When pericardial pressure exceeds intracardiac pressure of a particular chamber, that chamber will collapse. The collapse will occur in a maner that is dependent on the phase of the cardiac cycle. Because the right side of the heart has much more compliant walls than the left side of the heart, RA collapses occur well before LA collapse. The RA is the first chamber to give in. The RA collapse typically occurs at the end of ventricular diastole – near the QRS on the ECG – and persists until the point in systole where filling pressures into the RA increase the RA pressure above the pericardial pressure. Brief collapse may occur non-specifically in the absence of tamponade. You want to see the RA collapse take up at least 1/3 of the cardiac cycle. Note that this sign is less reliable if there is RA/RV pacing or RA hypertension. Apart from on A4C, RA diastolic collapse can also be seen in PSAX (figure below on left) and M-mode (figure below on right). Correlation with the EKG signal indicates that the RA collapse here persists into mid-systole.