EKG Intervals and Segments
Key measurements, normal ranges, and interpretations for PR, QRS, QT/QTc, and ST on the EKG.
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Questions Covered in This Set
12 cards to master
Interval vs. segment
An interval includes at least one wave; a segment is only the flat baseline stretch between waves (e.g., PR interval contains the P wave; PR segment does not).
Paper speed conversions at 25 mm/s
1 small box = 0.04 s (40 ms); 1 large box = 0.20 s (200 ms).
Normal PR interval
120–200 ms (3–5 small boxes), measured from earliest P deflection to earliest QRS deflection, usually in lead II.
PR interval > 200 ms
First-degree AV block — conduction is slow but every P wave gets through. Progressive lengthening with a dropped beat = Wenckebach.
PR < 120 ms with a delta wave
Pre-excitation (Wolff–Parkinson–White) — the impulse bypassed the AV node's braking delay. Short PR with a normal narrow QRS suggests junctional/low-atrial rhythm.
Normal QRS duration and where it ends
< 120 ms (ideally ≤ 110 ms), measured from Q (or R) onset to the J point, where the QRS abruptly changes slope into the ST segment.
Differential for a wide QRS (>120 ms)
Bundle branch block, ventricular origin (PVC/VT), paced rhythm, hyperkalemia, or sodium-channel blocker (drug) toxicity.
QT interval: what it measures and the tangent method
Total ventricular depolarization plus repolarization; draw a line along the steepest T-wave downslope and mark where it crosses baseline (ignore U waves). Use lead II or V5.
Bazett's formula and an example
QTc = QT / √RR (RR in seconds). QT 400 ms at HR 75 (RR 0.8 s): √0.8 ≈ 0.894, QTc ≈ 447 ms.
QTc upper limits and danger threshold
~450 ms in men, ~460 ms in women; above 500 ms torsades de pointes risk climbs sharply. Quick screen: QT should be less than half the RR interval.
Causes of QT prolongation vs. short QT
Prolonged: hypokalemia, hypomagnesemia, hypocalcemia, antiarrhythmics, antipsychotics, macrolides, congenital long-QT. Short (<340 ms): hypercalcemia or digoxin effect.
ST segment baseline and measurement point
Should sit at isoelectric baseline referenced to the TP segment (or PR segment if fast); measure deviation at the J point or 40–80 ms after it.
