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Bradycardia and Heart Block the ConducOn System

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Bradycardia and Heart Block the Conduc�On System Bradycardia and Heart Block The conduc3on system SA node (natural pacemaker) AV node (junction box) Left bundle branch posterior fascicle Left bundle branch anterior fascicle Right bundle branch The normal ECG Bradycardia • The conduc3on system can fail at any point • Bradycardia per se is not a bad thing • Bradycardia without symptoms is usually not an issue, except in select cases The conduc4on system – SA node SA node (natural pacemaker) Sinus Bradycardia Sinus Bradycardia – a problem? • Physically fit people have heart rates in the 50s • Athletes can have heart rates as low as 25 without a problem • If this is an 80 yr old lady, chances are this is sinus node dysfuncon • If she has lethargy, exercise incapacity, falls or blackouts, refer for consideraon of pacing Atrial Pacemakers • Atrial pacemakers can restore exercise capacity • Have not been shown to improve mortality, only morbidity What is happening with the rhythm? Sinus arrhythmia: Sinus arrhythmia • In itself not an issue • Usually mediated by vagal tone • Common in athletes What is unusual about this rhythm strip? Low atrial rhythm – usually benign Describe the rhythm strip Junconal ectopics (no p waves visible with the ectopics despite ‘room’ to see them) Profoundly hypothyroid paent Junconal bradycardia The conducon system AV node (junction box) Left bundle branch anterior fascicle The AV node • If problems occur within the AV node, they can form one of the three types of HB • First degree (conducts every beat) • Second degree (conducts some beats) • Third degree heart block (conducts none) • If the QRS width is less than 120ms (three small squares), conduc3on must be coming through the AV node and down both bundles First degree block, not usually a big deal First degree –what to do? • In very young person (<45), refer for inves3gaon • Anyone old enough for IHD, look for signs of old infarct to explain (par3cularly inferior) • Check for drugs that can do this and consider stopping these if alternaves available • In older person (>70) likely represents beginnings of conduc3ons system disease • Look out for symptoms of higher grade block • Don’t treat or inves3gate unless you suspect this • Check thyroid Second degree heart block Mobitz type I (Wenkebach) • Progressive prolongation of the PR interval followed by a non-conducted beat Mobitz type II with 2:1 block. • Every second beat is non-conducted Third degree (complete) heart block (for comparison) • No connection at all between p waves and QRS complexes Wenkebach Wenkebach – what to do • Usually nothing • Usually due to high vagal tone • Can occur post-infarct like any arrhythmia • Rarely symptomac • Refer symptomac paents • Consider referral of asymptomac paents aer seeing results of 24 hour tape Mobitz Type II block Mobitz Type II Heart Block • Always pathological • Strong possibility of needing paced • Can progress to complete heart block unpredictably • Post-infarct scenario complex and considered separately Third degree (Complete) Heart Block CHB • Post-infarct scenario aside, is usually paced • If in context of drugs, thyroid, hyperkalaemia, decision is made case by case • Outside these scenarios untreated CHB has 50% mortality at 1 yr • Sudden death a possibility • Admit Dual chamber pacing – modern pacing spikes can be very small Temporary pacing • There are five hard indicaons for a temporary pacing wire; • Syncope at rest • End-organ failure • Escape VT • HB with anterior infarct • Acutely alterna4ng leT and right bundle BB. – Some would include a broad escape, but external pacing is usually adequate for this – Pacing wire complicaon rates are significant – Consider carefully before inser3on and discuss where possible The conducon system Left bundle branch posterior fascicle Left bundle branch anterior fascicle Right bundle branch Right Bundle Branch Block: QRS ≥3 squares and V1 points up Right Bundle Branch Block • Many causes • May be normal variant (3% of populaon) • OPT referral if paent has symptoms to indicate an underlying cause – SOB – Chest pain – Dizzy spells N.B. RBBB should have a normal axis LeT Bundle Branch Block QRS ≥3 squares and V1 points down LeT Bundle Branch Block • Always pathological • Requires inves3gaon • Can exacerbate heart failure, but otherwise is asymptomac LeT Anterior Fascicular Block Left bundle branch posterior fascicle Left bundle branch anterior fascicle Right bundle branch LeT anterior fascicular block = LeT axis deviaon without cause LeT Anterior Fascicular Block • Part of the leY bundle not func3oning • Causes leY axis deviaon on the ECG, but not broadening of the QRS • Other things can cause leY axis deviaon on an ECG – LVH – Body habitus – Structural heart disease • No LVH, plus leY axis deviaon, is usually LAFB • The aide memoire is in the leders: Le Anterior fascicular block = Le Axis deviaon LeT posterior fascicular block = Right axis deviaon without cause Bifascicular Block Left bundle branch posterior fascicle Left bundle branch anterior fascicle Right bundle branch Fascicular Block • Clinically not essen$al to understand • However, worth knowing how to spot them as they are a clue as to the extent of your paent’s conduc3on disease • There are three fascicles: – The right bundle – The leY anterior fascicle of the leY bundle – The leY posterior fascicle of the leY bundle Fascicular Block – If the only the right bundle goes, we should see RBBB with a normal axis – If the leY anterior and leY posterior go we see LBBB – If only one of the leY fascicles goes, we only see axis deviaon on the ECG with no broadening of the QRS Therefore....... – If the right goes and one of the leY fascicles goes, we see RBBB with an abnormal axis Bifascicular block – RBBB & LeT axis Bifascicular block – RBBB & Right axis Trifascicular block • Badly named • This refers to a situaon where two fascicles are blocked and the remaining fascicle has abnormal conduc3on • i.e. bifascicular block plus first degree heart block The conducon system Left bundle branch posterior fascicle Left bundle branch anterior fascicle Right bundle branch Trifascicular block – RBBB (one), leT axis (two), first degree HB (hence tri-) Summary • The conduc3on system can fail at any point • Conduc3on block on a res3ng ECG is a pointer that more trouble may lie ahead • First degree, Wenkebach, Bifasc Block, Trifasc Block don’t themselves cause morbidity • They make it more likely that rhythms may also be intermidently occurring that can cause morbidity (CHB) • CHB and Mobitz Type II usually paced .
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