Cardiac Function Curve

Lecture Details

• Course Title: Quantitative Physiology
• Instructor: Prof. Cathal J. Kearney
• Module: I
• Lecture: 4
• Focus: Modeling the cardiovascular system as a complete unit
• Institution: College of Engineering, Department of Biomedical Engineering

Learning Outcomes

• At the end of this lecture, students should be able to:
  • Describe the cardiac function curve.
  • Define total peripheral resistance (TPR).
  • Define mean systemic pressure (Ps).
  • Derive the vascular function curve.
  • Identify the steady-state operating point within the cardiovascular system.

Overview of the Cardiovascular System

• The cardiovascular system consists of a closed loop that facilitates the circulation of blood throughout the body.
• Key components include:
  • Right and Left Atrium: Collect blood from the body and lungs, respectively.
  • Right and Left Ventricle: Pump blood to the lungs and body, respectively.
  • Valves: Ensure unidirectional blood flow (e.g., tricuspid and bicuspid valves).
  • Vessels: Including pulmonary arteries and veins, aorta, and vena cava, carry oxygenated and deoxygenated blood.

Cardiac Function Curve

• Describes the relationship between stroke volume and right atrial pressure:
  • Experimental Setup:
  • Central Venous Pressure (CVP) at entrance to right atrium (RA).
  • Pressure in pulmonary vein equals pressure at entrance to left atrium (LA).
  • As right atrial pressure increases, the stroke volume of both ventricles also increases due to the closed-loop nature of the system.*

Definitions of Key Terms

• Total Peripheral Resistance: The resistance to blood flow in the systemic circulation, often impacted by vessel diameter and blood viscosity.
• Mean Systemic Pressure (Ps): Pressure in the systemic circulation when the heart is stopped, and blood is instantaneously redistributed such that pressure is equal everywhere.

Vascular Function Curve

• Represents the relationship between venous return and mean systemic pressure.
• Important for determining steady-state operating conditions in the cardiovascular system.

Steady-State Operating Point

• This is reached when the cardiac output equals venous return, indicating efficient functioning of the cardiovascular system.

Changes in Ventricular Function

• Ventilatory Mechanisms:
  • Increased contractility leads to increased heart rate (HR) and decreased afterload.
  • Decreased contractility leads to decreased HR and increased afterload.

Preload and Afterload

• Preload: The initial stretching of the cardiac myocytes prior to contraction.
  • Increased Preload: Results in increased stroke volume (SV).
  • Decreased Preload: Results in decreased stroke volume (SV).
• Afterload: The pressure that the ventricles must overcome to eject blood.

Intrinsic Regulation of the Heart

• The heart's ability to regulate its function based on system demands, with afterload typically fixed during standard measurements.

Cardiac Output and Flow Dynamics

• The closed nature of the cardiovascular system can be described quantitatively:
  • Total blood flow (e.g., venous return) is equal to cardiac output.
  • Data:
  • Venous return averages 7200 L/day or 5 L/min.
  • Cardiac output mirrors these values under steady conditions.

Poiseuille’s Law and Flow in the Cardiac System

• Pressure Drives Fluid Flow: Simplified principle is expressed as: $( riangle P = Q imes R)$
  • Where $(Q)$ is flow and $(R)$ is resistance, aiding in understanding of blood circulation dynamics.

Filling the Vascular System

• The relationship between total blood volume and mean systemic pressure is graphically assessed:
  • Mean Systemic Pressure, Ps (mmHg) against Blood Volume (L) where stressed and unstressed volumes are distinguished.

Derivation of the Vascular Function Curve

• The flow in the vena cava ($Q_{veins}$) is calculated based on cardiac output and systemic vascular resistance.
  • Considerations include compliance of veins ($C<em>V$) and arteries ($C</em>A$) and total peripheral resistance ($TPR$).

Combining Curves to Solve for Cardiac Output

• The slope of the relationship between mean systemic pressure and venous return is given by:
  $( ext{slope} = -1 + \frac{C<em>V}{C</em>A} imes TPR)$
• Implications for cardiovascular physiology and interpretation of functional dynamics are critical to understanding heart performance under various conditions.

Summary

• Understanding the function and regulation of the cardiovascular system involves complex interactions between the heart, vessels, and peripheral resistance. Mastery of these concepts is crucial for biomedical engineering applications and overall physiological comprehension.