IDENTIFYING HEART BLOCKS, also called atrioventricular (AV) blocks, can be a challenge for nurses, even those working in a telemetry unit. Because not all patients require cardiac rhythm monitoring, many nurses lack enough current experience to keep their skills sharp. And even nurses on telemetry units encounter heart blocks less frequently than normal sinus rhythm or dysrhythmias such as atrial fibrillation. In addition, distinguishing types of heart block—for example, differentiating second-degree heart blocks from a third-degree heart block—can be difficult.
This article provides a three-question algorithmic method to help nurses identify first-, second-, and third-degree heart blocks more easily and consistently. To use this algorithmic method, nurses need a solid understanding of cardiac rhythm interpretation principles. (See Start with the basics.)
Ask three questions to identify heart blocks
Nurses need to know how to identify and differentiate heart blocks because patient management may vary. Although patients normally aren't symptomatic with first-degree heart block or Mobitz type I (Wenckebach) second-degree heart block, some patients may experience symptoms such as fatigue or dizziness, depending upon the underlying cause of the heart block. Patients may experience more serious symptoms, such as hypotension or change in mentation, with Mobitz type II second-degree heart block or third-degree heart block (complete heart block).
By asking yourself three questions presented here, you can quickly identify the different types of heart blocks. For this method to work, P waves and QRS complexes must be identifiable and the cardiac dysrhythmia should not be atrial, junctional, or ventricular in origin.
Question 1: If the PR interval is > 0.20 second consistently, does the number of P waves equal the number of QRS complexes?
If yes, the rhythm is most likely a first-degree heart block, which is characterized by a PR interval of greater than 0.20 second. (See Sinus bradycardia with first-degree heart block.) Electrical impulses from the sinoatrial (SA) node are delayed but not blocked at the atrioventricular (AV) junction, prolonging the PR interval.2
First-degree heart block can be seen with both sinus bradycardia and sinus tachycardia as well as a normal sinus rhythm. Both the atrial and ventricular rhythms are regular. The QRS complex is usually narrow with a duration of < 0.12 second, unless an intraventricular conduction defect is present.3 The key identifying characteristic of first-degree heart block is consistent prolongation of the PR interval (> 0.20 second).
If no, proceed to the next question in the algorithm.
Question 2: Does the PR interval lengthen from beat (QRS complex) to beat (QRS complex) until a QRS complex is dropped?
If yes, the rhythm is most likely second-degree heart block Type I (also called Mobitz I or Wenckebach). (See Second-degree heart block, Type I [Mobitz I or Wenkebach].) In second-degree heart block Type I, the conduction delay occurs below the SA node, usually within the AV node.3 Some electrical impulses from the SA node don't reach the ventricles, preventing generation of a QRS complex.
The key characteristic of this type of block is progressive lengthening of the PR interval until a P wave is blocked and no QRS complex is generated.3 After a QRS complex fails to occur, the process starts all over again. The atrial rhythm is regular. However, the ventricular rhythm is irregular and the atrial rate is just slightly faster than the ventricular rate (because of the dropped beats).2
If no, proceed to the next question.
Question 3: Are PR intervals present and consistent?
If yes, the rhythm is second-degree heart block Type II (also called Mobitz II). (See Second-degree heart block, type II [Mobitz II].) This type of block is characterized by too many P waves in comparison to QRS complexes. The conduction delay in this type of block is most often below the AV node, either at the bundle of His or at the bundle branches; therefore, a QRS complex doesn't follow the blocked P wave.4 When a P wave is followed by a QRS complex, the PR interval is usually constant. When the QRS is blocked or dropped after every second, third, or fourth P wave, the rhythm disturbance is called a 2:1, 3:1, or a 4:1 block.2,3 The heart rate may vary, but the atrial rate will always be greater than the ventricular rate because of the non-conducted P waves.
With this type of block, the atrial rhythm is regular and the ventricular rhythm is usually irregular because of the blocked impulses. However, the ventricular rhythm is regular if there is a consistent block pattern, such as 2:1 or 3:1 block.
If no, the rhythm is third-degree heart block, also known as AV dissociation or complete heart block. (See Third-degree heart block [complete heart block]). The block occurs at the AV node, bundle of His, or bundle branches. The atria and ventricles depolarize independently, with no relationship between the two.
The key characteristic of this type of heart block is that both the atrial rhythm and ventricular rhythm are regular but independent (“dissociated”) and there is no consistent relationship between the P waves and QRS complexes.
Fast, accurate identification is key
This simple algorithmic method can help nurses quickly identify heart blocks with any telemetry-monitored patient. Accurate and timely assessment is key to intervening appropriately as needed.
Start with the basics7
Here are the five basic steps of cardiac rhythm analysis:
- Determine the rhythm by measuring the distance between R waves and noting any variations in R wave regularity. Determine if a 0.12-second or greater variance exists between the shortest and longest R wave variations.
- Calculate the heart rate, using the rapid rate calculation method (counting the number of R waves in a 6-second strip and multiplying by 10 to calculate the heart rate per minute), for regular or irregular rhythms. For a regular rhythm, you can also use the precise rate calculation method: Count the number of small squares between two consecutive R waves, and divide this number into 1,500 (the number of small squares in a 1-minute rhythm strip) to obtain the heart rate in beats per minute. Report the atrial and ventricular rates separately if they're different.
- Identify and examine P waves to see if one precedes each QRS complex, and if they're all identical in size, shape, and position.
- Measure the PR interval, which should be 0.12 to 0.20 second.
- Measure the QRS complex, which should be 0.10 second or less.
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6th ed. Philadelphia, PA: Lippincott, Williams, & Wilkins; 2015.
5. McCutcheon K, Rogers KM. The principles and practice of interpreting cardiac rhythm strips: a seven-step model. J Perioper Pract
6. Wesley K. Huszar's Basic Dysrhythmias and Acute Coronary Syndromes: Interpretation & Management
. 4th ed. St. Louis, MO: Mosby/Jems; 2011.
Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved.
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