Diastolic dysfunction is characterized by elevated diastolic pressure in the left ventricle despite normal or sub-normal diastolic volume. Histologic evidence supporting diastolic dysfunction demonstrates hypertrophy of the cardiomyocytes, increased interstitial collagen deposition and/or infiltration of the myocardium. These influences collectively lead to a downhill spiral in distensibility of the myocardium. The ventricle then behaves as a balloon made from abnormally thick rubber. Despite filling with high pressure, the volume cannot expand adequately. If the heart cannot fill with blood easily, either the cardiac output becomes diminished or compensation ensues to increase the ventricular diastolic pressure to higher levels. When the left ventricular diastolic pressure is elevated, venous pressure in the lungs must also become elevated to maintain forward flow. Increased pulmonary venous pressure results in alveolar edema causing the patient to be short of breath.
It is crucial to note that a normal heart that is overfilled with blood may demonstrate increased stiffness and decreased compliance characteristics. This is analogous to a balloon that is over-filled with air. Blowing more air into the balloon becomes difficult because the balloon acts stiff and non-compliant at a filling volume it wasn't designed to hold. It is wrong to classify the volume overloaded heart as having diastolic dysfunction just because it is behaving stiff and non-compliant. The term diastolic dysfunction should therefore not be applied to the dilated heart. The term diastolic dysfunction is often erroneously applied in this circumstance when increased fluid volume retention causes the heart to be over-filled.
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