In cardiac tamponade, the primary abnormality is compression of all cardiac chambers due to increased pericardial pressure. The pericardium has some degree of elasticity; but once the elastic limit is reached, the heart must compete with the intrapericardial fluid for the fixed intrapericardial volume. As cardiac tamponade progresses, the cardiac chambers become smaller, and chamber diastolic compliance is reduced. This results in a collection of three medical signs associated with acute cardiac tamponade, known as the Beck’s Triad. The signs are low arterial blood pressure, jugular venous distention, and distant, muffled heart sounds.
The decrease in arterial blood pressure is due to inability of the ventricles to stretch to their maximum size. This is because the heart cannot relax due to the pressure that is being enforced on the heart by the fluid that is accumulated around it. This limits the diastolic expansion (filling) which results in a lower End Diastolic Volume, therefor reduces stroke volume, which a major determinant of systolic blood pressure.
The rising central venous pressure is demonstrated by distended jugular veins while in a non-supine position. This is due to reduced diastolic filling of the right ventricle, secondary to the fluid accumulation around the heart, which limit the diastolic expansion of the right ventricle. This results in a backup of fluid in the jugular veins.
The suppressed heart sounds occur due to the muffling effects of the fluid surrounding the heart.
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