Mar 20, 2008 ECG Rounds Yael Moussadji, R4

Case 1

Case 2

Pacemakers ECG manifestations of normally functioning pacemakers Pacing modes Ventricular paced rhythms vs LBBB Myocardial ischemia in VPR Pacemaker malfunction

Pacing Modes

Mode?

AAI

Mode?

VVI

Mode?

DDD

ECG Highlights Atrial Paced Rhythm Pacer spike just before the P wave P wave normal morphology Ventricular Paced Rhythm Pacer spike before QRS, morphology similar to LBBB with LAD QRS and T waves discordant Wide, mainly negative QS with poor R wave progression QS complexes in II, III, aVF; large R waves in I, aVL Dual-chambered pacing Pacing spikes before P and QRS

Which of these could be a dual-lead paced heart? a b c d all of the above

VPR vs LBBB Ventricular pacing lead is placed in RV, ventricles depolarize from right to left by slow conducting tissue rather than conduction system Manifest by wise, negative QS complexes with poor R wave progression with overall morphology similar to LBBB The ventricular lead is placed near the apex, causing the ventricles to contract from apex to base, yielding a LAD, and discordant ST segments and T waves Distinguishing between VPR and LBB is easiest in V6; in LBBB the QRS is upright; however, in VPR the QRS is below the isoelectric line because the pacemaker impulse is generated at the apex and travels in the opposite direction of the V6 lead axis

AMI in VPR

Cabrera’s Sign

AMI criteria 1. STE >/= 5mm discordant with QRS (normally discordant STE in VPR is < 3mm) 2. STE >/= 1mm concordant with QRS (violates discordance rule) 3. STD >/= 1mm in V1, V2, V3 (violates discordance rule)

ECG Pearls LBBB can be differentiated from VPR by QRS vector in V6 Application of magnet converts pacing mode to asynchronous pacing, allowing to asses capture Check for pacemaker spikes in multiple leads To evaluate AMI check old ECG’s and be suspicious of exaggerated discordance or inappropriate concordance of QRS segments and T waves

Case 3

Pacemaker Malfunction Disorders of sensing, capture, or rate Complaints of palpitations, dyspnea, irregular heartbeat, syncope, presyncope, dizziness, CP, orthopnea, PND, or fatigue should prompt an evaluation for pacemaker malfunction Application of a magnet allows for assessment of capture, but not sensing

Failure to Pace Pacemaker output failure occurs when the pacemaker fails to fire in a situation where pacing should occur Can occur as a result of oversensing (most common), lead fracture, battery failure, electromagnetic interference No visible pacing spikes on ECG Application of magnet reveals no pacing spikes

Failure to Capture Pacing stimulus is generated but fails to trigger myocardial depolarization Most common cause is exit block - elevations in the threshold voltage required for depolarization Exit block can occur due to maturation of tissues or tissue damage from defibrillation Other causes include lead dislodgement or fracture, AMI, electrolyte abnormalities (hyperkalemia), metabolic derangements Best assessed by applying a pacer magnet, switching to asynchronous mode; shows regular pacing spikes without 100% capture

Failure to capture

Undersensing Occurs when a pacemaker fails to sense native cardiac activity Difficult to diagnose on ECG; inferred from knowledge of expected pacemaker behaviour

Pacemaker mediated tachycardia Reentry dysrhythmia occurring in dual chamber pacemakers with atrial sensing where the pacemaker acts as the reentry circuit Occurs when a retrograde P wave (most commonly after PVC) is interpreted as native atrial stimulus, triggering ventricular pacing and another reptrograde P wave etc Pacemaker is the antergrade conductor and retrograde conduction occurs through the AV node Appears as a wide complex, regular, ventricular paced tachycardia at a rate at or less than maximal upper rate of pacemaker ( bpm) Apply magnet to initiate asynchronous pacing to break reentry circuit

Pacemaker syndrome Constellation of signs and symptoms in patients with suboptimal pacing modes Major factor appears to be suboptimal AV synchrony Look for retrograde P waves or other evidence of AV asynchrony SBP may drop 20 mmHg when paced vs during native rhythm Requires upgrade to dual chambered pacing to restore AV synchrony