3rd Degree AB Block Complete Heart Block

3rd Degree AB Block / Complete Heart Block Rate: Atrial rate is normal. Ventricular rate is slower. 40-60 bpm if back-up pacer is from the junction or 20-40 bpm if back-up pacer is from the ventricles PRI: No relationship between P waves and QRS complex. QRS :LGWKVHFRQGVLIFRQWUROOHGE\WKHMXQFWLRQ!VHFRQGVLISDFHGE\WKHYHQWULFOH. QT: May be < .40 seconds if it is a junctional rhythm or may be wider (> .40 sec) with slower ventricular rhythms. P waves: Upright and normal. Rhythm: Rhythm: P-P is regular; R-R is regular (but the two are independent functions) Source of pacer: Variable. Will be below the atria, since the connection between the SA Node and AV Node has been severed. Usually Junctional or Ventricular in origin. Characteristics: Third-degree AV block is also called complete heart block. This type of dysrhythmia indicates complete absence of conduction between atria and ventricles (the atria and the ventricles are not communicating with one another). The atrial rate is always equal to or faster than the ventricular rate in complete heart block. The block may occur at the level of the AV node, the bundle of His, or in the bundle branches. As in second-degree AV block, this distinction is not merely academic since pathogenesis, treatment, and prognosis may vary considerably, depending on the anatomic level of block. When third-degree AV block occurs at the AV node, a junctional escape pacemaker frequently will initiate ventricular depolarization. This is usually a stable pacemaker with a rate of 40 to 60 beats per minute. Since it is located above the bifurcation of the bundle of His, the sequence of ventricular depolarization usually is normal, resulting in a normal QRS. This type of third-degree AV block can result from increased parasympathetic tone associated with inferior infarction, from toxic drug effects (e.g., digitalis, propranolol), or from damage to the AV node. When third-degree AV block occurs below the junction, it is most often due to a block involving both bundle branches. The only escape mechanism available is in the ventricle distal to the site of block. Such a ventricular escape pacemaker has an intrinsic rate that is slow, less than 20-40 beats per minute. Like any depolarization originating in a ventricle, the QRS complex will be wide. It is not a stable pacemaker, and episodes of ventricular asystole are common. Remember that the rhythm strip reflects two separate processes that are taking place. The SA node continues to control the atria and typically fires at a rate of 60-80 bpm. Since the atria and the ventricles are not communicating, one of the two remaining back-up intrinsic pacemakers will take over. Either the junction will pace the ventricles (rate 40-60 bpm) or the back-up ventricular pacer will discharge (rate 20-40 bpm). When there are two separate pacemakers controlling the upper and lower chambers of the heart without regard to each other, the situation is called AV disassociation (this is not a “rhythm” but a “condition” and the umbrella term AV disassociation is often used). On the ECG, you will see normal P waves marching regularly across the strip. The P-P intervals are regular. You will also see QRS complexes at regular intervals. The unique feature is that the P waves and the QRS complexes will not be “talking to each other.” There is no relationship between the P and the QRS waveforms. The PR interval will be totally inconsistent and you may even see P waves superimposed in the middle of QRS complexes. There will be more P waves than QRS complexes (because the intrinsic rate of the sinus node is faster than either the junctional or ventricular rates). Since there is no connection between the atria and ventricles, it is possible to have an atrial rhythm such atrial fibrillation or flutter, and a junctional or idioventricular rhythm driving the ventricles. This is illustrated in the EKG strip below. Causes: 3rd degree AVB may be transient and reversible or permanent. Common causes are: acute inferior or right ventricle MI, ischemic heart disease in general, increased vagal tone, digitalis toxicity, amiodarone, beta-blockers, calcium channel blockers, electrolyte imbalances. Nursing Priorities: Check the patient’s blood pressure, assess for syncope, palpitations, or SOB. Your patient may need to lie down to prevent syncope and/or potential falls. Patient may have lower B/P due to low ventricular rate. Potential Treatment: Asymptomatic: Notify physician. Observation and monitoring only. Hold drugs that can slow AV node conduction. Obtain supplies for pacing should this become necessary. Symptomatic: Notify physician. If symptomatic bradycardia is present, administer atropine and apply transcutaneous pacemaker. Atropine may be effective if the QRS is narrow (AV node level of block) but has little or no effect on wide QRS (bundle-branch level) third-degree block rhythms. Administer a dopamine infusion if patient is hypotensive.