Caffeine and Diving

By Dr. Sawatzky

Caffeine is so ubiquitous in our culture that we often forget it is a drug that can have many negative side effects, especially in diving.

Caffeine is one of a group of compounds called methylxanthines. The common methylxanthines are theophylline (1,3-dimethlyxanthine), theobromine (3,7-dimethylxantine, and caffeine (1,3,7-trimethylxanthine). Methylxanthines naturally occur in coffee beans (caffeine), tea leaves (theophylline), chocolate, cocoa beans (theobromine), and cola nuts. In addition, they are often added to carbonated beverages (cola drinks) and many medications. In fact, they are so wide spread it is virtually impossible to avoid them and quite difficult to determine how much we consume each day. The word "caffeine" will be used to refer to all of the methylxanthines in this column.

The concentration of caffeine in beverages is highly variable but you can get some idea from the values in Table 1. In addition, people often consume significant amounts of caffeine in various medications. Table 2 lists the caffeine content of a selection of Canadian medications, many of which you might not expect to contain caffeine. Finally, some people take straight caffeine powder or pills to help them stay awake. Caffeine powder is sold as 200 mg packets in the USA but is not available in Canada. Caffeine tablets are sold in 75 mg, 100 mg, 150 mg and 200 mg strengths in the USA but only in 100 mg strength in Canada (Wake-Up).

Caffeine is rapidly and almost completely absorbed from the stomach and distributes to all parts of the body. The peak concentration is achieved 50-75 minutes after oral consumption (up to three hours in some individuals) and the half-time of elimination is three to seven hours. This means that after ingesting caffeine, the maximum effect will be felt approximately one hour later and every 3 to 7 hours, one half of the remaining drug will be eliminated from the body. There is a very large variation in the sensitivity of individuals to caffeine and people who consume moderate to large amounts of caffeine on a regular basis become quite resistant to its effects.

The primary effects of caffeine are stimulation of the central nervous system, augmentation of the analgesic effect of pain killers and stimulation of the respiratory system (breathing). Stimulation of the CNS is the primary reason people consume caffeine. Its ability to make pain killers more effective is the reason it is often added to analgesics (see Table 2). The respiratory stimulant effect of caffeine seems to be via enhancing the sensitivity of the medullary respiratory centre (in the brain) to carbon dioxide.

Caffeine has many other effects. It increases the heart rate, the force of cardiac contractions and cardiac output. In some individuals, it causes cardiac arrhythmias. Caffeine inhibits uterine contractions (useful for treating menstrual cramps). Caffeine reduces the flow of blood to the brain (useful for treating migraines) and increases blood pressure via increased systemic vascular resistance.

For information on how caffeine affects divers, pick up a copy of the February issue of DIVER Magazine.