Cardiac Output, Stroke volume, EDV, ESV, Ejection Fraction

Cardiac Output

• Definition: Amount of blood pumped by each ventricle in one minute.

• Formula:

  • Cardiac Output = Stroke Volume x Heart Rate

• Key Points:

  • Increase in stroke volume or heart rate results in increased cardiac output, and vice versa.

Stroke Volume

• Definition: Amount of blood pumped in one heartbeat.

• Typical Values:

  • End-Diastolic Volume (EDV): Approximately 100 ml (blood volume at the end of filling).

  • Ejection Fraction: Average 60% (only 60 ml is ejected during contraction).

  • End-Systolic Volume (ESV): 40 ml (amount remaining in ventricle after contraction).

• Calculation:

  • Stroke Volume = EDV - ESV

Factors Affecting Stroke Volume

1. Contractility

• Definition: Force of contraction of the heart muscle.

• Concept:

  • More forceful contractions lead to increased blood ejection.

2. Preload

• Definition: Degree of stretch of cardiac myocytes at the end of ventricular filling.

• Measured by:

  • End-Diastolic Volume (EDV) is used as a proxy for preload.

• Explanation:

  • Increased blood volume = more stretch (similar to inflating a balloon).

• Frank-Starling Mechanism:

  • Greater stretch = greater force of contraction.

3. Afterload

• Definition: Resistance the ventricle must overcome to eject blood.

• Components:

  • Vascular Pressure:

    • Left ventricle pressure must exceed systemic pressure for aortic valve to open.

    • Right ventricle pressure must exceed pulmonary pressure for pulmonary valve to open.

    • Hypertension raises vascular pressures, making valve opening difficult, resulting in less blood ejected.

  • Valve Damage:

    • Conditions such as stenosis increase resistance, lowering blood output.