Hemodynamics: Cardiac Function and Venous Return Curves

Hemodynamics: Cardiac Function and Venous Return Curves

Cardiac Function Curve

The cardiac function curve illustrates the relationship between cardiac output (CO) and right atrial pressure (RAP). Understanding this curve helps explain how various factors such as preload, afterload, and contractility affect cardiac output.

Preload

Preload refers to the initial stretching of the cardiac myocytes prior to contraction, which is related to the end diastolic volume (EDV). Increased preload leads to increased stroke volume (SV) according to the Frank-Starling mechanism. Here are the key points about preload: - Increased Preload: More blood enters the heart (e.g., blood transfusion, venoconstriction), leading to greater EDV and subsequently higher SV and CO. - Decreased Preload: Less blood enters the heart (e.g., blood loss, venodilation), leading to lower EDV, SV, and CO.

Contractility

Contractility is the heart’s intrinsic ability to contract at a given preload (EDV). It is influenced by the availability of calcium, which affects actin-myosin interaction in the cardiac myocytes. - Increased Contractility: Leads to a higher SV for a given EDV. This can be due to β1-adrenergic receptor activation or elevated [Ca2+]. Examples include exercise and catecholamine secretion. - Decreased Contractility: Results in a lower SV and CO for a given EDV. This can be due to β1-adrenergic receptor inhibition or heart diseases.

Afterload

Afterload is the resistance that the heart must overcome to eject blood. Increased afterload results in decreased SV and CO, whereas decreased afterload leads to increased SV and CO.

- Increased Afterload: Conditions like aortic stenosis and hypertension raise afterload, reducing SV and CO.

- Decreased Afterload: Vasodilators reduce afterload, enhancing SV and CO.

Shifts in Cardiac Function Curves

  • Increased Contractility: Shifts the curve upwards, resulting in higher SV and CO for any given RAP.
  • Decreased Contractility: Shifts the curve downwards, resulting in lower SV and CO for any given RAP.
  • Increased Afterload: Shifts the curve downwards, reducing SV and CO.
  • Decreased Afterload: Shifts the curve upwards, increasing SV and CO.

Venous Return Curve

The venous return (VR) curve describes the relationship between VR and RAP. MSFP (Mean Systemic Filling Pressure) is the pressure in the venous system that drives blood back to the right atrium.

Factors Affecting Venous Return

  • ‘Available’ Blood Volume: Determines preload and MSFP. Changes in blood volume shift the VR curve.
  • Venous Tone: Increased venous tone enhances VR, whereas decreased venous tone reduces VR.

Key Relationships

  • MSFP and RAP: VR = (MSFP – RAP) / Resistance (R). VR decreases as RAP approaches MSFP.
  • Intersection with Cardiac Function Curve: The point where the cardiac function and VR curves intersect represents the operating point of the heart, where CO equals VR.

Summary of CO-VR Relationship

  1. Increased Contractility: Shifts the cardiac function curve upwards, increasing CO without much change in RAP.
  2. Increased Preload: Shifts the VR curve to the right, increasing CO.
  3. Increased Afterload: Shifts the cardiac function curve downwards, decreasing CO.
  4. Decreased Afterload: Shifts the cardiac function curve upwards, increasing CO.
  5. Decreased Contractility: Shifts the cardiac function curve downwards, decreasing CO.
  6. Decreased Preload: Shifts the VR curve to the left, decreasing CO.

Solving CO-VR Problems

  1. Identify changes in preload, afterload, and/or contractility.
  2. Redraw/shift the relevant curve (cardiac function or VR curve).
  3. Determine the new intersection point to find the steady-state CO.

Examples:

  • Fluid Infusion: Shifts the VR curve to the right, increasing CO and RAP.
  • Exercise: Increases contractility and venous return, shifting the cardiac function curve up and VR curve to the right, maximizing CO.
  • Heart Failure: Decreases contractility, shifting the cardiac function curve down, necessitating compensatory mechanisms like increased preload to maintain CO.