Arrhythmias | Clinical Medicine

Arrhythmias — Main Ideas and Lessons

1. Classification and Underlying Mechanisms

Arrhythmias are broadly divided into:

• Bradyarrhythmias: Abnormally slow heart rhythms, discussed mainly as sinus bradycardia or AV block.
• Tachyarrhythmias: Abnormally fast rhythms, which may originate above the ventricles (supraventricular) or in the ventricles.

The lecture presents three broad mechanisms that can produce tachyarrhythmias:

1. Increased automaticity: The sinus node or another focus fires more rapidly, often because sympathetic stimulation increases.
2. Re-entry circuits: Electrical activity repeatedly travels around an abnormal pathway. Atrial stretching and remodeling can create this substrate; ventricular scarring, commonly after myocardial infarction or in heart failure, can do so in the ventricles.
3. Triggered activity: Premature electrical depolarizations provoke extra beats.
   • Early afterdepolarizations (EADs) are associated with prolonged repolarization and QT intervals, including from low potassium or magnesium and QT-prolonging medicines.
   • Delayed afterdepolarizations (DADs) are associated with excess intracellular calcium, which the lecture links to increased sympathetic activity, myocardial infarction, and digoxin toxicity.

The lecture stresses that serious rhythms—particularly atrial fibrillation/flutter and ventricular tachycardia/fibrillation—may result from a combination of a vulnerable electrical substrate, triggers, and increased sympathetic activity.

2. Bradyarrhythmias: Causes and Mechanisms

Sinus Bradycardia

Sinus bradycardia is attributed to reduced sinus-node activity. The lecturer uses a rate below 60 beats per minute as a common definition but regards a rate below 50 as more concerning.

Potential causes discussed include:

• Drugs that suppress nodal activity
  • Beta-blocker overdose, such as metoprolol, carvedilol, or labetalol.
  • Calcium-channel blocker overdose, especially verapamil or diltiazem.
  • Digoxin toxicity, which increases vagal effects at the heart.
• Hyperkalemia: Can disrupt conduction, producing peaked T waves, PR prolongation, and widened QRS complexes. Severe disturbances may progress to dangerous rhythms.
• Increased vagal tone: May occur with neurological emergencies such as subarachnoid hemorrhage, intracranial hemorrhage, or traumatic brain injury with raised intracranial pressure. The lecture connects this with Cushing’s triad: bradycardia, hypertension, and abnormal breathing.
• Reduced sympathetic effect: Severe hypothyroidism, including myxedema coma, can reduce cardiac stimulation. The lecture also notes possible hypothermia and skin changes and recommends thyroid testing.

AV Block

In AV block, the sinus node generates impulses, but conduction from the atria to the ventricles is impaired. The lecturer says many causes overlap with sinus-node suppression, and adds:

• Inferior-wall myocardial infarction, particularly when the right coronary artery is involved.
• Lyme carditis, which can cause high-grade block.
• Age-related fibrosis of the conduction system.

The lecture suggests considering ECG findings, troponin, and symptoms when assessing possible infarction, and looking for Lyme disease features when appropriate.

3. Tachyarrhythmias: Types and Causes

Supraventricular Rhythms

The lecture includes sinus tachycardia, multifocal atrial tachycardia (MAT), paroxysmal supraventricular tachycardia (PSVT), atrial flutter, and atrial fibrillation.

• Sinus tachycardia is generally framed as a response to an underlying cause, rather than a rhythm to treat in isolation. Potential causes include:
  • Low blood pressure or shock, including hypovolemic, septic, cardiogenic, or obstructive shock.
  • Hypoxemia, for example with pulmonary embolism, pneumonia, or severe COPD; severe anemia is also mentioned.
  • Hyperthyroidism, especially thyroid storm, and fever.
  • Sympathomimetic medicines or substances, including inhaled bronchodilators, catecholamines, cocaine, and methamphetamine.
  • Pheochromocytoma, particularly with episodic palpitations, headache, sweating, and hypertension.
  • Pain and anxiety.
• MAT is associated particularly with chronic hypoxemia and COPD; theophylline use is also highlighted.
• PSVT is linked to re-entry. The lecture describes:
  • An AV nodal re-entry mechanism involving functionally different pathways.
  • Wolff–Parkinson–White (WPW) syndrome, in which an accessory pathway—the bundle of Kent—can bypass the AV node. The classic ECG features given are a short PR interval, wide QRS, and delta wave. The lecture warns that atrial fibrillation conducted over an accessory pathway can be dangerous.
• Atrial flutter and atrial fibrillation are described as arising from a re-entry-prone atrial substrate, often related to atrial remodeling from conditions such as heart failure, hypertension, or mitral stenosis. Sympathetic stimulation, fever, hypoxemia, electrolyte abnormalities, and other triggers may precipitate an episode.

Ventricular Rhythms

• Ventricular tachycardia (VT) is commonly associated with re-entry around scarred or remodeled ventricular tissue, such as after myocardial infarction or in heart failure with reduced ejection fraction. It may be:
  • Monomorphic: QRS complexes have a similar appearance.
  • Polymorphic: QRS complexes vary in appearance.
• Ventricular fibrillation (VF) is described as chaotic ventricular electrical activity, often arising in a high-risk cardiac substrate.
• Torsades de pointes is linked to prolonged QT and triggered activity. Risk factors discussed include low potassium, low magnesium, and QT-prolonging medicines, including certain antiarrhythmics, antibiotics, antipsychotics, antidepressants, and antiemetics.

4. When an Arrhythmia Is Unstable

The lecture emphasizes identifying instability rather than focusing only on the rhythm name. A fast or slow rhythm can reduce cardiac output and impair organ perfusion:

• Severe bradycardia can directly lower cardiac output.
• Severe tachycardia can shorten ventricular filling time, lowering preload, stroke volume, and ultimately cardiac output.

Signs of instability described include:

• Altered mental status, fainting, or reduced consciousness.
• Angina or other evidence of myocardial ischemia.
• Shortness of breath, pulmonary edema, or hypoxia.
• Significant hypotension or shock, including the need for vasopressors.
• Elevated lactate and signs of poor peripheral perfusion.

5. Diagnostic Approach

Bradycardia: ECG Sequence

1. Obtain an ECG and assess whether the rhythm is sinus.
2. Check whether each P wave is followed by a QRS complex.
3. Assess the PR interval:
   • A regular sinus pattern with a normal PR interval is consistent with sinus bradycardia.
   • An abnormal PR relationship or missing QRS complexes suggests AV block.
4. Classify AV block:
   • First degree: PR interval prolonged, but every P wave conducts.
   • Second-degree Mobitz I (Wenckebach): PR interval progressively lengthens until a QRS complex is dropped.
   • Second-degree Mobitz II: PR intervals remain consistent, but a QRS complex is intermittently dropped.
   • Third degree: P waves and QRS complexes have no consistent relationship (complete AV dissociation).

Tachycardia: ECG Sequence

1. Assess QRS width:
   • Narrow: under 120 ms.
   • Wide: over 120 ms.
2. Assess whether the rhythm is regular or irregular.

Narrow, regular tachycardia:

• Consider sinus tachycardia, PSVT, or atrial flutter with 2:1 conduction.
• Sinus tachycardia has sinus-pattern P waves and a P wave before each QRS.
• PSVT may have absent or retrograde P waves.
• Flutter waves may be apparent in the inferior leads.

Narrow, irregular tachycardia:

• Consider atrial fibrillation or MAT.
• Fibrillatory activity may help identify atrial fibrillation.
• Three distinct P-wave morphologies support MAT.

Wide, regular tachycardia:

• The lecture advises treating this as VT until proven otherwise, while noting that PSVT with bundle-branch aberrancy can also appear wide and regular.

Wide, irregular tachycardia:

• Consider VF, torsades de pointes, or atrial fibrillation with aberrant conduction.
• Prolonged QT followed by a polymorphic, twisting-looking rhythm is described as suggestive of torsades.

6. Treatment Approaches Described

Bradyarrhythmias

For symptomatic, unstable bradycardia, the lecture outlines:

1. Give atropine.
2. If it does not work or the patient remains unstable, use an epinephrine infusion; isoproterenol or dopamine are also mentioned as alternatives.
3. Prepare for pacing:
   • Use transcutaneous pacing as an immediate measure.
   • Consider temporary transvenous pacing while stabilizing the patient.
   • A permanent pacemaker may be considered when an ongoing conduction problem requires it.
4. Investigate and treat the cause, such as medication toxicity, hyperkalemia, hypothyroidism, or myocardial infarction.

Tachyarrhythmias

• PSVT: Try vagal maneuvers first; if unsuccessful, the lecture describes adenosine. For recurrent episodes, it discusses beta-blockers or calcium-channel blockers and referral for electrophysiology assessment and possible ablation.
• Atrial fibrillation/flutter: Address contributing causes and control the rate, with beta-blockers or calcium-channel blockers discussed; digoxin may be added in some cases. Rhythm-control medicines and ablation are described as options in selected or persistent cases.
• Sinus tachycardia and MAT: Treat the underlying cause—for example, correct the trigger for sinus tachycardia or address COPD and theophylline exposure in MAT.
• Stable VT: The lecture discusses antiarrhythmic treatment, including amiodarone or lidocaine depending on the suspected context, and beta-blockers to reduce recurrence. An implantable cardioverter-defibrillator (ICD) may be considered after recurrent or resuscitated serious ventricular arrhythmia.
• Torsades: Remove contributing QT-prolonging drugs, correct electrolyte abnormalities, and give magnesium as described in the lecture. Recurrent episodes or cardiac arrest may prompt consideration of an ICD.

For unstable tachycardia, the lecture recommends immediate electrical treatment rather than delaying for medication:

• Use synchronized cardioversion for unstable atrial flutter, atrial fibrillation, PSVT, or VT when synchronization is appropriate.
• Use unsynchronized defibrillation for torsades or other rhythms where synchronization is not feasible.

Cardiac Arrest

The lecture separates arrest rhythms into:

• Non-shockable: Pulseless electrical activity (PEA) and asystole.
• Shockable: Pulseless VT and VF.

The described approach includes:

1. Begin high-quality CPR and obtain IV or intraosseous access.
2. Attach monitoring/defibrillation electrodes and reassess the rhythm at intervals.
3. For PEA or asystole, continue CPR and give epinephrine; do not defibrillate these rhythms.
4. For VF or pulseless VT, defibrillate, then resume CPR; the lecture also discusses epinephrine and amiodarone during ongoing resuscitation.
5. Search for and treat reversible causes using the H’s and T’s framework. Causes named include hypoxia, blood-pressure/volume problems, acidosis, potassium abnormalities, hypothermia, tamponade, tension pneumothorax, coronary thrombosis or myocardial infarction, pulmonary embolism, and toxins.
6. After return of spontaneous circulation, assess the patient’s responsiveness. The lecture discusses targeted temperature management for patients who remain unresponsive or have myoclonic activity.

Speakers and Sources Featured

• Speaker: Zach, the Ninja Nerd lecturer, as named in the subtitles.
• Source: A Ninja Nerd educational lecture with ECG examples and explanatory diagrams. No guest speakers or other named external sources are identified in the subtitles.