Heart Rate

Effect of heart rate on myocardial performance

An increase in HR increases CO and CI.

However an increase in HR:

• increases MVO₂
• reduces the time for ventricular filling (preload)
• reduces the time for coronary perfusion

Factors affecting heart rate

Under normal physiologic conditions heart rate is under autonomic control. Sympathetic stimulation increases heart rate by increasing the rate of firing of the sinoatrial (SA) node. Parasympathetic stimulation decreases heart rate by decreasing the rate of firing of the SA node.

The autonomic stimulation is influenced by:

• Cephalic stimulation – excitement, boredom, sleeping, physical activity
• Respiration: Hering-Brewer reflex. HR increases with inspiration and decreases with expiration. This rhythm is termed normal sinus arrhythmia. During inspiration the reflex is stimulated to inhibit the vagal center resulting in a relative increase in sympathetic activity.
• Baroreceptor activity: Involved in the minute-to-minute control of blood pressure which involves altering the tone on the arterial tree and altering heart rate. Any increase in BP activates the baroreceptors which stimulates the vasomotor center with increased vagal stimulation decreasing HR, decreasing CO, and returning BP to normal. Baroreceptors (or high pressure receptors) are located in the carotid sinus and aortic arch. These receptors respond to stretch and not pressure (mechanoreceptors). Activation of these receptors sends inhibitory impulses to the vasomotor center in the medulla of the brain via the vagus and glossopharyngeal nerves. The vasomotor center sends sympathetic nerve traffic to the body. Activation of the baroreceptor reflex inhibits sympathetic outflow and increases vagal tone. Reduced activation of these receptors, as with hypotension, increases sympathetic outflow (and thus HR) and inhibits vagal tone.
• Other reflexes involved in HR control are:
  • Bainbridge Reflex: Increased left atrial pressure (due to increased volume) causes an increase in HR.
    • Mechanoreceptors located at the junction of the right atrium and caval veins or at the junctions of the pulmonary veins and the left atrium.
    • Volume expansion causes a tachycardia. Result is sometimes the opposite.
    • It serves as a counterbalance to the baroreceptor reflex.
    • Causes withdrawal of parasympathetic tone.
• A number of disorders can induce either excessive heart rates (tachyarrhythmias) or excessively slow heart rates (bradyarrhythmias)

How might an abnormality of heart rate manifest?

It may manifest as either:

An increase in HR – a tachyarrhythmia
A decrease in HR – a bradyarrhythmia