II. EFFECTS A. Cardiovascular
Caffeine has mild and variable effects on the heart rate, the heart muscle, blood pressure, blood flow, and blood cholesterol.
1. Heart Rate
Caffeine generally increases the heart rate. After ingestion of caffeine a mild decrease in heart rate may be observed within the first hour followed by an increase in heart rate during the following two hours. Chronic caffeine use elevates the resting heart rate and significant reductions in heart rate are observed upon 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 rates may be precipitated. In these individuals cessation of caffeine intake is often beneficial. However, the most common arrhythmia encountered in medical practice, atrial fibrilla¬tion, is not adversely affected by caffeine intake (see the chapter Arrhythmias/Palpitations).