Lecture Notes: Stroke Volume and Cardiac Function

Definition of Stroke Volume

• Stroke Volume (SV): Volume of blood ejected from the ventricle during one contraction.
• Calculated as: End-Diastolic Volume (EDV) - End-Systolic Volume (ESV).

Pressure-Volume Loops and Wigger's Diagram

• End-Diastolic Volume (EDV): Volume of blood in the ventricle at the end of relaxation.
• End-Systolic Volume (ESV): Volume of blood left in the ventricle after contraction.
• EDV is depicted at the end of ventricular filling.
• ESV is depicted after blood is ejected.

Factors Affecting Stroke Volume

1. Sarcomere Length and Muscle Contraction

• Skeletal muscles operate typically around their optimal length for contraction.
• Cardiac muscle exhibits a wider range of tension due to varying sarcomere lengths.

2. Cardiac Function Curve

• Relation between EDV and SV: Known as the cardiac function curve.
• Increase in EDV leads to an increase in SV (Frank-Starling mechanism).
• Frank-Starling Mechanism: The heart's intrinsic ability to eject the same volume of blood returned to it.

3. Contractility and Sympathetic Stimulation

• Sympathetic Stimulation: Increases myocardial contractility via beta-1 receptors.
  • Results in more forceful and rapid contractions.
  • Known as a positive inotropic effect.
• Inotropic Effects vs. Chronotropic Effects:
  • Inotropic: Affect force of contraction.
  • Chronotropic: Affect heart rate.

4. Shifts in Cardiac Function Curve

• Sympathetic stimulation shifts the function curve upward, increasing SV at the same EDV.
• Reduction in sympathetic tone may shift the curve downward.

Measurement and Clinical Correlation

Ejection Fraction (EF)

• Definition: Ratio of SV to EDV, indicating contractility.
• Normal EF is around 55%.
• Calculation Example:
  • Normal: SV = 80 ml, EDV = 145 ml, EF = 55%.
  • With increased contractility: SV = 90 ml, EF = 62%.

Summary

• Stroke Volume: Blood ejected per contraction; modulated by:
  1. Sympathetic Stimulation: Positive inotropic effect, increased contractility.
  2. Frank-Starling Mechanism: Changes in EDV influence SV.