Cardiac arrhythmias - supraventricular tachycardia
Description
- Tachycardia (>100 bpm) arising at or above the His bundle, requiring atrial or AV nodal tissue for initiation/maintenance
- Conventionally excludes AF, atrial flutter and multifocal AT, which are discussed separately - "SVT" in practice means the paroxysmal regular SVTs
The three that matter
| Share | Circuit | |
|---|---|---|
| AVNRT | ~60% | Re-entry within the AV node using slow + fast pathways |
| AVRT | ~30% | Re-entry using an accessory pathway (bypass tract) |
| Atrial tachycardia | ~10% | Focal atrial automaticity/micro-re-entry; does not need the AV node |
- Others: junctional ectopic tachycardia, inappropriate sinus tachycardia, sinus node re-entry, permanent junctional reciprocating tachycardia (PJRT)
Epidemiology
- Prevalence ~2.25/1000; incidence ~35/100,000/yr
- AVNRT: F>M (~2:1), peak onset 3rd-4th decade
- AVRT: M>F, presents earlier (childhood to young adult)
- WPW pattern on ECG in 0.1-0.3% of the population
- Structurally normal heart in the majority - SVT is not a marker of cardiac disease
Aetiopathogenesis
AVNRT
- Dual AV nodal physiology: slow pathway (short refractory period) + fast pathway (long refractory period)
- Premature atrial beat blocks in fast pathway -> conducts down slow -> fast pathway recovered -> retrograde up fast
- = typical "slow-fast" AVNRT (90%) -> atrium and ventricle activated near-simultaneously
- Atypical "fast-slow" or "slow-slow" -> long RP tachycardia
AVRT
- Accessory pathway = embryological remnant of AV ring myocardium; conducts without decrement
- Orthodromic (95%): down AV node, up the pathway -> narrow complex
- Antidromic (5%): down the pathway, up AV node -> wide complex (mimics VT)
- WPW = pathway with antegrade conduction -> pre-excitation on resting ECG (short PR, delta wave)
- Concealed pathway - retrograde only -> normal resting ECG but still causes AVRT
Triggers
- Caffeine, alcohol, stimulants, thyrotoxicosis, pregnancy, emotional stress, exercise, sleep deprivation
Diagnosis
Presentation
- Abrupt on/off palpitations ("switch"), rapid and regular, 140-250 bpm
- Neck pulsation / "shirt flapping" - frog sign: cannon a waves from atria contracting against closed AV valves (characteristic of AVNRT)
- Polyuria after termination (ANP release)
- Chest discomfort, dyspnoea, presyncope; syncope uncommon and should prompt a wider look
ECG during tachycardia
| Finding | Suggests |
|---|---|
| Pseudo-R' in V1, pseudo-S in II/III/aVF (retrograde P buried in QRS) | Typical AVNRT |
| RP interval <70 ms | AVNRT |
| RP 70-p, P visible after QRS, inverted in inferior leads | AVRT (orthodromic) |
| Long RP (RP > PR) | Atypical AVNRT, AT, PJRT |
| QRS alternans | AVRT |
| P waves continue during AV block | Atrial tachycardia (AV node not part of the circuit) |
| Warm-up/cool-down in rate | Automatic AT |
- Rate-related ST depression is common and does NOT mean ischaemia
- Capture a 12-lead ECG during tachycardia and during vagal manoeuvre - the single most valuable investigation
Resting ECG
- WPW: PR <120 ms, delta wave, wide QRS, secondary ST-T changes
- Pseudo-infarct Q waves are common - do not call an old MI
- Otherwise usually normal
Wide-complex tachycardia
- *Assume VT until proven otherwise*, especially with structural heart disease or prior MI
- SVT with aberrancy or antidromic AVRT are the alternatives
- Never give verapamil to an undifferentiated wide-complex tachycardia
Other
- TTE if structural disease suspected or pre-ablation
- TFTs, FBE, electrolytes
- Ambulatory monitoring if not captured; EPS is diagnostic and therapeutic in one sitting
Management
A. Acute termination - haemodynamically unstable
- Synchronised DC cardioversion (sedation), starting 50-100 J
- Unstable = hypotension, ischaemia, pulmonary oedema, altered consciousness
B. Acute termination - stable
1. Vagal manoeuvres first
- Modified Valsalva (40 mmHg strain 15 s, then supine with passive leg raise) - success ~43% vs ~17% standard
- Carotid sinus massage (exclude bruit; avoid if recent TIA/stroke)
2. Adenosine 6 mg rapid IV push into a large vein + flush; then 12 mg, then 12 mg
- Terminates AVNRT/AVRT in >90%; unmasks flutter/AT rather than terminating it - diagnostic either way
- Warn about chest tightness, flushing, doom sensation (seconds)
- Avoid/reduce: asthma (bronchospasm), transplanted heart (denervation supersensitivity - halve the dose), dipyridamole/carbamazepine (potentiate), theophylline (antagonises)
- *Contraindicated in pre-excited AF* - AV block can accelerate pathway conduction -> VF
3. Verapamil 5-10 mg IV or diltiazem, or IV beta blocker
- Never with a beta blocker already on board; never in wide-complex tachycardia; never in HFrEF
C. Pre-excited AF / antidromic AVRT
- Procainamide, ibutilide, or DC cardioversion
- *AVOID all AV nodal blockers: adenosine, verapamil, diltiazem, beta blockers, digoxin* - shunt conduction down the accessory pathway -> VF
D. Long-term - the ablation decision
- Catheter ablation is first-line for symptomatic recurrent SVT
- Cures 90-95%; recurrence ~3-5%
- Risks: death/MI/stroke ~1 in 2000; AV block requiring pacing 1 in 200-500 (slow-pathway ablation near the compact node); vascular access complication ~1 in 100
- Pathway location drives risk: septal pathways carry the highest AV block risk; left free wall requires transseptal or retro-aortic access
- Drug therapy if ablation declined or deferred - reduces, does not abolish, recurrence
- Verapamil SR, atenolol or other beta blocker, digoxin
- Flecainide or sotalol if structurally normal heart and above fail (flecainide contraindicated with IHD or structural disease)
- "Pill-in-the-pocket" - single-dose diltiazem + propranolol, or flecainide, for infrequent well-tolerated episodes
- Teach the patient Valsalva technique
E. Asymptomatic WPW pattern
- Risk-stratify - not simply reassure
- Exercise test: abrupt loss of pre-excitation at higher rates = long pathway refractory period = low risk
- EPS if: high-risk occupation (pilot, driver, athlete), shortest pre-excited RR in AF <=250 ms, multiple pathways, syncope
- Ablate if high risk
- SCD risk in asymptomatic WPW ~0.1%/yr
F. Lifestyle
- Reduce caffeine, alcohol, stimulants; treat thyrotoxicosis
- Advise not to drive while symptomatic until controlled
Associations
- Ebstein anomaly - multiple right-sided accessory pathways; look for it in any right-sided WPW
- HOCM, and PRKAG2 / Danon / Fabry - pre-excitation with LVH
- Congenital heart disease, post atrial surgery (atrial tachycardia, incisional re-entry)
- Thyrotoxicosis, pregnancy (inc frequency; adenosine and metoprolol are safe; avoid amiodarone)
- Structural heart disease -> favours atrial tachycardia over AVNRT/AVRT
- Incessant SVT (esp. PJRT or AT) -> tachycardia-mediated cardiomyopathy - reversible if the arrhythmia is cured
Natural history & complications
- Benign in the structurally normal heart - mortality essentially that of the population
- Course: intermittent lifelong episodes, frequency variable; often increases with age in AVNRT
- Exceptions carrying real risk
- WPW with a rapidly conducting pathway + AF -> VF -> sudden death
- Incessant SVT -> tachycardia-mediated cardiomyopathy
- SVT superimposed on severe structural disease -> haemodynamic compromise
- Post-ablation: symptom-free in most; recurrence usually within the first 3 months
- Persistent palpitations after successful ablation are common and usually ectopy, not recurrence - document with monitoring before re-referring
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