What does an ECG show in pericardial tamponade due to fluid accumulation?

In pericardial tamponade, the accumulation of fluid in the pericardial sac can lead to characteristic changes on an ECG, one of which is electrical alternans. This phenomenon occurs due to the motion of the heart within the fluid-filled pericardial sac, leading to alternating amplitudes of the QRS complexes. As the heart shifts position with each heartbeat, there is a variation in the electrical axis, which results in this alternating pattern.

Electrical alternans is especially evident when there is a significant amount of fluid present, as the heart's ability to maintain a consistent electrical axis is disrupted. It's a classic sign of pericardial effusion and can be a helpful diagnostic indicator for clinicians.

Other potential findings such as ST elevation, tachycardia, and atrial fibrillation can occur in various cardiac conditions but are not as specific to pericardial tamponade as electrical alternans. ST elevation is typically seen in myocardial infarctions, tachycardia can be a general response to numerous conditions, and atrial fibrillation is related to atrial instability rather than fluid accumulation specifically affecting pericardial function.