A person's electrocardiogram (ECG) has no $\mathrm{P}$ wave, but has a normal QRS complex and a normal $\mathrm{T}$ wave. Therefore, his pacemaker is located in the

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[III A]. The absent $\mathrm{P}$ wave indicates that the atrium is not depolarizing and, therefore, the pacemaker cannot be in the sinoatrial (SA) node. Because the QRS and $\mathrm{T}$ waves are normal, depolarization and repolarization of the ventricle must be proceeding in the normal sequence. This situation can occur if the pacemaker is located in the atrioventricular (AV) node. If the pacemaker were located in the bundle of His or in the Purkinje system, the ventricles would activate in an abnormal sequence (depending on the exact location of the pacemaker) and the QRS wave would have an abnormal configuration. Ventricular muscle does not have pacemaker properties.

A Person's Electrocardiogram (ECG) Has No P\mathrm{P}P Wave, but Has a Normal QRS Complex and a Normal

A Person's Electrocardiogram (ECG) Has No $\mathrm{P}$ Wave, but Has a Normal QRS Complex and a Normal