Rhythm Analysis: Ventricular/Blocks Flashcards

How do you calculate the corrected QT interval (QTc), and what value is considered prolonged in adults?

Answer: QTc = QT ÷ √RR (Bazett's formula); QTc > 450 ms in men and > 460 ms in women is considered prolonged

Bazett's formula corrects the QT interval for heart rate: QTc = QT(sec) divided by the square root of the RR interval (sec). Normal upper limits are 450 ms in men and 460 ms in women; > 500 ms is considered at high risk for TdP.

Ventricular fibrillation (VF) appears as which pattern on the ECG?

Answer: Completely chaotic, irregular, undulating waveforms with no identifiable QRS complexes, P waves, or T waves at an indeterminate rate

VF produces completely disorganized electrical activity with chaotic, irregular waveforms of varying amplitude and frequency, with no recognizable QRS complexes, P waves, or T waves — the heart is in electrical chaos and not pumping.

What is the significance of the 'R-on-T' phenomenon in the setting of myocardial infarction?

Answer: A PVC that falls on the T wave of the preceding beat can trigger ventricular tachycardia or fibrillation because it occurs during the vulnerable period of ventricular repolarization

R-on-T PVCs occur during the vulnerable period (relative refractory period, near the T wave apex) when ventricular muscle cells have varying degrees of repolarization, creating the ideal substrate for reentrant ventricular tachycardia or fibrillation.

Accelerated idioventricular rhythm (AIVR) differs from VT in what important way?

Answer: AIVR has a ventricular rate of 40-100 bpm (slower than VT's >100 bpm) and is generally benign, often seen in reperfusion after MI

AIVR (rate 40-100 bpm) is a slow ventricular rhythm that does not reach the threshold for VT (>100 bpm). It is characteristically seen during successful reperfusion after MI (a reperfusion arrhythmia) and is generally hemodynamically well-tolerated.

What is the ECG appearance and clinical significance of a 'fusion beat' during ventricular tachycardia?

Answer: A QRS morphology intermediate between the sinus QRS and VT QRS, caused by simultaneous activation from both the sinus impulse and the ventricular ectopic focus; it is pathognomonic of VT

A fusion beat occurs when a supraventricular impulse and a ventricular ectopic impulse simultaneously activate the ventricles, producing a QRS with intermediate morphology. It is pathognomonic (diagnostic) of VT because it proves AV dissociation — the ventricle was partly captured by a sinus beat.

When interpreting a wide-complex tachycardia, which ECG finding most strongly supports a diagnosis of ventricular tachycardia rather than supraventricular tachycardia with aberrant conduction?

Answer: AV dissociation (P waves and QRS complexes are independent with P rate slower than QRS rate)

AV dissociation — seeing independent P waves firing at a slower rate than the wide QRS complexes — is the most specific ECG finding for VT. It proves that the impulse originates in the ventricles, independent of atrial activity.