EKG 2 - Intervals

Question 1

heart block - defined

Answer 1

*an obstruction in the conduction system of the heart, including the pacemakers
*commonly leads to an arrhythmia as the heart seeks to circumvent the blocked conducting system by beating from an abnormal focus, rather than the SA or AV node

Question 2

first degree AV block

Answer 2

*characterized by PROLONGED PR INTERVAL (> 200 msec)
*delayed conduction through or from the AV node → prolonged time between atrial depolarization and ventricular depolarization
*asymptomatic; no tx required

Question 3

second degree AV block, Mobitz type I (Wenckeback)

Answer 3

*characterized by PROGRESSIVE LENGTHENING OF THE PR INTERVAL UNTIL A QRS COMPLEX IS DROPPED (P wave not followed by a QRS complex)
*variable R-R interval with a pattern (regularly irregular)
*anatomic site of block = AV node
*asymptomatic; no tx required

Question 4

second degree AV block, Mobitz type II

Answer 4

*characterized by CONSTANT PR INTERVALS WITH RANDOM, INTERMITTENT DROPPED QRS COMPLEXES
*anatomic site of block = bundle of His
*risk of progression to 3rd-degree heart block because it usually indicates a structural abnormality (ischemia or fibrosis)
*tx = pacemaker

Question 5

third degree AV block (complete heart block)

Answer 5

*characterized by RHYTHMICALLY DISSOCIATED P WAVES AND QRS COMPLEXES
*occurs when none of the atrial impulses are conducted to the ventricles → a separate, ectopic pacemaker forms in the ventricles, producing a slow escape rhythm
*rate of atrial depolarizations > rate of ventricular depolarizations (AV dissociation)
*tx = permanent pacemaker

Question 6

bundle branch blocks (general)

Answer 6

*interruption of conduction of normal left or right bundle branches
*affected ventricle depolarizes via slower myocyte-to-myocyte conduction from the unaffected ventricle (which depolarizes via the faster His-Purkinje system)
*both characterized by widened QRS complexes (>120 msec)

Question 7

right bundle branch block

Answer 7

*characterized by widened QRS complex, RsR’ (M-shaped) QRS morphology in V1 or V2, and prominent prolonged S wave in V6
*occurs because of delay through the right bundle → transmission of impulses in RV come from LV

Question 8

left bundle branch block

Answer 8

*characterized by widened QRS complex, notched wave without a Q wave in lead V6; QS or rS pattern in lead V1
*occurs because of delay in impulses down left bundle → impulses first travel to RV then to LV via interventricular septum → tall R waves in lateral leads (I, V5, V6) and deep S waves in V1-V3