Study Notes – Preload vs. Afterload

Cardiac Output

• Definition: The amount of blood the heart pumps throughout the body per minute.

• Importance: Maintains tissue perfusion.

• Formula:

  $$\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$$

Stroke Volume

• Definition: The amount of blood pumped by a ventricle with each beat.

• Determinants:

  1. Contractility – strength of cardiac muscle contraction.

  2. Preload – ventricular stretch at end-diastole.

  3. Afterload – pressure ventricles must overcome to eject blood.

Preload

• Definition: Ventricular stretch at the end of diastole (end-diastolic volume).

• Cardiac Cycle Phase: Ventricular filling phase (atria empty into ventricles via AV valves).

• Analogy: Like stretching a balloon as it fills.

Ways to Increase Preload:

• IV Fluids: Increase venous return → increases ventricular filling.

• Sympathetic Nervous System / Vasopressors: Vasoconstriction increases venous return.

Ways to Decrease Preload:

• Diuretics (e.g., Furosemide/Lasix): Reduce blood volume → less ventricular filling.

• Vasodilation (e.g., Nitroglycerin): Dilates vessels → blood pools → reduced venous return.

Afterload

• Definition: Pressure the ventricles must overcome to open semilunar valves and eject blood.

• Determinant: Vascular resistance (systemic or pulmonary).

Right Ventricle:

• Pumps against pulmonary vascular resistance (pulmonic valve → pulmonary artery → lungs).

Left Ventricle:

• Pumps against systemic vascular resistance (aortic valve → aorta → systemic circulation).

Ways Afterload Increases:

• Vasoconstriction: Narrowed arteries ↑ systemic or pulmonary resistance.

• Pulmonary Hypertension: Damaged pulmonary arteries → ↑ RV afterload.

• Aortic Stenosis: Narrowed valve increases LV pressure requirement.

• Systemic Hypertension: High BP increases LV workload.

Ways Afterload Decreases:

• Vasodilators: Widen vessels → ↓ vascular resistance → ↓ ventricular workload.

Key Point:

• Preload = volume (stretch before contraction).

• Afterload = pressure (resistance to ejection).