What is Cardiac tamponade ?
The normal pericardium is a double-layered sac; the visceral pericardium is a serous membrane that is separated by a small quantity (15–50 mL) of fluid, an ultrafiltrate of plasma, from the fibrous parietal pericardium.
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The normal pericardium, by exerting a restraining force, prevents sudden dilation of the cardiac chambers, especially of the right atrium and ventricle, during exercise and with hypervolemia. It also restricts the anatomic position of the heart, minimizes friction between the heart and surrounding structures, prevents displacement of the heart and kinking of the great vessels, and probably retards the spread of infections from the lungs and pleural cavities to the heart.
There are both limitation of ventricular filling and reduction of cardiac output. The quantity of fluid necessary to produce this critical state may be as small as 200 mL when the fluid develops rapidly or >2000 mL in slowly developing effusions when the pericardium has had the opportunity to stretch and adapt to an increasing volume. The volume of fluid required to produce tamponade also varies directly with the thickness of the ventricular myocardium and inversely with the thickness of the parietal pericardium.
Tamponade is characterized by elevated intrapericardial pressure (>15mmHg) , which restricts venous return and ventricular filling . As a result , the stroke volume and pulse pressure fall, and the heart rate and venous pressure rise . shock and death my result .
Tamponade is a spectrum of hemodynamic derangements that can be divided into three phases : Phase I is characterized by equalization of right atrial and intrapericardial pressures , but not right ventricular or pulmonary capillary wedge pressure (PCWP) . In phase II , an equilibration of right atrial and right ventricular pressure but not PCWP exists , so that cardiac output is not significantly affected .
Phase III is the clinically evident syndrome of hypotension , tachycardia, tachypnea and pulsus paradoxus (typically exceeds 20mmHg) . At phase III , intrapericardial pressures have equalized with right atrial , right ventricular pressures and PCWP with a significant decrease in cardiac output . thus phase III represents the most severe hemodynamic abnormality in the spectrum of pericardial compression and is characterized by pressure and flow abnormalities . Phase II is charactherized predominantly by pressure abnormality and a modest degree of flow abnormality (pulsus paradoxus , if present , is usually less than 20 mm Hg) , whereas phase I consists of only pressure abnormality and is at the mildest end of the spectrum ( it may not be clinically evident).
Echocardiography helps to identify these phases . for example , when right –sided heart collapse ( phase II) occurs , the patient may be mildly symptomatic (tachypnea and tachycardia may be present , but not pulsus paradoxus). Performing urgent pericardiocentesis may not be necessary in all such cases . However, shen hupotension , pulsus paradoxus , and electrical alternans are present , and urgent need exists to tap the pericardial fluid . thus the decision to perform pericardiocentesis should incorporate clinical and echocardiographic findings .
Low-pressure tampnade occurs when the right atrial pressure is less than 10mmHg , usually because of hypovolemia . in such case. Low intracardiac pressures equilibrate with intrapericardial pressures , compromising cardiac output . cautious fluid replenishment is usually sufficient , although a subgroup of patients with low-pressure Tamponade benefit from pericardiocentesis .
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