Clinical Note

In some acute forms of heart failure, left atrial pressure can become quite elevated. If this continues for more than a day, the heart can dilate such that the chamber diameter may double or even triple. Dilation is caused by a slippage of the points at which one fiber attaches to its neighbor, in response to sustained high wall tension. With a large chamber radius and a thin wall, the geometry may become so unfavorable that an adequate stroke volume can no longer be generated by the heart. This is a grave complication for the already failing heart. The dilated heart has a normal mass of muscle and should not be confused with a hypertrophied heart in which the muscle mass has been increased.

can be measured by a variety of methods in the human heart. These now include ultrasound, nuclear medicine methods, and x-ray angiography. Although the ejection fraction is strongly influenced by contractility, the ejection loop analysis reveals that it is also influenced by filling pressure and aortic pressure. Nevertheless, the ejection fraction is the most important index of contractility in the clinic today, primarily because of its ease of measurement. Normally, the ejection fraction should be about 0.6 for a healthy heart. Ejection fractions below 0.5 suggest disease and those below 0.3 are associated with high mortality.

End-Systolic Pressure-Volume Relationship

The best measurement of contractility would be to analyze the systolic pressure-volume curves for that ventricle. It is now possible to obtain a continuous ventricular volume recording with what is called a conductance catheter. By having a computer plot the ventricular volume signal against ventricular pressure, an ejection loop for each beat can be displayed. If aortic pressure is varied over several beats, the pressure and volume at the end of systole can be determined for each of those beats, as shown in Fig. 12. Connecting those points plots a segment of the systolic pressure-volume curve. The end-systolic pressure-volume relationship is clearly the most accurate measurement of contractility available today. Unfortunately, the method still requires relatively invasive instrumentation, which limits its clinical utility.

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