Normal EKG Waveform Components
Flashcards covering the P wave, QRS complex, ST segment, T wave, and U wave of a normal cardiac cycle.
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Questions Covered in This Set
12 cards to master
What physiological event does the P wave represent?
Atrial depolarization — the impulse spreading from the SA node across the right then left atrium. The SA node's own firing is too small to see.
Normal P wave duration and amplitude?
Duration < 0.12 s (under 3 small boxes) and amplitude < 2.5 mm; smooth, rounded, and monophasic.
What P wave polarity indicates sinus rhythm?
Upright in lead II and inverted in aVR, because normal atrial activation travels down and leftward, straight at lead II's positive pole.
Does the EKG record contraction or electrical activity?
Electrical activity only — depolarization and repolarization of muscle. Electricity precedes mechanical contraction by a few milliseconds.
Normal QRS duration, and why is it narrow?
0.06–0.10 s. The His–Purkinje system fires both ventricles almost simultaneously, so the massive ventricles depolarize quickly.
What does a QRS ≥ 0.12 s suggest?
The impulse traveled slowly through muscle instead of the Purkinje system: bundle branch block, ventricular ectopy, or a paced beat.
Strict naming rules for Q, R, and S waves?
Q = first downward deflection only if it precedes any upward deflection; R = any upward deflection (second = R′); S = downward deflection following an R.
What is a QS complex?
An entirely negative complex with no upward deflection at all.
Why is the ST segment normally at the baseline?
It's the plateau phase when all ventricular myocardium is depolarized together, so no voltage difference exists between regions.
Why is the T wave usually upright (concordant with the QRS)?
Repolarization is electrically opposite but travels in the opposite direction (epicardium to endocardium); the two reversals cancel out.
When are Q waves considered pathologic?
When > 40 ms wide or > 25% of the R wave height — suggesting old infarction. Small septal q waves in I, aVL, V5, V6 are normal.
What do peaked vs. wide notched P waves in lead II suggest?
Tall, peaked = right atrial enlargement ('P pulmonale'); wide and notched = left atrial enlargement ('P mitrale').
