Regulation of Heart rate and myocardial performance

Cardiac output is the product of…

HR (heart rate) and SV (stroke volume)

The 3 major determinants of cardiac output

• Preload

• Afterload

• Contractility

Preload

• Stretch/tension on relaxed myocytes pre-contraction

• Determined by end-diastolic volume and end-diastolic pressure

Afterload

• Tension on myocytes during systole

• Resistance that myocardium must overcome to push blood out of the ventricle

• Muscle “feels” this load after it starts to contract

• Determined by resistance of ventricular outflow tract + aorta and systemic vascular resistance

• Approximated by mean arterial pressure

Contractility (inotropic state)

• Intrinsic ability of heart to generate the force required to eject blood, independent of preload and afterload

• Increased contractility (inotropy) = increased velocity and peak force of contraction (heart generates force faster and to a greater final degree)

The ______ system is the major way that the body regulates HR

Autonomic

Which tone dominates in a patient that is healthy and at rest?

Parasympathetic

Regulation of heart rate (reflex control)

• As MAP increases the stretch and firing rate of arterial baroreceptors increases.

• Signals are delivered to the vasomotor center which then decreases sympathetic flow and increases parasympathetic flow.

• Results in a decreased HR

Intrinsic properties of stroke volume

• Length-dependent activation (The Frank-Starling mechanism)

• Response to changes in preload and afterload; important for beat-to-beat regulation

Extrinsic mechanisms of stroke volume

• Effects of the autonomic nervous system and other hormones/substances

• Allow adaptation to sustained/substantial hemodynamic challenges (exercise)

Frank-Starling Mechanism

• The greater the stretch/tension on resting myocytes at end-diastole (greater the preload), the greater the force of contraction

• Increased contractility = Increased stroke volume = increased cardiac output

• When the amount of blood returning to the heart increases, this built-in mechanism allows it to adapt by matching output volume to input volume

• Also explains how the heart adjusts to acute changes in afterload

• Increases in afterload do not substantially reduce stroke volume and cardiac output, unless heart is diseased or afterload is severely increased.

Extrinsic myocardial regulation (sympathetic nervous system effects)

• If greater increases in cardiac output are needed, the heart needs help

• Sympathetic stimulation increases contractility = increases in stroke volume = increases in cardiac output = increase in preload