If you have a heart arrhythmia you should not take Ritalin. It has the side effect of causing or worsening heart arrhythmias. This is a definite side effect of Ritalin. It also causes tachycardia and hypertension.
My guess is that you should not be on this medication. However, if your doctor prescribed it maybe s/he thinks it is okay? It should definitely not be taken without a prescription.