Caffeine and the Heart - technical
Caffeine is a psychoactive drug that occurs naturally inmany foods and beverages including coffee, tea, chocolate, and cocoa. It has found itsway into many commercial soft drinks because of its provenmild stimulant effect and relative safety when consumed in small amounts. But caffeine can produce beneficial effects in one individual and potentially harmful effects in another. Many pain relievers contain caffeine added to aspirin or acetaminophen to potentiate the effects of these agents. Levels of caffeine (mg/5 oz serving) in common beverages (Blount and Cox 1997) are as follows:
Coffee, drip 150, percolated 110, instant 50-100, decaffeinated <8
Black tea 1 min brew, <20, 3 min brew 20-40, instant tea 40 60
Cocoa beverages <20
Caffeinated cola drinks 12 oz 30-70
Mountain dew 40-50
Caffeine is the most widely used psychoactive substance. More than 100 billion doses of caffeine are consumed annually in North America. Caffeine occurs naturally in more than 60 plant species worldwide; the drinking of tea which contains small quantities of caffeine dates back to around 2500 BC in China.
Caffeine is a xanthinederivative 1, 3, 7 trimethyl xanthine. Other naturally occurring xanthines include the well known theophylline (the major constituent in tea) and theobromine. Coffee is a major source of caffeine and chlorogenic acid, and also contains a substantial amount of magnesium and other micronutrients.
Caffeine is a powerful central nervous stimulant, and moderate doses of 200 mg contained in two cups of coffee activate the cerebral cortex sufficiently to slow changes in an individual’s electroencephalogram (EEG). Caffeine is also a mild cardiac stimulant.
Caffeine has mild and variable effects on the heart rate, the heart muscle, blood pressure (BP), blood flow, and blood cholesterol.
Caffeine generally increases the heart rate. After ingestion of caffeine, a mild decrease in heart ratemay be observedwithin the first hour followed by an increase in heart rate during the following 2 h. Chronic caffeine use elevates the resting heart rate, and significant reductions in heart rate are observed upon the cessation of caffeine intake. Caffeine also stimulates the medullary vagal nerve nuclei and thus causes a mild decrease in heart rate depending on the dose and time of ingestion.
A genuine tachycardia, heart rate greater than 100 beats per minute, is not often observed. Arrhythmias that cause a sensation of palpitations may be precipitated, however, by two or three cups of coffee daily in susceptible individuals. Patients with paroxysmal atrial tachycardia (AV nodal reentrant tachycardia) and those with atrial or ventricular premature beats may note an increase in the frequency of these beats and pronounced increase in heart ratesmay be precipitated. In these individuals, the cessation of caffeine intake is often beneficial.
The most common arrhythmia encountered in medi cal practice, atrial fibrillation, is not adversely affected by caffeine intake.
Caffeine has a direct stimulant effect on the heart muscle and causes an increase in the force of myocardial contraction (i.e., an inotropic effect). The inotropic effects are, however, minimal compared to that of digoxin, the mild inotropic agent commonly used to treat heart failure. Because of the increase in the force of myocardial contractility and increase in heart rate, the cardiac output increases slightly. This effect is important in patients with heart failure in whom two cups of coffee daily may induce a minimal benefit without harm.