Preload: load imposed on a muscle before the onset of contraction
Muscle stretches to new length
Stretch in cardiac muscle determined by end diastolic volume
At bedside, use EDP as surrogate for ventricular preload
i.e. assume EDV = EDP

How can we measure EDP? - Pulmonary Capillary Wedge Pressure

How does wedge pressure work?

A balloon catheter is advanced into PA
Balloon at the tip is inflated
Creates static column of blood between catheter tip and left atrium
Thus, pressure at tip = pressure in LA

Ventricular function is mostly determined by the diastolic volume
Relationship between EDV/EDP and stroke volume illustrated by ventricular function curves

The ability of the cardiac muscle to contract (i.e. the contractile state)
Reflected in ventricular function curves

Afterload: Load imposed on a muscle at the onset of contraction

Wall tension in ventricles during systole

Determined by several forces
Pleural Pressure
Vascular compliance
Vascular resistance

Pleural Pressure
Pleural pressures are transmitted across the outer surface of the heartNegative pressure increases wall tension. Increases afterload
Positive pressure Decreases wall tension. Decreases afterload.

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