London 2003
Page 5 of 7

<Back to Abstract Home Page>   <Previous Page>     <Next Page>  <Program>

 

MEASUREMENT OF INHALATION AGENT CONCENTRATION

-         PAST, PRESENT AND FUTURE –

Prof. Dr. Linda VERSICHELEN   

University of Gent

 

Data displayed by gas monitors appear to be similar, but as different technologies are used for the analysis of inhalation agent concentration a basic understanding of the principles of measurement is necessary. Some analysers display data in mmHg (partial pressure) and others in volume percent (a proportion).

 

About 40 years ago the only practical analysers were the ultra-violet halothane meter and the silicone rubber analyser, which could be used in closed systems.

 

About 30 years ago more modern and reliable technology became available. Basically the used sampling techniques can be divided in side-stream systems and main-stream systems. If we compare the analysing techniques, only the proportion analysis with mass spectrometry can give measurement of all respiratory gases. With partial pressure measurement only Raman spectrometry comes close, whereas the popular IR techniques measure anaesthetic agents and CO2, and piezoelectric resonance technique only the potent inhalation agents.

 

Mass spectrometry is an expensive side-stream proportioning system, whereby in a high vacuum chamber large molecules by electron bombardment are cracked into smaller positively charged ions; these are focused by electrostatic fields, accelerated and directed by permanent magnet according to their mass/charge ratio. Individual collectors are used for detecting the ion current. Individual collector voltage is expressed as % of total voltage.

In Raman spectroscopy gas molecules of 2 or more atoms absorb monochromatic light and re-emit it at longer wave-length. This technique is agent specific.

With piezoelectric resonance technique a lipophilic coated quartz crystal undergoes changes in natural resonance frequency by the potent inhalation anaesthetics, proportional to partial pressure.

In the infrared light spectroscopy technique energy is absorbed from a narrow band of wave-lengths of infrared light. IR analysers measure partial pressure: CO2 at 4.2-4.4 µm, N2O at 4.4-4.6 (also 3.9) µm, volatile anaesthetics at 3.3 µm (no differentiation, but agent identification possible by sweeping spectrum analysis) and at 9-12 µm (agent specificity).

The infrared photoacoustic spectrometer uses similar wavelengths but infrared beams pulse on and off at particular frequencies. Each gas absorbs energy and expands and contracts, resulting in sound waves detected by a microphone. Partial pressure = proportional to the amplitude of sound.

 

Recently sophistication with IR techniques allowed agent identification with a spectrum scan between 3.24 and 3.39 µm. Newer sensors use 7 narrow-band filters: 2 in the 3-5 µm low-end band (CO2 and N2O) and 5 in the 8-9 µm far-end band (anaesthetic identification).There is no influence of trace concentrations of methane and alcohol. In an IR rapidly identifying analyser (with filterwheel) non dispersive measurement is done in the NIR 4 µm spectrum and the MIR 8-10 µm spectrum (anaesthetic agents). In a miniaturised multi-spectral detector the filterwheel is replaced by pulsed IR light source and fixed interference filters; a multi-spectral detector with 4 miniaturised pyrodetectors is present. It can be used for anaesthetic concentration control.

The future will bring maybe a compact reasonable cost main-stream analyser for all respiratory gases (including N2 and Xe

 

 

 

 

The Use of Xenon as a Sedative in Intensive Care

 

JM Murray, Department of Anaesthetics and Intensive Care, The Queen’s University of Belfast.

 

Introduction:

Xenon is a noble gas with potent sedative, anaesthetic and analgesic properties (1). It does not undergo metabolism and has been shown to have minimal effects on the myocardium (2). Xenon may be a useful sedative for critically ill patients.

Methods:

Following informed consent, 21 patients requiring elective ventilation following major thoracic surgery were studied. A double blind, randomised cross-over study was used. Following admission to the intensive care unit, patients were stabilised and then randomly allocated to either regimen X (xenon in oxygen) or regimen P (intravenous propofol (0-5 mg/kg/h) and alfentanil 30µ/kg/h). After 8 hours the patient received the other regimen. The concentration of xenon gas and the rate of propofol infusion were adjusted at the direction of a blinded observer to attempt to maintain a Ramsay sedation score of 2 or 3. A Bispectral index monitor recorded the subjects’ EEG and estimated depth of sedation throughout the study. Additional analgesia when required was provided using 250µ boluses of alfentanil. The patients’ lungs were ventilated using a specially designed bellows-in-bottle breathing system and a Puritan-Bennett 7200A ventilator. After each sedative regimen the sedation was stopped and the time taken for the Ramsay score to change to 1 was measured. All patients in the study had standard monitoring which also included a pulmonary artery catheter.

Results:

Twenty-one patients with a mean Apache III score of 82 (range 39-110) were studied. Sedation with xenon was well tolerated and without adverse effect. Examination of the trends in arterial systolic, diastolic and mean blood pressure showed a greater tendency for negative gradients in patients receiving regimen P (t=2.24 p< 0.05; t=1.74 p<0.1; t=2.84; p<0.01 respectively). Cardiac output increased in the first eight hours of sedation. No significant change in cardiac output in the second period of sedation was seen with time or sedative regimen. Measured haematology and serum biochemistry did not differ between the two groups. Recovery from sedation was significantly faster in the xenon group (p<0.0001).

Discussion:

We report the first use of xenon as a sedative for patients receiving intensive care. It is now necessary to further this work in certain sub-groups of ICU patients. Patients such as those with poor myocardial function, sepsis or multiple organ dysfunction and patients in whom a rapid assessment of their true neurological state is desirable may be those who will benefit most from the unique properties of xenon.

Conclusions:

Sedation of patients in intensive care is possible using xenon and is well tolerated and without adverse effect. Xenon requirements varied widely between and within patients. Xenon appears to be biologically inert based on the evidence from this study and provided an extremely rapid recovery from sedation.

References:

  1. Boomsma F et al. Anaesthesia 1990; 45: 273-8.
  2. Luttropp HH et al. Anaesthesia 1993; 48: 1045-9.
  3. Dingley J et al. Crit Care Med 1999; 27: 2435-41.