Electrocardiography

Question 1

What is the orientation of the six frontal plane limb leads?

Answer 1

Question 2

What is the placement of the six precordial limb leads?

Answer 2

Question 3

What is the morphology and the duration of a normal P wave?

Answer 3

The contour of a normal P wave is smooth and entirely positive or entirely negative except in V1 and possible V2. The duration of a normal P wave is 120 ms.

Question 4

What is the relationship between current flow and ECG deflections?

Answer 4

If a wave of depolarization flows towards a (+) electrode or ECG lead, then the deflection on the ECG will be positive. If a wave or depolarization flows away from a (+) electrode or ECG, then the deflection on the ECG with be negative.

Question 5

What is the PR interval, and what is the normal duration?

Answer 5

The PR interval is the time it takes for an electrical impulse to travel from the atrial myocardium adjacent to the sinoatrial node (SA node) to the ventricular myocardium adjacent to the Purkinje fibers. The normal value of the PR segment is 120-200 ms.

Question 6

What is the duration of a normal QRS complex?

Answer 6

The duration of a normal QRS complex is <100 ms.

Question 7

What is a Q wave?

Answer 7

A Q wave represents the initial depolarization of the interventricular septum, and it is defined as any initial negative deflection following the P wave and occurring before the R wave.

In some leads (V1, V2, V3) the presence of any Q wave should be considered abnormal. In all other leads (except rightward oriented leads III and aVR) a normal Q wave is very small.

Question 8

What is the R wave?

Answer 8

The R wave reflects depolarization of the main mass of the ventricles, and it is defined as any positive deflection in the QRS complex.

Large R waves in leads V1 and V2 can be produced by right-ventricular hypertrophy, while large R waves in leads V5 and V6 can be produced by left-ventricular hypertrophy.

Question 9

What is the S wave?

Answer 9

The S wave signifies the final depolarization of the ventricles as the base of the heart, and it is defined as any negative deflection after the R wave.

Question 10

What is the ST segment?

Answer 10

The ST segment, which is also known as the ST interval, is the time between the end of the QRS complex and the start of the T wave. It reflects the period of time from the end of ventricular depolarization to the start of repolarization. Normally, the ST segment is isoelectric with the electrical baseline (the TP segment).

Slight up-sloping, down-sloping, or horizontal depression of the ST segment may occur as a normal variant.

Question 11

What is the T wave?

Answer 11

The T wave represents ventricular repolarization. The contour of the T wave is usually smooth and rounded, and waveform is positively in all leads except aVR, where they are negative, and in lead V1, where they may be biphasic.

Question 12

What is the QT interval and the QTc interval.

Answer 12

The QT interval represents the total time it takes for depolarization and repolarization of the ventricles. The normal duration of the QT interval is <460 ms.

The QTc interval is the QT interval corrected for heart rate. The normal duration of the QTc interval is <50% the RR interval.

Question 13

What is the standard calibration of an electrocardiogram?

Answer 13

The standard calibration of an electrocardiogram is defined by a paper speed of 25 mm/s and an amplitude with units of 10 mm/mV.

One small box on the horizontal time axis = 1 mm = 40 ms (0.04 sec).
One big box on the horizontal time axis = 5 mm = 200 ms (0.2 sec).
Five big boxes on the horizontal time axis = 25 mm = 1000 ms (1 sec).

Question 14

What are the two methods of calculating heart rate from an electrocardiogram?

Answer 14

The first method of calculating heart rate can be used for regular rhythms. The steps are the following:
1) Count the # of big boxes between QRS complexes
2) Note the following: HR = 300 / (# of big boxes between the QRS complexes)
3) See the image below for reference values.

The second method of calculating heart rate can be used for slow of irregular rhythms. The steps are the following:
1) Count the # of QRS complexes in the 10 second rhythm strip
2) Multiply the # of QRS complexed in the 10 second rhythm strip by 10

Question 15

What are the criteria for sinus rhythm?

Answer 15

The criteria for sinus rhythm are the following:

1) P wave for every QRS complex, QRS complex for every P wave
2) P wave is upright in I, II; P wave is inverted in aVR
3) P wave morphology is the same from beat to beat
4) PR interval is constant
5) HR 60-100 bpm

Question 16

What is the normal electrical axis of the heart?

Answer 16

The normal electrical axis of the heart is between -30 degrees and 90 degrees. The more muscular left ventricle generates more depolarizing current, so the left ventricle typically dominates the vector and the QRS axis can be used to determine the overall electrical axis.

If the QRS complex is mostly positive in Leads I & II, then the electrical axis of the heart is normal.

Question 17

What is a pathologic Q wave?

Answer 17

A pathologic Q wave indicate that the myocardium has had irreversible myocardial cell death, i.e. that there has been a myocardial infarct.

The criteria for an infact (pathologic Q waves) are the following:
1) Q wave with width > 0.03-0.04 (~1 small box)
2) Q wave with depth >25% of the height of the QRS complex
3) Pathologic Q waves in two or more contiguous leads

Question 18

What is atrioventricular nodal reentrant tachycardia (AVNRT), and what are the characteristic findings of it on electrocardiogram (ECG)?

Answer 18

Atrioventricular nodal reentrant tachycardia (AVNRT) is a type of paroxysmal supraventricular tachycardia that results due to the presence of a re-entry circuit within or adjacent to the AV node.

In most cases, the electrocardiogram (ECG) will show a heart rate between 140-280 beats per minute (bpm), and in the absence of an aberrant conduction, a QRS complex with a duration <120 milliseconds (a narrow QRS complex).

Question 19

What is the treatment for atrioventricular reentrant tachycardia (AVNRT)?

Answer 19

The treatment options for atrioventricular reentrant tachycardia include the following:

1) Vagal maneuvers
2) Medications that block electrical conduction at the atrioventricular (AV) node such as:
Adenosine, digoxin, beta-lockers, & calcium channel blockers
3) Catheter ablation (curative)

Question 20

What is Wolff Parkinson White syndrome (WPW), and what are the characteristic findings of it on electrocardiogram (ECG)?

Answer 20

Wolff Parkinson White syndrome (WPW) is a congenital cardiac preexcitation syndrome that arises from abnormal cardiac electrical conduction through an accessory pathway that can result in symptomatic and life-threatening arrhythmias.

The hallmark electrocardiographic (ECG) finding of WPW pattern or preexcitation consists of a short PR interval and a prolonged QRS complex with an initial slurring upstroke (“delta” wave) in the presence of sinus rhythm. WPW syndrome refers to an ECG pattern that is consistent with the above findings along with a tachyarrhythmia such as atrial fibrillation or atrioventricular reentry tachycardia.

Question 21

What is the treatment for Wolff Parkinson White (WPW) with preexcitation and atrial fibrillation?

Answer 21

The treatment for Wolff Parkinson White (WPW) syndrome with preexcitation and atrial fibrillation is procainamide.

Question 22

What is atrial flutter, and what are the characteristics of it on electrocardiogram (ECG)?

Answer 22

Atrial flutter is a supraventricular arrhythmia that is characterized by a fast atrial rate with a fixed or variable ventricular rate. Atrial flutter is a macro-reentrant tachycardia, and typically the rhythm originates in the right atrium at the level of the tricuspid valve annulus and the atrial rate is around 300 beats per minute (bpm).

The characteristic finding of atrial flutter is the undulating “sawtooth” pattern on electrocardiogram (ECG).

Question 23

What are the treatments for atrial flutter?

Answer 23

The treatments for atrial flutter include the following:

1) An agent to decrease heart rate
-Atrioventricular (AV) nodal blocker such as a beta blocker, nondihydropyridine calcium channel
blockers, or digoxin

2) An agent to control rhythm
-Antihythmic drugs or catheter ablation

3) An agent to prevent stroke
-Anti-coagulant

Note that catheter ablation is the procedure of choice and is curative.

Question 24

What is ventricular tachycardia, and what are the characteristics of it on electrocardiogram (ECG)?

Answer 24

Ventricular tachycardia is a monomorphic tachyarrhythmia that is characterized by a wide QRS complex (greater than 120 milliseconds) and a heart rate greater than 100 beats per minutes (bpm).

Question 25

What are the treatment options for ventricular tachycardia?

Answer 25

The treatment options for ventricular tachycardia are the following:

1) Electrical cardioversion (if the patient is unstable)

2) Anti-arrhythmic drugs
Note that Amiodarone is a drug of choice for VT originating from myocardial scar after a myocardial
infarct (MI).

3) Catheter ablation

4) Implantation of a cardioverter-defibrillator

Question 26

What is left axis deviation of the heart, and what is the characteristic finding of this on electrocardiogram (ECG)?

Answer 26

A left axis deviation of the heart is between -30 degrees and -90 degrees.

The characteristic finding of left axis deviation on electrocardiogram (ECG) is a positive P wave in Lead 1 and a negative P wave in Lead II.

Question 27

What is right axis deviation of the heart, and what is the characteristic finding of this on electrocardiogram (ECG)?

Answer 27

A right axis deviation of the heart is between 90 degrees and 160 degrees.

The characteristic finding of right axis deviation on electrocardiogram (ECG) is a negative P wave in Lead I and a positive P wave in Lead II.