###### Created: 2024-06-12 14:39 ###### Areas & Topics: #medicine ###### Note Type: #permanent ###### Connected to: [[Atrial Fibrillation]] [[Adult Advanced Life Support (AALS)]] [[Acute Coronary Syndrome (ACS)]] - The term 'tachycardia' refers to an abnormally fast heart rate, usually defined as more than 100bpm. - Abnormally fast heart rhythms are generally referred to as tachyarrhythmias. - Tachyarrhythmias which are transient are referred to as paroxysmal tachyarrhythmias. - Generally, tachyarrhythmias are divided into supraventricular (originating above the ventricles) or ventricular (originating in the ventricles). - However, atrial tachycardia, atrial flutter and [[Atrial Fibrillation]] are usually considered separately from the other supraventricular tachycardias/tachyarrhythmias. The supraventricular rhythms include: - Sinus tachycardia - Atrial tachycardia - Atrial flutter - Atrial fibrillation - Atrioventricular nodal re-entrant tachycardia (AVNRT) - Atrioventricular re-entrant tachycardia (AVRT) The ventricular rhythms include: - Ventricular tachycardia - Polymorphic ventricular tachycardia ### Pathophysiology - In normal conditions, when a wave of depolarisation goes through the heart it will start in the sinoatrial (SA) node, go through the atrioventricular (AV) node and finish at the Purkinje fibres, causing organised contraction of the atria and ventricles. - However, other mechanisms can lead to issues with mainly either the conduction of or formation of electrical impulses in this circuitry. #### Re-Entry - A re-entry mechanism is where after depolarisation reaches an area of the heart, abnormal circuitry and/or an accessory pathway causes the impulse to go back to its origin (known as retrograde conduction). - This causes impulses to be propagated repeatedly earlier than they should be, leading to tachycardia. - Re-entry pathways that occur in a localised area of cardiac tissue are known as micro-reentrant. - Re-entry pathways that occur over a larger area of cardiac tissue are known as macro-reentrant. N.B. this mechanism mainly underlies Atrioventricular Nodal Reentrant Tachycardia (AVNRT), Atrioventricular Reentrant Tachycardia (AVRT) and some types of atrial tachycardia ![[reentrant_circuits.jpeg]] #### Increased Automaticity - This is where cells in the heart generate spontaneous action potentials at an increased rate to normal due to pathological changes, which dominate the normal rhythm produced by the SA node. - The SA node can experience increased automaticity, or groups of cardiac cells outside the SA node can as well. N.B. this mechanism is responsible for more than 70% of cases of focal atrial tachycardia. #### Triggered Activity - After cardiac cells depolarise, they can produce extra-depolarisations immediately after the cells re-polarise. - These extra depolarisations are known as 'after-depolarisations'. - If after-depolarisations reach a high enough amplitude to stimulate the membrane to depolarise again, another action potential is produced spontaneously. - These spontaneous action potentials are known as a triggered response, and go on to initiate extra systoles, leading to tachycardias. N.B. this mechanism is mainly responsible for [[Atrial Fibrillation]] and Atrial Flutter, and the remaining 30% of focal atrial tachycardias. ### Differentiating Supraventricular and Ventricular Tachyarrhythmias - In normal conditions, electrical depolarisation going through the heart will travel through the normal circuitry leading to fast depolarisation of the ventricles and thereby cause a narrow QRS (less than 0.12 seconds). - If electrical activity begins in the ventricles and travels abnormally, this will cause depolarisation across the ventricles much more slowly, leading to a broad QRS (more than 0.12 seconds). - Therefore, the simplest way to determine the origin of a tachyarrhythmia is by assessing whether the QRS is narrow (supraventricular) or broad (ventricular). - However, if a patient has an accessory pathway connecting the atria and ventricles then this can also cause a broad QRS (due to slow depolarisation occurring through this abnormal pathway) - This would be supraventricular in origin and thereby an exception to the rule above, but if a patient presents with a broad QRS it should be assumed to be ventricular until proven otherwise. ### Ectopic Beats - An ectopic beat is simply any heart beat which originates somewhere besides the SA node. - It can be identified on ECG by heart beats which come earlier or out of rhythm with others. - Ectopic beats are often considered benign and are not necessarily indicative of cardiac pathology. ![[NSR_and_Ectopic_beats_(PVC_and_PAC).svg.png]] ### Atrial Tachycardia - Atrial tachycardia can either be focal or multi-focal, depending on where impulses are being generated - Focal atrial tachycardia is where a single focus of atrial tissue is creating rapid action potentials and causing the tachycardia. - Multi-focal atrial tachycardia is where multiple ectopic foci in the atria cause the tachycardia. #### ECG Atrial tachycardia will typically present with: - Tachycardia - Narrow QRS - Abnormal P Waves N.B. A multi-focal atrial tachycardia may also produce an irregular rhythm. ![[Pasted image 20240612155022.png]] ### Atrial Flutter - Atrial flutter is a specific type of atrial tachycardia and a classic example of a macro-re-entrant circuit. - This re-entrant circuit commonly involves the cavotricuspid isthmus in the right atrium. - Rapid circular depolarisation occurs leading to a rate of depolarisation around 250 bpm or more. - This causes the isoelectric line on ECG to no longer be flat (as depolarisation is occurring so quickly), creating a 'sawtooth appearance'. - Additionally, this rapid atrial rate leads to a physiological response where the AV node will 'block' some impulses to allow adequate ventricular filling. - This is represented at the ratio of flutter waves to QRS complexes (e.g. if there are 2 flutter waves followed by 1 QRS complex, then the degree of block is 2:1) - The clinical features and management of atrial flutter is incredibly similar to [[Atrial Fibrillation]]. - Diagnosis is primarily done with ECG. #### ECG - Sawtooth Appearance (best seen in the inferior leads II, III and aVF) - AV Block (typically 2:1 but can vary) ![[Pasted image 20240612160057.png]] ### Atrial Fibrillation (AF) - The most common type of cardiac arrythmia that is characterised by atria fibrillating due to multiple waves of spontaneous depolarisation. - See the [[Atrial Fibrillation]] page for more details. #### ECG - Irregularly irregular pulse. - Absence of P-Waves. - Chaotic baseline. ![[af.jpg]] ![[Pasted image 20240604170939.png]] ### Supraventricular Tachycardias (SVT) - Abnormal tachyarrhythmias that are derived from re-entrant circuits involving the AV node are generally known as SVTs. #### Atrioventricular Nodal Re-entrant Tachycardia (AVNRT) - AVNRT is the most common paroxysmal SVT. - It typically occurs in patients with structurally normal hearts who have additional conduction pathways within or involving their AV node (allowing for a re-entrant retrograde circuit). ECG - Tachycardia - Narrow QRS - Absent P waves ![[Pasted image 20240612183308.png]] #### Atrioventricular Re-entrant Tachycardia (AVRT) - AVRT is the second most common SVT. - It is caused by an accessory pathway between the atria and ventricles which is separate or away from the AV node. - This accessory pathway can allow antegrade conduction (atria to ventricles) or retrograde conduction (ventricles to atria), creating a re-entry circuit. - Stimulation of this re-entry circuit then leads to a tachycardia. - In cases where this accessory pathway does allow antegrade conduction, impulses are able to quickly travel to the ventricles and cause pre-excitation (i.e. early depolarisation) by bypassing the control of the AV node. - The classic pre-excitation syndrome that leads to episodes of AVRT is Wolff-Parkinson-White (WPW) Syndrome. #### [[Wolff-Parkinson-White (WPW) Syndrome]] - WPW Syndrome is a pre-excitation syndrome. - It is characterised by a congenital accessory pathway causing AVRT. - This accessory pathway is typically referred to as the 'Bundle of Kent' and is both antegrade and retrograde. - The antegrade conduction is what allows impulses to bypass the AV node and pre-excite the ventricles. ##### ECG - Short PR Interval (due to there being no AV conduction delay from the accessory pathway) - Delta Wave (early slowed upstroke in the QRS complex due to pre-excitation of the ventricles) N.B. the remaining part of the QRS complex following the delta wave will be normal as normal conduction via the AV node and other circuitry will eventually catch-up with the pre-excited impulse (see the [[Wolff-Parkinson-White (WPW) Syndrome]] page for further details) ![[Pasted image 20240612185328.png]] ### Ventricular Tachycardia (VT) - Ventricular tachycardia occurs due to a focus or foci of electrical activity in the ventricles leading to rapid recurrent depolarisation. - Managing VT is important as it has the potential to turn into ventricular fibrillation. - It is split into monomorphic and polymorphic VT (in reference to the morphology of the QRS complexes seen). Monomorphic VT - Occurs due to a single focus of electrical activity in the ventricles - It is most commonly due to scarring of the ventricles following a myocardial infarction (MI) - It will be seen on ECG as a rapid, broad-complex tachycardia. - Each QRS will be stable with no variation between beats (as it originates from a single focus) ![[Pasted image 20240612193207.png]] Polymorphic VT - Occurs due to multiple foci of electrical activity in the ventricles, causing variable QRS complexes. - It is typically secondary to myocardial ischaemia. - It will be seen on ECG as a ventricular tachycardia where each QRS has beat-to-beat variation and different morphologies. [[Torsades de Pointes]] - Torsades de Pointes is a subtype of polymorphic VT. - It occurs secondary to a prolonged QT interval. - It is characterised on ECG by a ventricular tachycardia which 'twists' around the isoelectric line. - It is managed by attempting to shorten the QT interval IV magnesium sulphate (or DC cardioversion in a unstable patient). ![[Pasted image 20240612194318.png]] ### Ventricular Fibrillation (VF) - Ventricular Fibrillation is an arrythmia which is incompatible with life. - Independent contraction of ventricular fibres cause VF, showing a chaotic, fibrillating baseline on ECG. - See [[Adult Advanced Life Support (AALS)]] for details on management. ![[Pasted image 20240613163129.png]] ### Resources Patti L, Ashurst JV. Supraventricular Tachycardia. [Updated 2023 Aug 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK441972/ Foth C, Gangwani MK, Ahmed I, Alvey H. Ventricular Tachycardia. 2023 Jul 30. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan–. PMID: 30422549. Tachycardias - PulseNotes https://app.pulsenotes.com/clinical/ecgs/notes/6-tachycardias Supraventricular Tachycardia - PassMed Ventricular Tachycardia - PassMed Atrial Fibrillation - NICE CKS https://cks.nice.org.uk/topics/atrial-fibrillation/diagnosis/diagnosis-of-atrial-fibrillation/