Assessment of Stroke Volume and Cardiac Output in Response to Varying Heart Rates

Permanent pacemakers have sometimes been used to treat patients with recurrent vasovagal syncope. This is because patients with vasovagal syncope often experience bradycardia at the time of their syncope. Unfortunately, pacemakers are often ineffective in preventing syncope.

Classical cardiovascular hemodynamics would suggest that increasing the heart rate (HR; via pacemaker) should increase the cardiac output (CO). This is because:

CO = HR x Stroke volume (SV).

The assumption is that the SV is fixed during manipulation of the HR. However, this might not be the case. As the number of beats per minute (HR) increases, the cardiac cycle length (R-R interval [RRI]) shortens, as a function of simple math. This means that the time in cardiac systole shortens, and the time in cardiac diastole shortens. Since cardiac diastole is when the heart fills up with blood, faster HR can be associated with decreased cardiac filling times. This, in turn, could compromise the SV. However, there is a paucity of data as to what happens to SV and CO at different HR. Further, it is possible that these effects will be different when a person is lying down supine versus when a patient is upright (when stroke volume will be lower).

The investigators will seek to determine the effects of different HR on both SV & CO. In order to safely manipulate the HR, they will study patients with permanent pacemakers in whom HR manipulation can be done in a safe and non-invasive manner.

Inclusion criteria

• Age 18-70 years (inclusive)
• Implanted permanent pacemaker or implanted defibrillator capable of atrial and ventricular pacing
• LV ejection fraction >50%

Exclusion criteria

• Unable or unwilling to provide informed consent
• Clinical need for a cardiac resynchronization device