Breath alcohol concentration determined with a new analyzer using free exhalation predicts almost precisely the arterial blood alcohol concentration

Abstract

A new breath alcohol (ethanol) analyzer has been developed, which allows free exhalation, standardizes measured exhaled alcohol concentration to fully saturated water vapor at a body temperature of 37 °C (43.95 mg/L) and includes a built-in self-calibration system. We evaluated the performance of this instrument by comparing standardized alcohol concentration in freely expired breath (BrAC) with arterial (ABAC) and venous (VBAC) blood alcohol concentrations in fifteen healthy volunteers who drank 0.6 g of alcohol per kg body weight. The precision (coefficient of variation, CV) of the analyzer based on in vivo duplicate measurements in all phases of the alcohol metabolism was 1.7%. The ABAC/BrAC ratio was 2251 ± 46 (mean ± S.D.) in the post-absorptive phase and the mean bias between ABAC and BrAC × 2251 was 0.0035 g/L with 95% limits of agreement of 0.033 and −0.026. The ABAC and BrAC × 2251 were highly correlated (r = 0.998, p < 0.001) and the regression relationship was ABAC = 0.00045 + 1.0069 × (BrAC × 2251) indicating excellent agreement and no fixed or proportional bias. In the absorption phase, ABAC exceeded BrAC × 2251 by at most 0.04 ± 0.03 g/L when tests were made at 10 min post-dosing (p < 0.05).

The VBAC/BrAC ratio never stabilized and varied continuously between 1834 and 3259. There was a proportional bias between VBAC and BrAC × 2251 (ABAC) in the post-absorptive phase (p < 0.001). The pharmacokinetic analysis of the elimination rates of alcohol and times to zero BAC confirmed that BrAC × 2251 and ABAC agreed very well with each other, but not with VBAC (p < 0.001).

We conclude that this new breath analyzer using free exhalation has a high precision for in vivo testing. The BrAC reflects very accurately ABAC in the post-absorption phase and substantially well in the absorption phase and thereby reflects the concentration of alcohol reaching the brain. Our findings highlight the magnitude of arterio-venous differences in alcohol concentration and support the use of breath alcohol analyzers as a stand-alone test for medical and legal purposes.

Introduction

Breath alcohol (ethanol) measurements are widely used in law enforcement as evidence for prosecution of drunk drivers. Breath-testing eliminates the need for taking blood samples, can be used at the roadside, and the result is immediately available making it possible for police officers to charge the suspect on the scene without delay.

Breath alcohol concentration (BrAC) is often converted to the corresponding blood alcohol concentration (BAC) by multiplying by a BAC/BrAC ratio. Many breath alcohol analyzers assume a constant BAC/BrAC ratio of 2100 for this conversion. Owing to both inter- and intra-individual variations in the BAC/BrAC ratio this conversion has been challenged [1]. In many countries, the breath alcohol concentration is already admissible as evidence in court without reference to the blood-alcohol concentration [2]. The reason for the wide variability in the BAC/BrAC ratio has been a topic of several scientific articles [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13].

The physiological basis of the breath alcohol test depends on the equilibration of alcohol within hundredth of a second between the pulmonary capillary blood and the alveolar air [14]. Since the alcohol (ethanol) molecule moves by simple diffusion, it is the kinetic energy of the alcohol molecules that makes alcohol to diffuse into the air of the airways and into different body tissues. At equilibrium the average kinetic energy of the alcohol molecules is the same in arterial blood, tissues and alveolar air and depends on body temperature. However, average velocities of alcohol molecules and alcohol concentrations vary between compartments and depend on solubility. Since pulmonary capillary blood flows directly from the lung into left atrium the kinetic energy of alcohol can be assumed to be similar to that in arterial blood, which also perfuses the airways by way of the bronchial circulation. The benefit of using the breath alcohol test relies on the assumption that alveolar alcohol concentration can be measured or estimated in expired air. Since the partial pressure of alcohol in the alveoli is believed to reflect the average kinetic energy in arterial blood, it should also in turn reflect the brain alcohol concentration at equilibrium. This equilibration of alcohol in arterial blood with the brain tissue is likely to occur rapidly, since the brain receives a large cardiac output and has a low distribution volume owing to its high fat content [2], [15], [16], [17].

This paper concerns a new breath alcohol technology, which has been developed to improve the usefulness and performance of the breath alcohol test. It is designed to operate as a stationary unit in-door or as a mobile unit. The alcohol concentration in expired air is standardized to the water vapor concentration known to occur in the alveoli at normal body temperature [18]. This technique makes it possible to use free exhalation, making the measurements almost independent of the flow rate and the standardization reduces the temperature dependence to a minimum. Simultaneous measurement of carbon dioxide in breath is included as a way to control that alveolar air is exhaled so that the measurements process can begin.

The aim of the present study was to evaluate the performance of this new technique by the ultimate test comparing the BrAC after free exhalation with the ABAC in healthy volunteers. Measurements of venous BAC (VBAC) were included in the comparison, since presently this serves as a standard in forensic practice for the prosecution of drunk drivers and as a measure of their impairment and drunkenness.