U01.07.011 Cardiac and vascular function curves

The cardiac and vascular function curves illustrate the relationship between cardiac output (CO) and venous return (VR) as they interact to determine right atrial pressure (RAP). The intersection of these curves represents the heart’s operating point, where venous return and cardiac output are equal in the closed circulatory system.

Key Variables in Cardiac and Vascular Function Curves

Variable	Definition & Effects
Volume, Venous Tone	Affects venous return (VR) and mean systemic pressure (MSP). Higher volume or venous tone increases VR.
Total Peripheral Resistance (TPR)	Changes CO and RAP. Increased TPR reduces CO, while decreased TPR raises CO.
Right Atrial Pressure (RAP)	Determines preload. Influences CO and venous return.
Inotropy	Affects contractility, stroke volume (SV), and CO. Positive inotropes enhance CO, while negative inotropes reduce CO.
Mean Systemic Pressure (MSP)	Reflects the equilibrium pressure in the systemic circulation. Affected by blood volume and venous tone.

Graph Interpretation

Cardiac Output/Venous Return Interactions

Vascular Function Curve: Shows how changes in venous return affect RAP.
Cardiac Function Curve: Represents how CO varies with changes in RAP.
The intersection of these curves represents the operating point of the heart where CO and VR are balanced.

Effects of Physiological and Pathological Changes

Change	Effect on Curves	Examples
Inotropy (↑ or ↓ contractility)	Alters SV → Changes CO and VR.	⊕ Catecholamines, digoxin, exercise ⊝ Heart failure (HF) with reduced EF, narcotic overdose, sympathetic inhibition
Venous Return (↑ or ↓ circulating volume or venous tone)	Alters RAP → Changes SV and CO.	⊕ Fluid infusion, sympathetic activation ⊝ Acute hemorrhage, spinal anesthesia
Total Peripheral Resistance (TPR)	Alters CO; effect on RAP is variable.	⊕ Vasopressors ⊝ Exercise, arteriovenous (AV) shunt

Compensatory Mechanisms

Changes in these parameters often occur simultaneously:

Reinforcing Effects:
- Exercise → Increased inotropy + Decreased TPR → Maximized CO.
Compensatory Mechanisms:
- Heart Failure → Decreased inotropy → Fluid retention → Increased preload to sustain CO.

Your voice matters – comment below!Cancel reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

USMLE Step 1 Review Series

Curriculum