for example, usually indicates an atrial (or a ventricular) premature contraction, whereas one not
preceded by a premature beat usually indicates a heart block.
A premature beat associated with a cannon “A” wave on venous exam usually indicates an atrial contraction against a closed tricuspid valve and, therefore, a ventricular rather than an atrial premature contraction. Cannon “A” waves might also consistently occur in paroxysmal atrial tachycardia due to the almost
simultaneous contraction of atria and ventricles in this condition. Conversely, they might occur
randomly in complete heart block.
A very loud, almost “cannon-like,” S1 occurring at times in patients with regular rhythm usually suggests
the coincidental contraction of atria just before ventricles. This argues in favor of an escape
ventricular rhythm from complete heart block.
A “regular” pulse deficit (for example, a rate at the wrist that is exactly half of that at the apex) argues in
favor of bigeminy, with the premature beats being always unable to achieve ejection. This needs to be
differentiated from pulsus alternans