Ventricular function can be analyzed and graphically displayed with a pressure-volume diagram. Both systolic and di-astolic pressure-volume relationships during a single cardiac cycle are displayed in Fig. 2. Pressure-volume assessment of myocardial function on intact myocardium involves multiple factors, such as preload, afterload, heart rate, and contractility. The area inside the pressure-volume loop is an estimate of the myocardial energy (work = pressure x volume) utilized for each stroke volume (stroke volume = end-diastolic volume - end-systolic volume). The shape of the normal pressure-volume loop changes with alterations in myocardial compliance, contractility, and/or valvular and myocardial disease.
Pressure-volume loops are displayed by plotting ventricular pressure (y axis) against ventricular volume (x axis) during a single cardiac cycle (Fig. 2). Points and segments along the pressure-volume loop correlate with specific mechanical events of the ventricle. The width of the pressure-volume loop is the stroke volume. Myocardial contractility is represented by the slope of the end-systolic pressure-volume relationship; this relationship defines the maximal pressure generated over time with a given myocardial contractility state. Contractility is proportional to change in pressure over time (dP/dt). The passive ventricular filling during diastole is defined by the end-dias-tolic pressure-volume relationship, and ventricular compliance is inversely proportional to the slope of the end-diastolic pressure-volume relationship.
The effect of heart rate on the pressure-volume relationship cannot be assessed with a single pressure-volume loop. Instead, multiple pressure-volume loops must be obtained to assess effects of heart rate on the pressure-volume loop. By altering variables such as afterload, contractility, and preload, the mechanical events and pressure-volume relationship are displayed.
The pressure-volume diagram shows events of a single cardiac cycle (Fig. 2):
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.