18.4
Dysrhythmias IV: Characteristics of Bradyarrhythmias
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Q1: What defines a bradyarrhythmia and how is it measured?
A bradyarrhythmia is a cardiac rhythm disorder with a heart rate below 60 beats per minute, resulting from issues with the heart's electrical conduction system. These slower-than-normal rhythms originate from abnormalities in how electrical impulses travel through the atria and ventricles, disrupting normal cardiac function and potentially causing symptoms like fatigue and dizziness.
Q2: How does sinus bradycardia appear on an ECG?
Sinus bradycardia shows a heart rate below 60 beats per minute with normal P waves preceding each QRS complex. The ECG displays a normal PR interval of 0.12 to 0.20 seconds and normal QRS duration of 0.06 to 0.10 seconds, with a regular rhythm originating from the SA node.
Q3: What is the key difference between Mobitz I and Mobitz II second-degree AV blocks?
Mobitz I features progressive PR interval lengthening until a beat is dropped, creating grouped beating with irregular ventricular rates. Mobitz II shows constant PR intervals with some P waves not followed by QRS complexes, occurring below the AV node in the His-Purkinje system without prior PR prolongation.
Q4: How does first-degree AV block affect conduction between atria and ventricles?
First-degree AV block is characterized by a prolonged PR interval greater than 0.20 seconds, indicating delayed conduction from the atria to the ventricles. Despite this delay, each P wave is still followed by a QRS complex, and the heart rate remains normal with a regular rhythm.
Q5: What happens to the atrial and ventricular rates in complete heart block?
In third-degree or complete AV block, the atria and ventricles beat independently with no conduction between them. The atrial rate typically remains 60 to 100 beats per minute, while the ventricular rate depends on block location: 40 to 60 beats per minute if in the AV node, or 20 to 40 beats per minute if in the His-Purkinje system.
Q6: How does the QRS complex appearance indicate the location of a complete heart block?
In complete heart block, a normal QRS complex indicates the escape rhythm originates at or above the bundle of His. A widened QRS complex signals the rhythm originates below the bundle of His, representing a ventricular escape rhythm that generates a slower, less reliable heart rate.
Q7: Why does third-degree AV block often require urgent pacemaker intervention?
Third-degree AV block causes complete dissociation between atrial and ventricular activity, resulting in dangerously slow ventricular rates and severe symptoms including fatigue, dizziness, and syncope. Nursing management of dysrhythmias includes pacemaker placement to restore coordinated heart function and maintain adequate cardiac output.