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Question of the day

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.

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