Physiological Laws Of Alcohol Breath Testing

by Michael P. Hlastala, Ph.D.
Division of Pulmonary and Critical Care Medicine
Box 356522
University of Washington
Seattle, WA 98195-6522

True Alcohol Partition Ratio

The scientific relationship governing the solution of gases in liquids is Henry's Law (Henry, 1803) which relates the equilibrium concentration of a dissolved gas in a liquid to its concentration in the air above the liquid. The proportionally constant between the liquid and air concentration in Henry's Law is the partition coefficient (partition ratio). Henry's Law is a general scientific principle which relates to dilute solutions of all ideal gases dissolved in liquids, not just to ethyl alcohol in blood. The partition ratio is different for various gases, various liquids, and is strongly temperature dependent. The partition ratio defines the distribution of molecules between two media, such as gas and liquid.

If the true partition ratio of the blood sample is different from the value assumed by the breath testing instrument, then an error will occur. Halving the partition ratio would cause a doubling of the number of molecules in the air. A breath testing instrument would measure twice as many alcohol molecules and would estimate a BAC which is twice the true value. The error in the breath testing instrument is directly related to the ratio of the actual partition ratio to the assumed value of 2100. Another way of thinking about the effect of changes in partition ratio is to consider the partition ratio as the number of molecules of alcohol in the blood compared to the number of molecules of alcohol in the breath. If the partition ratio is increased, then more molecules will stay in the blood, and fewer will come out in the breath. Thus, the breath test will be too low. If the partition ratio is decreased, then fewer molecules will stay in the blood, a few more will come out in the breath, and the breath test will be too high.

It is important to realize that the partition ratio term is incorrectly used in the breath testing literature. The partition ratio (or partition coefficient, as use din chemistry or physics) defines the distribution at equilibrium of a material (such as alcohol) between two media (such as blood and air). It is a physicochemical property of the substances involved at the interface between the two materials. If the alcohol concentration is altered in any way in either the air or the blood as they are being sampled or analyzed, then the term, partition, should not be applied. In fact, the true blood-air partition ratio is quite different when measured directly in an external blood sample than it is when measured from the human breath and a blood sample drawn at the same time (Jones, 1983). In the exhaled breath, the alcohol concentration clearly changes during exhalation (Adrian, 1981; Jones, 1982; Russell and Jones, 1983; Slemeyer, 1981). The alcohol concentration in the breath at the mouth is different from the alcohol concentration in the air within the lungs. Therefore, the term partition ratio does not apply to the relationship between breath and blood after the breath has left the alveolar spaces in the lung. Nevertheless, the partition ratio has been applied before to the measurement of breath and blood. In this chapter, the term blood-breath ratio (BBR) will represent the ratio of alcohol between the blood and a sample of breath. The term partition ratio (PR) is used to represent the equilibrium ratio of alcohol between a liquid and air.

Body Temperature

Variations in temperature have a profound influence on the partition ratio and the breath alcohol concentration. The partition ratio for alcohol in blood decreases by about 6 1/2% for each 1° C increase in temperature. This implies a 6 1/2% increase in breath alcohol concentration when the body temperature increases by 1° C (equal to 1.8° Fahrenheit).

The average body temperature for humans is 37°C (98.6°F) and varies as much as ± 1°C amongst different individuals. In addition, every person has a normal diurnal (daily) variation of 1°C. Females also have a temperature variation of about 1°C with the menstrual cycle. These temperature variations are normal. In addition, there are factors that can elevate body temperature above the normal range. Many diseases (such as influenza) can cause fever. Physical or emotional trauma can elevate body temperature.

In order to determine the blood alcohol concentration (in a blood "per se" state) or the breath alcohol concentration (in a breath "per se" state) and thus the level of intoxication or driving impairment, it is imperative that the body temperature is known. Breath testing procedures do not require measurement of body temperature. Therefore, any breath test is an inaccurate means of determining level of intoxication.

Blood Hematocrit

Variations in hematocrit (blood cell concentration) also have a significant influence on the partition ratio and the breath alcohol concentration. Blood is composed of plasma (mostly water) and cells (mostly red cells). The hematocrit is the fraction of blood that is cellular. The male hematocrit normally varies from 0.42 to 0.52 and averages 0.47. The female hematocrit normally varies from 0.37 to 0.47 and averages 0.42. Various types of diseases or other environmental influences can increase or decrease hematocrit beyond the normal range.

When alcohol dissolves in blood, it tends to go into the plasma more than the blood cells. The result is that individuals with a higher hematocrit have a lower partition ratio and a higher breath alcohol concentration.

In order to determine the blood alcohol concentration or the breath alcohol concentration and thus, the level of intoxication or driving impairment, it is imperative that hematocrit is known. Breath testing procedures do not require measurement of hematocrit. Therefore, any breath test is an inaccurate means of determining level of intoxication.

Development of the single breath test for alcohol (Harger et al, 1950; Borkenstein and Smith, 1961) took place in the early 1950's when the field of respiratory physiology was just beginning. At that time, it was generally understood that the first air exhaled from the lungs contained air from the airways and had little "alveolar air". It was thought that further exhalation would result in exhalation of air from the alveoli containing gas in equilibrium with pulmonary capillary blood. These concepts were held in the respiratory physiology community (Rahn et al, 1946; Fowler, 1948) and followed from data obtained with low solubility gases, such as nitrogen. Without the advantage of having present-day analytical equipment, the profile of exhaled alcohol could not be measured, but was expected to be identical to nitrogen (after a single breath of oxygen) and to appear as shown in Figure 1. The first part of the exhaled air was thought to come from the airways and was called the anatomic dead space and the later part of the exhaled air (with higher gas concentration) was thought to come from the alveolar regions. This later part of the exhaled gas profile was termed the alveolar plateau (Rahn et al, 1946; Fowler, 1948). With a presumed flat exhaled alcohol profile, it was thought that endexhaled alcohol concentration would be independent of exhaled volume after exhalation beyond anatomic dead space volume. It was further assumed that alveolar alcohol concentration was precisely related to the arterial blood alcohol concentration by virtue of the physical-chemical relationship known as the partition coefficient (Henry, 1803). The implicit assumption was that the alcohol concentration remained unchanged as alveolar air passed through the airways. Viewed through the limited perspective of respiratory physiology of the 1940's, the breath alcohol test seemed to be reasonable in principle and further development as a non-invasive measure of blood alcohol concentration was justifiable.

The alcohol breath test is a single exhalation maneuver. The subject is asked to inhale (preferably a full inhalation to total lung capacity, TLC) and then exhale (preferably a full exhalation to residual volume, RV) into the breath testing instrument. Very few restrictions (i.e., exhaled volume, exhaled flow rate, inhaled volume, pre-test breathing pattern, air temperature, etc.) are placed on the breathing maneuver. The constraints applied vary among the different breath testing instruments and among the operators administering the test, and the level of cooperation varies among subjects, resulting in substantial uncontrolled variation in the precise maneuver used for the breath test. In the last decade, there have also been several advances in understanding of the processes which govern pulmonary gas exchange of soluble gases. This new information provides a means for understanding errors associated with the alcohol breath test. This review presents a discussion of pulmonary alcohol exchange from the perspective of a respiratory physiologist using recent data relating to soluble gas exchange.

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