Những phản ứng đền bù bao gồm co mạch hệ thống, khối lượng máu tăng lên, và tăng nhịp tim và inotropy. Thích nghi tim, chẳng hạn như phì đại hoặc giãn nở, cũng sẽ làm thay đổi tâm thất làm đầy thụ động và do đó ảnh hưởng đến động lực học tim. | Cardiovascular Physiology Concepts 11 and aortic valve stenosis and mitral and aortic valve insufficiency. The descriptions are for acute changes that directly alter cardiac dynamics and therefore do not include cardiac and systemic compensatory mechanisms that attempt to maintain cardiac output and arterial pressure. These compensatory responses include systemic vasoconstriction increased blood volume and increased heart rate and inotropy. Cardiac adaptations such as hypertrophy or dilation would also alter the passive ventricular filling and thereby affect the cardiac dynamics. Furthermore severe valve disease usually leads to heart failure which further modifies intracardiac pressures and volumes. Valve Stenosis Stenosis can occur at either an outflow valve aortic or pulmonic valve or inflow valve mitral or tricuspid valve . Stenosis increases the resistance to flow across the valve which causes a high pressure gradient as blood flows across the valve. The pressure gradient across a valve is the pressure difference on either side of the leaflets. For the aortic valve the pressure gradient is the intraventricular pressure minus the aortic pressure for the mitral valve the pressure gradient is the left atrial pressure minus the left ventricular pressure. In normal valves the pressure gradient is only a few mm Hg when the valve is open. The following equation is the general hemodynamic expression that relates pressure gradient A P flow F and resistance R under laminar non-turbulent flow conditions DP F R A reduced valve orifice increases the resistance to flow across the valve because resistance is inversely related to the radius r of the valve orifice to fourth power equivalent to valve orifice area A to the second power because A n r2 see Chapter 5 . If the average valve radius is reduced by 50 equivalent to a 75 reduction in area the valve resistance is increased 16-fold which increases the pressure gradient 16-fold if flow remains unchanged. In reality the .
