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

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.

Physiology Of The Lungs

The lungs are located within the chest. The 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 seventeenth 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. The alveolar region is where alcohol comes from the blood into the air in the lungs. But in order to be tested for alcohol, the breath must first pass from the alveoli along the branching network of airways to the mouth. During this journey through the airways, a great deal happens to the air changing the alcohol concentration.

The lung does not actively participate in the breathing process. The muscles which cause breathing are located outside the lung in the chest wall (intercostal muscles) and the abdomen (diaphragm). There are small muscles in the lung around the blood vessels and airways which assist in controlling the way in which blood flow and air flow are distributed to different alveolar groups (acini). To initiate inspiration, the external intercostal muscles are contracted. This pulls the ribs upward and outward increasing the chest size. In addition, the diaphragm pulls downward, also increasing the chest size. The change in chest size causes a decrease in pressure around the lung which causes the lung to expand and air to move into the lung. To initiate exhalation, the diaphragm and external intercostal muscles are relaxed. The previously stretched lung and chest wall then relax and shrink down increasing the pressure of the air in the alveoli causing air to flow out of the lung. If a rapid exhalation or an exhalation against a resistance, such as that caused by a breath testing instrument, is required, then the internal intercostal muscles can also be used to pull the ribs down assisting with the exhalation.

The number of molecules that leave the blood and enter the alveolar air is dependent on the blood alcohol concentration and the partition ratio (PR). In order to calculate the BAC from an alveolar sample, the alveolar alcohol concentration and the partition ratio each must be known precisely. However, it is impossible to sample air directly from the alveoli because of the small size of the airways. Therefore, all breath testers attempt to take a sample from the end of the breath for analysis under the assumption that the concentration of alcohol in the end-exhaled breath is the same as the concentration of alcohol in the air within the alveoli. In other words, it is assumed that nothing happens to the alveolar air sample as it is passing through the airways to the breath tester. However, changes do occur to the breath as it is exhaled which serve to alter the breath alcohol concentration.

Diffusion in the Lung

The process of diffusion governs the exchange of gas between blood and air in the alveolus. As blood enters the capillary in the alveolus, it is exposed to the alveolar air. If alcohol is present in the blood, some of the alcohol will diffuse out through the cells separating the blood from the air and increase the alcohol concentration in the alveolar air. The amount of the decrease of alcohol in the blood is extremely small and is governed by the relative amount of ventilation and blood flow to the alveolus.

The exchange of alcohol across the alveolar-capillary membrane is not limited by diffusion as has sometimes been stated in the alcohol breath testing literature. Blood is in the pulmonary capillary for a long enough time such that the alcohol in the alveolar air is in equilibrium with the alcohol in the blood after it has traversed only a small fraction of the alveolar capillary. This equilibrium is maintained until the end of the capillary. Under such circumstances, the distribution of alcohol is governed by the partition ratio for alcohol in blood at the temperature in the alveolus (normally 37° C). This is true for a very soluble gas like alcohol in a normal alveolus with nearly the same amount of blood flow and air flow.

Gases diffuse very easily within the air of the lung. So the concentration of alcohol is virtually identical throughout the alveolar acinus (containing many alveoli). This addresses another common misconception in the alcohol breath testing literature. The changing alcohol concentration during exhalation is not because the gas near the alveolar surface is in better equilibrium with the blood. Almost all of the gas in the lung (except that in the dead space) is in equilibrium with the blood before exhalation. The changing alcohol concentration during exhalation is caused by another mechanism related to the changing of temperature of exhaled air.