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?
If it is just from the sinus node, I don't think it would be. However tachycardia from atrial arrhythmia is dangerous.– CatersFeb 11, 2018 at 23:44
@Caters Describing atrial tachycardias as dangerous is overly broad and misleading. Atrial fibrillation and flutter create an increased risk of stroke, which can be prevented with anticoagulants, but other than that atrial tachycardias are not dangerous. Ventricular tachycardias, on the other hand, are. Perhaps you're mixing them up?– Carey Gregory ♦Feb 12, 2018 at 15:54
It would depend entirely on the cause of the tachycardia so the question is too broad as stated. For example, there is some evidence that caffeine actually has beneficial qualities for patients with atrial fibrillation, contrary to longstanding common wisdom and medical practice.– Carey Gregory ♦Feb 12, 2018 at 15:59
@CareyGregory Seems like that could make a good answer, no? I agree the question is a bit broad but we get so many poor questions here that I think we can be a little less picky. "It depends on the source of tachycardia, for example for X __________ while for Y ___________" is a fairly parsimonious answer.– Bryan Krause ♦Feb 12, 2018 at 16:50
Atrial (sinus) tachycardia is not danger-free. Elevated rates (especially >140) decreases diastolic filling time, thus reduces cardiac output and can lead to hypotension, ischemia, etc depending on the situation. Stimulants including caffeine can precipitate arrythmias, depending on dose and the individual's physiology/pathophysiology. Problems in answering this question include: we don't know the underlying cause of resting HR of 110, whether there is any conductivity abnormality, the individual's dose response to the stimulant, history of ischemia or sensitivity to hypotension...– DoctorWhomFeb 13, 2018 at 0:50
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.
Tolerance to the Humoral and Hemodynamic Effects of Caffeine in Man (PubMed Central, 1981)
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.
A prospective placebo controlled randomized study of caffeine in patients with supraventricular tachycardia undergoing electrophysiologic testing (Wiley Online Library, 2015)
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].
Caffeine and Cardiac Arrhythmias: A Review of the Evidence (The American Journal of Medicine, 2011)
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.