Basic ECG Readings

5.  Atrial depolarization ( activation)
 normal amplitude/height is 0.5 to
2.5 mm
 normal duration/width is up to
0.10s (2 ½ small squares)
 biphasic (with upward and
downward)

7.  represents time interval for
impulse to reach ventricles
from SA node
 normal is 0.12-0.20s or 3- 5
small square or 1 big square

9. Serves as the reference
Isoelectric line of the ECG
End of P wave and onset of
QRS complex

11.  Ventricular depolarization
 Normal duration of complex is
< 0.12 sec (3 small squares)

13. Rapid repolarization of
contractile cells
usually upright in LI, LII
and diphasic or
inverted in LIII,V1

15.  represents period from end of
ventricular depolarization to start of
ventricular repolarization
 between end of QRS and start ofT
wave
 clinically important if elevated or
depressed as it may represent infarction
or ischemia
 usually isoelectric

17.  represents electrical systole
 measured by counting number of small
boxes
 time required for ventricular
depolarization and repolarization
 varies with age, sex , and heart rate
 normal QT = 0.35-0.44s

19. Not always observed as
a result of its small size
Represent
repolarization of the
Purkinje fibers

21. 1. Identify the rhythm if regular or
irregular.
2. Determine the heart rate.
3. Measure the important intervals.
4.Calculate the electrical axis.
5. Inspect P waves for atrial
enlargement.

22. 6. Inspect QRS complexes for ventricular
hypertrophy or low voltage.
7. Inspect QRS complexes for bundle
branch block or fasicular block.
8. Assess Q waves and determine
significance.
9. Assess ST segments andT waves.
10. Measure QT interval for specific
diagnosis.

23. 11. Check for miscellaneous ECG
findings

24.  Determined by the sinus node which
fires at 60-100 beats per minute
 If the rhythm is regular, the RR interval
should be constant throughout the ECG
 Check to see if a P wave is present
before each of the QRS complexes
 P wave is normally upright in Lead II
(and usually in leads I, AVL and AVF)

26. A. Regular Rhythm
 Method 1:
In determining the heart rate in
regular rhythm, count the number
of small squares between two
successive QRS complexes

27. The numerator (1500) is a constant, and when
divided by 20 (number of small squares between
successive QRS complexes), yields a heart rate of 75
beats per minute.

28.  Method 2:
Sequence method: 300, 150, 100, 75, 60, 50

29. B. Irregular rhythm
count the number of QRS complexes
within the 6 second strip.This value should
be multiplied by 10 in order to yield the
heart rate in 1 minute.

32. Step 4.A -The mean QRS
axis refers to the average
orientation of the heart's
electrical activity. It uses
the Lead I and avF

33.  An ECG with the QRS axis oriented to the
quadrant between 0° and 90° is said to
be normal.
 An ECG with the QRS axis oriented to the
quadrant between -1° and -90° is said to
have left axis deviation.

34.  An ECG with the QRS oriented to the quadrant
between +91° and 180° is said to have right axis
deviation.
 An ECG with the QRS oriented to the quadrant
between -91° and -180° is an extreme axis
deviation because one cannot tell if it represents
right or left axis deviation.

35.  Step 4.B-The mean QRS axis is oriented
towards the lead with the greatest net QRS
deflection.To calculate the net QRS
deflection, add up the number of small
squares that correspond to the height of the
R wave (positive deflection), and subtract the
number of small squares that correspond to
the height of the Q and S waves (negative
deflection)..

37.  Step 4.C- Approximate the net QRS
deflection for leads I and aVF.
Remember that the mean QRS axis will
be oriented towards the lead with the
greatest positive net QRS deflection. If
the net deflection is positive for both,
the axis lies between leads I and aVF (0-
90°) and is therefore normal.

40. Quadrant Method:

41. . Three lead analysis (lead I, avf and lead ii)
This is a refinement of the quadrant method
and highlights the utility of lead ii in
distinguishing between normal axis and lad.

42.  P waves in leads I, II, III andV1
should be inspected for evidence
of right or left atrial
enlargement. Usually, lead II will
have the clearest P wave.

43. P wave amplitude should not
exceed 3 small squares (3 mm
or 0.3mV). If it does, this
represents right atrial
enlargement.

44.  In leadV1, the terminal negative
deflection of the P wave
represents left atrial
depolarization and should not
exceed 1 mm (0.1mV). If it does,
this is indicative of left atrial
enlargement.

47.  In LVH, leadsV5,V6 and aVL will have tall R
waves, while leadsV1 andV2 will have deep S
waves.

48.  normal QRS interval is 0.12 seconds (3 mm or
3 small squares) on the ECG.To correctly
determine the QRS interval, use the lead with
the widest QRS complex. If the QRS complex
is less than or equal to 0.12 seconds, then no
further analysis is necessary. If it is greater
than 0.12 seconds, then you should try to
determine the reason for the abnormally long
QRS interval.

49.  Q waves should be assessed and their
significance determined, particularly in
regard to the diagnosis of myocardial
infarction. SmallQ waves are commonly a
normal finding in the inferior leads III and
aVF, and in the anterolateral leads aVL, I,V5
andV6. Q waves of 0.04 seconds (1 mm)
duration and greater than one third the R
wave's amplitude in the same lead may be
pathological.

51.  QT interval can be prolonged secondary to
metabolic disorders and drug effects. It must
be corrected for heart rate since it is rate
dependent.The corrected QT interval is
calculated using the following formula:
QTI corrected = (QTI observed) / (square root
of RR interval)
 The QTI corrected is often reported with
computerized ECG interpretation.

52. Low voltage complexes
 QRS complexes <5 small squares in limb leads or <10
small squares in precordial leads
 Ex: COPD, anasarca, obesity, myocarditis, moderate-
sized to massive pericardial effusions
Electrolyte abnormalities
 Hypokalemia: prominent U waves + flattenedT waves
 Hyperkalemia: peakedT-waves >10mm, widened
QRS, prolonged PR interval, loss of P waves, eventually
sine wave pattern
 Hypocalcemia: prolonged QT interval
 Hypercalcemia: shortened QT interval