Well, caffeine increases the resting heart rate. If the resting heart rate of a person with tachycardia is 110 bpm, it may increase to 125 bpm after consuming 200 mg caffeine. My questions: Is having a constantly high resting heart rate (because of caffeine) dangerous?
A regular caffeine consumer will likely develop a partial or complete tolerance to many caffeine effects and side effects, including increased heart rate. Tolerance develops within few days of regular caffeine consumption and disappears within few days of caffeine discontinuation.
Acute caffeine in subjects who do not normally ingest methylxanthines leads to increases in blood pressure, heart rate, plasma epinephrine, plasma norepinephrine, plasma renin activity, and urinary catecholamines. Using a double-blind design, the effects of chronic caffeine administration on these same variables were assessed. Near complete tolerance, in terms of both humoral and hemodynamic variables, developed over the first 1-4 days of caffeine. No longterm effects of caffeine on blood pressure, heart rate, plasma renin activity, plasma catecholamines, or urinary catecholamines could be demonstrated.
A single caffeine dose may increase blood pressure but not necessary heart rate in individuals with supraventricular tachicardia.
Caffeine, at moderate intake (5 mg/kg), was associated with significant increases in systolic and diastolic blood pressures, but had no evidence of a significant effect on cardiac conduction and refractoriness. Furthermore, no effect of caffeine on SVT induction or more rapid rates of induced tachycardias was found.
the resting heart rate was not significantly different between both groups [caffeine vs placebo].
Overall, the data suggest that in most patients, even those with known or suspected arrhythmia, caffeine in moderate doses is well tolerated and there is therefore no reason to restrict ingestion of caffeine. Care should be taken to avoid caffeine in situations in which catecholamines are thought to drive the arrhythmia, as well as in patients who note sensitivity to caffeine.
Now, I personally cannot claim if caffeine consumption is or is not dangerous for individuals with tachycardia, but most doctors, for example, from American Heart Association) still advise them to cut it down.