Introduction
25. Major determinants of myocardial function: preload, afterload, contractility. Stroke volume and cardiac output.
Preload: The Stretching Force Before Contraction.
Definition: Preload refers to the tension in the ventricular myocardium just before contraction. Imagine it as the stretching force exerted on the heart muscle fibers at the end of diastole, preparing them for the upcoming contraction.
Key Factor: The primary factor influencing preload is the volume of blood filling the ventricles, known as the End-Diastolic Volume (EDV).
Relationship to Ventricular Compliance: Ventricular compliance, the ability of the ventricle to stretch and accommodate the incoming blood, also plays a role in preload. A while a less compliant ventricle requires higher pressure to accommodate the same volume.
Estimating Preload: Clinically, preload is estimated by assessing the EDV or the End-Diastolic Pressure (EDP).
Frank-Starling Law: The Frank-Starling law of the heart states that the greater the preload (stretch) on the ventricular muscle fibers, the stronger the force of contraction they generate. This intrinsic mechanism ensures that the heart pumps out the volume of blood it receives, maintaining a balance between cardiac output and venous return.
Afterload: The Resistance to Ejection
Definition: Afterload is the pressure that the ventricles must overcome to eject blood into the aorta. Think of it as the resistance the heart faces when pushing blood out.
Key Factor: The primary determinant of afterload is the pressure in the arteries above the semilunar valves (aortic pressure for the left ventricle).
Relationship to Stroke Volume: Increased afterload directly reduces stroke volume (SV). The higher the pressure the heart needs to overcome, the less blood it can effectively eject with each beat.
Contractility:The Intrinsic Pumping Power
Definition: Contractility, also known as inotropism, is the intrinsic ability of the heart muscle to contract, independent of preload and afterload. It's the inherent strength of the heart's pump.
Positive Inotropic Agents: Factors that increase contractility are known as positive inotropic agents. Examples include:
Hypercalcemia: Higher calcium levels enhance the binding of calcium to troponin C, facilitating the interaction between actin and myosin, leading to stronger contractions.
Glucagon: This hormone stimulates higher intracellular calcium levels by activating adenylyl cyclase, leading to increased contractility.
Negative Inotropic Agents: Factors that decrease contractility are called negative inotropic agents. Examples include:
Hypocalcemia: Lower calcium levels hinder the contraction process, leading to weaker contractions.
Hyperkalemia: High potassium levels disrupt the electrical activity of the heart, impairing its ability to contract effectively.