Overview
This lecture explains the key differences between cardiac preload and afterload, their effects on cardiac output, and how various interventions can modify them.
Cardiac Output and Stroke Volume
- Cardiac output is the amount of blood the heart pumps per minute, crucial for tissue perfusion.
- Cardiac output = heart rate × stroke volume.
- Stroke volume is the amount of blood pumped by a ventricle with each beat.
- Stroke volume is affected by contractility, preload, and afterload.
Preload
- Preload is the stretch of the ventricles at the end of diastole, also called end-diastolic volume.
- It represents how much blood fills the ventricles during the relaxation phase.
- Increasing preload (more stretch) can increase stroke volume and cardiac output.
- IV fluids and vasoconstriction (sympathetic stimulation or vasopressors) increase preload by raising venous return.
- Decreasing preload is useful in fluid overload situations (like heart failure).
- Diuretics (e.g., furosemide/Lasix) and vasodilators (e.g., nitroglycerin) reduce preload by decreasing blood volume or increasing vessel diameter.
Afterload
- Afterload is the pressure the ventricles must overcome to open semilunar valves and eject blood.
- It is mainly determined by vascular resistance (systemic and pulmonary).
- Right ventricle afterload is affected by pulmonary vascular resistance; left ventricle by systemic vascular resistance.
- Increased afterload means the heart works harder to pump blood out.
- Vasoconstriction, high blood pressure, pulmonary hypertension, and valve stenosis (e.g., aortic stenosis) increase afterload.
- Vasodilators decrease afterload by lowering vascular resistance and pressure.
Key Terms & Definitions
- Cardiac Output — blood volume pumped by the heart per minute.
- Stroke Volume — blood volume pumped by a ventricle per beat.
- Preload — ventricular stretch at end diastole (end-diastolic volume).
- Afterload — pressure ventricles must overcome to open semilunar valves.
- Vasoconstriction — narrowing of blood vessels, increasing resistance and pressure.
- Vasodilation — widening of blood vessels, lowering resistance and pressure.
- Diuretics — drugs that increase urine output to reduce blood volume.
- Aortic Stenosis — narrowing of the aortic valve, raising afterload.
Action Items / Next Steps
- Take the free quiz on preload and afterload.
- Review related cardiac physiology concepts for better understanding.
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