AAI and VVI pacemakers are single-chamber pacemakers. Typically, the pacing electrode for an AAI is placed in the right atrium and in a pacing lead in the right ventricle for a VVI pacemaker. These rhythm strips show how each pacemaker works. AAI pacemaker Note how the AAI pacemaker senses and paces the atria only. Each spike is immediately followed by a P-wave complex, depicting consistent capture. The QRS complex that follows occurs as a result of the heart’s own AV conduction. 
VVI pacemaker The VVI pacemaker senses and paces the ventricles. Each spike is immediately followed by a wide QRS complex, depicting consistent capture. When all QRS complexes are preceded by a spike, as shown here, the rhythm is said to reflect 100% pacing. 
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