Supraventricular rhythms are cardiac rhythms originating above the ventricles but not from the sinoatrial (SA) node. These rhythms can significantly alter the morphology of P-waves on an electrocardiogram (EKG), making their identification crucial for accurate diagnosis and management. This guide provides an analytical overview of various supraventricular rhythms, their EKG characteristics, and key differentiating features. Note that stating someone has SVT isn’t specific enough- stating the type of SVT is crucial in terms of best next steps.
Premature Atrial Contractions (PACs)
Premature Atrial Contractions occur when an ectopic focus within the atria initiates a heartbeat earlier than expected. On the EKG, PACs are identified by P-waves with different morphology and axis compared to normal P-waves. These P-waves appear sooner than anticipated in the cardiac cycle, disrupting the regular rhythm established by the SA node. Recognizing PACs is important as they can be precursors to more serious arrhythmias.
Atrial Escape Beats
Atrial escape beats arise when non-SA nodal atrial sites generate impulses later than expected, often due to a temporary failure of the SA node. These beats typically occur at a rate of 60–75 beats per minute. The EKG will show altered P-wave morphology and axis, distinguishing them from normal sinus beats. Atrial escape beats serve as a protective mechanism to maintain heart rhythm when the primary pacemaker fails.
Atrioventricular Nodal Reentrant Tachycardia (AVNRT)
AVNRT is characterized by a regular rhythm with a rapid rate of 150–250 beats per minute. On the EKG, P-waves may be located before, during, or after the QRS complex, and they might be hidden within it. The P-wave morphology changes, appearing inverted in lead II and upright in lead aVR due to the retrograde conduction of impulses. Carotid massage can be effective in slowing down or terminating AVNRT by increasing vagal tone, which influences the AV node.
Atrioventricular Reentrant Tachycardia (AVRT)
Often associated with Wolff-Parkinson-White (WPW) syndrome, AVRT involves an accessory pathway that allows impulses to bypass the AV node. WPW is characterized by a short PR interval and a delta wave on the EKG. AVRT presents as a regular tachycardia with either broad or narrow QRS complexes.
Atrial Flutter
Atrial flutter features a regular atrial rate of approximately 300 beats per minute. The ventricular rate is usually slower due to a conduction block (e.g., 2:1 or 3:1 block) at the AV node. The EKG displays characteristic “sawtooth” flutter waves. If the conduction block varies, the ventricular rate may become irregular. Carotid massage can increase the degree of block, further slowing the ventricular rate. Atrial flutter can distort the baseline, potentially leading to pseudo-Q waves or ST-T changes on the EKG.
Atrial Fibrillation
In atrial fibrillation, the atrial rate exceeds 350 beats per minute, resulting in no distinct P-waves on the EKG. The baseline appears chaotic due to erratic electrical activity. The ventricular response is irregularly irregular, a hallmark of this arrhythmia. Coarse atrial fibrillation can distort the EKG baseline, causing pseudo-Q waves or ST-T changes. Effective management is crucial to prevent complications like stroke.
Atrial Tachycardia
Atrial tachycardia presents as a regular rhythm with a rate of 100–200 beats per minute. The P-wave morphology and axis differ from those of normal sinus rhythm, indicating an ectopic atrial focus.
Can be difficult to differentiate from other SVTs but the presence of the following suggest atrial tachycardia:
- Rate: Atrial tachycardia has a slower atrial rate compared to atrial flutter.
- Warm-Up Period: usually paroxysmal and may exhibit a “warm-up” period where the heart rate gradually increases.
- Minimal response to carotid massage
- Isoelectric Line: An isoelectric line is present between P-waves in atrial tachycardia, while atrial flutter lacks this due to continuous atrial activity.
Wandering Atrial Pacemaker
Wandering atrial pacemaker is characterized by an irregular rhythm with a heart rate below 100 beats per minute. The EKG shows at least three different P-wave morphologies and variable PR intervals, indicating multiple atrial pacemaker sites. This rhythm is generally benign and often seen in healthy individuals, particularly athletes or during sleep.
Multifocal Atrial Tachycardia
Similar to wandering atrial pacemaker but with a faster rate, multifocal atrial tachycardia has a heart rate of 100–200 beats per minute. The EKG reveals at least three different P-wave morphologies and varying PR intervals. This arrhythmia is commonly associated with pulmonary diseases and is significant in elderly patients with respiratory conditions. It is irregularly irregular and is often mistaken for atrial fibrillation on clinical exam as well as on EKGs.
Junctional Rhythms
Junctional rhythms originate near the atrioventricular (AV) node. On the EKG, P-waves may appear before, during, or after the QRS complex or may be hidden within it. The P-wave morphology changes, appearing inverted in lead II and upright in lead aVR due to retrograde atrial activation. Types of junctional rhythms include:
Junctional Premature Complexes
Occur earlier than expected due to premature impulses from the AV node.
Junctional Escape Rhythm
Occurs later than expected with a rate of 40–60 beats per minute, serving as a backup pacemaker.
Accelerated Junctional Rhythm
Similar to junctional escape rhythm but with a rate of 60–100 beats per minute.
Junctional Ectopic Tachycardia
A rapid rhythm exceeding 100 beats per minute. Differentiating it from AVNRT may require electrophysiological studies. Vagal maneuvers like carotid massage typically help in AVNRT but have minimal effect on junctional ectopic tachycardia.
Conclusion
Recognizing the differences in P-wave morphology, rhythm regularity, and response to interventions like carotid massage can aid in differentiating between the various supraventricular rhythms, leading to better patient outcomes.
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