Cardiac and vascular function curves illustrate the relationship between cardiac output (CO), venous return (VR), and right atrial pressure (RAP) in the circulatory system. The heart generates pressure to circulate blood, and the balance between cardiac function and vascular function determines overall hemodynamics.
Key Concepts:
- Right Atrial Pressure (RAP): Reflects venous return and preload.
- Cardiac Function Curve: Shows how changes in contractility (inotropy) affect CO.
- Vascular Function Curve: Represents how venous return is influenced by blood volume, venous tone, and total peripheral resistance (TPR).
- Mean Systemic Pressure (MSP): The equilibrium pressure when cardiac output stops.
- Operating Point: The intersection of the curves where CO = VR in a closed circulatory system.
Factors Affecting Cardiac and Vascular Function Curves
1. Inotropy (Contractility)
- Increased inotropy ⬆ (positive effect on CO):
- Catecholamines (epinephrine, norepinephrine)
- Dobutamine, milrinone, digoxin
- Exercise
- Decreased inotropy ⬇ (reduced CO):
- Heart failure with reduced ejection fraction (HFrEF)
- Narcotic overdose
- Sympathetic inhibition
2. Venous Return (Blood Volume & Venous Tone)
- Increased venous return ⬆ (higher RAP & CO):
- Fluid infusion (increased circulating volume)
- Sympathetic stimulation (venoconstriction)
- Arteriovenous (AV) shunt
- Decreased venous return ⬇ (lower RAP & CO):
- Acute hemorrhage (blood loss)
- Spinal anesthesia (venodilation)
3. Total Peripheral Resistance (TPR)
- Increased TPR ⬆ (reduced CO, unpredictable RAP change):
- Vasopressors (e.g., norepinephrine, phenylephrine)
- Decreased TPR ⬇ (higher CO, unpredictable RAP change):
- Exercise
- Arteriovenous shunt
How These Factors Interact
- Reinforcing Effects: Exercise increases inotropy and decreases TPR to optimize CO.
- Compensatory Effects: In heart failure, reduced inotropy triggers fluid retention to increase preload and maintain CO.