Atrioventricular nodal reentrant tachycardia and cannon A waves.

Regular, narrow complex tachycardia with a ventricular rate around 150 can be challenging. The differential includes sinus tachycardia, atrioventricular nodal reentrant tachycardia (AVNRT), atrioventricular reentrant tachycardia (AVRT), and atrial tachycardia (focal or macro re-entrant - i.e. flutter). We present a case of a 90-year-old woman presenting with shortness of breath in which the ECG was not diagnostic, but the presence of regular neck pulsations helped secure the diagnosis of AVNRT. In AVNRT, atria and ventricular contractions occur nearly simultaneously. When the right atrium attempts to contract against a closed tricuspid valve, an abrupt increase in venous pressure is encountered. This increase in venous pressure manifests as prominent neck pulsations termed "cannon A waves." The patient was ultimately successfully electrically cardioverted resulting in resolution of her presenting symptoms, neck pulsations, and tachycardia. While irregular "cannon A waves" can be seen in conditions of AV dissociation, regular "cannon A waves" strongly favor the diagnosis of AVNRT.