coronary artery disease (narrowing of the arteries of the heart by atherosclerosis) who has a very rapid heart rate may experience chest pain similar to angina; this pain is band- or pressure-like around the chest and often radiates to the left arm and angle of the left jaw. Symptoms often occur without any specific trigger, although some find that their palpitations often occur after lifting heavy items or bending forwards. The onset of palpitations is sudden, with the acceleration of the heart rate occurring within a single beat, and may be preceded by a feeling of the heart skipping a beat. The heart may continue to race for minutes or hours, but the eventual termination of the arrhythmia is as rapid as its onset. During AVNRT the heart rate is typically between 140 and 280 beats per minute. Close inspection of the neck may reveal pulsation of the jugular vein in the form of "cannon A-waves" as the right atrium contracts against a closed tricuspid valve. Mechanisms The fundamental mechanism of AVNRT is a presence of a dual atrioventricular node physiology (present in half of the population), which acts as a re-entrant circuit within the atrioventricular node. This can take several forms. "Typical", "common", or "slow-fast" AVNRT uses the slow AV nodal pathway to conduct towards the ventricle (the anterograde limb of the circuit) and the fast AV nodal pathway to conduct to the atria (the retrograde limb). The re-entrant circuit can be reversed such that the fast AV nodal pathway is the anterograde limb and the slow AV nodal pathway is the retrograde limb, referred to as "atypical", "uncommon", or "fast-slow" AVNRT. Atypical AVNRT may also use the slow AV nodal pathway as the anterograde limb and left atrial fibres that approach the AV node from the left side of the inter-atrial septum as the retrograde limb, and is sometimes referred to as "slow-slow" AVNRT. Typical AVNRT In typical AVNRT, the anterograde conduction is via the slow pathway and the retrograde conduction is via the fast pathway ("slow-fast" AVNRT). Because the retrograde conduction is via the fast pathway, stimulation of the atria (which produces the inverted P wave) occurs very soon after stimulation of the ventricles (which causes the QRS complex). As a result, the time from the QRS complex to the P wave (the RP interval) is short, less than 50% of the time between consecutive QRS complexes.