Electrical alternans:

Electrical alternans is an electrocardiographic phenomenon of alternation of QRS complex amplitude or axis between beats. Also a wandering baseline may be seen. It is seen in cardiac tamponade and is thought to be related to changes in the ventricular electrical axis due to fluid in the pericardium.

Cardiac tamponade:

Cardiac tamponade, also known as pericardial tamponade, is an emergency condition in which fluid accumulates in the pericardium (the sac in which the heart is enclosed). If the fluid significantly elevates the pressure on the heart it will prevent the heart's ventricles from filling properly. This in turn leads to a low stroke volume. The end result is ineffective pumping of blood, shock, and often death.

Causes:

Cardiac tamponade occurs when the pericardial space fills up with fluid faster than the pericardial sac can stretch. If the amount of fluid increases slowly (such as in hypothyroidism) the pericardial sac can expand to contain a liter or more of fluid prior to tamponade occurring. If the fluid occurs rapidly (as may occur after trauma or myocardial rupture) as little as 100 ml can cause tamponade.

Causes of increased pericardial effusion include hypothyroidism, physical trauma (either penetrating trauma involving the pericardium or blunt chest trauma), pericarditis (inflammation of the pericardium), iatrogenic trauma (during an invasive procedure), and myocardial rupture.

Cardiac tamponade is caused by a large or uncontrolled pericardial effusion, i.e. the buildup of fluid inside the pericardium. This commonly occurs as a result of chest trauma (both blunt and penetrating), but can also be caused by myocardial rupture, cancer, uraemia, pericarditis, or cardiac surgery, and rarely occurs during retrograde aortic dissection, or whilst the patient is taking anticoagulant therapy. The effusion can occur rapidly (as in the case of trauma or myocardial rupture), or over a more gradual period of time (as in cancer). The fluid involved is often blood, but pus is also found in some circumstances.

Myocardial rupture is a somewhat uncommon cause of pericardial tamponade. It typically happens in the subacute setting after a myocardial infarction (heart attack), in which the infarcted muscle of the heart thins out and tears. Myocardial rupture is more likely to happen in elderly individuals without any previous cardiac history who suffer from their first heart attack and are not revascularized either with thrombolytic therapy or with percutaneous coronary intervention or with coronary artery bypass graft surgery.

One of the most common settings for cardiac tamponade is in the first 24 to 48 hours after heart surgery. After heart surgery, chest tubes are placed to drain blood. These chest tubes, however, are prone to clot formation. When a chest tube becomes occluded or clogged, the blood that should be drained can accumulate around the heart, leading to tamponade. Nurses will frequently milk clots from the tubes, or strip the tubes, but even with these efforts chest tubes can become clogged. Thus, after heart surgery it is critical to be on the watch for chest tube clogging.