An interval is a portion of the baseline and at least one wave. We measure an interval on the horizontal axis in seconds. The PR, QRS, and QT are the intervals which should be routinely scanned on each ECG. For measuring intervals, look at the widest form in any lead.

Figure 23: Intervals.

  1. PR interval (beginning of P wave to the beginning of the next QRS). Normally, < .2 seconds or one large box. If it is > .2 seconds, it is a first degree block. (Note: this concept was introduced under blocks).

    Figure 24: Note the prolonged PR interval (.28 seconds), especially at the second beat.

  2. QRS interval (beginning of Q to the end of the S wave) should be < .12 seconds (< 3 small boxes). If QRS is > .12, check for bundle branch block.

    A QRS > .12 and RR (2 peaks or R waves in QRS) occurring in the right chest leads (V1-V2) indicates a right bundle branch block.

    Figure 25: RBBB.

    If QRS is > .12 and RR occurs in the left chest leads (V5-V6), this indicates a left bundle branch block.

    Figure 26: LBBB.

    Very important: For left bundle branch block, the ECG is often unreliable for identifying infarct. Infarct is a concept explained later in this tutorial, but don't forget that with left bundle branch block, one cannot rely on the ECG for diagnosis of infarct.

  3. QT interval (beginning of QRS to end of T wave) should be less than half of the preceding RR interval - this varies with the rate. For normal rates, QT < .4 seconds (2 large boxes). "QT prolongation" (too long) can lead to a refractory form of ventricular tachycardia called torsades de pointes.

    Figure 27: The QT interval is greater than half the preceding RR interval. Look at lead I.

    Rhythm Guidelines
    1. Check the bottom of the rhythm strip for regularity, i.e. - is it completely regular, mostly regular with a few extra beats, or totally irregular?
    2. Check for a P wave before each QRS, QRS after each P.
    3. Check RR interval (for AV blocks) and QRS interval (for bundle branch blocks). Check for prolonged QT.
    4. Continue to recognize "patterns" such as atrial fibrillation, PVCs, PACs, escape beats, ventricular tachycardia, paroxysmal atrial tachycardia, AV blocks and bundle branch blocks.

Take the Rhythm and Intervals Quiz.