This is a CHEST X-Ray of a patient fitted with a PAcemaker...the pacemaker senses every P- wave and awaits for a QRS complex to be formed. If none are formed in a given time it will send an impulse to the ventricles to contract thus inducing a QRS complex.Eg.. in case of a heart block u find 2-3 P waves followed by 1 QRS complex.. So once a pacemaker is in for every P wave u have 1 QRS complex. This inturn increases the heart rate.... Cool huh.. its placed subcutaneously inder the skin over the Left chest wall