Ventricular Fibrillation and Heart Block: ECG Presentation and Diagnosis

Ventricular Fibrillation (VF)

Ventricular Fibrillation (VF) is a life-threatening cardiac arrhythmia characterised by chaotic, rapid, and irregular electrical activity in the heart's lower chambers, known as the ventricles. It results in ineffective ventricular contractions, leading to the inability to pump blood and the risk of cardiac arrest or sudden cardiac death.

ECG Features of Ventricular Fibrillation

• Chaotic Rhythm: VF presents as a disorganised and chaotic rhythm without any identifiable pattern.
• Rapid Rate: Electrical activity in VF is rapid and irregular.
• Absent P Waves, QRS Complexes, and T Waves: In VF, the normal components of the ECG waveform, including P waves, QRS complexes, and T waves, are absent and replaced by irregular undulating waveforms of varying amplitude and morphology.

Heart Block

Heart block refers to a delay or interruption in the normal electrical conduction between the atria and ventricles, often occurring at the AV node. Heart block can be classified into different degrees, each with distinct ECG characteristics.

First Degree Heart Block

• Prolonged PR Interval: The PR interval, measured from the start of the P wave to the beginning of the QRS complex, is greater than 0.2 seconds, indicating a delay in conduction from the atria to the ventricles.
• All P Waves Followed by QRS Complexes: Every atrial impulse is conducted to the ventricles, resulting in a QRS complex following each P wave.

Second Degree Heart Block Type 1 (Wenckebach)

• Progressive PR Interval Prolongation: The PR interval progressively lengthens with each successive beat until a P wave is not followed by a QRS complex.
• Pattern of Dropped QRS Complexes: After a dropped QRS complex, the PR interval resets, and the cycle repeats.

Second Degree Heart Block Type 2 (Mobitz II)

• Fixed PR Interval: The PR interval remains constant.
• Regular Pattern of Dropped Ventricular Beats: Not all P waves are followed by QRS complexes, resulting in a regular pattern of dropped ventricular beats. The ratio of P waves to QRS complexes is typically fixed (e.g., 2:1 or 3:1), but it can be variable in some cases.

Third Degree Heart Block (Complete Heart Block)

• Full AV Dissociation: Atrial and ventricular activities are completely independent, with no correlation between P waves and QRS complexes.
• Atrial Rate Faster Than Ventricular Rate: The atrial rate is typically faster than the ventricular rate, resulting in more P waves than QRS complexes on the ECG.

Importance of Accurate Diagnosis

Accurate diagnosis of heart block is crucial as the severity and clinical implications of different types of heart block can vary. Appropriate management strategies must be employed based on the specific type of block.