An evaluation of the Datex Normac anaesthetic agent monitor

Can Modern Infrared Analyzers Replace Gas Chromatography to Measure Anesthetic Vapor Concentrations?

Background

Gas chromatography (GC) has often been considered the most accurate method to measure the concentration of inhaled anesthetic vapors. However, infrared (IR) gas analysis has become the clinically preferred monitoring technique because it provides continuous data, is less expensive and more practical, and is readily available. We examined the accuracy of a modern IR analyzer (M-CAiOV compact gas IR analyzer (General Electric, Helsinki, Finland) by comparing its performance with GC.

Methods

To examine linearity, we analyzed 3 different concentrations of 3 different agents in O₂: 0.3, 0.7, and 1.2% isoflurane; 0.5, 1, and 2% sevoflurane; and 1, 3, and 6% desflurane. To examine the effect of carrier gas composition, we prepared mixtures of 1% isoflurane, 1 or 2% sevoflurane, or 6% desflurane in 100% O₂ (= O₂ group); 30%O₂+ 70%N₂O (= N₂O group), 28%O₂ + 66%N₂O + 5%CO₂ (= CO₂ group), or air. To examine consistency between analyzers, four different M-CAiOV analyzers were tested.

Results

The IR analyzer response in O₂ is linear over the concentration range studied: IR isoflurane % = -0.0256 + (1.006 * GC %), R = 0.998; IR sevoflurane % = -0.008 + (0.946 * GC %), R = 0.993; and IR desflurane % = 0.256 + (0.919 * GC %), R = 0.998. The deviation from GC calculated as (100*(IR-GC)/GC), in %) ranged from -11 to 11% for the medium and higher concentrations, and from -20 to +20% for the lowest concentrations. No carrier gas effect could be detected. Individual modules differed in their accuracy (p = 0.004), with differences between analyzers mounting up to 12% of the medium and highest concentrations and up to 25% of the lowest agent concentrations.

Conclusion

M-CAiOV compact gas IR analyzers are well compensated for carrier gas cross-sensitivity and are linear over the range of concentrations studied. IR and GC cannot be used interchangeably, because the deviations between GC and IR mount up to ± 20%, and because individual analyzers differ unpredictably in their performance.

Intrathecal neurokinin-1 receptor antagonist reduces isoflurane MAC in rats

PURPOSE

To study the effects of intrathecal administration of a neurokinin-I(NK-I) receptor antagonist (CP96,345) on the minimum alveolar anaesthetic concentration (MAC) of isoflurane in anaesthetized rats, and on the locomotive activity of conscious rats.

METHODS

Wistar rats (n = 36) were fitted with indwelling intrathecal catheters, and the MAC of isoflurane was determined following the intrathecal administration of saline (control group) or the NK-I receptor antagonist CP96,345 (CP) at 1, 10 and 100 micrograms. Subsequently a reversal dose of intrathecal Substance P (SP) at 1, 10 and 100 micrograms was administered and MAC isoflurane was redetermined. Conscious rats (n = 35) were also examined for the presence of locomotor dysfunction following intrathecal administration of CP and SP. Animals were randomly assigned to each treatment group and the investigators were blinded.

RESULTS

CP at 10 and 100 micrograms reduced MAC isoflurane by 9.9% and 15.3%, respectively (P < 0.05). Intrathecal administration of SP reversed the decreases in MAC by CP; however, locomotive activity was not changed.

CONCLUSION

These results suggest that the NK-1 receptor plays an important role in determining the MAC of isoflurane by inhibition of pain transmission in the spinal cord.

Intrathecally administered NMDA receptor antagonists reduce the MAC of isoflurane in rats

PURPOSE

We studied the effects of intrathecal administration of an N-methyl-D-aspartate (NMDA) receptor antagonist and an antagonist of the glycine site of the NMDA receptor on the minimum alveolar anaesthetic concentration (MAC) of isoflurane in rats, and on locomotor activity in conscious rats.

METHODS

In Wistar rats fitted with indwelling intrathecal catheters, we determined the MAC of isoflurane after the administration of saline (control group); the competitive NMDA receptor antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosponic acid(CPP) at 0.01, 0.1, and 1.0 nM; and the selective antagonist of the glycine site on the NMDA receptor complex 7-chlorokynurenic acid (7CKA) at 0.1, 1.0, and 10 nM. After measurement of MAC following administration of the antagonist, the equipotent reversal dose of NMDA or D-serine was administered. The rats were examined for the presence of locomotor dysfunction by intrathecal administration of NMDA receptor antagonists, NMDA and D-serine in conscious rats. All of the experiments were performed using randomization and masking of drugs.

RESULTS

CPP at 0.1 and 1.0 nM decreased the MAC of isoflurane by 9.9-17.6% (P < 0.05). 7CKA at 1.0 and 10 nM reduced MAC from 10.5-15.5% (P < 0.05). Intrathecal administration of NMDA or D-serine reversed the decreases in MAC to control values. Locomotor activity was not changed.

CONCLUSIONS

We believe that NMDA receptor plays an important role in determining the MAC of isoflurane in the spinal cord.

Infrared analysis of volatile anesthetics: impact of monitor agent setting, volatile mixtures, and alcohol

Infrared analysis can determine exhaled concentrations of the three volatile anesthetics in common use because each absorbs infrared light. Many infrared analyzers use a single source of infrared light at a wavelength of 3.3 microns for measurements of all three agents but cannot identify which agent is in use. Organic gases such as ethanol also absorb infrared light. This study determined the effects on the accuracy of a single-wavelength infrared anesthetic monitor (Capnomac, Puritan-Bennett PB254) of (1) incorrect anesthetic agent setting, (2) mixtures of volatile anesthetics, and (3) ethanol vapor in the analyzed gas. Changing the agent setting on the monitor during steady-state delivery of an agent resulted in readings for the halothane setting five times higher than those for the enflurane setting, and six times higher than those for the isoflurane setting. These ratios reflect the monitor's fixed internal gain for each agent setting. Mixtures of anesthetics had a simple additive effect on the monitor's accuracy. With the monitor set for halothane, 0.2 vol% isoflurane mixed with halothane caused readings 1.2 vol% higher than the true halothane concentration. Conversely, with the monitor set for isoflurane, 1 vol% halothane mixed with isoflurane resulted in readings 0.2 vol% too high. In a model simulating alveolar gas, ethanol vapor corresponding to blood alcohol levels of 0.10, 0.30, and 0.50% had a slight but not clinically significant effect on readings for enflurane and isoflurane but increased readings with the halothane setting 3.5 times the corresponding level of blood alcohol. Clinicians can test for an interfering gas such as ethanol before induction by checking the reading in the halothane setting during preoxygenation.

Parameters influencing the response time of volatile anesthetics monitors

In a given inhalational anesthetic analyzer, response (RT) is usually thought to be a constant value, however, several factors may influence RT. RT's measured under ideal conditions for the Beckman LB 2, the Normac (Datex), the Servo S 120 (Siemens) and the Irina (Dräger) were 107 +/- 5, 589 +/- 14, 538 +/- 30, and 166 +/- 15 msec, respectively. In addition, we investigated the RT of a Beckman LB 2 analyzer under conditions which may occur in clinical practice (non ideal conditions). Increasing aspirating flow (AF) resulted in shorter RT's, the effect being most pronounced when AF was below 200 ml/min. Interposing a filter prolonged RT by 80%. The type of the inhalational anesthetic (halothane or isoflurane), humidity and temperature of the carrier gas as well as size and direction of the concentration step change did not influence RT. Increasing length or internal diameter (ID) of the sample tube resulted in longer RT's. Changing the sample tube material from glass to Teflon or polyethylene resulted in a slight increase of RT, but the increase was dramatic when rubber or silicone tubes were used. The partition coefficient of halothane in the material of a particular sample tube was directly correlated to the corresponding RT in this sample tube. The influence of different sampling places was studied by interposing copper or corrugated rubber tubing between the place where the concentration step change occurred and the place where the gas was sampled, the measured time was called total response time (TRT). Using corrugated rubber tubes instead of copper tubes increased TRT two to four times. More distal gas sampling and/or lower flow rates caused longer TRT's. Compared with sampling in its center, gas sampling near the walls of the tube resulted in an increase in TRT of 13-45%. It is concluded that the response time of an infrared inhalational anesthetic analyzer is not a constant parameter, but varies between 100 and 4000 msec depending on the characteristics of the analyzer, the sample line, and the place of gas sampling.

'Anestheticography': on-line monitoring and documentation of inhalational anesthesia

The safe practice of inhalational anesthesia requires control over the amount of volatile anesthetic delivered to the patient. With minimal fresh gas flow this is facilitated by continuous monitoring and recording of the agent's concentration ('Anestheticography'). Alterations brought about by routine clinical maneuvers become visible. We recorded the course of the inspiratory and expiratory concentration of volatile anesthetic (Isoflurane) by infrared absorption and a trend recorder. Changing the carrier gas composition during high flow from 75% to 25% nitrous oxide in oxygen resulted in a 10% increase of the inspiratory isoflurane concentration. Activating the oxygen bypass or exchanging the soda lime canisters was followed by a prolonged disturbance of concentrations, most pronounced with minimal flow. Initiating emergence by closing the vaporizer during minimal flow led to a slow decrease in concentration whilst at a flow of 61/min the inspiratory isoflurane concentration rapidly decreased to subanesthetic levels. Insertion of a charcoal filter into the inspiratory limb of the breathing circuit immediately dropped the inspiratory concentration to undetectable levels. 'Anestheticography' is a useful means of monitoring and documentation of inhalational anesthetic. With the use of a charcoal filter all advantages of minimal flow anesthesia can be realized throughout the entire anesthetic, including emergence.

Dedicated monitoring of anesthetic and respiratory gases by Raman scattering

The monitoring of respiratory and anesthetic gases in the operating room is important for patient safety. This study measured the accuracy and response time of a multiple-gas monitoring instrument that uses Raman light scattering. Measurements of oxygen, carbon dioxide, nitrogen, nitrous oxide, halothane, enflurane, and isoflurane concentrations were compared with a gas mixer standard and with measurements made with an infrared anesthetic agent analyzer. Correlation coefficients were all greater than 0.999, and probable errors were less than 0.43 vol% for the gases and less than 0.03 vol% for the volatile anesthetics. Response time was 67 ms with a sample flow rate of 150 ml/min. There was some signal overlap between nitrogen and nitrous oxide and between the volatile anesthetic agents. Such overlap can be compensated for by linear matrix analysis. The Raman instrument promises a monitoring capability equivalent to the mass spectrometer and should prove attractive for the monitoring of respiratory and anesthetic gases in the operating room.