Waveforms and Measurements

P wave

• Wave that precedes (comes before) the QRS wave complex

• represents atrial depolarization

• smooth and rounded

• positive deflection

  • above baseline

• 0.5 - 2.5 mm in amplitude

P wave variations:

• peaked

• wide

• notched

• inverted/negative

  • below baseline

  • occurs when ectopic pacemaker is below the AV junction

Ta wave

• Lost in QRS complex

  • cannot measure / “see” on EKG strip

• represents atrial repolarization

PR Segment

• Segment → a line between waveforms

• represents delay in conduction through the AV node

• ~0.1 s

• normally an isoelectric / flat line

  • b/c the conducting structures within the heart are so small the electricity within them is not detected

PR Interval (PRI)

• Interval - a waveform + a segment

• P wave + PR segment

• ends w/ onset of QRS

• 0.12 - 0.20 s in duration

  • 3-5 small boxes

QRS Complex

• Complex - consists of several waveforms

• Represents ventricular depolarization

  • atrial repolarization usually takes place at this time (Ta wave)

  • QRS complex overshadows Ta wave)

• 0.06 - 0.10 s

• < 0.12 s in duration

  • direction of QRS complex may be predominantly positive, predominantly negative, or biphasic (part positive, part negative)

    • will see normal variatins of waveforms

Q wave

• initial (first) negative (downward) deflection following P wave

• precedes R wave

R wave

Initial positive (upward) deflection following P wave

S wave

• negative deflection

• follows R wave

ST Segment

• end of QRS to beginning of T wave

• represents early portion of ventricular repolarization

• “J point”

  • point at which QRS meets ST segment

• normally isoelectric w/ J point

  • sensitive indicator of MI and ischemia

Abnormal ST segments:

• ST segment elevation

• ST element depression

ST segment depression

Suggestive of chronic MI

ST segment elevation

Suggestive of acute MI

T wave

• represents ventricular repolarization

• the absolute refractory period is still present during the beginning of the T wave

• relative refractory period begins at the peak of the T wave

  • during RRP, a stimulus may produce a new impulse and result in arrhythmias

• sensitive indicator of MI and ischemia

Normal T wave appearance:

• upright

• rounded

• larger than P wave

• slightly asymmetrical

• ends when return to baseline

QT Interval

• beginning of QRS complex to end of T wave (all)

• represents total ventricular activity

  • ventricular depolarization and repolarization

QT interval varies w/:

• heart rate

• age

• gender

Measuring the Heart Rate

• HR = BPM (beats per minute)

  • Atrial rate

    • look at P waves

  • Ventricular rate

    • look at R waves, or peak of QRS complex

• Record time and count beats per second

  • or 30 s and mult by 2

  • or 15 s and mult by 4

3 methods to calculate / estimate HR:

• 6 second method

• R-R method / P-P method

• Sequence method

6 second method

• count number of complete QRS complexes within 6 second period and multiply by 10

  • 30 large boxes

    • usually marked on strip chart

      • we can assume all strips are 6 sec in length

• simplest, quickest, most common method

R-R method / P-P method

• count number of large squares between two consecutive R waves / P waves and divide into 300

  • R waves give ventricular rate

  • P waves give atrial rate

Sequence Method

• assigns rate (number) to each vertical line

• locate R wave that falls on (or near) thick vertical line)

• count each thick line

  • 300-150-100-75-60-50-43-38-33-30

  • until next R wave

To determine the Rhythm / Regularity:

• use calipers to compare distance of “intervals” on same strip

  • variations up to 0.12 s is technically WNL

    • 3 small boxes

  • the slower the HR, the more acceptable the variation

• classify as:

  • regular

    • normal rhythm

  • irregular

    • arrhythmia or dysrhythmia

To determine Ventricular Rhythm:

• measure distance between 2 consecutive R-R intervals and compare with other R-R interval

  • regular if close to same distance between each R-R interval

    • normal rhythm

    • variation of up to 0.12 s is acceptable

  • irregular if different measurement between each R-R interval

    • arrhythmia / dysrhthmia

To determine Atrial Rhythm:

Do same as determining ventricular rhythm but with P-P intervals

Analyzing P wave:

• one should precede each QRS complex

  • present and uniform in appearance?

  • is the atrial rate equal to the ventricular rate?

Analyzing PRI:

• use calipers to measure interval

  • normal length → 0.12 - 0.2 s

  • should not vary from beat to beat

Analyzing QRS complex:

• use calipers to measure interval

  • normal length → < 0.12 s

  • should all be uniform

Determine Rate:

• use one of 3 methods

  • 6 second - easiest

  • Ventricular rate

    • R waves counted

  • Atrial rate

    • P waves counted

• Report the BPM

• Classify as:

  • Normal : 60-100 BPM

  • Bradycardic : <60 BPM

  • Tachycardic : >100 BPM

Determine Rhythm:

• use calipers

  • Ventricular

    • R to R intervals

  • Atrial

    • P to P intervals

• Classify as:

  • regular / normal

  • irregular / arrhythmic / dysrhythmic