ECG Exigency 011
He also complains of some anterior sternal discomfort that is worse with deep breathing and coughing. He is otherwise stable with BP 130/70, MAP 95, clear chest, normal mental status and good urine output. The nurse asks you: “Are you sure his pacemaker is working ok?” His ECG is shown below:
Q1. Describe the ECG.
- Atrially paced rhythm with atrial pacing spikes at around 90 bpm
- Narrow QRS (90 ms)
- Each pacing spike is followed by a small P wave, indicating 100% capture of the atria
- There are intermittent non-conducted P waves (dropped QRS complexes) with overall ventricular rate around 72 bpm
- The PR interval increases with each successive beat until a QRS complex is dropped
- PR interval is longest immediately before the dropped beats (460 ms) and shortest immediately after the dropped beats (240 ms)
- No acute ST changes to indicate ischaemia as the cause of his pain
Q2. What is the diagnosis?
Q3. What are the possible causes of this rhythm?
- Increased vagal tone (athletes, young children, painful stimuli)
- Valvular heart surgery (especially mitral valve sugery)
- AV nodal blocking drugs (beta-blockers, calcium channel blockers, amiodarone, digoxin)
- Inferior MI
- Congenital heart disease (e.g. Tetralogy of Fallot)
AV block following mitral valve surgery
- AV block is fairly common following mitral valve surgery, as the AV node lies close to the posterior leaflet of the mitral valve.
- In a recent study of 391 patients following mitral valve surgery (replacement or repair), the incidence of AV block was 23.5%.
- The most common type of AV block was first degree (in 18% of patients), followed by third degree (4%), with second degree block the least common (1.3%).
- A large proportion (44%) of these AV blocks are transient, resolving prior to hospital discharge.
Possible mechanisms for AV block following mitral valve surgery include:
- Direct damage to the AV conduction system during surgery
- Damage to the AV nodal artery causing ischaemia of the AV node
- Prolonged cross-clamp time causing ischaemia of the AV node
- Peri-operative use of AV-nodal blocking drugs
Q4. How would you manage this patient?
- Withhold any AV nodal blocking drugs
- Correct any electrolyte abnormalities
- Treat his pain (most likely due to his recent sternotomy – but be on the lookout for any signs of myocardial ischaemia)
- Given that this patient is currently haemodynamically stable, there is no need to alter his pacemaker settings at present
- If hypotension developed secondary to a low heart rate, he could be switched over to VVI pacing to maintain cardiac output
- Permanent pacing would need to be considered if haemodynamically unstable AV block persisted beyond the first week post-op
Indications for permanent pacing would include:
- Progression to complete (third degree) AV block
- Development of Mobitz II AV block
- Persistent Mobitz I AV block with an inadequate ventricular rate (causing haemodynamic compromise)
- AF with a slow ventricular response
Q5. Can you guess what happened next?
- The patient remained in Mobitz I for the next 12 hrs without any drop in BP or deterioration to 3rd degree AV block.
- The following morning he was trialled off pacing and found to be in sinus bradycardia at 56 bpm with a first degree heart block (PR 240 ms); BP was stable at 130/80, so he was switched over to backup VVI pacing with the pacing wires capped after a further few hours.
- The first degree heart block was not present on his pre-op ECG.
- It appears that he had developed some post-op AV dysfunction that resulted in a first degree heart block at slower heart rates and progressed into Mobitz I when paced at a faster rate.
- At present, his first degree heart block persists but he is recovering well from surgery.
- Berdajs D, Schurr UP, Wagner A, Seifert B, Turina MI, Genoni M. Incidence and
pathophysiology of atrioventricular block following mitral valve replacement and
ring annuloplasty. Eur J Cardiothorac Surg. 2008 Jul;34(1):55-61. Epub 2008 May
15. PMID: 18482844.
- Meimoun P, Zeghdi R, D’Attelis N, Berrebi A, Braunberger E, Deloche A, Fabiani
JN, Carpentier A. Frequency, predictors, and consequences of atrioventricular
block after mitral valve repair. Am J Cardiol. 2002 May 1;89(9):1062-6. PubMed