This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Overall action of caffeine on hea rate is ? 
Select and explain. 
A. Increases, B. Decreases, C. No effect, D. First increases then decreases
B: Decreases
Answer: B: Decreases 
Explanation: Ans. is'b' i.e., Decreases (Ref: KDT Vh/e P. 4881)CaffeineCaffeine 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.