The Influence of Temperature on Breath-Alcohol AnalysisGünter Schoknecht

For determination of breath-alcohol concentration (BrAC) the examined subject has to perform a breath test with forced expiration. During this test essential physical quantities change continuously. Ethanol concentration, breath temperature, and breath flow are such important quantities. Significant results can only be obtained if concentration (BrAC) and temperature have reached final values and the expired volume (integral value upon breath flow) has exceeded a certain minimum value. According to our own experiences this minimum value depends on sex and age of the examined person.

An important problem in measuring the BrAC is the influence o f temperature. The passage of ethanol into the inspired air occurs in the alveoli of the lungs. According to the law of HENRY a concentration is established which is proportional to the concentration of ethanol in the blood o f the lung-capillaries and depends on a function of temperature in the lungs. The exchange of ethanol takes place through the alveolar-capillary membrane (see GRUNER 1985). The dependency of the equilibrium between BrAC and blood-alcohol concentration (BAC) was carefully studied by HARGER et al. 1950, DUBOWSKI 1980, WELLS 1981, JONES 1983, SCHOKNECHT and KOPHAMEL 1988. According to the results BrAC changes nearly 7 % per 1°C (DUBOWSKI: 5.58 %; SCHOKNECHT and KOPHAMEL: 6.61 % at a temperature of 34°C) when blood-alcohol concentration has a constant value.

With forced expiration of breath through the mouth the content o f alveolar air increases in course of time. This alveolar air cools down several degrees on its way from the alveoli to the mouth. Under normal conditions the temperature of lungs is equal to core body temperature o f about 37°C and the mouth temperature is about 34°C. Because the decrease of temperature takes place in a wet environment (mucous membranes, saliva etc.) an over saturation of ethanol concentration occurs connected with a loss o f ethanol concentration. Thus a measurement o f temperature is distinctly recommended by FOX and HAYWARD 1987 when BrAC determination is performed.

Measurements of WRIGHT (1962), DUBOWSKI (1974, 1980), JONES (1982), FOX and HAYWARD (1987) and SCHOKNECHT and KOPHAMEL (1988) have shown that the temperature of an end-expired sample o f breath differs inter- and intraindividually in the range of several degrees. Besides this

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there are important influences by breathing technique and by the environmental temperature.

According to these physical facts a working group, which was formed at the Federal Health Office to investigate the forensic applicability of breath alcohol analysis, has recommended to measure besides the end-expired BrAC the end- expired temperature of breath at the same time. The measured BrAC should then be converted into a value which corresponds to a temperature o f 34°C using coefficient o f temperature given above.

For measuring the breath temperature a measuring device has been developed which is schematically shown in Fig. 1. A thermistor is the element used for the measurement o f the temperature. Due to the small capacity of heat in breath difficulties exist to warm up the thermistor during a breath test from the environmental temperature to the end-expired temperature. Therefore the thermistor is situated in a heated housing (SCHOKNECHT et al. 1989), which also is capable o f keeping the mouth piece. The temperature of preheating corresponds to body temperature. Thus a negative gradient of temperature in the mouth piece is avoided that could lead to the effect o f condensing. The final temperature is reached asymptotically as shown in Fig.2.

temperature (*C)

Fig. 1 Heated housing for thermistor and m outh piece Fig.2 Course o f temperature during expiration

Fig.3 presents the BrAC as a function of time measured during a drinking experiment with a commercial breath alcohol instrument (Alcomat FA 113). In this case the pattern o f alcohol uptake was a short time drinking. The measuring apparatus was equipped with an additional device o f the described kind to measure the breath temperature.

The measuring points marked by K were obtained after cooling the mouth of the subject with water. Before measuring points marked by H were obtained the subject was asked to hyperventilate vehemently several times. In Fig.3 the influence o f mouth cooling and hyperventilation can clearly be recognized by the decreased measured values. The BrAC values slowly increase in course of time.

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Fig.3 BrAC during a drinking experiment

with hyperventilation and mouth cooling

Fig.4 Breath temperature during the drinking

experiment o f Fig.3

Fig.5 BrAC values o f Fig.3 related to the

reference temperature o f 34°C

BrAC [mg/l] A lcomat FA 113

time [h]

te m p era tu re [°C] A lcom at FA113

time [h]

BrAC [mg/l] Alcomat FA113

time [h]

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Fig.4 presents the end-expired temperatures which correspond to the BrAC values of Fig.3. BrAC values measured after mouth cooling (K) or hyperventilation (H) meet extremely small breath temperatures which differ more than -2°C from the uninfluenced values. Further measurements following mouth cooling and hyperventilation also demonstrate the deviations of temperature.

Fig.5 shows the BrAC as a function o f time after converting the BrAC (Fig.3) to the reference temperature of 34°C using temperature values (Fig.4). The rise of the BrAC curve in the absorption period, the fall of BrAC according to diffusion in the post peak phase, and the linear decrease of BrAC are now clearly indicated. The influences of mouth cooling and hyperventilation are mostly eliminated by standardisation of the BrAC to the reference temperature.

Besides this other experimental investigations indicate that by measuring of the breath temperature and computing the BrAC to reference temperature the influence of hypoventilation can also essentially be eliminated. Hypoventilation events before BrAC measurement lead to a warming up of the mouth and therefore to increased BrAC measured values.

The significant influence of temperature on BrAC function can be seen by the measurements of a subject represented in Fig.6. Especially at high environmental temperatures we found breath temperatures which clearly deviate from the reference temperature of 34°C. In the special case shown in Fig.6 medium breath temperature increased within 5 hours from 34.5°C to more than 36°C. The course of directly measured BrAC values and the course of corrected values (related to 34°C) show definite differences (see Fig.7). These differences even appear with such measurements which are not influenced by special breathing techniques (e.g. hyper- and hypoventilation).

o 1 2 3 4 5 6 7 — corrected a uncorrectedhours after end of drinking

Fig.6 Course of breath temperature

at high environmental temperature

Fig.7 Measured BrAC values and values

related to 34°C according to Fig.6395

In conclusion it can be stated that the measurement of breath temperature and the conversion of BrAC to the reference temperature is necessary for the following reasons:

- to obtain comparable BrAC values despite of different body and mouth temperature,

- to obtain BrAC values which are independent from temperature o f the environment,

- to correct conscious or unconscious influences of breathing technique.

The results of FOX and HAYWARD (1987, 1989) show that the influence of increased body temperature (fever) which results according to the law of HENRY in an enhancedinated by conversion to the reference temperature.

Even at normal measuring conditions the deviations o f uncorrected BrAC values from values related to 34°C may reach values between 10 and 20 %. Such a magnitude of the deviation is in our opinion unacceptable in forensic breath analysis. Breath alcohol measurements without determining the temperature can only be regarded as measurements for pretesting.

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