Objectives: Pericardial effusion has long been associated with low voltage of the surface electrocardiogram and its clinical utility is well known. The cause of this reduced QRS voltage has usually been attributed to short circuiting of the impulses by the pericardial fluid, but its precise mechanism has not been clearly elucidated. Therefore, we investigated this phenomenon by instilling various fluids with different resistances in increasing volumes into the pericardial space to finally produce acute cardiac tamponade.
Methods: Acute cardiac tamponade was produced 65 times in 25 closed chest pigs with a mean weight of 57(SEM 1.2) kg. A sufficient quantity (mean = 200 ml) of plasma, saline, blood, and blood with varying packed cell volume was introduced into the pericardial space in 60 ml increments to decrease systemic and pulmonary pressures to 50% of control to produce cardiac tamponade. At that point intrapericardial pressure had increased to 15-20 mm Hg. Resistivity values for the instilled fluids are 150 omega-cm for blood, 60 omega-cm for plasma, and 50 omega-cm for saline.
Results: Mean limb and precordial lead QRS voltage fell significantly, with no significant differences among these fluids. Electrocardiograph recordings from a unipolar electrode catheter in the right ventricle showed an increase in R wave voltage, but body surface recordings of stimuli introduced into the right atrium via a bipolar electrode catheter showed no amplitude change.
Conclusions: In considering possibilities such as internal shunting of cardiac currents by intrapericardial fluid, distance of the heart from body surface electrodes, and change in cardiac position, the results are best explained by a reduction of cardiac volume and size during cardiac tamponade. This conclusion confirms Brody's hypothesis.