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Myriad medical texts describe the medical consequences of acute and chronic ethanol use. The purpose of this discussion is to describe the history and use of assessments of ethanol induced impairment, primarily in a legal context. Some of the principles described may, however, be extrapolated from the legal to the medical setting. Throughout this chapter, the terms ethanol and alcohol are used interchangeably to retain the forensic, legislative, and medical contexts in which the terms are utilized.


Numerous well designed laboratory studies assessing alcohol induced impairment have been published. However, application of study results to actual cases of possible driving impairment or over service of alcohol in a drinking establishment may be inaccurate without an understanding of the origin of laws governing these activities. Although the intoxicating effects of ethanol have been known for centuries, the advent of mechanized transportation spawned increased public and legal scrutiny. Concern was expressed over the potential adverse safety implications of driving while intoxicated, not just for the impaired driver, but also for other persons (passengers, other drivers, pedestrians) and property. Although railroads had regulations against the operation of equipment while intoxicated dating back to the 19th century, the first arrest for drunk driving in an automobile was that of a London taxicab driver in 1897.20 The realization that alcohol intoxicated motor vehicle operators were a public health issue worthy of legal scrutiny soon followed, and legislation was enacted to combat “drunken driving,” with New York being the first US state to enact such a law in 1910.20

In early laws, “drunken driving” was poorly defined and relied heavily on observable signs of gross intoxication. Attempts at legislative clarification of language resulted in terms that were nearly as nebulous, such as “alcohol impaired” and “under the influence.” Prior to the landmark works in Sweden by Widmark and in the United States by Heise, evaluations of driver impairment were predominantly based on the “expert” testimony of an evaluating physician and the arresting police officer, as well as behaviors reported by witnesses. These evaluations were observational rather than scientifically based, and such nonsystematic and nonobjective testimony was often dubious and frequently plagued by exaggeration of behaviors (eg, staggering gait, incoherent speech).

The development of analytical technology capable of measuring the concentration of alcohol in blood gave rise to the idea that diagnosis of intoxication might be assisted by an objective chemical test result. The theory behind this idea being simply, the higher the blood alcohol concentration (BAC), the more drinks the individual must have consumed and therefore, the greater the degree of impairment. It was, therefore, reasoned that there may also exist a legally punishable limit of alcohol in blood or other biological matrix. Although the assignment of a specific clinical effect to a given BAC is not rigorously applicable to evaluation of an individual case, the general trends when ...

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