Pump Function of the Myocardium

Myocardium

• The myocardium must pump effectively to maintain adequate blood pressure and perfusion.

• The heart can increase its pumping ability significantly when the body needs more blood flow.

• Cardiac output (CO) = amount of blood pumped by the left ventricle in 1 minute.

• Normal resting cardiac output in an adult is about 5 L/min.

Cardiac Output Formula

Cardiac Output = Heart Rate × Stroke Volume

So cardiac output depends mainly on:

• Heart rate (HR)

• Stroke volume (SV)

Heart Rate

• Heart rate = number of heart contractions per minute.

• The heart has automaticity, meaning it can generate its own electrical impulses.

• The SA node is the heart’s primary pacemaker.

The autonomic nervous system affects heart rate:

• Sympathetic stimulation ↑ → heart rate ↑

• Parasympathetic stimulation ↑ → heart rate ↓

The sympathetic nervous system can also cause release of epinephrine and norepinephrine.

• Beta₁ stimulation increases:

  • Heart rate

  • Strength of contraction

  • Cardiac output

Stroke Volume

Stroke volume = amount of blood ejected from the left ventricle with each contraction.

Stroke volume depends on 3 main factors:

1. Preload

• Preload = amount/pressure of blood filling the ventricle at the end of diastole.

• Primarily influenced by venous return and blood volume.

• Generally:

  • ↑ Blood volume → ↑ preload → ↑ stroke volume → ↑ cardiac output

  • ↓ Blood volume → ↓ preload → ↓ stroke volume → ↓ cardiac output

Frank-Starling Law

• The more the ventricle fills, the more its muscle fibers stretch.

• Increased stretch normally produces a stronger contraction.

• However, if the ventricle becomes overstretched, contraction can become weaker.

2. Myocardial Contractility

• Contractility = strength of the heart muscle's contraction.

• ↑ Contractility → ↑ stroke volume → ↑ cardiac output

• ↓ Contractility → ↓ stroke volume → ↓ cardiac output

• Conditions such as heart failure or a large heart attack can reduce contractility.

3. Afterload

• Afterload = resistance the left ventricle must overcome to eject blood into the aorta.

• High blood pressure, especially high diastolic pressure, can increase afterload.

• ↑ Afterload → heart must work harder.

• Chronically high afterload can eventually weaken the left ventricle.

Important Relationships

• ↓ Heart rate → ↓ cardiac output

• Moderate ↑ heart rate → ↑ cardiac output

• ↓ Blood volume → ↓ preload → ↓ stroke volume → ↓ cardiac output

• ↑ Contractility → ↑ stroke volume → ↑ cardiac output

• Sympathetic stimulation → ↑ heart rate + contractility

• Parasympathetic stimulation → ↓ heart rate and cardiac output

• Beta blockers reduce beta₁ effects → ↓ heart rate and contractility

Extremely Fast Heart Rate

• A very fast heart rate can actually decrease cardiac output.

• In adults, rates around >160 bpm may leave too little time for the ventricles to fill.

• Less filling → ↓ preload → ↓ stroke volume → ↓ cardiac output.