Supraventricular Tachycardia

In the setting of regular, narrow complex tachycardia, P waves can aid the diagnosis but are often absent. At faster rates, sinus tachycardia can be obscured when P waves are buried within the T waves. P waves in a sawtooth pattern favors atrial flutter (2:1 conduction usually has a ventricular response rate around 150 bpm). While most cases of AVNRT do not have visible P waves, up to one third of AVNRT cases will show retrograde P’ waves immediately following the QRS complex, giving the appearance of a “pseudo-S wave” in the inferior limb leads, or a “pseudo-R wave” in V1. Rarely, atypical “fast-slow” AVNRT can produce retrograde P’ waves that precede the QRS complex. A regular, fixed R-R interval without respiratory variation would oppose the diagnosis of sinus tachycardia, while a labile heart rate that changes with positions or respirations favors sinus tachycardia. While the traditional equation for calculating maximal heart rate (220 – age) has come under scrutiny and other equations exist, it remains a simple bedside tool for helping to determine the likelihood of sinus rhythm.