Cardiac Tamponade Eval
Cardiac tamponade is a medical emergency as a cause of obstructive shock. It is defined as a decompensated cardiac compression caused by pericardial fluid accumulation and rising intrapericardial pressure. The good news here is that we can make the diagnosis of tamponade based on focused cardiac ultrasound (FoCUS).
During spontaneous ventilation the negative intrathoracic pressure generated increases RV preload. The blood in the RV restricts full expansion of the LV limiting LV preload. The pulmonary veins also receive less blood as a consequence of the negative intrathoracic pressure. This also causes less LV preload. As a consequence stroke volume and CO drops to lower levels on inspiration when breathing spontaneously. This is the paradoxical pulse. The opposite occurs in a patient undergoing mechanical ventilation as the inspiration causes a drop of RV preload. Under normal circumstances the respirophasic differences of BP due to inspiration is less than 10mmHg.
Cardiac Tamponade Features
Pericardial fluid accumulates around the heart but it is not the volume per se but the change in pressure that is important that impairs filling of the heart. The right sided heart structures normally operate at a lower pressure than those on the left to keep fluid moving forward. With pericardial fluid, the pressure differential between the right sided chambers and the pericardial fluid restricts movement of flow into the RV. In essence once the pericardial sack generates more pressure than the pressure needed to deliver preload to the RV we have tamponade. The RV then receives less preload which ultimately results in hemodynamic compromise. Keep in mind that a pericardium that accumulates blood slowly has more time to adapt to the changes in pressure than one that does so fast. The importance here is that the volume in the pericardium is not as important as its physiologic effect on the heart and cardiac ultrasound can help us with this.
Patients with tamponade have thus exaggerated respirophasic features. As mentioned, the increased intrapericardial pressure restricts flow to the right sided structures. On inspiration thus the RV tries to accommodate this restriction of flow by invaginating the interventricular septum into the LV. This ultimately results in a lower stroke volume and cadiac output and clinically seen as a drop in BP greater than 10mm Hg. This is called pulsus paradoxus.
Sonographic features of Cardiac Tamponade.
Echocardiography and recently cardiac ultrasound is the primary diagnostic modality for the diagnosis of cardiac tamponade.
Early on right atrial collapse is seen in late diastole is an early sign of tamponade physiology and 100% sensitive. As pericardial pressure increases, the right ventricle presents with diastolic collapse in early diastole. We may also observed bowing of the IVS towards the LV during inspiration and towards the RV during expiration.
We can also observe the IVC diameter and its changes with respiration. With tamponade, the IVC is enlarged and does not vary with inspiration.
On the images above we see pericardial fluid. The heart rate is fast as a response to the decreased CO. This makes it difficult to asses the heart movement during diastole. The interrogation here lies on the right sided structures so we need to slow down the speed so that we can time diastole correctly with the cardiac cycle. The features above are typical of cardiac tamponade including diastolic collapse and a plethoric non collapsible IVC.
Slow speed to look at RV
In the images below the same clip has been slowed so that we can have a better appreciation at the RV and the effects of the surrounding pericardial fluid. The label Diastole on the clip marks the duration of diastole (opening to closing of the mitral valve). RV collapse is seen when the dot appears on the screen. Chronologically the Diastole appears and we observe that the RV collapses almost immediately in the early stages of and throughout almost all diastole in this clip.