{"id":67148,"date":"2026-05-21T18:27:53","date_gmt":"2026-05-21T12:57:53","guid":{"rendered":"https:\/\/johnsonfrancis.org\/professional\/?p=67148"},"modified":"2026-05-21T18:28:08","modified_gmt":"2026-05-21T12:58:08","slug":"differential-diagnosis-of-narrow-qrs-tachycardia","status":"publish","type":"post","link":"https:\/\/johnsonfrancis.org\/professional\/differential-diagnosis-of-narrow-qrs-tachycardia\/","title":{"rendered":"Differential diagnosis of narrow QRS tachycardia"},"content":{"rendered":"<iframe loading=\"lazy\" width=\"560\" height=\"315\" src=\"https:\/\/www.youtube.com\/embed\/vlqNoVnnB_E?si=ate-mMgUerrFBFaI\" title=\"YouTube video player\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe>\n\n<p class=\"wp-block-paragraph\">When a patient presents with a regular or irregular tachycardia and a narrow QRS complex (less than 120 milliseconds), it tells us that ventricular depolarization is occurring via the normal, rapid His-Purkinje system. This almost always points to an origin above or within the Bundle of His\u2014collectively categorized as <strong>Supraventricular Tachycardias (SVT)<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The most practical clinical approach to narrowing down the differential diagnosis is to first assess <strong>regularity<\/strong>, and then evaluate the relationship between the P waves and QRS complexes.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">1. Regular Narrow QRS Tachycardia<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">If the R-R intervals are perfectly regular, the differential largely narrows down to three primary culprits, alongside a few distinct sinus variants.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">The Big Three (Paroxysmal SVTs)<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li class=\"\"><strong>AV Nodal Reentrant Tachycardia (AVNRT):<\/strong> The most common regular SVT. It utilizes a dual-pathway system within the AV node (slow and fast pathways). Because ventricles and atria are activated nearly simultaneously, P waves are typically buried inside the QRS complex or appear as a &#8220;pseudo-R'&#8221; in lead V1 or a &#8220;pseudo-S&#8221; in the inferior leads.<\/li>\n\n\n\n<li class=\"\"><strong>AV Reentrant Tachycardia (AVRT):<\/strong> This relies on an anatomically distinct accessory pathway (like in Wolff-Parkinson-White syndrome) creating a macro-reentrant circuit.\n<ul class=\"wp-block-list\">\n<li class=\"\"><em>Orthodromic AVRT:<\/em> Conduction goes down the AV node (keeping the QRS narrow) and back up the accessory pathway. P waves are usually clearly visible in the ST segment, retrogradely inverted in inferior leads, with an RP interval greater than 70 ms.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li class=\"\"><strong>Atrial Flutter (with fixed block):<\/strong> Typically presents with a classic ventricular rate of 150 bpm (2:1 conduction of a ~300 bpm atrial rate). Look closely at leads II, III, aVF, and V1 for the characteristic sawtooth &#8220;F&#8221; waves, especially if the rate refuses to budge.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Sinus and Atrial Variants<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li class=\"\"><strong>Sinus Tachycardia:<\/strong> A normal physiologic response to stress, pain, hypovolemia, or fever. It features a normal, upright P-wave morphology in leads I, II, and aVF preceding every QRS.<\/li>\n\n\n\n<li class=\"\"><strong>Inappropriate Sinus Tachycardia (IST):<\/strong> An elevated resting heart rate greater than 100 bpm or exaggerated response to minimal exertion without an identifiable physiological trigger, keeping identical normal sinus P-wave morphology.<\/li>\n\n\n\n<li class=\"\"><strong>Sinoatrial Nodal Reentrant Tachycardia (SANRT):<\/strong> A paroxysmal micro-reentrant tachycardia arising near the sinus node. It mimics sinus tachycardia perfectly on the ECG but starts and stops abruptly.<\/li>\n\n\n\n<li class=\"\"><strong>Focal Atrial Tachycardia (AT) with fixed block:<\/strong> Arises from a distinct atrial focus outside the SA node. The P wave morphology will differ from normal sinus rhythm (e.g., inverted in lead I or aVL if originating from the left atrium).<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">2. Irregular Narrow QRS Tachycardia<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">When the R-R intervals are variable, the differential shifts completely toward atrial arrhythmias with variable conduction through the AV node.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li class=\"\"><strong>Atrial Fibrillation (AFib):<\/strong> The quintessential &#8220;irregularly irregular&#8221; rhythm. There are no discernible, organized P waves; instead, the baseline shows chaotic fibrillatory (&#8220;f&#8221;) waves.<\/li>\n\n\n\n<li class=\"\"><strong>Atrial Flutter with Variable Conduction:<\/strong> The macro-reentrant atrial circuit remains completely regular (~300 bpm), but the AV node blocks the impulses unpredictably (e.g., alternating between 2:1, 3:1, and 4:1 conduction), creating an irregular ventricular response.<\/li>\n\n\n\n<li class=\"\"><strong>Multifocal Atrial Tachycardia (MAT):<\/strong> Defined by a heart rate greater than 100 bpm with <strong>three or more distinct P-wave morphologies<\/strong> within a single lead, and varying PP, PR, and RR intervals. This is highly associated with severe underlying pulmonary disease (like severe COPD exacerbations).<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">The Electrocardiographic Roadmap (Regular R-R)<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">To quickly differentiate the regular narrow QRS tachycardias on an ECG, we look at the location of the retrograde P wave relative to the QRS complexes:<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td><strong>RP Classification<\/strong><\/td><td><strong>Timing<\/strong><\/td><td><strong>Primary Differentials<\/strong><\/td><td><strong>Key ECG Clue<\/strong><\/td><\/tr><\/thead><tbody><tr><td><strong>P buried \/ Pseudo-R&#8217;<\/strong><\/td><td>RP &lt; 70 ms<\/td><td>Typical AVNRT<\/td><td>Pseudo-R&#8217; in V1 or pseudo-S in inferior leads that disappears in sinus rhythm.<\/td><\/tr><tr><td><strong>Short RP<\/strong><\/td><td>70 ms &lt; RP &lt; PR<\/td><td>Orthodromic AVRT, Atrial Tachycardia<\/td><td>Retrograde P wave visible in the ST segment, well after the QRS ends.<\/td><\/tr><tr><td><strong>Long RP<\/strong><\/td><td>RP > PR<\/td><td>Atypical AVNRT, Atrial Tachycardia, Permanent Junctional Reciprocating Tachycardia (PJRT)<\/td><td>P wave falls <em>before<\/em> the next QRS, often masquerading as a prolonged sinus PR interval.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">Diagnostic Bedside Maneuvers<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">When the baseline ECG doesn&#8217;t provide an absolute answer because the rate is too fast to clearly map the P waves, two main interventions can unmask the underlying rhythm:<\/p>\n\n\n\n<ol start=\"1\" class=\"wp-block-list\">\n<li class=\"\"><strong>Vagal Maneuvers:<\/strong> Valsalva or carotid sinus massage increases vagal tone to transiently slow AV nodal conduction.<\/li>\n\n\n\n<li class=\"\"><strong>Adenosine:<\/strong> A rapid intravenous bolus blocks conduction through the AV node for a few seconds.<\/li>\n<\/ol>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>The Diagnostic Response:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li class=\"\">If the rhythm <strong>abruptly terminates<\/strong> back to sinus rhythm, the AV node is an essential limb of the reentrant circuit (highly suggestive of <strong>AVNRT<\/strong> or <strong>AVRT<\/strong>).<\/li>\n\n\n\n<li class=\"\">If the rhythm <strong>transiently slows down<\/strong>, unmasking underlying rapid atrial activity (sawtooth waves or flutter waves) without stopping the atrial arrhythmia itself, the mechanism is <strong>Atrial Flutter<\/strong> or <strong>Atrial Tachycardia<\/strong>.<\/li>\n<\/ul>\n<\/blockquote>\n","protected":false},"excerpt":{"rendered":"<p>When a patient presents with a regular or irregular tachycardia and a narrow QRS complex (less than 120 milliseconds), it tells us that ventricular depolarization is occurring via the normal, rapid His-Purkinje system. This almost always points to an origin above or within the Bundle of His\u2014collectively categorized as Supraventricular Tachycardias (SVT). The most practical [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":67149,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"nf_dc_page":"","footnotes":""},"categories":[9],"tags":[],"class_list":["post-67148","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-general"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Differential diagnosis of narrow QRS tachycardia - All About Cardiovascular System and Disorders<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/johnsonfrancis.org\/professional\/differential-diagnosis-of-narrow-qrs-tachycardia\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Differential diagnosis of narrow QRS tachycardia - All About Cardiovascular System and Disorders\" \/>\n<meta property=\"og:description\" content=\"When a patient presents with a regular or irregular tachycardia and a narrow QRS complex (less than 120 milliseconds), it tells us that ventricular depolarization is occurring via the normal, rapid His-Purkinje system. This almost always points to an origin above or within the Bundle of His\u2014collectively categorized as Supraventricular Tachycardias (SVT). 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