values displayed by the monitors were constant within 10 seconds, the error range of each set o f data was too small for graphic pre- sentation. Furthermore, each gas mixture produced was measured three times and essentially no variation be- tween measurements occurred.

To assess the effect o f an increasing respiratory rate on the accuracy of the monitors, we plotted the mea- sured value in percent o f the actual value as a function of the shifting frequency, thus showing the percent in- accuracy o f increasing the "respiratory rate."

RESULTS

The linearity and accuracy for all AAs and all 3 moni- tors are shown in Fig 1, the regression coefficients being close to 1. All lines were close to the ideal fit, with a slope o f 1. The values f rom the BK 1304 deviated f rom the ideal by - 3 to - 5 % relative, the N-2500 by 2% to - 2% relative, and the U L T I M A by - 10% to - 12% relative.

The linearity and accuracy in determination of 0 2, CO2, and N 2 0 showed all lines to be almost ideal. Re- gression coefficients (r), slope (a), and intercept (b) are as follows:

BK 1304: to2 = 1.0000, Yo2 = 0.07 + 1.00X, rco 2 = 0.9997,

Yco2 = -0.1 + 1.01X, rN20 = 1.0000, YN20 = - 0 . 1 6 + 1.00X

ULTIMA: to2 = 1.0000, go2 = 0.02 + 1.001, rco 2 = 0.9998,

Yco2 = 0.05 + 0.99X, rN20 = 0 .9999 , YN20 = 0.58 + 0.99X

N-2500: rco 2 = 1.0000, Vco2 = 0.06 + 1.00X, rN20 = 0.9999,

YN20 = -0 .56 + 1.00X

The relationship between increasing respiratory rate and accuracy is presented in Fig 2. Figure 2A illustrates the ideal situation: constant flow and almost no dead space. These conditions lead to a square wave shift in gas composit ion at the sampling site. The monitors gave correct readings at the lower frequencies, but when the rate increased, a decrease in accuracy was seen. This happened at a lower frequency and was more pronounced with the U L T I M A and N-2500 compared with the BK 1304. The effects o f adding a dead space consisting of an endotracheal tube with an internal di- ameter o f 5 m m and a length o f 16 cm, as well as the effects o f using the recommended sampling devices, are shown in Fig 2B. The same pattern o f a decrease in accuracy was noted, although the decrease occurred ear- lier and was more pronounced.

In all instances the displayed values reached the con- trol concentrations when the frequency, was again re- duced to 0, thereby verifying constant gas composit ion during the experiment.

The consequences o f monitor ing an AA other than the one delivered are shown in Table 1. The results o f delivering a mixture of ISO and HAL are shown in Table 2.

To investigate whether the N-2500 was able to detect the agents in a mixture o f three agents, we constructed a mixture o f ISO 0.68 vol%, then added HAL 1.11 vol% and successively ENF 0.82 vol%, supplying them to the moni tor in the same sequence. In all three in- stances the N-2500 identified the agents and the concen- trations correctly.

No effect o f water vapor on the displayed values yeas found. When changing the units during sampling, the BK 1304 and N-2500 correctly displayed the values in BTPS when kilopascal or millimeters of mercury was chosen and in STPD when vol% was chosen: BTPS values were 6% (relative) lower compared with STPD values. The U L T I M A displayed the values in STPD in all modes, thus disregarding the effect o f water vapor in the alveoli o f the patient.

Neither the BK 1304 nor the N-2500 was influenced

Nielsen et al: Evaluation o f Multigas Anesthetic Monitors 95

Vol% BK 1 3 0 4 5

3

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I -~11~- ISOFLURANE -B- ENFLURANE + HALOTHANE I

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Fig 1. Linearity and accuracy m determination o f A A s . Measured volume percent plotted against calculated volume percent. Regres- sion coefficients (r), slope, and intercept (linear model Y = a + bX, where a represents the intercept and b represents the slope) are as follows: B K 1304: rls o = 0.9999, r~N,: = 0.9993, rHa c = 1.0000, Yiso = 0.00033 + 0.95X, YENF = 0.01 + 0.96X, YHA*, = 0.02 + 0.97X; UL T I M A : rls o = 1.0000, rEN F = 9985, rHA L = 0.9993, YxSO = - 0 . 0 3 + 0.89X, YEzvF = 0.01 + 0.882*2, YHAL = - -0 .03 + 0.90X; N-25400: rls o = 0.9997, reN F = 0.9999, rHA L = 0.9997, YIso = - 0 . 0 2 + 1.02X, YENF = --0.03 + 0.98, YHAL = - -0 .0 I + 0.98X.

by alcohol up to 1,000 ppm. The U L T I M A was not influenced by 02 alcohol levels associated with nor- mally occurring blood alcohol contents, but alcohol concentration in 0 2 o f 1,000 ppm led the ULTIMA to an erroneous AA reading, especially in the HAL setting, where the displayed value was 2.4 vol%.

After being shut off for 8 hours, the BK 1304 mea- sured 02, N20, and AA correctly after 2 minutes, whereas the correct CO 2 value was obtained after 15 minutes (increasing from 5.3 to 5.6 vol%). With the ULTIMA, correct values o f 02, CO2, and N20 were displayed after 2 minutes, whereas the correct AA value was obtained after 15 minutes (ENF; increasing from 2.7 to 2.8 vol%). With the N-2500, correct values of CO2 and N 2 0 were displayed after 2 minutes, whereas no AA could be monitored for 40 minutes. Thus, all monitors were able to detect C O 2 within 2 minutes fol- lowing a cold start and, thus, to confirm appropriate placement of an endotracheal tube. The Nellcor com- pany advises users to keep the agent analyzer main switch in the " o n " position during periods o f no moni- toring. If this was done, the AA was displayed correctly within 2 minutes. The company warns that discon- nection o f the N-2500 from the main electrical supply

for even a short period of time may cause the agent analyzer to be unable to display AA values for up to 40 minutes. To assess this warning, we disconnected the N-2500 from the mains for 5 minutes. When the device was turned on again, CO2 and N 2 0 were measured correctly after 2 minutes and AAs were measured cor- rectly after 5 minutes.

DISCUSSION

When describing the accuracy o f a measuring device, one must ask the question, "With what standard are the results to be compared?" This question usually remains unanswered. In previous investigations o f AA moni- tors, investigators have used a mass spectrometer [9], a gas chromatograph [7,8], or an interferometer [6]. Since the true values remain unknown, the accuracy of the comparison device or standard must be known and must be superior to the one undergoing testing. To help achieve these goals, we created a method and a set-up for the production of gas mixtures with known concen- trations. We applied this method to investigate the lin- earity and accuracy o f the three monitors in determining the concentrations o f AA, O 2, CO2, and N2 O.

The linear regression coefficients were close to 1, that is, there was an excellent linearity of all the variables measured by these devices. Concerning the accuracy as partially expressed by the slope o f the lines, this should ideally be 1.00. With the BK 1304, the curves o f the AAs deviated from the ideal by - 3 to - 5 % (rela- five). With the N-2500, the curves o f the AAs deviated from the ideal by 2 to - 2 % (relative), whereas with the U LTIMA the slope deviation was - 1 0 to - 1 2 % (relative), leading to an underprediction o f the same

96 Journal of Clinical Monitoring Vol 9 No 2 April 1993

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Fig 2. The effect on accuracy of increasing "respiratory rate," which is depicted as a function of the displayed gas concentration in percent of the actual concentration (I00%). Isoflurane was used as the AA. (A) No dead space, constant gas flow, and inspiratory to expiratory ratio of 1:1. (B) Added dead space of an endotracheaI tube 16 cm in length with an internal diameter of 5.0 am, con- stant gas flow (3 L/min), and inspiratory to expiratory ratio of i:1.

magni tude . This re la t ive ly large devia t ion in accuracy was caused b y inaccuracy in the cal ibra t ion gas mix tu re suppl ied b y the manufac ture r . A n addi t iona l measure - men t us ing a ca l ibra t ion gas canister w i t h a different lot n u m b e r y ie lded an o v e r p r e d i c t i o n o f the same m a g n i - tude as the init ial underpred ic t ion . W h e n the l inear i ty is excel lent the accuracy o f the m o n i t o r depends p r i m a r - i ly on the ca l ibra t ion gas and ca l ibra t ion procedure . A p - parent ly , the D a t e x c o m p a n y has a p r o b l e m concern ing inaccuracy o f the A A fract ion in the cal ibra t ion gas; this is n o w being addressed (Mona Grons t r and , personal

Table I. The Consequences of Monitoring an Anesthetic Agent Other Than the One Delivered

Displayed Concentrations (vol%)

Delivered Monitored AA (vol%) AA BK 1304 ULTIMA N-2500

ISO 0.95 1.0 1.0 A ISO 0.96 ENF 0.75 1.2 - -

HAL 0.90 6.0 - - ISO 1.25 1.0 - -

B ENF 0.98 ENF 0.95 1.0 1.0 HAL 1.25 4.9 - - ISO 0.85 0.1 - -

C HAL 0.83 ENF 0.65 0.2 - - HAL 0.80 0.8 0.8

Delivered AA versus concentration at different monitor AA settings (BK 1304 and ULTIMA), or detected AA and displayed concentration (N-2500). There was no variation between the three consecutive sam- plings.

Table 2. Delivery of Mixed Anesthetic Agents (AA)

Example: Nearly empty isoflurane-vaporizer refilled with halothane. Assumed vaporizer setting: 1.2 vol%

Delivered mixture (vol%): ISO 0.67 and HAL 1.10 Monitored AA (vol%)

BK 1304 ULTIMA N-2500

ISO 1.75 0.9 0.7 ENF 1.30 1.0 - - HAL 1.70 5.2 1.1

Displayed values at different AA settings (BK 1304 and ULTIMA), or detected AA and displayed value (N-2500). There was no variation between the three consecutive samplings.

communica t i on , 10/1/91). Conc e rn ing the accuracy in de t e rmin ing 0 2 (BK 1304 and U L T I M A ) , C O > and N 2 0 , this was excel lent for all three mon i to r s , the slopes be ing 1 or close to 1.

W h e n character iz ing the speed o f funct ion, manufac - turers usual ly state t w o t imes: rise t ime and de lay t ime. The rise t ime is the t ime requi red to measure f rom 10 to 90% o f the actual value o f the s ampled gas concent ra - tion, whi le the de lay t ime is the t ime f rom a change in anesthetic gas concen t ra t ion at the sampl ing site to the ach ievement o f 10% o f that change in anesthet ic g a s value in the analyzer . T h e sum o f these t w o t imes is the total sys t em response t ime. T h e shor te r the total sys tem response t ime, the h igher the r e sp i ra to ry rate at w h i c h the m o n i t o r can be used w i t h o u t loss o f accuracy. It is not obv ious to the clinician, howeve r , h o w to relate the total sys tem response t ime to the resp i ra to ry rate at which loss o f accuracy begins. To de te rmine the accu- racy o f these m o n i t o r s in re la t ion to a d y n a m i c clinical s i tuation, w e used a se t -up s imula t ing increasing respi -

Nielsen et al: Evaluation of Multigas Anesthetic Monitors 97

ratory rate and expressed the accuracy as the percent decrease o f the actual concentration as a function o f the respiratory rate. The increase o f a gas concentration in the measuring chamber can be described by an expo- nential model with an initial rapid rise, the rate of which decreases as the value approaches maximum. The faster the respiratory rate the less time is left for wash-in of gas into the measuring chamber. Thus, an exponential decrease in precision with increasing respiratory rate was to be expected. The shape o f the curves in Fig 2 is in good accordance with this. The stepwise decline is produced by the electrically rounding off of the dis- played values. With the addition of the dead space in an endotracheal tube, the decline is accentuated, as ex- pected. The aim of this part of the investigation was not to state specific respiratory rates at which the accu- racy had fallen below a certain limit, but to point out the fact that the dynamic accuracy o f a monitor is influ- enced by the total dead space (in patient and system), the ventilatory volume, and the respiratory rate. From the results we conclude that the BK 1304 provides the greatest accuracy at high respiratory rates, followed by the N-2500 and the ULTIMA. When high respiratory rates are expected, as in pediatric anesthesia, Badgwell et al recommended that gas for analysis be sampled via a small catheter with the tip placed inside the endotracheal tube [13]. The sampling tubes o f the three monitors can be connected to such a catheter via the Luer-lok connector.

Ideally, the AA monitor should be able to detect which AA is being used. The N-2500 is the first infrared analysis-type monitor equipped with an agent detection system for ISO, ENF, or HAL. Our results confirm this system to be functioning accurately in the laboratory setting. In each instance it correctly determined the AA being used, alone as well as in mixtures o f 2 or 3 gases. With the BK 1304 and U L T I M A it is possible to moni- tor for one AA but actually be using one of the other two. The consequences o f this error depend on several factors: the MAC value of the AA used and the AA believed to be used, the hardware (infrared filters) and software (gain factors). The monitor displays a gas con- centration computed from the infrared absorption with an absorption-dependent gain factor specific for the se- lected AA. Two questions must be answered: will the displayed value be different from the vaporizer setting to such an extent that it will cause the anesthetist to be alerted and, if not, to what extent will the MAC value actually delivered differ f rom the one believed to be given? When such a situation occurs one also must ask whether it is clinically relevant, i.e., whether it will lead to administering too high a concentration. One way to answer this could be to determine to what extent the MAC factor (the MAC value o f the delivered AA di-

vided by the MA C value o f the monitored AA) exceeds the relationship between MAC9s and MACs0, this being approximately 1.3 (MACgs/MACs0) [14]. de Jong and Eger suggested that when the actual MA C value devi- ates by more than 40 to 50% from the MAC value thought to be given, a risk o f overdose exists [14]. The bigger the difference, the bigger the risk in terms of speed and depth o f anesthesia. In the discussion below, we assumed that the monitor AA setting indicates what the anesthetist believes is being delivered.

For example, consider situation C in Table 1. In this example, 0.83 vol% HAL is delivered (vaporizer setting assumed to be 0.8%). With the BK 1304 set for ISO, the displayed value is acceptable (0.85 vol%) and the resulting MAC factor is 1.4, a value that is most likely to have no clinical significance. I fEN F is set, the display shows 0.65 vol%. The difference between this value and the vaporizer setting probably will be accepted by most anesthetists, but the MAC factor is 2.7, a value that may lead to an overdose. Using the ULTIMA set for ISO or ENF during normal flow, the displayed val- ues are 0.1 and 0.2 vol%, respectively, values that would cause most anesthetists to be alerted. In the low- flow situation this difference from the vaporizer setting is likely to be accepted, leading to MA C factors o f 14.9 and 8.7.

Another example is the situation in which an almost empty ISO vaporizer has been refilled with HAL. This is known to lead to an increased vaporization of both agents [15]. We assume that the vaporizer is set at 1.2 vol% and the actual output is ISO 0.67 vo1% and HAL 1.10 vol%. With the BK 1304, using the AA settings ISO and HAL leads to display o f values (1.75/1.70 vol%) that would alert the anesthetist. If the monitor is set for ENF, the displayed value is 1.30 vol% and the resulting MAC factor will be 2.5 times the MAC as- sumed to be delivered (MAC values are added) [16,17]. With the U L T I M A set for ISO or ENF, the displayed values are 0.9 or 1.0 vol%, respectively. The ISO set- ting might alert the anesthetist during normal flow, whereas the displayed value in the ENF setting would probably be accepted by most anesthetists, resulting in a MAC factor o f 3.3. With the HAL setting, a value o f 5.2 vol% will always cause suspicion.

In the continuing debate over agent identification it has been argued that in situations with a discrepancy between monitored and delivered AA leading to an overdose, the resulting cardiovascular depression most likely would be recognized and promptly dealt with by turning the vaporizer down or off. Although human error is the quantitatively greatest contributor to anes- thetic mishaps [18], we are not aware o f any specific reports describing patient injury resulting from the above-mentioned theoretical situations.

98 Journal of Clinical Monitoring Vol 9 No 2 April 1993

A future problem concerns the new inhaled AAs0 des- flurane and sevoflurane, which are currently being in- troduced. Assuming that monitors follow suit and ac- quire the capability o f measuring 5 agents, this increase in the number of AAs will increase the probability of an error in setting the monitors correctly. Analyzers that are non-agent-specif ic can be adjusted more easily to be used with new agents by simple software correc- tions. With an agent-specific analyzer, however, the monitoring o f new agents will demand far more exten- sive corrections both in hardware and software.

None o f the monitors was influenced by water vapor in the expired air. The BK 1304 and N-2500 correctly computed the displayed values o f vo lume percent in STPD and of kilopascals and millimeters o f mercury in BTPS. The U L T I M A displayed the values in STPD in all modes, thus leading to an overestimation (6% relative) o f the actual C O 2 concentration or other gases in the alveoli and in the blood [19].

None of the monitors were influenced by alcohol in air at concentrations associated with normally occurring blood alcohol levels. I f the concentration in the air was high, as during and after the use of an alcohol-contain- ing detergent, the U L T I M A displayed values at all AA settings even though no AAs were present. The reason for this is that the U L T I M A uses infrared wavelengths at which alcohol also absorbs the light.

In conclusion, we found that all three monitors had excellent linearity and accuracy, sufficient for clinical use, although the U L T I M A gave an underprediction o f the AA of 10 to 12% (relative) because of the calibration mixture supplied with the device. Given an accurate calibration mixture, the U L T I M A is also accurate. The BK 1304 is probably best suited for pediatric anesthe- sia because o f better accuracy at high respiratory rates. When using the BK 1304 and U L T I M A , one should be aware of the consequences when the monitored AA differs f rom the one actually used. This may theoreti- cally lead to M A C factors up to 2.7 (BK t304) and 14.9 (ULTIMA). Finally, users should be aware that accurate values are not available immediately after power-up, with a delay of 15 minutes with the BK 1304 (CO2) and the U L T I M A (AA) and 40 minutes for the N-2500 (AA).

Presented in part at the Annual Meeting of the American Soci- ety of Anesthesiologists, Las Vegas, NV, 1990.

The authors thank professor and chairman S. H. Johansen and J. Qvist M.D. (Herlev Hospital, University of Copenhagen, Denmark) for their helpful discussion of the manuscript. We are grateful to Bruel & Kjaer (Naerum, Denmark), Datex Instrumentarium Corp. (Helsinki, Finland), and Nellcor Corp (Hayward, CA) for providing the gas monitors for the study.

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