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Second-degree atrioventricular block.

Atrioventricular (AV) block refers to dysrhythmias involving a delay or failure in conduction of a sinoatrial (SA) node-originated impulse from the atria to the ventricles. First-degree AV block occurs when all SA node-originated impulses are conducted from the atria to the ventricles, but the conduction is delayed (prolonged PR interval). Second-degree AV block is characterized by intermittent conduction of these impulses from the atria to the ventricles. Third-degree or complete AV block occurs when there is a total absence of conduction of SA node-originated impulses from the atria to the ventricles (Huff, 2011).

Two types of second-degree AV block have been identified: Mobitz Type I and Mobitz Type II. However, when every other sinus node-originated P-wave is not followed by a QRS, it is impossible to determine from a rhythm strip if the heart block is Mobitz Type I or Mobitz Type II. This manifestation of AV block is known as 2:1 AV block (Sauer, 2012). Because second-degree heart block occurs only occasionally on most telemetry units, differentiating between the types of second-degree AV block can be challenging.

Mobitz Type I Second-Degree AV Block

Also known as Wenckebach, Mobitz Type I seconddegree AV block results when there is an intermittent block of SA node-originated impulse conduction within the AV node (nodal) followed by complete failure of the impulse conduction from the atria to the ventricles. It is characterized by a progressive slowing of successive SA node-originated impulses in the AV node (progressive prolongation of the PR interval) until one impulse is not conducted at all (no QRS following the P-wave) (Sauer, 2013).

On Figure 1, QRS complexes are grouped (groups of two in this instance). When groupings of QRS complexes are seen, Mobitz Type I heart block always should be considered in the rhythm analysis. In this particular example of Mobitz Type I AV block, the PR interval for the first QRS complex is 0.36. The PR interval for the second QRS complex lengthens to 0.48. The T-wave after the second QRS complex has more amplitude and is wider than the T-wave after the previous QRS complex. This indicates the third P-wave is actually hidden within the T-wave and is not conducted to the ventricles (not followed by a QRS complex) (Huff, 2011).

Using a caliper, you can determine the P-to-P interval is a constant 0.8 seconds throughout the rhythm strip (including the one hidden in the T-wave). Constant P-to-P intervals are an important characteristic of all types of second-degree heart block and result from the impulse originating in the SA node. If a P-wave before a missing QRS is early (P-to-P interval not constant), a non-conducted premature atrial complex (PAC) would be indicated--a more benign finding (Surawicz & Knilans, 2008).

Mobitz Type I second-degree AV block is usually asymptomatic unless the heart rate is very slow, resulting in decreased cardiac output. Mobitz Type I second-degree AV block has been noted in 2%-10% of long-distance runners. However, it also occurs as a result of drugs that slow AV node conduction (beta blockers or calcium channel blockers) or in persons with intrinsic AV nodal disease, myocarditis, acute inferior myocardial infarction or ischemia, and after cardiac surgery (Sauer, 2013). When Mobitz Type I AV block is noted, vital signs should be taken to determine if the rhythm has resulted in a change in blood pressure or heart rate. The nurse then must determine the presence of symptoms (e.g., fatigue, lightheadedness, syncope, presyncope, chest pain/pressure, shortness of breath) prior to contacting the physician or initiating the emergency response team as indicated.

Mobitz Type II Second-Degree AV Block

Mobitz Type II second-degree AV block almost always results with episodic failure of impulse conduction in the His-Purkinje system (infranodal). It is characterized by unchanged PR intervals for conducted beats (those followed by a QRS), with intermittent sinus P-waves without conduction through to the ventricles (evidenced by the absence of a QRS complex) (Surawicz & Knilans, 2008).

On Figure 2, the P-to-P intervals are again constant (although every third one is just before the T-wave). Unlike Mobitz Type I second-degree AV block in which the PR interval progressively lengthens, the PR interval for the conducted beats (those followed by a QRS) in Mobitz Type II AV block is constant (0.34 in this rhythm strip). Additionally, Mobitz Type II AV block often is accompanied by a widened QRS complex (Surawicz & Knilans, 2008).

Mobitz Type II second-degree AV block may be symptomatic or asymptomatic. A slow heart rate (fewer conducted beats) with this rhythm can lead to signs and symptoms of reduced cardiac output. Mobitz Type II AV block can occur from age-related degeneration of the conduction system, myocardial ischemia or infarction, or as a result of drugs that block the AV node (e.g., digoxin [Lanoxin[R]], beta blockers, calcium channel blockers). Because it is often permanent and frequently progresses to third-degree or complete AV block, Mobitz Type II AV block is considered a more serious dysrhythmia than Mobitz Type I (Sauer, 2012). When Mobitz Type II AV block is noted, vital signs should be taken to determine if the rhythm has resulted in a change in blood pressure or heart rate. In addition, the presence of symptoms (e.g., fatigue, lightheadedness, syncope or presyncope, chest pain/pressure, shortness of breath) should be assessed. A physician should be notified immediately and the emergency response team should be activated.

2:1 AV Block

When every other P-wave is not followed by a QRS complex in second-degree AV block (the impulse does not conduct through to the ventricle), it is not possible to differentiate Mobitz Type I from Mobitz Type II AV block using a rhythm strip. In this situation, the nurse is unable to observe for the progressive lengthening of consecutive PR intervals that is characteristic of Mobitz Type I second-degree AV block. Thus, the rhythm is identified as 2:1 AV block (Surawicz & Knilans, 2008).

In Figure 3, as in all second-degree AV block rhythms, the P-to-P intervals are constant. Similar to Mobitz Type II AV heart block, the PR intervals for the conducted beats (those followed by a QRS) are constant.

When 2:1 AV block is noted, a lengthy rhythm strip can sometimes give clues as to whether the rhythm is Mobitz Type I (nodal pathology) or Mobitz Type II (infranodal pathology). When 2:1 AV block (with narrow QRS) is found in the same recording strip as an obvious Mobitz Type I AV block, Mobitz Type II AV block essentially is excluded as an interpretation because the coexistence of both types of block is extremely rare (Surawicz & Knilans, 2008).

Rhythm strips from Figures 1 and 3 are actually a contiguous strip from the same patient! The QRS is narrow (<0.12). Thus, the 2:1 block seen in this patient is likely Mobitz Type I and nodal rather than infranodal in origin. This is important, as the treatment for infranodal and nodal rhythms differs.

Conclusion

The differences between second-degree AV blocks are summarized in Table 1. This can be photographed with a smartphone so it is available for reference. Because the clinical significance for Mobitz Type I and Mobitz Type II differs, the ability to determine the correct rhythm is an essential skill for nurses who are responsible for monitored patients.

REFERENCES

Huff, J. (2011). ECG workout, exercises in arrhythmia interpretation (6th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.

Sauer, W.H. (2012). Second degree atrioventricular block: Mobitz type II (Wenckeback block). Retrieved from http://www.uptodate.com/contents/second-degree-atrioventricular-block-mobitz-type-i-wenckebach-block?source=search_result&search=second+degree+heart+block&selectedTitle=2%7E62

Sauer, W.H. (2013). Second degree atrioventricular block: Mobitz type I. Retrieved from http://www.uptodate.com/contents/second-degree-atrioventricular-block-mobitz -type-ii?source=search_result&search=second+degree+heart+block&selectedTitle=1%7E62

Surawicz, B., & Knilans, T. (2008). Chou's electrocardiography in clinical practice (6th ed.). Philadelphia, PA: Elsevier Sanders.

TABLE 1.

Second-Degree AV Blocks

Type of AV Block         Rhythm               P-Waves

Second-Degree      Atrial: regular       Sinus origin
Mobitz Type 1      Ventricular:
                   irregular

Second-Degree      Atrial: regular       Sinus origin with
Mobitz Type II     Ventricular:          two or more
                   regular or            P-waves before
                   irregular             each QRS

2:1                Atrial: regular       Sinus origin
                   Ventricular:          Every other P-wave
                   regular               is not followed by
                                         a QRS complex.

Type of AV Block       PR Interval          QRS Complex

Second-Degree      Progressively         Normal (<0.12)
Mobitz Type 1      lengthens until one
                   P-wave does not
                   conduct through to
                   the ventricle
                   (the QRS complex
                   is missing).

Second-Degree      Remains constant      Normal or widened
Mobitz Type II     for conducted beats
                   (those followed
                   by a QRS).

2:1                Remains constant      If normal width,
                   for conducted beats   rhythm is likely
                   (those followed       Mobitz Type I; if
                   by a QRS).            widened (>0.12),
                                         more likely to be
                                         second-degree
                                         Mobitz Type II.

Source: Surawicz & Knilans, 2008
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Title Annotation:Tackling Telemetry
Author:Palmer, Beth; Carroll, Karen
Publication:MedSurg Nursing
Article Type:Report
Date:Jul 1, 2014
Words:1454
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