What you will learn
• Understanding preload, afterload, and
• Phases of the cardiac cycle (revision)
• Linking the ‘phases’ to the ECG
• How electrolyte imbalances affect ECG’s
• Acute AF,MI,
Testing your cardiac knowledge!
Preload, afterload &
• If you think of the
heart as a balloon, it
will help you
Phases of the Cardiac Cycle.
• What can you
• Do not worry how
many phases there
are, as long as you
• Are the A-V Valves open or closed?
• Are the Semilunar Valves open or closed?
• Is this polarization or depolarization of the
• The pressure within the atrial chambers
increase or decrease? (remember balloon)
• What part of the ECG does this represent?
Where do I start!
• P-wave is the depolarization of the Atrium
• QRS-wave is the depolarization of the Ventricles
• There should be one P-wave to each QRS
• Normal P to R interval should be less than 0.2
seconds or five little boxes
• Normal QRS interval should be less than 0.12
seconds or three little boxes
• Normal heart rate is 60-100
• Bradycardia is heart rate less than 60
• Tachycardia is heart rate greater than 100
• Step 1. Is the speed of the rhythm
Step 2. Is it regular?
Step 3. Is the complex narrow?
Step 4. Is it preceded by a P-wave?
Step 5. Do all the complexes look the
• Vertical and
perspective of the
leads. The limb
leads “view” the
heart in the vertical
plane and the chest
leads in the
Unipolar, Bipolar and Precordial leads.
• The word ‘leads’ is
Sometimes it is used
to mean the pieces of
wire that connect the
patient to the ECG
recorder. Properly, a
lead is an electrical
picture of the heart.
• Sinus denotes that the
rhythm of the heart is
still being generated
by the sinuatrial node,
so the P-wave and
QRS complex are
How electrolyte imbalances
• The electrolytes sodium, potassium, and calcium,
with the help of magnesium, shift back and forth
across myocardial cell membranes.
• This shifting of electrolytes causes alternating
periods of activity (depolarization) and rest
(repolarisation), which allow for normal
• Serum potassium < 3.5
• Ventricular repolarisation
• The ECG shows a
characteristic U wave
• Ectopic impulses develop
in the myocardium
• A rhythm strip may show
• Potentially fatal
arrhythmias such as
• Serum potassium > 5.5 mEq/L
• A tall tented T wave
• AV or ventricular block may develop
• Flattened P wave
• A prolonged PR interval
• A widened QRS complex
• Depressed ST segment
• Severe Hyperkalemia > 9 mE/L causes the P
wave to disappear, the QRS complex to widen,
and sine waves to form. Hyperkalemia may end
in lethal arrhythmias.
• A 58-year-old male, presents to ED with C/O
chest pain, with radiation to the Left arm and jaw.
He reports feeling anxious, diaphoretic, and
SOB. His PMH is significant of type II diabetes
mellitus and hyperlipidemia. O/E, the patient
appears to be in moderate distress and anxious.
His ECG shows evidence of Acute MI in the
inferior leads. The Dr. suspects that the left
anterior descending artery is involved.
Case study continued
• What would the ST segment of this ECG
• On which leads would you see this ST
• What does the T wave represent
• The normal
pattern consists of a P
wave, a QRS complex, and
a T wave (A).
• In patients who have an ST
infarction (MI), the ST
segment is elevated above
the baseline (B).
• In patients who have a
non-ST elevation MI, the
ST segment is not
elevated, and instead other
patterns are seen (for
example, ST depression)
• ST segment appearance: Elevation of the
• Inferior leads: II, III, and aVf
• T wave: Represents ventricular polarizion.
ECG recognition quiz
• Choose one of the following answers (A – E)
• A. – P wave
• B. – PR wave
• C. – QRS complex
• D. – ST segment
• E – T wave
1. Period when ventricular action potentials are in their
2. Prolonged during first-degree heart block
3. Produced by depolarization of atrial fibers
• To summarise:
• We have reviewed the
• Wetted your appetite
for ECG recognition.