35. Forms and consequences of paroxysmal tachycardia

How can we see atrioventricular nodal re-entry tachycardia (AVNRT) on an ECG?

- Tachycardia - P-wave can be before, inside or after QRS - Negative P-wave in leads II, III, aVF - QRS complex is normal

What is a paroxysmal tachycardia?

- Tachycardia which begins and ends suddenly and violently - Most of these are supraventricular, meaning that the stimulus originates at above the level of the ventricle

How can we see atrial tachycardia on an ECG?

- The P-wave morphology is abnormal - Frequency is between 100 - 250/min - QRS complex normal

What happens in atrial tachycardia?

- The atria are stimulated by a heterotopic focus inside the atria, most commonly due to re-entry - It can occur in people with healthy hearts - The heterotopic focus often requires time to "warm-up", so tachycardia takes some time to develop

Where can we find re-entry circuit, and why is it developed?

- The re-entry circuit most commonly lies between the atria and ventricles, so the four chambers of the heart contract almost simultaneously - It developes due to an early electrical beat that comes and reaches the old beat, so the other side goes round and round and just give endless beats

What is Torsade de pointes?

- Torsade de pointes (TdP) is a type of polymorphic ventricular tachycardia - The QRS axis changes continuously, giving the ECG an irregular spindle-like morphology

Which types of paroxysmal ventricular tachycardias do we have?

- Ventricular tachycardia - Torsade-de-pointes

What happens in ventricular tachycardia?

- Ventricular tachycardia (VT) is a potentially life-threatening arrhythmia that originates from the ventricles - A re-entry circuit lies in the background. It's often initiated by a ventricular extrasystole - As the atria and ventricles are beating asynchronously ventricular filling is impaired - As the ventricles are contracting with such high frequency (100 - 250/min) the contractions are no longer haemodynamically significant - The result is that the cardiac output decreases significantly Ventricular tachycardia can progress into ventricular flutter or fibrillation

How can we see ventricular tachycardia on an ECG?

- Wide QRS complexes - Ventricular frequency 100 - 250/min

How can the atrioventricular nodal re-entry tachycardia (AVNRT) develop?

- AVNRT is a micro re-entry, meaning that it develops within the AV node - It develops because some conducting fibres in the AV node have different conduction speed and refractory period than others - A supraventricular extrasystole is necessary to trigger AVNRT

How can the atrioventricular re-entry tachycardia (AVRT) develop?

- AVRT is a macro re-entry, so for it to develop an accessory conducting fibre between the atria and ventricles must be present - Because of this, they're most commonly associated with the pre-excitation syndrome WPW - A supraventricular extrasystole is necessary to trigger AVRT

Which types of paroxysmal supraventricular tachycardias do we have?

- Atrioventricular re-entry tachycardia (AVRT) - Atrioventricular nodal re-entry tachycardia (AVNRT) - Atrial tachycardia

How can we differentiate between supraventricular and ventricular tachycardias?

- By looking at the response of carotid massage - Supraventricular tachycardias should have their frequency decreased by carotid massage, while ventricular tachycardias don't

Treatment for paroxysmal supraventricular tachycardia?

- Haemodynamically unstable patients may require cardioversion - Haemodynamically stable patients can be treated with carotid massage or can perform the Valsalva manoeuvre - Patients with identifiable accessory conducting fibres can have these fibres burnt off with catheter ablation

Pathomechanism in antidromic atrioventricular re-entry tachycardia (AVRT)?

- In antidromic AVRT the stimulus is conducted in retrograde direction (up) through the AV node and in anterograde direction (down) through the accessory fibre - The ventricles will not be depolarized normally, so the QRS will be wide

Pathimechanism in orthodromic atrioventricular re-entry tachycardia (AVRT)?

- In orthodromic AVRT the stimulus is conducted in anterograde direction (down) through the AV node and in retrograde direction (up) through the accessory fibre - The ventricles will be depolarized normally, so the QRS will be normal

Causes of ventricular tachycardia?

- Ischaemic heart disease - Cardiomyopathy - Long QT syndrome - Electrolyte abnormalities --> Hypokalaemia - Hyperkalaemia

What is micro/macro re-entry and why are they important?

- Micro re-entry - This re-entry circuit can exist entirely within the AV node - Macro re-entry - If the re-entry circuit involves an accessory conducting fibre (like the bundle of Kent)

Which types of atrioventricular re-entry tachycardia (AVRT) do we have?

- Orthodromic AVRT - Usually seen in WPW. - Antidromic AVRT

What is re-entry and why is it important?

- Re-entry is an important mechanism in paroxysmal tachycardias - Not all conducting fibres in the conducting system of the heart has the same properties; some have higher conduction speed than others, and some have longer refractory period than others - This creates the basis for a re-entry circuit - In a re-entry circuit an extrasystole occurs at a specific time, which causes the action potential to go in an endless cycle, essentially producing a new pacemaker that excites tissues around it

How can we see atrioventricular re-entry tachycardia (AVRT) on an ECG?

- Tachycardia - P-wave can be before, inside or after QRS - In orthodromic AVRT - QRS is normal - In antidromic AVRT - QRS is wide (> 120 ms) - Antidromic AVRT can be difficult to differentiate from ventricular tachycardia

What does AVNRT stand for?

Atrioventricular nodal re-entry tachycardia

What does AVRT stand for?

Atrioventricular re-entry tachycardia

How can we classify the ventricular tachycardia?

Based on duration - Non-sustained - stops within 30 seconds - Sustained - lasts more than 30 seconds Based on QRS morphology - Monomorphic - Only one heterotopic focus, all QRS look similar - Polymorphic - More than one heterotopic focus, all QRS look different - Torsade de pointes

Treatment of ventricular tachycardia?

Cardioversion or defibrillation should be performed

Clinical importance of paroxysmal supraventricular tachycardia?

Episodes of paroxysmal supraventricular tachycardia cause symptoms like palpitations, dizziness, chest pain, dyspnoea and potentially syncope