Effects of Heart Rate on Cardiac Output

Heart rate affects CO in the following ways:

• An increase in heart rate will lead to a progressive increase in CO up to about 140 bpm. When heart rate increases, SV decreases due to shorter diastolic filling time, but this only becomes significant at higher rates > 140 bpm

• When HR > 150 bpm the decrease in diastolic filling time reduces the CO significantly because it encroaches on rapid filling time. Diastolic time is affected by a tachycardia to a much greater extent than systolic time

• The Bowditch phenomenon ("Treppe' or staircase effect) is increased inotropy in response to increased chronotropy. The increased contractility that occurs at higher heart rates, is due to the greater availability of intracellular calcium for excitation-contraction coupling. This follows the reduced diastolic time available for calcium re-uptake (Figure HE.32)

• During a tachycardia in a normal heart, EDPVR will move to the left and downwards, i.e. smaller filling volumes at lower EDP. In an ischaemic heart, however, a tachycardia will move EDPVR to the left, but upwards, i.e. smaller filling volumes at higher EDP

• Tachycardia will increase the ventricular systolic elastance (Ees) due to the increased contractility

• A pronounced bradycardia < 40 bpm will cause a fall in CO, because the compensatory increase in SV is not enough to make up for the decrease in ejection rate

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Figure HE.32 Treppe or Bowditch effect of heart rate on cardiac output

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