Why Breath Tests Don't Work
by Michael P. Hlastala, Ph.D.
Division of Pulmonary and Critical Care Medicine
Box 356522
University of Washington
Seattle, WA 98195-6522
Over the years, breath testing has become a widely used method for quantitative determination of the level of intoxication of individuals suspected of driving while under the influence of alcohol. After recognition of the need for quantitative assessment of intoxication, blood alcohol concentration was considered as the single most important variable. However concern about the invasiveness requirements of drawing a blood sample led to the development of the breath test as a non-invasive means of assessing level of intoxication. The breath test is an indirect test, but has been considered to be a good estimate of the blood alcohol concentration because of the assumption that an end-exhaled breath sample accurately reflects the alveolar (or deep-lung) air which is in equilibrium with the blood. In spite of the great deal of effort that has gone into the studies attempting to validate the breath test, forensic scientists and toxicologists still have a very rudimentary understanding of the breath alcohol test and its limitations.
Anatomy of the Lungs
The lungs are located within the chest. This organ allows inspired air to come into close proximity with the blood so gases (such as oxygen and carbon dioxide) can exchange between the air and the blood. The lung is made up of over 300 million small air sacs called alveoli. Outside air comes to the alveoli from the mouth or nose via the airways. The major airway leading to the lungs from the throat is the trachea. The trachea divides into the left and right "mainstem bronchi" (going to the left and right lungs) which divide further into the "lobar bronchi". This division goes on about 23 times until the alveoli are reached. Actually, some alveoli begin to appear at about the 17th generation airways. Surrounding each alveolus are small blood vessels. The thinness (less than 0.001 millimeter) of the membrane separating blood from the air in the lungs allows oxygen and carbon dioxide to exchange readily between the blood and air. Because of the large number of very small alveoli, there is a very large surface area (70 square meters) for this gas exchange process.
Anomalies
Since 1950, many studies have been performed to quantify the relationship between breath alcohol concentration (BrAC) and blood alcohol concentration (BAC) with the goal of defining a precise relationship between the two for accurate non-invasive determination of BAC. These studies, undertaken to validate the use of breath tests by comparing BrAC and BAC in normal subjects, have shown a surprising amount of variability which has not been improved in spite of advances in instrument technology. The physiology of the lungs and of the body as a whole remains as the primary explanation for this variability.
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.
The Partition Ratio: The False Foundation
The evolution of scientific understanding depends on the continuous development of new ideas that form the bases for experimentation. This concept has been termed "scientific revolution" by Kuhn, who sees science as the shift from one paradigm to another. The term, "paradigm" refers to a set of universally recognized scientific achievements that for a time provide a model or conceptual framework for a phenomenon. This paradigm represents the core principles that define the scientific understanding.
A paradigm is established after a number of initial observations are obtained. Experiments are then carried out to test hypotheses related to the paradigm. Usually, these experiments provide data that reinforce the paradigm. Occasionally, these experiments result in anomalies, or results that do not fit within the framework of the original paradigm, and are inconsistent with the predictions of the paradigm.
The accumulation of anomalies leads the scientist to develop a new paradigm which provides a new framework for interpreting experimental results which accounts for the anomalies of the old paradigm as well as new observations. At that point, the new paradigm undergoes scrutiny through newly suggested experiments that provide data to reinforce the new paradigm. The new paradigm must account for the new observations as well as the prior observations. The transition from the old paradigm anomalies to the new paradigm always encounters enormous resistance to change. This resistance is crucial for this scientific progress to occur.
Eventually, it is likely that another set of anomalies with the new paradigm will lead to yet a third paradigm. This will occur as new technologies reveal new anomalies. Kuhn, a physicist turned philosopher, cites a number of paradigms that have evolved in his field in the form of scientific revolutions: Copernican astronomy, Newtonian physics, the wave theory of light, and quantum physics. These same ideas apply to different fields in very different scales. The concept of the paradigm can also be applied to the Alcohol Breath Test.
The Old Paradigm
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Figure 3. Assumed exhaled alcohol profile in the 1950's. |
Development of the single breath test for alcohol 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 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 3. 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. With a presumed flat exhaled alcohol profile, it was thought that end-exhaled 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. 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.