Breath Alcohol Concentration and Breath Temperature

Transcription

1 Breath Alcohol Concentration and Breath Temperature A. L. Flores U. S. Department of Transportation Volpe National Transportation Systems Center Cambridge, Massachusetts, USA Abstract Breath alcohol concentration, breath temperature, and body temperature are used to extrapolate the breath alcohol concentration to the concentration of alcohol in the deep lung air, which is in equilibrium with the pulmonary blood. Introduction The variability that is seen in the breath alcohol test (note: alcohol means ethyl alcohol) has always been more or less ignored as inevitable biological variability. It has been known from the beginning of the use of breath test for numerical evidence of drunk driving that breath temperature plays a part in the test result. With the intention of reducing breath test variability, authorities in Germany and Alabama have recently begun extrapolation of the test result from the measured breath temperature to a standard breath temperature, 34 o C. The procedure is described by Schoknect and Stock (1) and is based on Henry s Law. Extrapolation to a standard temperature does not take into account pulmonary blood temperature, which varies from person to person and with time of day and with it, breath alcohol concentration. Therefore, extrapolation to body temperature, the procedure described below, should further reduce the variability of the breath test as well as to make the BrAC more equitable with the BAC. The procedure assumes equilibrium between pulmonary blood and breath in the deep lung region; but in the upper lung region only near equilibrium is assumed between the breath and the water in the mucous surface that lines the airways. Recent Work In recent years, several workers have investigated the process leading to a breath alcohol measurement. Lubkin et al (2) have developed a model of the breath alcohol profile, but it does not have a temperature component. Curiously, Lefranc and Montamat (3) concluded that the breath test is not influenced by breath temperature, although they did not measure it. They noticed the effect of hyperventilation on breath alcohol but ascribed it to a temporary thinning of the arterial blood in the alveoli. Tsu et al (4) have developed a detailed model of the interaction of alcohol with the lung surfaces that has

2 been tested by George et al (5). It predicts alcohol profiles, but is too complex for police use. Methods The inlet end of the breath hose of a National Patent Analytical Systems, Inc. Datamaster infrared type police evidential breath alcohol tester was fitted with a thermostated (at 34 o C) fast response gas thermistor temperature probe assembly. The thermistor was calibrated by passing air saturated at 37 o C through a 10 foot long 3/8 inch diameter copper tube coiled into a water bath thermostated at temperatures from 30 o C to 36 o C. Breath alcohol concentration calibration data was obtained immediately before and after each human subject test run using a Repco Marketing simulator operating at 34 o C to provide reference alcohol-in-air samples. The flow thermistor was calibrated by use of a Warren Collins, Inc. spirometer connected to the Datamaster sample vents. Compressed air saturated at 34 o C was used for this calibration. Oral temperatures to the nearest tenth of a degree Fahrenheit using a digital oral thermometer were obtained from each of 30 drinking human subjects just before being tested with the apparatus. All temperature sensors used were checked with a NIST traceable digital thermometer. The output of the breath temperature thermistor, and the alcohol and flow outputs of the Datamaster, were fed to a data logger, and then to a computer for spreadsheet analysis. The breath temperature probe is shown in Figure 1. Four types of breath sample were obtained from each subject in the following order: Brass tube Mouth pc Breath thermistor Breath hose Control thermistor Figure 1. Breath temperature probe A normal police type sample where the subject takes a deep breath before blowing fully into the Datamaster. A hyperventilated sample where the subject inhales deeply then fully exhales. This is repeated ten times before finally inhaling and blowing into the Datamaster as above. The purpose of this maneuver is to aggressively cool the airway surfaces of the lung. A hypoventilated sample where the subject takes a deep breath and holds it for 20 seconds before blowing into the Datamaster. The purpose of this maneuver is to allow the airway surfaces to warm. A re-breathed sample where the subject inhales deeply, then blows his breath fully into an unheated 4 liter plastic bag, then re-inhales the same air from the bag, all the while keeping his nostrils pinched shut to prevent fresh air from entering his lungs. After the 7 th inhale, the subject blows into the Datamaster. The purpose of this maneuver is to stabilize the interaction of alcohol vapor in the breath with the airway surfaces. Results The relationship of BrAC (g alcohol/210l air) to breath temperature of a breath sample

3 b hypo hyper normal ba= liters t t2=35.8 nomal hypo 33 hyper liters Figure 2a. b1 vs volume (ba=0.105). Figure 2b. t1 vs volume (t2=35.8 o C) can be seen in Figure 2a and Figure 2b and are typical of the results obtained. Breath flow rate data where used to calculate breath volume. The profiles are from a single subject and were obtained within a period of less than 10 minutes. Comparison of the alcohol and the temperature profiles clearly shows the effect of cooling the lung airway surfaces on the breath alcohol concentration. The sharp rise seen in the alcohol profiles over the first 150 ml or so corresponds the displacement of air in the Datamaster plus air from the conducting airway volume (from the mouth to the bronchi of the subject). The slower increase that follows corresponds to air from the lungs. The order of the tests had a small influence in the test result, indicating that it takes a few minutes for the lung surfaces to stabilize after the previous blow. It is surprising that the temperature of hypo samples does not rise to deep lung temperature, assumed to be equal to oral temperature. This may reflect the disturbance of heat balance at the mucous water/air interface due to O 2 /CO 2 /water condensation/evaporation during breath holding. Re-breathed samples showed similar behavior. Re-breathed samples were obtained specifically for use as a surrogate for the pulmonary blood alcohol concentration, pbac (g/100ml blood). Following Harger s work (6) to determine a more stable relationship between BrAC and (venous) BAC, an earlier study in this laboratory (7) involving 30 subjects who had passed into the post-absorption period (at least 2 hours after drinking had stopped) yielded the following empirical relationship between re-breathed BrAC and BAC (at this stage in the post absorption period, BAC=pBAC): Eq. 1 pbac = BrAC(re-breathed) for which R 2 = , and the standard error for predicted pbac = Since one cannot be certain whether or not the subject is fully post-absorptive unless several hours have elapsed after drinking had stopped, which was not the case in the present study, re-breathed BrAC was used instead of BAC in the calculations below. Fingertip blood, which is equivalent to arterial blood and hence to pulmonary blood, could have been used but this requires specialized equipment and techniques that were not available to this study. With the assumption that Henry s law holds at the mucous water/air interface in the upper lung to a close enough approximation, an extrapolated deep lung alcohol concentration

4 can be obtained from measurement of BrAC, breath temperature, and deep lung temperature. The following notation is used: Alcohol conc. in lung air (g/210l) Upper lung/conducting airway Deep lung b1 (BrAC) types: Normal, Hyper, Hypo, Re-breathed b2 (BrAC extrapolated from t1 to t2) ba (calculated from pbac) Temperature ( o C) Alcohol conc. in mucous water (g/210l) t1 (Breath temp) c1 k1 t2 (oral temp) c2 k2 Henry s law water/air constant for alcohol (Oral temperature was used as an estimate of deep lung temperature. No doubt differences the temperature at these two sites in the body exist, and the differences may be variable. In any case, a better measure of deep lung temperature is not available.) By Henry s Law: b2 = k2c2 at t2, and b1 k1c1 at t1, so that Eq. 2 b2 b1(k2/k1)(c2/c1), Or equivalently: Eq. 3 ba/b1 (k2/k1) (c2/c1) where ba has been substituted for b2. Jones (8) has determined the water content of whole blood (in terms of grams/100 grams) for men (n=20) to be 79.3 ±0.25 and for women (n=15) 81.1 ±0.13 by a freeze dry method. Then c2 equals 1.189pBAC for men and 1.163pBAC for women in grams per 100ml mucus water. Expressing c2 in grams/210 liter mucus water leads to: Eq. 4: for men for women ba = c2k2 = 2498 pbac k2 ba = c2k2 = 2443 pbac k2 ba/b k2c2/k1c1 = (t1/t2) R 2 = t1/t2 where k2 can be obtained from Harger s data (9) and pbac can be obtained by use of equation 1. Using equation 4 to obtain ba (the value of ba was raised by 5% to account for the effect of the estimated salt content of mucus), and experimental values of b1, the ratio ba/b1 is plotted against t1/t2 in Figure 3. Regression of the data yields equation 5. Figure 3. Graphical evaluation of (k2/k1)(c2/c1)

5 Eq. 5 ba/b1 = (k2/k1) (c2 / c1) = (t1/ t2) The ratio shows a weak dependence on temperature (R 2 = ). The instability seen is possibly due to instability of t2 due to heat balance effects during blowing (see below) steyx: b2.009 b ba Substituting the right hand side of equation 5 into equation 2 provides a value of the deep lung alcohol concentration extrapolated from the measured breath temperature for each b1 that can be compared with the same quantity calculated from pbac in equation 4. Consistency of the deep lung concentration obtained by these two Figure 4. ba vs b1,b2. methods supports the validity of the extrapolated value. Results obtained from the 30 human subjects are summarized in Table 1. b1 and b2 are plotted against ba in Figure 4. Discussion The data in Table 1 show the increase in accuracy for estimation of deep lung alcohol concentration with reduced variability obtained by extrapolation to deep lung temperature. The procedure seems justifiable on the following grounds: Table 1. Data summary (n=40). ba-b1 ba-b2 normal ave sd hyper ave sd hypo ave sd b2 b1 If there were no mucus water/air interaction in the upper lung, alcohol profiles having a distinctly different appearance from what is seen in Figure 2a would be expected. The portion of the profile following the steeply rising conducting airway portion would have a slope closer to zero, and breathing pattern would not produce the effects on concentration and temperature that are seen. It is clear that interaction of alcohol vapors with the surfaces of the upper lung results in loss of alcohol as the breath moves up through the bronchioles. The final concentration is variable, depending on the temperature of the airway surfaces, surface concentration, and the volume of breath. Hyperventilation prior to blowing causes the greatest cooling and lowest concentrations. Hypoventilation and rebreathing limits cooling, but O 2 /CO 2 /water heat balance during breath holding (rebreathing is a form of breath holding) may prevent full warming of the mucous water/air interface to body temperature. (Perhaps measurement of breath humidity and CO 2 would yield a means for a better estimate of deep lung temperature.) Still, highest concentrations are obtained by breath holding and rebreathing, probably because c1 approaches c2 more closely than would otherwise be the case. At equilibrium, the alcohol concentration in the lung air is dependent only on the concentration of alcohol in, and the temperature of, the mucous layer. But during the dynamic process of breathing, true equilibrium is not obtained, except presumably in the deep lung region. However, the difference between deep lung temperature and breath

6 temperature is only a few degrees (about 37 o C vs. about 33 o C to 36 o C). Furthermore, diffusion of alcohol through the mucus is probably much slower than diffusion through the lung air. In an experiment to demonstrate how quickly the air-water-alcohol system reaches equilibrium, the bubble tube of a simulator was adjusted from the normal depth of 3 inches below the surface of the solution in ½ inch increments to a final depth of ¼ inch. The bubble diameter was kept at about ¼ inch by limiting airflow at about 0.1 liters/sec, a flow rate comparable to the lower range of flow rates encountered in police testing. It was found that the alcohol concentration decreased uniformly from a maximum of g/210l at 3 inches depth, to at ¼ inch. This indicates near equilibrium even at the briefest contact times between air inside the bubble and the bubble surface as it passes from the bubble tube to the surface of the solution. Even at the maximum depth, bubbles passed to the surface within less than 1 second, whereas it takes 5 to 10 seconds to deliver the breath sample from the lungs to the breath tester. Calculation shows that, for equivalent volumes, the surface area available for interaction is significantly greater for air in the lungs than air in the ¼ inch bubbles. The above considerations suggest the validity of the estimation of b2 via equation 2 above in the case of normal and hyperventilated breath samples, a procedure that does not require individual anatomical data for each subject. The breath alcohol concentration extrapolated to deep lung concentration, b2, would provide a less variable, more direct measure of alcohol load on the brain. The procedure can be performed easily using available and relatively inexpensive technology and without any additional requirement of the test operator. References 1. Schoknecht G, Stock B. The technical concept for evidential breath testing in Germany. In: Kloeder CN, McLean AJ, eds. Proceedings 13 th Int. Conf. Alc. Drugs & Traf. Safety, Adelaide Aus, RARU Adelaide Univ Lubkin SR, Gullberg RG, Logan BK, Maini PK, Murray JD. Simple vs sophisticated models of breath alcohol exhalation profiles. Alcohol & Alcoholism 1996;31: LeFranc J, Montamat M. Evidential Breath Analyzers: The influence of temperature and alcohol in the mouth for road controls. OIML Bulletin 1995; 36:28: Tsu ME, Babb AL, Ralph DD, Hlastala MP. Dynamics of heat, water, and soluble gas exchange in the human airways: A model study. Ann Biomed Eng 1988; 16: George SC, Babb AL, Hlastala MP. Dynamics of soluble gas exchange in the airways III. Single-exhalation breathing maneuver. J Appl Physiol 1993; 75: Harger RN, Forney RB. Estimation of the level of blood alcohol from analysis of breath. II. Use of rebreathed air. Quart J Stud on Alcohol 1956; 17: Flores AL. Rebreathed air as a reference for breath alcohol testers. National Technical Information Service, Springfield VA. DOT-TSC-NHTSA-74-4, Jones AW. Determination of liquid/air partition coefficients for dilute solutions of ethanol in water, whole blood, and plasma. J Anal Tox 1983; 7: Harger RN, Raney BB, Bridwell EG, Kitchel MF. The partition ratio of alcohol between air and water, urine, and blood. J Biol Chem 1950; 183: