Supraventricular tachycardias are a frequent complaint in emergency departments. The modified Valsalva maneuver is a simple procedure for heart rhythm diagnosis and cardioversion. It does, however, require a good understanding of the cardiorespiratory physiology and patient's cooperation. The procedure carries few risks, but must be performed in a suitable environment. Particular care must be taken with patients suffering from heart disease. In the case of severe valvulopathy or advanced heart disease, abstention from the Valsalva maneuver may prevent serious adverse events.