Assessment of Pericardial diseases
Although the purpose of the pericardium is mainly to protect the heart and provide lubrication, it also has important hemodynamic effects on the cardiac chambers. The nondistensible pericardium limits the acute distension of the ventricles and contributes to ventricular coupling: the distension of one ventricle alters the filling of the other, an effect that is important in the pathophysiology of cardiac tamponade and constrictive pericarditis. The pericardium is not seen normally on an echocardiogram neither is pericardial fluid. Therefore the visualization of either of these is pathological.
Filling of the potential pericardial space with fluid or blood results in a pericardial effusion. When the volume of fluid is small, it can be seen as a black echo-free space present only posterior to the heart in the parasternal short and long axis view, and may be present only in the systolic phase (fig.1 and 2). When the volume of fluid is more than 25ml, an echo free space exists all around the heart throughout the cardiac cycle. When the amount of fluid is massive, the heart may have a swinging motion in the pericardial cavity (fig.3).
While this fluid may be visible in all cardiac echo views, measurements of fluid thickness need to be made anteriorly and posteriorly on the PLAX or SAX views, apically in the A4C, A3C or A2C views and inferiorly in the subcostal view. Small effusions have an echo free space of <5mm, moderate sized effusions range between 5mm and 10mm and are circumferential and more than 10mm indicates a large effusion. Fluid adjacent to the right atrium is an early sign of pericardial effusion.
Pericardial versus pleural effusion
Pericardial effusion is usually located circumferentially. If the echo free space is seen only anteriorly, it is more likely an epicardial pad of fat or a pleural effusion. Posteriorly a pericardial effusion is anterior to the descending thoracic aorta whereas a pleural effusion is posterior to it. A pericardiaal effusion is rarely >4cms thick. If pericardial and pleural effusions co-exist, then a linear echo (the pericardium) separates them. A pleural effusion on the left side allows cardiac imaging from the back.
The other structure to differentiate from pericardial effusion is epicardial fat. Epicardial fat is tissue is more prominent anteriorly but may appear cicumferentially, thus mimicking an effusion. Fat is slightly echogenic and moves in concert with the heart, two characteristics that help to differentiate it from an effusion which is echolucent and motionless.
The fluid in the pericardial sac can cause hemodynamic alterations depending on the volume of fluid and the speed at which it accumulates. The echocardiographic signs of tamponade are
2-D echo and M-mode
1.Diastolic right ventricular collapse
2.Right atrial collapse / inversion
3.IVC plethora with a lack of change with breathing
1.Exaggerated respiratory variation in tricuspid and mitral inflows
2.Increased LVOT VTI variation
3.Exaggerated IVC flow variation
The first three are important to look for.
Right ventricular collapse
This occurs when the intrapericardial preesure exceeds the intraventricular pressure and occurs in early diastole. This can be observed in PLAX and A4C views, but being a rapid movement, may need to be resolved with M-mode through the RVOT or RV free wall in a PLAX view (see fig.4 ). The right ventricular outflow tract (RVOT) has the most compliance and is the first part of the RV to collapse. When the entire body of the RV collapses (see video), then it is an indicator of a more substantial elevation in intrapericardial pressure.
Although this sign is a relatively sensitive and specific marker for tamponade, RV diastolic collapse is sensitive to alterations in ventricular loading conditions and may not be seen in the presence of right ventricular hypertrophy.
Right atrial collapse
The raised intrapericardial pressure also impedes atrial filling and causes the right atrium to remain collpapsed even after atrial systole. Atrial collapse therefore is a late diastole, early systole phenomenon. With increasing intrapericardial pressures, the atrium remains collapsed throughout diastole as well and buckles inward, reversing the normal wall curvature.
While this is best seen in the A4C and subcostal views, M-mode through the RA wall may be necessary to clearly identify the collapse, particularly when the heart rate is high.
Right atrial collapse is virtually 100% sensitive for tamponade but less specific. Duration of this collpase exceeding one third of the cardiac cycle increases specificity without sacrificing sensitivity. Left atrial collapse is seen in about 25% of patients and is very specific for tamponade.
Because of the elevated filling pressures of the right heart, the IVC becomes distended (>2cms diameter) and has less than 12% variation in diameter with respiration.
Marked variations with respiration (>12%) in mitral and tricuspid inflow velocities (E and A) as well as LVOT and RVOT VTIs is commonly seen.
These changes may not be overtly manifest in the presence of a hypertrophic right ventricle as seen in pulmonary hypertension, thickening of the ventricular walls due to malignancy, overlying inflammatory response or overlying thrombus and in severe hypovolemia - the so called low pressure tamponade.
Video 1. 2D and Doppler features of tamponade (Source: Youtube)
Video 2. RV collapse in tamponade
Echocardiography guided pericardiocentesis
While a detailed description of the procedure is out of the scope of this tutorial, a few key points are noted below
a) The procedure should be performed by a cardiologist or an intensivist with experience in the procedure
b) ECG monitoring is mandatory
c)A long 18 gauge catheter over needle can be used
d)The entry site could be just left of the xiphisternum or the apex. The site with the least distance from the skin to the pericardium as determined on the echo can be used.
e) The needle is advanced toward the left shoulder if the entry point is the xiphisternum or directly downward if it is the apex.
f) The needle should be advanced under direct visualization on the echo
g) If there is a doubt about having entered a cardiac chamber, agitated saline can be injected through the needle and this will delineate the pericardial space or cardiac chamber the tip is in
h) If a cardiac chamber has been entered, the catheter should be passed into it, locked and the patient should be transported urgently to the cardiac catheterization lab to plug the leak
I) Frond-like, band-like or shaggy intrapericardial echoes should alert one to the possibility of a difficult and potentially less therapeutic pericardiocentesis.
See this brief video on pericardiocentesis
Video 3. Steps of pericardiocentesis in brief (Source: Youtube)