Determine the following:
P waves: _______________________________________
PR interval: ____________________________________
QRS complex: __________________________________
What's your interpretation?
(Answers on next page)
Name that strip: Answers
Rhythm: Basic rhythm regular; irregular with premature junctional contraction (PJC)
Rate: 75 beats/minute
P waves: Sinus P waves with basic rhythm; inverted P wave before PJC
PR interval: 0.16 second (basic rhythm); 0.08 to 0.10 second (PJC)
QRS complex: 0.06 to 0.08 second (basic rhythm and PJC)
Comment: ST segment depression is present
Interpretation: Normal sinus rhythm with one PJC.
A PJC is an early beat that originates in an ectopic pacemaker site in the atrioventricular (AV) junction, interrupting the regularity of the basic rhythm, which is usually a sinus rhythm. Like the premature atrial contraction (PAC), the PJC is characterized by a premature, abnormal P wave followed by a normal duration QRS complex and a pause that is usually noncompensatory. The premature beat occurs in addition to an underlying rhythm. Therefore, both the underlying rhythm and the premature beat must be identified (for example, normal sinus rhythm with a PJC). Some differences exist, however, between the two premature beats. Because atrial depolarization occurs in a retrograde fashion with the PJC, the P wave associated with the premature beat will be negative in lead II (a positive lead). The inverted P waves will occur immediately before or after the QRS complex or will be hidden within the QRS complex. The PR interval will be short (0.10 second or less).
PACs are much more common than PJCs. As a result, narrow complex premature beats are more likely to be PACs. PJCs occur in the same pattern as PACs: as a single beat; in bigeminal, trigeminal, or quadrigeminal patterns; or in pairs. A series of three or more consecutive junctional beats is considered a rhythm (junctional rhythm, accelerated junctional rhythm, or junctional tachycardia). Differentiation of the rhythm depends on the heart rate. Like PACs, the premature junctional impulse may be conducted to the ventricles abnormally (aberrantly). This results in a wide QRS complex. Because of the wide QRS complex, PJCs with aberrancy must be differentiated from premature ventricular contractions (PVCs).
Conditions associated with PJCs include ingestion of substances such as caffeine, alcohol, or tobacco; electrolyte imbalances; hypoxemia; heart failure; coronary artery disease; and enhanced automaticity of the AV junction caused by digitalis toxicity (the most common cause). PJCs may also occur without apparent cause. Frequent PJCs (more than 6 per minute) are best treated by correcting the underlying etiology.
Frequent PJCs may precede the development of a more serious junctional dysrhythmia, such as junctional tachycardia. Occasionally, an ectopic beat will occur late instead of early. These junctional escape beats usually occur during a pause in the underlying rhythm (following sinus arrest or block, after premature beats or nonconducted PACs, or during the pause associated with second-degree AV block Type I). The pause in the rhythm allows a focus in the AV junction to “escape” and pace the heart. Escape beats act as an electrical backup to maintain the heart rate and require no treatment.
Premature junctional contraction: Identifying ECG features
Rhythm: Underlying rhythm usually regular; irregular with PJC
Rate: That of the underlying rhythm
P waves: P waves associated with the PJC will be premature, will be inverted in lead II, will occur immediately before or after the QRS complex, or are hidden within the QRS complex
PR interval: Short (0.10 second or less)
QRS complex: Premature; normal duration (0.10 second or less)