4. Atrial Arrhythmias - 12-Lead ECG Interpretation (Notes)

Recap: ECG basics

• The shape of the waveform represents the pathway of electrical activity across the chamber.

• If electrical activity originates from the same location, the shape remains the same; if it originates from a different location, the shape changes and may look abnormal.

• Each lead (view) displays the overall vector of electrical activity.

• Vector direction and deflection: if the vector travels toward a lead, the deflection is positive; away from a lead, deflection is negative; parallel to a lead yields a biphasic (or flat) deflection.

• ECGs that will not be examined are shown by a dash; this does not imply these scenarios are unimportant. Failure to recognise can still lead to serious complications.

Sinus rhythms (basic framework)

• In all sinus rhythm variants, each atrial contraction is initiated at the SA node, so lead II shows upright P waves and the QRS complex is normal.

Normal Sinus Rhythm

• Normal upright P waves in lead II; all complexes initiated at the SA node.

• Regular rhythm.

• HR ≈ 60\text{ bpm}.

• Normal PR interval.

• Normal QRS complex.

• Green Flag: OK to Exercise.

Sinus Bradycardia

• Everything is normal, but heart rate is below 60\text{ bpm}.

• Most common cause of bradycardia in young adults is physiological adaptation to exercise.

• Most common cause of bradycardia in older adults is beta-blockers lowering heart rate.

• These indications are normal and OK for exercise.

• When there is no obvious cause for bradycardia, then this person should not exercise. These people will typically not be able to increase heart rate appropriately in response to exercise.

• Green Flag: OK to Exercise.

Sinus Tachycardia

• Everything is normal, except heart rate is above 100\text{ bpm}.

• Tachycardia during exercise is normal. Tachycardia at rest is typically not a normal observation, unless the individual is acutely stressed or anxious (perhaps about the impending exercise!).

• Like bradycardia, if there is no known cause, then you should consider not commencing exercise. Their HR is too fast to allow sufficient filling of the ventricles, and so exercise will not be sustainable.

• Green Flag: OK to Exercise.

Respiratory Sinus Arrhythmia (sinus arrhythmia)

• Everything is normal apart from the rhythm:

  • Normal sinus P waves (upright in lead II) with a constant morphology.

  • PR interval is constant.

  • P-P and R-R interval varies widely.

  • QRS complex is normal, and there is 1 QRS complex for each P-wave.

• This arrhythmia is normal in young, healthy adults. It is explained by HR changing in parallel with respiration (inspiration causes an acute decrease in HR due to reduced vagal tone, and vice versa).

• Sinus arrhythmia is not usually normal in older adults. An older adult with sinus arrhythmia should not exercise.

• Green Flag: OK to Exercise.

• Example HRs: 94\text{ bpm} and 58\text{ bpm} during inspiration/expiration.

Sinoatrial Block (Sinus Block)

• Amber Flag: Get medical clearance; exercise with caution.

• Characterised by skipped or missing P waves. The key is that the underlying P-wave rhythm doesn’t change.

• In examples, the next P wave following the block occurs exactly where it would with a normal underlying rhythm.

• Medical clearance is required. The cause of the block needs to be investigated. The person may not be able to sufficiently increase HR to meet exercise demands.

Sinus Arrest/Pause

• Amber Flag: Get medical clearance; exercise with caution.

• There is a long pause, but the underlying P-P interval (and hence R-R interval) is interrupted.

• This can be a normal finding in a young, healthy individual in times of high emotional stress or pain.

• It is recommended this is referred on to ensure the underlying cause is not pathological.

Atrial Arrhythmias (supraventricular)

• Huff, J. (2006). ECG Workout (5th Ed.)

• In all these rhythms, there will be P-waves that do not occur at the SA node.

• What does that mean about those P-waves? The underlying atrial rhythm might still be classified as normal sinus rhythm.

• In some cases, there will be no visible P waves at all.

Premature Atrial Complex (PAC)

• Green Flag: OK to Exercise.

• Underlying rhythm is normal sinus rhythm. There is a single occurrence of a PAC (P wave occurs early and is of a different shape).

Non-conducted PAC

• Green Flag: OK to Exercise.

• Suspected P-wave on the terminal portion of the T-wave. The premature beat is not conducted (no ensuing QRS) because AV conduction is refractory at that early phase in the cycle.

Atrial bigeminy

• Underlying rhythm is sinus. However, every second P-wave is a PAC. When this occurs every second beat, it is called atrial bigeminy. If it occurs every 3rd, it is atrial trigeminy.

• You should refer back to check for underlying pathology.

• Get medical clearance. Exercise with caution.

• Amber Flag

PACs – key features

• An abnormal (non-sinus) P-wave is followed by a normal QRS complex.

• The abnormal P-wave may be hidden in the preceding T-wave, producing a peaked or camel hump appearance.

• The P-wave morphology/axis typically differs from the sinus P waves.

• In lead II, PACs can show inverted or biphasic P-waves.

• PACs arising close to the AV node (low atrial ectopics) cause retrograde activation of the atria, producing an inverted P-wave in lead II.

Sustained Atrial Tachycardia or Supraventricular Tachycardia (SVT)

• Red Flag: Do not Exercise.

Paroxysmal Atrial Tachycardia (PAT)

• Red Flag: Do not Exercise or stop exercise.

• Characterised by a Run of PACs.

Atrial Fibrillation (AF)

• If AF is known and treated and resting HR < 100\text{ bpm}, exercise with caution (Amber Flag).

• No discernible P-waves. Instead, the baseline is shaky or quivering.

• Rhythm is irregularly irregular.

• QRS complexes are normal and consistent in appearance.

• Red Flag: If new, unknown, or resting HR > 100\text{ bpm}.

Uncontrolled Atrial Fibrillation

• Ventricular rate is > 100\text{ bpm} at rest.

• Red Flag: Do not Exercise.

Atrial Fibrillation – clinical implications

• One of the most common arrhythmias in individuals with heart disease.

• Increased risk of thromboembolic stroke; commonly treated with aspirin or anticoagulants like warfarin.

• Risk of arrhythmogenic heart failure.

• Beta-blockers are often prescribed to slow ventricular rate; if unsuccessful, digoxin may be prescribed (watch for digoxin/digitalis toxicity).

• SOB during exercise; why? Rate-related reductions in ventricular filling and cardiac output.

AF – types and severity

• Two main types:

  • Paroxysmal: intermittent AF; usually lasts < 48\text{ days}.

  • Persistent or permanent: continuous AF lasting > 7\text{ days}; permanent AF is when sinus rhythm is not expected to be restored.

• Two severity levels:

  • < 100\text{ bpm}: controlled; OK for exercise with caution.

  • > 100\text{ bpm}: uncontrolled; do not exercise.

• Onset triggers include:

  • Previous/recent/current MI

  • Atrial abnormalities

  • Electrolyte imbalance

  • Recent cardiac surgery

  • Excessive alcohol/drug use (holiday heart syndrome)

Atrial Fibrillation – treatment and exercise considerations

• Rate control and anticoagulation as indicated.

• Beta-blockers or digoxin to slow rate; monitor during exercise for side effects.

• Anticoagulation considerations for thromboembolism risk during activity.

• Exercise recommendations depend on rate control and symptom burden.

Atrial Flutter (Variable block)

• P-waves take a sawtooth (jagged) appearance; atrial rate typically 250\text{ to }300\text{ bpm}.

• The AV node blocks these impulses, so the ventricular rate is slower; with variable block, the ventricular rhythm can be irregular (e.g., 4:1 to 2:1).

• Red Flag: Do not Exercise.

• An example shows 6 small boxes between P waves corresponding to an atrial rate of 250\text{ bpm}.

Supraventricular Tachycardia (SVT)

• SVT is a tachycardia whose origin is above the bundle of His.

• Can arise from:

  • SA Node (sinus tachycardia)

  • Atrial ectopic focus/foci

  • AV Node

  • Atrial fibrillation

  • Atrial flutter

• Non-sinus forms of SVT at rest, or sudden onset of SVT during exercise, are Red Flags.

• Red Flag: Do not Exercise.

Case study (ECG exercise case)

• Case: 75-year-old patient exercising; ECG shown as Green Flag: OK to continue exercise.

Case study – Rhythm details (Page 26)

• Rhythm: Atrial – Regular; Ventricular – Regular.

• Atrial rate: 100\text{ bpm}; Ventricular rate: 100\text{ bpm}.

• P-wave: Normal – upright in lead II.

• PR interval: 0.2\text{ s}.

• QRS complex: 0.08\text{ s}.

• ST segment: On the isoelectric line in all leads.

• T wave: Flat.

• Limb leads: aVL, V2, V3 flat; aVR and V1 inverted.

• Axis: Right axis deviation.

• Green Flag: OK to continue exercise.

Case study – continued (Page 27)

• Cast study continued… Exercise intensity has not changed. What has happened? Do you continue?

Case study – additional rhythm interpretation (Page 28)

• Rhythm: Atrial: ; Ventricular: ; P-wave: Normal – isoelectric in all leads; ST segment: Flat; Axis: Red Flag: Stop Exercise.

Wandering Atrial Pacemaker (WAP) and Multifocal Atrial Tachycardia (MAT)

• Wandering Atrial Pacemaker:

  • Rate < 100\text{ bpm}.

  • Multifocal atrial rhythm (originating from atria).

  • At least 3 different P-wave morphologies.

  • Red Flag: Do not Exercise.

  • Pacemaker site shifts between the SA node, atria, and AV node.

• Must have 3 different, distinct morphologies. These may be abnormal if other pathology is present.

Multifocal Atrial Tachycardia (MAT)

• Definition: A rapid, irregular atrial rhythm arising from multiple ectopic foci within the atria.

• Key features:
  1) The SA node is not pacing the heart.
  2) Several groups of excitable cells in the atria compete to pace the heart.
  3) MAT has at least three different P-wave morphologies.
  4) MAT is an irregular rhythm with rate > 100\text{ bpm}.
  5) MAT has irregular P-R, P-P and R-R intervals.

• Red Flag: Do not Exercise.

Quick review

• Today’s focus: atrial/ supraventricular abnormalities including sinus rhythm variants, SA node dysfunction, ectopic complexes, atrial fibrillation, atrial flutter, and non-sinus SVT.

• Key diagnostic cues: changes in timing, shape of the P wave (emphasising lead II), and PR interval are essential for detecting these.

• All these abnormalities can be identified in the rhythm lead (most commonly lead II).

• Ventricular abnormalities will be covered later.

• Atrial and ventricular abnormalities can co-exist.