Bradycardia causes hypertension because the slower heart rate allows excess time for ventricular filling, which results in greater ventricular stretch and consequent increased force of contraction, as identified by Starling’s Law. By reviewing Starling’s Law, which identifies that “the greater the ventricles are stretched during diastole (rellaxed phase) the greater the force of contraction during the systolic phase” it can be seen that so long as bradycardia results in a greater filling of the ventricles, the contraction phase should be more forceful. This results in the patient having a higher than normal systolic blood pressure and often an unusually low diastolic blood pressure.
In general, patients with a signifcant bradycardia (less than 40 beats of the heart per minute), will usually have a very high systolic blood pressure as high as 220-240mmHg (the pressure during cardiac contraction), but a very low diastolic blood pressure as low as 40mmHg.
Bradycardia and Cardiac Output
Cardiac Output can be calculated by multiplying the heart rate (HR) by the stroke volume (SV). In bradycardia, the heart rate is reduced, but the stroke volume is increased. In a minor bradycardia, this can often result in hypertension (high blood pressure). However, as the heart rate is reduced greatly, such as 30-40 beats per minute, the stroke volume is higher than normal, but the heart rate is so slow that the person’s total cardiac output is much lower than normal or not effective at all.
Can Hypertension Cause Bradycardia?
Yes, hypertension causes the increased systolic blood pressure to stretch the baro receptors in a persons neck, which then stimulates a negative feedback mechanism that causes the brain to the SA node in the heart to reduce heart rate and force of contraction.