Bradycardias Flashcards
normal sinus rhythm
60-100 bpm
sinus bradycardia
*HR < 60 bpm
*usually seen during rest or sleep (40-50 bpm)
*due to low sympathetic or high vagal tone
*common due to aging, disease, or medications
*not always pathologic
sinus tachycardia
*HR > 100 bpm
*seen with exercise, infection, blood loss
*sympathetic stimulation (ex. anxiety) or in infants
*inappropriate sinus tachycardia (IST) > 90 bpm average HR on 24h ambulatory monitoring without identified cause
how can bradycardia happen?
*problems with SA node
*problems with AV node
*age (degenerative fibrosis)
*drugs
*other systemic disorders
why is the sinus node the pacemaker?
*SA node has the fastest automaticity; SA overdrive suppresses all foci (since all foci have a slower inherent pacing rate)
*SA node inherent rate: 60-100bpm
*AV node inherent rate: 60-80 bpm
*junctional foci inherent rate: 40-60 bpm
*ventricular foci inherent rate: 20-40 bpm
concept of escape rhythms
*if there is an interruption of the normal mechanism of conduction, usually another electrically active area of the heart will take over and beat
mechanisms of sinus node problems
- failure of impulse formation (sinus node doesn’t fire)
- SA node first but the signal is blocked from exiting the SA node/perinodal tissue (sino-atrial exit block)
atrio-ventricular (AV) block
*any scenario when a P wave (atrial depolarization) FAILS TO GENERATE A QRS COMPLEX (ventricular depolarization)
*could be due to:
1. block at level of AV node; common, sometimes physiologic and sometimes pathologic
2. block is His-Purkinje system; usually abnormal due to disease or aging
comparing the different types of AV block
- first-degree AV block: every P makes a QRS, but with a LONG PR INTERVAL
- second-degree AV block: regular P waves; some but not all P waves make QRS complexes [dropped QRS complexes] (note - there are 2 types of second-degree heart block)
- third-degree heart block: constant and independent atrial and ventricular rates; no P conducts to a QRS
first degree heart block - EKG
*prolonged PR interval (>200 msec or > 1 big box)
*no dropped beats
first degree heart block - causes
*increased vagal tone (sleeping)
*drugs that slow AV node conduction
*aging (degeneration/scarring of the conduction system)
*myocardial disease/ischemia
first degree heart block - symptoms & treatment
*usually asymptomatic; benign
*treatment: none required
second degree heart block: Mobitz type I (Wenckebach) - EKG
*progressive lengthening of PR interval until a beat is “dropped” (P wave not followed by a QRS)
*variable RR interval with a pattern (regularly irregular)
*typical location of the block: within the AV node
second degree heart block: Mobitz type I (Wenckebach) - causes
*increased vagal tone (sleeping)
*drugs
*acute inferior MI (usually transient)
second degree heart block: Mobitz type I (Wenckebach) - symptoms & treatment
*usually asymptomatic
*treatment: none required
second degree heart block: Mobitz type II - EKG
*dropped beats that are not preceded by a change in PR interval
*consistent PR interval, with some P waves not followed by a QRS
*typical location of block: below the AV node, in the His-Purkinje system
third degree (complete) heart block - EKG
*P waves and QRS complexes rhythmically dissociated
*atria and ventricles beat independently of each other
*regular interval between the P waves & regular interval between QRS complexes, but no relationship between the two
*atrial rate > ventricular rate
*no stimuli from the atria are transmitted to the ventricles
