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A. Waveforms
[Figure] "Normal Sinus Rhythm" navigator

  1. P wave: atrial depolarization. ~0.8 sec duration
  2. QRS: Ventricular depolarization. 0.12-0.16sec after P wave. Duration 0.4-0.8sec
  3. T: Ventricular repolarization. 0.25-.35sec after ventricular depolarization; duration 0.2sec
  4. U: Unknown basis. Appears more prominent in hypokalemia

B. Cardiac Action Potentials And ECG [1] navigator

  1. Sinoatrial (SA) Node
    1. Normally initiates the electrical conduction through the heart
    2. SA node has intrinsic automaticity (SA nodal firing rate)
    3. Sympathetic (adrenergic) and parasympathetic (acetylcholine, vagal) SA firing rate
    4. T- and L-type calcium channels play modulatory role as well
    5. Cell firing in the SA node primarily dependent on Calcium (Ca2+)
    6. Little sodium (Na+) role at SA or AV node
    7. The Ca2+ fluxes in the SA node are low voltage
    8. There is no ECG wave which shows SA node firing (rate of SA firing is P wave rate)
  2. Atrial Firing
    1. Depends on large influx of Na+ ions into atrial cells
    2. P wave shows net atrial (initially right, then left atrial) firing
    3. Calcium ions play a role as well
  3. Atrioventricular (AV) Node Conduction
    1. Slow conduction dependent on Ca2+ with little Na+ ion flux
    2. Corresponds essentially to PR interval on ECG
    3. Prolongation of PR interval (block) is due to AV nodal dysfunction
  4. Ventricular Firing
    1. Large Na+ currents into ventricular cells
    2. Calcium involved in prolonging the action potential
    3. Corresponds to QRS interval duration on ECG
    4. Prolonged QRS interval usually due to slowed ventricular repolarization (K+ currents)

C. Leadsnavigator

  1. Unipolar: leads I (LA), II (RA), III (LL)
  2. Bipolar: leads aVR (RA), aVL (LA), and aVF (LL)
  3. Chest: leads V1-V6 (from center of chest to left side in order)

D. Typical 12-Lead ECGnavigator

LeadNormal Wave Shapes
Ilow pR(s)T
IIgood PRT
IIImid P(q) ~Rt or -t
aVR-PQ or Qr-t
aVLp or -Rs or rSt
aVFlow pRt
V1low pRS or Qno or -t
V2no PRS or rST
V3PRS or rST
V4P(q)Rs or RST
V5PRST
V6PR or RsT

E. Intervalsnavigator
  1. PR Interval
    1. Normal: 0.12-0.2 sec (3-5mm)
    2. Prolonged PR (>200ms) interval is defined as primary heart block
  2. QRS Duration
    1. Normal: 0.06-0.1 sec (1.5-2 mm)
    2. Prolonged QRS: hemiblock, hypertrophy, ventricular beat
  3. QT interval
    1. Corrected QT is corrected for heart rate or QTc = QT/SquareRoot(R-R interval)
    2. Normal <0.5 sec.

F. Heart Axisnavigator

  1. Determine which quadrant axis is in by looking at QRS sign in leads I and aVF
    1. Normally, QRS is positive in both I and aVF
    2. Therefore axis is 0-90°.
  2. Determine which of the uni- and bipolar leads has the least vectorial (that is, net) QRS value
  3. Then add 90° to this vector direction into the quadrant determined in step (1)

G. ST Segment [2]navigator

  1. Level of ST segment measured in relation to end of PR segment
  2. Do not measure ST segment level from TP segment
  3. Normal ST Segments
    1. In women and ~10% of men, normal ST segment has no elevation (same as PR segment)
    2. ~90% of men have normal ST elevation
    3. Most normal variant ST segment elevation occurs in leads V1 - V3
    4. Normal ST segment with elevation has a concave ST segment
    5. Black men have the most prominent ST segment elevation, up to 1-4mm midprecordium
    6. Early repolarization has notch at the J point
  4. Terminal T wave inversion can also occur

References navigator

  1. Katz AM. 1998. Am J Med. 104(2):179 abstract
  2. Wang K, Asinger RW, Marriott HJL. 2003. NEJM. 349(22):2128 abstract