Volume ventricular chambers

SAGAWA I also disagree with Dr. Feigl. I don t see any reason why the word elastance should be used strictly as a passive property. Even the so-called passive property of muscle is quite active. It is very difficult to draw a sharp line between passive and active muscle. In addition, in engineering, the suffix ance is not used to represent a material property. Instead it represents a systems property. For example, the elastance of the balloon at a given volume is not a measure of the elasticity of the balloon material. Instead, the term elasticity, or modulus of volume elasticity, is used to describe the material property. So I think it s right to use the term elastance for the ventricular chamber regardless of its state of activation. 

To answer the question of optimal matching between the ventricle and arterial load, we developed a framework of analysis which uses simplified models of ventricular contraction and arterial input impedance. The ventricular model consists only of a single volume (or chamber) elastance which increases to an endsystolic value with each heart beat. With this elastance, stroke volume SV is represented as a linearly decreasing function of ventricular endsystolic pressure. Arterial input impedance is represented by a 3-element Windkessel model which is in turn approximated to describe arterial end systolic pressure as a linearly increasing function of stroke volume injected per heart beat. The slope of this relationship is E. Superposition of the ventricular and arterial endsystolic pressure-stroke volume relationships yields stroke volume and stroke work expected when the ventricle and the arterial load are coupled. From theoretical consideration, a maximum energy transfer should occur from the contracting ventricle to the arterial load under the condition E = Experimental data on the external work that a ventricle performed on extensively varied arterial impedance loads supported the validity of this matched condition. The matched condition also dictated that the ventricular ejection fraction should be nearly 50%, a well-known fact under normal condition. We conclude that the ventricular contractile property, as represented by is matched to the arterial impedance property, represented by a three-element windkessel model, under normal conditions. 

Myocardial hypertrophy The heart increases in size, and the chambers dilate. Initially, stretching of the heart muscle leads to a stronger contraction of the heart. However, excessive elongation of the fibers results in weaker contractions. This type of failure is termed systolic failure and is a result of a ventricle unable to pump effectively. Less commonly, patients with CHF may have diastolic dysfunction—a term applied when the ventricles ability to relax and accept blood is impaired by structural changes, such as hypertrophy. The thickening of the ventricular wall and subsequent decrease in ventricular volume decreases the ability of heart muscle to relax. In this case, the ventricle does not fill adequately, and the inadequacy of cardiac output is termed diastolic heart failure. 

Two-dimensional ECHO employs multiple windows of the heart, and each view provides a wedge-shaped image. Windows most commonly used include parasternal long- and short-axis and apical two-and four-chamber views (Fig. 11-6). These views are processed onto a videotape to produce a motion picture of the heart. 2D ECHO renders increased accuracy in calculating ventricular volumes, wall thickness, and degree of valvular stenosis compared with M-mode ECHO. Patient-specific calculated parameters such as ejection fraction and wall thickness are compared with standardized values (population-... 

Shroff SG, Janicki JS, Weber KT (1983a) The importance of internal resistance in the description of ventricular mechanics. Bull Philadelphia Phys Soc 2 32-43 Shroff SG, Janicki JS, Weber KT (1983b) Left ventricular systolic dynamics in terms of its chamber mechanical properties. Am J Physiol 245 H110-H124 Shroff SG, Weber KT, Janicki JS (1984) End systolic relations Their usefulness and limitations in assessing left ventricular contractile state. Int J Cardiol 5 253-259 Suga H, Sagawa K, Shoukas AA (1973) Load independence of the instantaneous pressure-volume ratio of the canine left ventricle and effects of epinephrine and heart rate on the ratio. Circ Res 32 314-322... 

Similar changes are noted when the right ventricle s contractile state is halved (c = 0.5). The right ventricular ejection fraction drops from 46 to 24%, root pulmonary artery pulse pressure decreases from 49/15 to 29/12 mmHg, and right stroke volume decreases from 64 to 44 ml, with an increased end-diastoHc volume of 164 ml, from 141 ml. Conversely, c can be increased in any heart chamber to depict administration of an inotropic drug. Although not plotted, pressures, flows, and volumes are available at any circuit site, all as functions of time. 

FIGURE 17.10 Left ventricular pressure-volume work loops computed for the complete human circulation model (Figure 17.6) for control and varied inotropy, achieved by changing the contractile parameter c in Equation 17.8 for each heart chamber 25%. 

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