How to read ECG

1. BASIC ECG
INTERPRETATION

2. • It is an electrogram of the heart which is a graph of voltage versus
time of the electrical activity of the heart using electrodes placed on
the skin.
• These electrodes detect the small electrical changes that are a
consequence of cardiac muscle depolarization followed
by repolarization during each cardiac cycle (heartbeat).

3. History of ECG William Einthoven (1860-1927)

8. PHASE 4: THE RESTING PHASE
• The resting potential in a cardiomyocyte is −90 mV due to a constant outward leak
of K+ through inward rectifier channels.
• Na+ and Ca2+ channels are closed at resting TMP.
PHASE 0: DEPOLARIZATION
• An action potential triggered in a neighboring cardiomyocyte or pacemaker cell
causes the TMP to rise above −90 mV.
• Fast Na+ channels start to open one by one and Na+ leaks into the cell, further
raising the TMP.
• TMP approaches −70mV, the threshold potential in cardiomyocytes, i.e. the point
at which enough fast Na+ channels have opened to generate a self-sustaining
inward Na+ current.
• The large Na+ current rapidly depolarizes the TMP to 0 mV and slightly above 0 mV
for a transient period of time called the overshoot; fast Na+ channels close (recall
that fast Na+ channels are time-dependent).
• L-type (“long-opening”) Ca2+ channels open when the TMP is greater than −40 mV
and cause a small but steady influx of Ca2+ down its concentration gradient.

9. PHASE 1: EARLY REPOLARIZATION
• TMP is now slightly positive.
• Some K+ channels open briefly and an outward flow of K+ returns the TMP
to approximately 0 mV.
PHASE 2: THE PLATEAU PHASE
• L-type Ca2+ channels are still open and there is a small, constant inward
current of Ca2+. This becomes significant in the excitation-contraction
coupling process described below.
• K+ leaks out down its concentration gradient through delayed
rectifier K+ channels.
• These two countercurrents are electrically balanced, and the TMP is
maintained at a plateau just below 0 mV throughout phase 2.

10. PHASE 3: REPOLARIZATION
• Ca2+ channels are gradually inactivated.
• Persistent outflow of K+, now exceeding Ca2+ inflow, brings TMP back
towards resting potential of −90 mV to prepare the cell for a new cycle of
depolarization.
• Normal transmembrane ionic concentration gradients are restored by
returning Na+ and Ca2+ ions to the extracellular environment, and K+ ions to
the cell interior. The pumps involved include the sarcolemmal Na+-
Ca2+ exchanger, Ca2+-ATPase and Na+-K+-ATPase.

15. The standard ECG has 12 leads. Six of the leads are considered “limb
leads” because they are placed on the arms and/or legs of the
individual. The other six leads are considered “precordial leads”
because they are placed on the torso (precordium). The six limb leads
are called lead I, II, III, aVL, aVR and aVF.

16. • A 3-lead configuration requires the placement of three electrodes; one
electrode adjacent each clavicle bone on the upper chest and a third
electrode adjacent the patient's lower left abdomen. It is Typical Bipolar
Lead form and Monitor reads as Lead I,II & III.
• A Five(5) lead configuration requires the placement of the three electrodes
in the Three (3)-lead configuration with the addition of a fourth electrode
adjacent to sternum (right side of Fourth intercostal space) and a fifth
electrode on the patient's lower right abdomen. It is Bipolar Lead form and
Monitor reads as Lead I,II & III along with Chest lead “C”
• The 6 Lead ECG method consists of 6 electrodes including four limb and
two chest electrodes. It can help us monitor Bipolar and augmented leads.
The Ca and Cb should be placed in two of the positions of C1 to C6,The
following combinations may be used
C1&C3,C2&C5,C3&C5,C1&C4,C2&C4,C3&C6,C1&C5

18. Posterior leads; V7-V9Leads V7-9 are placed on the posterior chest wall in the following
positions (see diagram below):V7 – Left posterior axillary line, in the same horizontal plane as V6. V8 – Tip of
the left scapula, in the same horizontal plane as V6. V9 – Left paraspinal region, in the same horizontal plane
as V6.
Right side chest leads V1R-V6RThe most useful lead is V4R, which is obtained by placing the
V4 electrode in the 5th right intercostal space in the mid-clavicular line. ST elevation in V4R has a sensitivity of
88%, specificity of 78% and diagnostic accuracy of 83% in the diagnosis of RV MI
Lewis Lead S5; The Lewis lead configuration (S5-lead placement) is used to better detect atrial
activity in relation to that of the ventricles.

22. 1. Rate
• If the rhythm seems regular
See the figure on the side
• If the rhythm seems irregular
(as in Atrial fibrillation) Count all
the R waves on the rhythm strip
and multiply by 6
• A rhythm strip is at least a 6-
second tracing printed out on
graph paper which shows activity
from one or two leads.

23. 2. Rhythm
• Regularity:
Is the spacing between R
waves regular or irregular
Is the ECG morphology the
same
• Sinus rhythm:
Check if there is a P-wave before
every QRS complex or a QRS
complex after every Pwave
• Arrhythmias

24. 3. Axis Deviation
normal vs. deviation
The axis is the average vector
of ventricular depolarization
Consider the size and direction
of the depolarization wave

25. right axis deviation
Causes of RAD:
• Normal in newborns
• Right ventricular hypertrophy
• Pulmonary embolism
• Cor pulmonale
• Left posterior fascicles block
• Hyperkalemia
• Pre-excitation (Wolff-Parkinson
White Syndrome)

26. left axis deviation
Causes of LAD
• LBBB
• Left ventricular hypertrophy
• Inferior MI
• Pre-excitation (WPW
syndrome)
• Left anterior fascicular block

27. 4. P- wave morphology
• The P-wave corresponds to atrial
depolarization
• Normal P-wave duration is <0.12s (< 2.5 small
squares)
• The first 1/3 of the p-wave corresponds to right
atrial activation and the final third corresponds
to left atrial activation
• P pulmonale (peaked P-wave)
• Right atrial hypertrophy
• Pulmonary hypertension
• P mitrale (notched P-wave)
• Due to left atrial enlargement

28. 5. PR interval
• It is the time from the onset of
atrial depolarization, to the start
of ventricular depolarization
• It also reflect conduction
through the AV node, through
the PR segment
• Normal PR-interval is between 3-
5 small squares (0.12s - 0.20s)
• Abnormal in
• Prolonged ; AV blocks
• Shorter; commonly caused by
preexcitation syndromes:
• Wolff-Parkinson White
• Lown-Ganong-Levine

29. 6. QRS morphology
The QRS-complex corresponds to
ventricular depolarization
Normal QRS duration is < 3 small
boxes (0.08s - 0.10s)
Abnormal in;
• Bundle branch blocks
• Ventricular rhythms

30. 7. QT interval
• Measured in either lead II or V5 or
V6
• Normal duration is 0.40s - 0.44s
• Corrected QT (cQT) interval
estimates the QT interval at a
standard heart rate of 60bpm
and allows for comparison of QT
values over time at different rates
and improved detection of
patients at increased risk of
arrhythmia (use Bazett Formula)
Short QT Interval Causes
• Hypercalcaemia
• Congenital short QT-syndrome
• Digoxin effect
Prolonged QT Causes
• Hypokalemia
• Hypomagnesaemia
• Hypocalcaemia
• Hypothermia
• ROSC post-cardiac arrest

31. 8. ST segment
• Represents the interval between
ventricular depolarization and
repolarization
• Check for displacement
• Elevation or depression
• Comment on the contour
• Horizontal
• Upsloping
• Downsloping
• Displacement of ≥ 1 small box is
counted as an elevation or
depression

32. ST-segment depression
Causes
• MI / NSTEMI
• Reciprocal change in STEMI
• Posterior MI
• Hypokalemia
• SVT
• RBBB
• RVH
• LBBB
• LVH
ST-segment elevation
Causes
• Acute MI
• Prinz metal angina
• Pericarditis
• Benign early repolarization
• LBBB
• LVH
• Brugada syndrome
• The lead in which the STEMI is in,
may point towards the offended
vessel

35. 9. T wave
• The T-wave corresponds to
ventricular repolarization
• Usually upright in all leads
except aVR and V1
• Amplitude < 5 small squares
(5mm)