James G. Wigmore, Forensic Toxicologist, Speaker, Author

In my 49th blog (posted November 3rd, 2013), the effect of acute tolerance to alcohol (first described by Sir Edward Mellanby in 1919) was discussed. Acute tolerance is the apparent decrease in physical impairment when the BAC is on the declining phase compared to the rising phase. Even though the BACs may be the same, the drinker feels more intoxicated when the BAC is rising than when it is declining. The degree of acute tolerance depends partly on the speed at which the BAC is increasing. When alcohol is consumed rapidly on an empty stomach and absorption is rapid there will be greater acute tolerance than when alcohol is consumed over hours with a meal and the rate of increase in BAC is slower.

Intoxication vs Impairment

Acute tolerance, although it affects the signs of intoxication, does not significantly affect the impairment of driving ability.  As my colleagues at the Centre of Forensic Sciences stated in a recent paper (Martin et al, 2013, WOA50103)

“While an intoxicated person will definitely be impaired in their ability to operate a motor vehicle, an absence of intoxication should not be taken as an absence of impairment.”

Acute Tolerance in Arrested Drinking Drivers

In one study in Germany, doctors assessed the degree of obvious intoxication and the time since drinking stopped for 2,815 arrested drinking drivers. The citation of this study is:

Gerchow, J., “Statistical and Experimental Studies on the Differing Assessment in Subjects during Rising and Falling BAC”, Heifte Unfallheik, 66: 90-95, 1961 (4 tables, 2 figures, 16 references), WOA50613

The following table shows the effect of the time since drinking ended and the percentage of drivers showing obvious marked drunkenness.

The effect of acute tolerance on actual drivers is seen with the decreasing percentage of obviously intoxicated drivers with increasing time since the end of drinking.  This effect complicates the enforcement and prosecution of drinking drivers.

For example, the police officer who just stopped a driver may see obvious intoxication, but and hour or so later at the police station the breath technician may not see any obvious intoxication.  It may appear that these two observations are in conflict and that the arresting officer may have “exaggerated” or “gilded” the signs of impairment, when in fact both observations could be correct.

Who Should the Judge Believe?

Another study in the Netherlands (which again shows the international nature of forensic alcohol toxicology) comes to the same conclusion:

Froentjes, W., “An Analysis of 10,000 Blood Tests in the Netherlands”, in Proceedings of the 3rd International Conference of Alcohol and Road Traffic, London, BMA House, 179-188, 1963 (4 tables, 6 figures, 8 references), WOA50614

This study concludes:

“It is therefore essential that the judge should be aware of this phenomenon since, in special cases, misunderstanding and contradictions in the statements of witnesses may be explained in this way.  It often happens that the police officer who arrests an intoxicated driver declares the latter to be obviously influenced, whereas another police official at a later point in time, for instance at the police station, may judge him to be slightly influenced.  Finally the physician who, at still later time performs the blood test may give the diagnosis of not influenced. However, as a result of the intervals between these three phases of observation, it may happen that all three observers are right.”

This shows the importance of videotaping drivers at the scene or the time of driving, as the physical observations of drunkenness may decrease dramatically at the time of the videotaping at the police station.

Acute Tolerance and Alcohol Poisoning

The most extreme type of alcohol tolerance occurs in alcohol poisoning cases, where recovery occurs at a much faster rate than the rate of elimination of alcohol.  In one case a 125 pound female alcoholic consumed 26 oz of bourbon (50%v/v alcohol) over 2 hours on an empty stomach at a local tavern (over-serving perhaps!).  She drove from the tavern and was involved in a motor vehicle collision several minutes later.  She was arrested and taken to jail where she rapidly lost consciousness.  As a result she was taken to hospital, where her admission BAC was 0.730 g/100mL and she was in a class 1 coma (i.e. responds to deep pain only).

She was treated in hospital for alcohol poisoning.  Three hours after admission, her BAC was 0.520 g/100 mL and she was conscious and able to respond to questions.  Eleven hours later when she was fully coherent and demonstrated no evidence of alcohol intoxication she was discharged.  Her BAC at that time was 0.190 g/100mL.  This case represents an extreme form of acute tolerance.  In addition, since she was an alcoholic, she would also have had substantial chronic tolerance to alcohol as well (Hammond et al, 1973, WOA506U2).

Webinar 4 (Scientific and Legal Issues of Impairment of Driving Ability due to Alcohol: “A Great Success!”

In the fourth of our series of webinars on the “Forensic Aspects in Civil and Criminal Cases”, broadcast on November 14th 2013, Justice Rick Libman and I discussed among other issues, the effect of chronic and acute tolerance on impairment of driving ability due to alcohol.  We would like to thank Sherry Colbourne, Andrew Bell, Tracey Gauley and Andrea Sesum and Gerri Camus  who contributed to the success of this webcast.  If you weren’t able to make the webinar, you can still access the recording here and receive CLE credits for an additional 180 days.