LECTURE: Basic Components of Cardiac Complex

Overview

• QQ interval: connects beginning of 2 consecutive QRS complexes
• Composed of 3 segments that in absence of tachycardia may be nearly isoelectric

3 Segments of Cardiac Complex

1. ST segment: from end of QRS to onset of T wave

• Corresponds plateau of ventricular action potential
• Isoelectric only in central portion, as beginning plateau potentials overlap with ventricular depolarization & end plateau potentials overlap with rapid ventricular repolarization
• Common causes of ST segment deviation:
• Tachycardia (from atrial repolarization overlap)
• Delayed repolarization 2/2 slow depolarization (eg, ventricular hypertrophy, preexcitation, bundle branch block)
• Myocardial ischemia can shift ST segment (systolic injury current) or TQ segment (diastolic injury current)
• Cause of ST segment shift can be seen with ECG recorded with direct-current (DC) coupled with amplifiers or magnetocardiograph
• Reference for measuring ST segment shift = line connecting 2 consecutive at beginning of QRS

2. TP segment (TP interval): from end of T wave to onset of P wave

• Useful baseline when abnormal PQ segment deviation from systolic or diastolic atrial injury current suspected (eg, during acute pericarditis or atrial infarction) [1]
• Note: not guaranteed that diastolic ventricular potential remained unchanged in such cases

3. PQ segment (PQ interval): from end of P wave to onset of QRS

• No advantage over TP segment as baseline (both represent same ventricular diastolic potential level)
• Accuracy as baseline may be diminished by presence of atrial repolarization

REFERENCES

1. Charles MA, Benziner TA, Glasser SP: Atrial injury current in pericarditis. Arch Intern Med 131:657, 1973.