The definition of preload is the volume of blood that remains in the left ventricle at the end of diastole. Preload is generally a reflection of the volume status of a patient. The afterload is defined as impedance or resistance to ventricular contraction. There are two factors that determine the afterload. They are vascular tone and changes in the intrathoracic pressure.
Myocardial contractility is defined as the ability of the myocardial muscle to shorten itself.
It is important in the treatment of shock states and in the use of fluids and inotropic agents. Shock is defined as the acute disruption of circulatory function leading to inadequate delivery of nutrients to the tissues. It is a common misconception that shock is diagnosed solely on the basis of the blood pressure, but this is not true. For example, a decrease in the cardiac output can lead to activation of the renin-angiotensin and sympathetic nervous systems, which cause and increase in the systemic vascular resistance with a resulting normal blood pressure, even though the cardiac output was decreased. Therefore, a patient in shock can have decreased, normal or increased blood pressure.