12-Lead ECG Intro

Question 1

Explain how cardiac electrical activity results in positive and negative deflections (waveforms) on an electrocardiogram (ECG).

Answer 1

• At rest and during plateau phase: baseline value of zero
o Everywhere on surface of heart positive (rest)
o Everywhere on surface negative (plateau)
• Depolarization toward recording (+) electrode or away from reference (-) electrode → positive deflection
• Repolarization toward recording (+) electrode or away from reference (-) electrode → negative deflection

Question 2

Describe each of the 12 ECG leads

Answer 2

• Standard limb leads (form Einthoven’s triangle with heart in middle)
o Lead I: left arm (+) → right arm (-)
o Lead II: left leg (+) → right arm (-)
o Lead III: left leg (+) → left arm (-)
• Augmented limb leads:
o Compare one lead (active) to electrical average of other two leads
o aVF (augmented voltage foot)
o aVR
o aVL
• Precordial leads (V1 – V6)
o Horizontal plane (anterior and posterior surfaces of heart)

Question 3

Describe the normal T wave

Answer 3

o Ventricles repolarize
o Normally has same polarity as QRS complex in same lead
o Repolarization is epi- to endocardium, opposite of direction of depolarization (endo- to epidardium)

Question 4

Describe the normal ST segment

Answer 4

o Period from end of QRS to start of T wave

o When ventricles are fully depolarized until repolarization begins

Question 5

Describe the normal P wave

Answer 5

o Atrial depolarization

Question 6

Describe the normal PR interval

Answer 6

o From start of P wave to start of QRS complex

o Time for conduction of AV node

Question 7

Describe the normal QRS complex

Answer 7

o	Ventricles depolarize
o	Q wave = first negative deflection 
o	R wave = first positive deflection 
o	S wave = second negative deflection 
o	Duration indicates time to fully depolarize (normally <120 ms)

Question 8

Describe the normal QT interval

Answer 8

o Period from start of QRS to end of T wave
o Total time that some part of ventricle is depolarized
o Affected by body temperature, pH, HR, and genetics
o Abnormally long QT interval associated with increased risk for lethal arrhythmias

Question 9

Use ECG to calculate heart rate

Answer 9

• Rule of 300: divide 300 by number of big boxes between QRS complex
• Ex. 1 box = 300; 2 boxes = 150
• Normal: 60 (or 50) – 100 bpm
• HR > 100 bpm = tachycardia
• HR < 60 (or 50) = bradycardia

Question 10

Use ECG to measure PR and QT intervals and QRS duration

Answer 10

•	PR: 0.2 s (< Half the R-R interval with normal HR
o	Corrected QT (QTc): Bazett’s formula:
•	QT/ √(R-R)
o	Causes of prolonged QTc intervals:
•	Drugs (K channel blockers)
•	Hypocalemia, hypomagnesemia, hypokalemia
•	Hypothermia
•	Acute myocardial ischemia/infarction
•	Congenital (inherited channelopathies)
•	Increased ICP

Question 11

Use ECG to recognize left ventricular hypertrophy

Answer 11

• LV depolarization creates S wave in leads V1 and V2 and the R wave in V5 and V6
• LVH: S wave in either V1 or V2 (whichever is larger) plus larger R wave in either V5 or V6 is greater than 35 mm
• Careful because sensitivity is low

Question 12

Utilize the vector orientation of the leads to calculate the average QRS vector (axis) and explain causes of axis shifts

Answer 12

• Vectors:
o V1 and V2 for right ventricle and interventricular septum
o V2-V4 for interventriclar septum and LV anterior wall
o V4-V6 for anterolateral surface of LV

• QRS axis: represents overall direction of heart’s activity
• Normal: between -30 and +90 degrees
• Axis more negative than -30 = left shifted
• Axis more positive than 90 = right shifted
• Methods:
o Plot deflection numbers from leads I and aVF
• Determined by amplitude of R wave (all positive deflection from baseline) minus the sum of the amplitude of Q and S waves (all negative deflection)