Toxicology Rounds

Figure: breath alcohol, blood alcoholFigureA blood alcohol level can be helpful in focusing the differential diagnosis in certain emergency patients, especially those with signs of intoxication or altered mental status and head trauma. Institutions such as mental health facilities may sometimes request an alcohol level before accepting a patient, even without clear signs of intoxication. A breath alcohol level is often obtained using a portable device such as a breathalyzer in these cases. A common misconception, however, is to assume that the actual number detected by a breath alcohol measurement is the same as the blood alcohol level. This is not true, as revealed by the following medicolegal case. John Doe was drinking at a bar for about three hours on a weekday afternoon. Video surveillance and a credit card receipt indicated that he consumed three beers during that time. He later admitted in his deposition that he drank several beers at home before arriving at the bar, but he could not remember how many. Mr. Doe left the bar, and an hour later was involved in a two-vehicle collision in which the other driver sustained a severe head injury that left him with persistent neurological deficits. State police administered a breath alcohol test to Mr. Doe approximately an hour after the collision, which reported a level of 0.080 g/210 L in duplicate samples. The injured man sued the bar under dram shop laws, claiming that the staff knew or should have known that Mr. Doe was intoxicated and should not have been served alcohol. Expert Opinions The plaintiff's expert noted that Mr. Doe's breath alcohol concentration was 0.080 g/210 L two hours after he left the bar. Using retrograde extrapolation, the expert calculated that his blood alcohol concentration at the time of the collision was 0.098 gm/dL. He estimated that this level represented consumption of at least five beers in a man of Mr. Doe's weight. He also said Mr. Doe would have exhibited characteristic signs of impairment associated with intoxication at that level, and these should have been recognized by the server and he should not have been served alcohol at the bar. The defendant's expert stated in rebuttal that the plaintiff's expert had erroneously assumed that the breath alcohol reading was equivalent to the blood alcohol concentration at the time the test was performed. He cited supporting medical literature that found breath alcohol measurements used to calculate the amount of alcohol present in whole blood often differed substantially from those obtained by direct measurement. (Br Med J. 1976;2[6050]:1479; http://bit.ly/3ucFPCg.) The plaintiff's expert responded that his report clearly defined BAC as breath alcohol concentration, and he argued that his opinion reflected well accepted principles in toxicology that breath alcohol and blood alcohol are correlated. Confusing Acronyms The plaintiff's expert said in the beginning of his report that he was using BAC to mean breath alcohol concentration, but stated later in that report that Mr. Doe's BAC at the time of the collision was 0.098 gm/dL. That makes no sense if BAC referred to breath alcohol concentration. Breath alcohol measures grams of ethanol in 210 liters of breath. Blood alcohol measures grams of ethanol in 100 cc of whole blood. By manipulating the units, the plaintiff's expert made the implicit and unwarranted assumption that these are equivalent, and that the breath alcohol measurement produces the same number as the concomitant blood alcohol concentration. A small amount of ingested alcohol is eliminated in the breath. Ethanol in arterial, not venous, blood rapidly diffuses across the alveolar membrane and equilibrates with air in the alveoli. But this is not the sample that is measured. By the time alveolar air is expelled through the mouth, it has interacted with alcohol in upper airway mucous and undergone temperature changes that affect the final reading. Breath alcohol analyzers used in the United States and Canada assume a partition ratio of 2100:1 to reflect the relationship between the concentration of ethanol in blood and breath. But this is a moving target, and can change with body temperature, ambient temperature, whether ethanol kinetics are in the stage of absorption, distribution, or elimination as well as many other factors. The actual partition ratio can vary between 1400:1 and 3500:1 in practice. Because of this, the alcohol level measured by the breath analyzer can differ from the actual blood alcohol level by as much as 50 percent either way. The plaintiff's expert said breath alcohol and blood alcohol are correlated, which is true but somewhat misleading. Correlation in statistics measures the strength of the linear relationship between two variables. Weight, for example, tends to be positively correlated with height: Weight generally increases as height increases. But this does not mean that the numbers are the same: A person 72 inches tall rarely weighs 72 pounds. As a person's blood alcohol concentration increases, the breath alcohol level also tends to increase, but this does not mean that the actual numbers are the same. Forensic toxicologist A.W. Jones noted that the relationship between blood and breath alcohol levels has a “considerable scatter of individual data points around the regression line as indicated by the residual standard deviation. This speaks against trying to make a quantitative evaluation of BAC [blood alcohol concentration] indirectly by analysis of alcohol in the breath, and this practice is not recommended.” (Garriott's Medicolegal Aspects of Alcohol, 5th edition, 2008.) Many jurisdictions have avoided this problem by redefining the standard for presumed intoxication to include a quantitative level of breath alcohol without having to assume that this indicates a specific blood alcohol level. The case against the bar was settled for an undisclosed sum. The important point for emergency physicians who use breath alcohol concentrations is that these are not equivalent to blood alcohol concentrations. Breath alcohol measurements tend to underestimate the actual blood alcohol concentration, and considerable variability can be seen. Measuring breath alcohol can provide a rough estimate of whether significant amounts of ethanol are on board or if the alcohol level is rising or falling, but don't make the common mistake of assuming that the blood level and the breath level are the same. Dr. Gussowis a voluntary attending physician at the John H. Stroger Hospital of Cook County in Chicago, an assistant professor of emergency medicine at Rush Medical College, a consultant to the Illinois Poison Center, and a lecturer in emergency medicine at the University of Illinois Medical Center in Chicago. Follow him on Twitter@poisonreview, and read his past columns athttp://bit.ly/EMN-ToxRounds.

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