Overview
This lecture covers cardiac mechanics, focusing on the definitions, influencing factors, and calculations of preload, afterload, contractility, and related hemodynamic values, as well as the Frank-Starling curve.
Preload
- Preload is the LV wall tension or sarcomere length at end-diastole, best estimated by LV end diastolic pressure or volume.
- Influenced by total blood volume, heart rate/rhythm, ventricular and pericardial compliance, myocardial relaxation (lusitropy), and intrathoracic pressure.
- High heart rate reduces preload by shortening diastolic filling time; arrhythmias like atrial fibrillation decrease preload due to lost atrial kick.
Afterload
- Afterload is the force opposing ventricular contraction at systole, often equated with wall stress during systole.
- Determined by systemic vascular resistance, arterial compliance, LV chamber size, and outflow tract obstruction (e.g., aortic stenosis).
- Governed by the Law of Laplace: wall stress = transmural pressure × radius ÷ (2 × wall thickness).
- Increased by hypertension, vasopressors, larger LV, decreased arterial compliance; decreased by vasodilators, diuretics (smaller LV), and positive pressure ventilation.
Contractility
- Contractility is the intrinsic strength of myocardial contraction, independent of preload and afterload.
- Increased by sympathetic activation, inotropes (e.g., dobutamine), and higher heart rate (Bowditch effect).
- Decreased by parasympathetic activation, beta/calcium channel blockers, ischemia/infarction, myocarditis, severe acidosis/alkalosis, and hypothermia.
Stroke Volume, EF, Cardiac Output, and Cardiac Index
- Stroke Volume (SV) = End Diastolic Volume (EDV) – End Systolic Volume (ESV).
- Ejection Fraction (EF) = SV ÷ EDV × 100%; normal EF is 55-70%.
- Cardiac Output (CO) = SV × Heart Rate; Cardiac Index (CI) = CO ÷ Body Surface Area (BSA); normal CI is 2.5–4.0 L/min/m².
- EF is used for classifying and managing heart failure and monitoring certain cardiac conditions, but has limitations, especially with valvular disease.
Measurement Techniques
- EF mostly measured by echocardiography (biplane method of disks).
- Cardiac output measured invasively (Fick method, thermodilution) or non-invasively, each with pros and cons.
- Fick method uses O₂ consumption and arterial/venous O₂ content; thermodilution uses temperature changes via pulmonary artery catheter.
Frank-Starling Curve
- Illustrates how stroke volume increases with LVEDV until it plateaus at higher volumes.
- Increased contractility shifts the curve upward; decreased contractility or increased afterload shifts it downward.
- EF decreases at extremely high EDV due to curve flattening, independent of contractility.
Key Terms & Definitions
- Preload — The LV wall tension or sarcomere length at end-diastole.
- Afterload — The force opposing ventricular contraction during systole.
- Contractility — The intrinsic myocardial strength, independent of preload and afterload.
- Ejection Fraction (EF) — Percentage of EDV ejected during systole.
- Stroke Volume (SV) — Volume of blood pumped per beat.
- Cardiac Output (CO) — Volume of blood ejected per minute.
- Cardiac Index (CI) — CO adjusted for body surface area.
- Law of Laplace — Wall stress formula: transmural pressure × radius ÷ (2 × wall thickness).
- Frank-Starling Curve — Graph showing the relationship between LVEDV and SV.
Action Items / Next Steps
- Review and memorize the four key formulas (SV, EF, CO, CI).
- Practice drawing the Frank-Starling curve and labeling shifts with changes in contractility/afterload.
- Read about invasive and non-invasive cardiac output measurement techniques for clinical application.