Heart Rate

How do changes in heart rate (HR) change cardiac performance?

Cardiac Output = HR x Stroke Volume

Chronotropy refers to changes in heart rate.

Increases in HR cause an increase in cardiac output. This becomes self limiting because when HR is excessive cardiac output falls due to: 

• reduced diastolic period which causes:
  • reduced time for ventricular filling (preload)
  • reduced time for coronary perfusion
• increased myocardial oxygen consumption per beat

As an example of the negative effect of sustained increases in heart rate, an experimental model of heart failure involves forced pacing of a normal heart at 240-250 bpm. Heart failure is produced in 3-4 weeks. Similarly, if heart rate is too low, signs of reduced cardiac output and heart failure can occur.