<prompt> Overall action of caffeine on hea rate is ?IncreasesDecreasesNo effectFirst increases then decreases  <response>: Ans. isb i.e., Decreases Ref: KDT Vhe P. 4881CaffeineCaffeine out of the three naturally occurring methylxanthines, only caffeine is used as a CNS stimulant.Methylxanthines directly stimulate the hea and increase force of myocardial contractions.They tend to increase hea rate by cardiac action, but decrease it by causing vagal stimulation- net effect is variable.Tachycardia is more common with theophylline, but caffeine generally lowers hea rate. 