Increased preload is seen in all of the following except: September 2011
Ans. B: RestPreloadIt is the end volumetric pressure that stretches the right or left ventricle of the hea to its greatest geometric dimensions under variable physiologic demand.Preload is theoretically most accurately described as the initial stretching of a single cardiomyocyte prior to contraction.The term end-diastolic volume is better suited to the clinic, although not exactly equivalent to the strict definition of preload.Atrial pressure is a surrogate for preload.Quantitatively, preload can be calculated as: LVEDP.LVEDR/ 2hWhere LVEDP = Left ventricular end diastolic pressure, LVEDR = Left ventricular end diastolic radius (at the ventricle's midpoint), and h = thickness of the ventricle.This calculation is based on the Law of Laplace.Preload is affected by venous blood pressure and the rate of venous return.These are affected by venous tone and volume of circulating blood.Preload is related to the ventricular end-diastolic volume; a higher end-diastolic volume implies a higher preload.Preload increases with exercise (slightly), increasing blood volume (over transfusion, polycythemia) and neuroendocrine excitement (sympathetic tone).An aeriovenous fistula can increase preload AfterloadIt is the tension or stress developed in the wall of the left ventricle during ejection.Following Laplace's law, the tension upon the muscle fibers in the hea wall is the product of the pressure within the ventricle, multiplied by the volume within the ventricle, divided by the wall thickness.Therefore, a dilated left ventricle has a higher afterload.Conversely, a hyperophied left ventricle has a lower afterload.When contractility becomes impaired and the ventricle dilates, the afterload rises and limits output.This may sta a vicious circle, in which cardiac output is reduced as oxygen requirements are increased.Afterload can also be described as the pressure that the chambers of the hea must generate in order to eject blood out of the hea and thus is a consequence of the aoic pressure (for the left ventricle) and pulmonic pressure or pulmonary aery pressure (for the right ventricle).The pressure in the ventricles must be greater than the systemic and pulmonary pressure to open the aoic and pulmonic valves, respectively.As afterload increases, cardiac output decreases. Preload best describes the maximum viscous blood volume of end diastole while afterload better describes the maximum tension of the myocardial muscle mass in end systole.
Sympathetic stimulation
Rest
Aeriovenous fistula
Over transfusion of blood