Carol Jacobson

Understanding atrioventricular blocks, part I: first-degree and second-degree atrioventricular blocks.

T electrocardiographic diagnosis of AV blocks presents a challenge to health care professionals who monitor cardiac rhythms. Much of the confusion exists as a result of differing definitions of the “degrees” of AV blocks in the literature, especially surrounding second-degree block. Many misconceptions exist about the definitions of type I and type II blocks and how these “types” differ from the “degrees” used in the AV block classification system. Additional sources of confusion and difference of opinion are (1) the concept of 2:1 conduction and (2) where to put blocks that involve failed conduction of 2 or more consecutive P waves. With this much uncertainty and difference of opinion among the experts, it is no wonder that instructors have a hard time explaining and teaching these concepts to students in cardiac arrhythmia interpretation classes. The term heart block or AV block refers to the problems associated with the conduction of an electrical impulse from the atria to the ventricles. The conduction can be delayed, intermittent, or absent. The classification system typically used to describe these “degrees” of conduction delay is still widely used in textbooks:

Narrow QRS complex tachycardias.

Narrow QRS complex tachycardias are either atrioventricular (AV) nodal passive or AV nodal active. AV nodal passive tachycardias do not require the participation of the AV node in maintenance of the tachycardia. Examples are atrial tachycardia, atrial flutter, and atrial fibrillation. Treatment is directed at ventricular rate control with calcium channel blockers or beta-blockers. AV nodal active tachycardias require active participation of the AV node in maintaining the tachycardia. Examples include AV nodal reentry tachycardia and circus movement tachycardia using an accessory pathway. Treatment with a vagal maneuver or adenosine usually terminates the tachycardia. Recognition of these tachycardias is reviewed.

Tools for teaching arrhythmias: wide QRS beats and rhythms--Part I. P waves, fusion, and capture beats.

T last issue of AACN Advanced Critical Care discussed mechanisms of aberration and situations that cause aberrant conduction. This issue begins a 2-part series on ECG clues that are helpful in differentiating aberrant conduction from ventricular beats and rhythms. The focus of this column is on the value of P waves and the concept of fusion and capture beats in helping to make the distinction. The next issue will focus on QRS morphology and why lead V1 is the recommended monitoring lead for differentiating ventricular beats from supraventricular beats with aberrant conduction.

Tools for teaching arrhythmias: wide QRS beats and rhythms--part II, QRS morphology clues.

Lead Selection Cardiac monitoring is done to diagnose complex arrhythmias, detect myocardial ischemia, and identify QT interval prolongation. There are 12 standard ECG leads available for cardiac monitoring, and each lead offers a different view of electrical activity traveling through the heart. Depending on the type of bedside monitoring equipment used, it is possible to display 1, 2, 3, or all 12 ECG leads at a time. Since different leads are used for different indications, lead selection should be made on the basis of the patient’s clinical situation and the goals of monitoring for each individual patient. For more than 40 years the published information on best leads to use for differentiating wide QRS beats based on morphology indicates that leads V1 and V6 are the best. 2–8 Despite this, many nurses still use lead II as their preferred bedside monitoring lead. Figure 1 illustrates why lead II is not helpful in identifying the origin of a wide QRS rhythm. Note that the QRS complexes in lead II look almost identical in all 4 examples of wide QRS beats, while lead V1 displays morphological clues that help differentiate the cause of each wide QRS beat. Whenever there is any doubt about an arrhythmia, “the best lead is a 12 lead”; but if differentiating wide QRS complex beats and rhythms is a goal of monitoring, then lead V1 (or V6 if V1 is unavailable for any reason) should be continuously monitored at the bedside. If the monitoring equipment offers the choice of a true V lead using a 5-wire cable or a modified chest lead (MCL) using a 3-wire cable, use a 5-wire cable and choose a true V lead rather than the MCL substitutes. Experienced clinicians are in an excellent position to ensure that appropriate leads are selected on the basis of the patient’s clinical situation and to change practice at the bedside if needed. The American Association of Critical-Care Nurses practice protocol on bedside cardiac monitoring and the Practice Alert dysrhythmia monitoring offer recommendations for best practices in bedside monitoring, and summarize the research that supports these recommendations. These documents, along with Practice Standards for Electrocardiographic Monitoring in Hospital Settings, can serve as research-based resources to support needed changes in practice. AACN Advanced Critical Care

Tools for teaching arrhythmias: wide QRS beats and rhythms.

I keeping with the theme, Tools for Teaching (and understanding) Arrhythmias, the department article for this issue focuses on wide QRS beats and rhythms and tools to help learners understand mechanisms of wide QRS complexes. The QRS complex represents ventricular depolarization, and the width of the QRS represents intraventricular conduction time (the time it takes for the impulse to travel through the ventricles once it gets to them). A normal QRS width is 0.04 to 0.11 sec and represents depolarization of the ventricles through the normal His-Purkinje system, which spreads the impulse rapidly through both ventricles simultaneously. A QRS of 0.12 sec or greater represents an intraventricular conduction delay. Whereas ischemia, hyperkalemia, and certain antiarrhythmic drugs can cause slowing of conduction through the ventricles and widen the QRS, and ventricular paced beats result in a wide QRS, there are 3 “arrhythmic” causes of a wide QRS: