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Thread: Preload and Afterload

  1. #1

    Exclamation Preload and Afterload

    Definition of Preload

    If one end of a muscle fiber is suspended from a rigid strut and a weight is attached to the free end of the muscle, the muscle will be stretched to a new length. The added weight in this situation represents a force called the preload. Preload is thus defined as the force imposed on resting muscle that stretches the muscle to a new length. According to the length-tension relationship of muscle, an increase in the length of a resting muscle will increase the strength of muscle contraction (because more cross-bridges are formed between contractile elements in the muscle). Therefore, the preload force acts to augment the strength of muscle contraction.

    Clinical Measures

    Ventricular end-diastolic volume is not easily measured at the bedside, and end-diastolic pressure is used as the clinical measure of ventricular preload. Although the end-diastolic pressure is the clinical measure of preload, clinical studies have shown a poor correlation between end-diastolic pressure and end-diastolic volume (preload). These studies indicate that ventricular filling pressures (i.e., CVP and wedge pressures) are unreliable as surrogate measures of ventricular filling.

    CVP=RAP=RVEDP (all are intravascular pressures)

    Notices:

    The CVP and wedge pressure measurements can be misleading because recorded pressure differs from the physiologically relevant pressure. The pressure in the superior vena caca is recorded as an intravascular pressure; i.e., the pressure in the blood vessel relative to atmospheric (zero) pressure. However, the pressure that distends the ventricles to allow ventricular filling is the transmural pressure, which is the difference between the intravascular pressure and the surrounding intrathoracic pressure. Therefore, the recorded (intravascular) pressure will reflect the relevant (transmural) pressure only when the intrathoracic pressure is equivalent to atmospheric pressure. This normally occurs at the end of expiration. Therefore, the CVP and wedge pressure should be measured at the end of expiration.

    When intrathoracic pressure changes, the pressure change can be transmitted into the lumen of the veins within the thorax, resulting in a change in the measured (intravascular) pressure without a change in the relevant (transmural) pressure. Although the recorded (intravascular) pressure is changing, the relevant (transmural) pressure is unchanged. Therefore, respiratory variations in the CVP (and wedge pressure) do not represent changes in ventricular filling pressure.
    Last edited by admin; Tue 21st June '16 at 3:50pm.
    Clinical Pharmacy Specialist - Hematology

  2. #2

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    Definition of Afterload

    When a weight is attached to one end of a contracting muscle, the force of muscle contraction must be enough to lift the weight before the muscle begins to shorten. The weight in this situation represents a force called the afterload, which is the load imposed on a muscle after the onset of muscle contraction. Unlike the preload force, which facilitates muscle contraction, the afterload force opposes muscle contraction. In the intact heart, the afterload force is equivalent to the peak tension developed across the wall of the ventricles during systole. Afterload is thus the wall stress associated with ejection of the stroke volume.

    According to the Law of Laplace,

    Wall Tension = (pressure*radius)/[2*(wall thickness)]

    We get three conclusions,

    1.The greater the peak transmural pressure during systole, the greater the wall stress.
    2.The larger the ventricular chamber size, the greater the wall stress.
    3.The greater the ventricular hypertrophy, the less the wall stress.

    Pleural pressure will affect the afterload. When the pleural pressure is negative, the contraction of the heart is impeded because the negative pleural pressure opposes the inward movement of the ventricular wall during systole. When the pleural pressure is positive, it facilitates the contraction of the heart because the positive pleural pressure is along the same direction of ventricular contracting.

    The other three factors affecting afterload include End-diastolic volume, aortic impedance (against pulsatile flow), and system vascular resistance (against steady flow).

    Preload and Afterload-screen-shot-2015-06-21-at-3-13-56-pm-png
    Last edited by CheneyHsiung; Sun 21st June '15 at 3:14pm.
    Clinical Pharmacy Specialist - Hematology

  3. #3
    PharmD Year 1 TomHsiung's Avatar
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    Because vascular impedance is not easily measured, vascular resistance is often used as a clinical measure of ventricular after load. However, animal studies have shown a poor correlation between direct measures of ventricular wall tension (true afterload) and the calculated vascular resistance.
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease.

  4. #4
    PharmD Year 1 TomHsiung's Avatar
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    End-diastolic ventricular pressure is referred to as ventricular preload because it sets the end-diastolic ventricular volume and therefore the resting length of the cardiac muscle fibers at the end of diastole.

    At the onset of systole, the ventricular muscle cells develop tension isometrically and intraventricular pressure rises accordingly. After the intraventricular pressure rises sufficiently to open the outlet valve, ventricular ejection begins as a consequence of ventricular muscle shortening. Systemic arterial pressure is often referred to as the ventricular afterload because it determines the tension that must be developed by cardiac muscle fibers before they can shorten.
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease.

  5. #5
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    Default Re: Preload and Afterload

    Determinants of stroke volume

    1.Preload

    Preload and Afterload-screen-shot-2016-06-07-at-12-50-50-pm-png

    2.Afterload

    Preload and Afterload-screen-shot-2016-06-07-at-12-52-16-pm-png

    3.Contractility

    Preload and Afterload-screen-shot-2016-06-07-at-12-53-06-pm-png
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease. Chengdu, Sichuan, China.

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