Introduction to Cardiac Output

Introduction to Cardiac Output

  • Focus on stroke volume and factors influencing it.

  • Components to understand:

    • Preload

    • Afterload

    • Frank Starling Law

    • Definition of cardiac output:

    • Cardiac output = Heart rate × Stroke volume

Overview of Factors Regulating Cardiac Output

  • Cardiac output regulation through two main pathways:

    • Intrinsic control: Specific to the heart itself.

    • Extrinsic control: Involves external mechanisms affecting the heart.

Intrinsic Control

  • Includes:

    • Total peripheral resistance (Afterload)

    • Defined as the resistance against systolic contraction, which correlates to aortic or arterial pressure.

    • End diastolic volume (Preload)

    • Defined as the volume of blood in the ventricle before contraction.

  • Stroke volume is impacted by both preload and afterload.

Definition of Stroke Volume

  • Defined as the volume of blood ejected by the ventricle during contraction (per stroke).

  • Determined by three main factors:

    • Preload: Degree of myocardial stretch before contraction.

    • Afterload: The resistance against the contraction.

    • Contractility: The strength of contraction at a given end diastolic volume.

The Pressure-Volume Loop

  • Visualization of the relationship between pressure and volume during the cardiac cycle.

  • Axes described as:

    • Y-axis: Pressure

    • X-axis: Volume

  • Four significant points identified within the loop:

    1. Point D to A (Diastole/Ventricular Filling):

    • Transition from Point D to A indicates the filling of the ventricle (preload).

    1. Point A to B (Isovolumetric Contraction):

    • Pressure rises sharply with no change in volume (volumes remain constant).

    • Aortic and mitral valves are closed during this phase.

    1. Point B to C (Ejection Phase/Systole):

    • Blood is ejected as you move from Point B to C.

    • The aortic valve opens, allowing blood to leave the heart.

    1. Point C to D (Isovolumetric Relaxation):

    • Both valves are closed again as the heart relaxes.

Role of Preload and Afterload in Stroke Volume

  • Increasing preload:

    • Results in increased end diastolic volume.

    • Greater myocardial stretch leads to a larger stroke volume (more blood ejected).

    • All other conditions being equal, such as heart rate and contractile timing.

  • Increasing afterload:

    • Elevates pressure against the ventricle.

    • Slower isovolumetric contraction phase, resulting in a reduced ejection time.

    • Causes a decrease in stroke volume, as less blood can be ejected during contraction.

Venous Return and its Effect on Stroke Volume

  • Venous return: Blood that returns to the heart from the peripheries before being ejected.

  • Increase in venous return leads to:

    • Higher preload, resulting in the ability to eject more blood (increased stroke volume and cardiac output).

  • Graphical representation of:

    • Cardiac output curve showing the relationship between preload and output.

    • Venous return curve indicating how low venous pressure enhances venous return.

Frank Starling Law

  • Definition: The energy of contraction is proportional to the initial length of the myocardial fibers.

  • Relationship with sarcomere stretch:

    • Cardiac contraction is based on optimal overlap between actin and myosin filaments.

    • At normal lengths, muscle fibers do not function at optimal lengths but can still increase contraction strength through stretch.

  • Implications of increased filling:

    • Increased stretch leads to better filament overlap, enhancing force of contraction.

  • Significance:

    • The heart cannot recruit additional fibers but instead works based on intrinsic tension adjustments.

Summary of Stroke Volume Influencing Factors

  • Increased preload correlates positively with stroke volume (via Frank Starling law).

  • Increased afterload correlates negatively with stroke volume and requires more effort for contraction, thereby reducing output if extrinsic modulations are not present (i.e., sympathetic nervous system input).

  • Importance of understanding:

    • How alterations in preload, afterload, and cardiac contractility influence the heart's ability to maintain effective cardiac output under varying physiological conditions.