Electrocardiography

# Electrocardiography
Every electrocardiogram (ECG) has nine features that should be examined systematically:      
1. Rate and regularity.
2. P wave morphology.
3. PR interval.
4. QRS interval
5. ST segment morphology.
6. T wave morphology.
7. U wave morphology.
8. QT interval.
9. Rhythm.
Rate, regularity, and rhythm are commonly grouped together. However, to accurately assess rhythm, it is necessary to consider not only rate and regularity, but also the various waveforms and intervals on the ECG.

# The Normal Adult EKG
- Majority QRS complexes are positive (have tall R waves)
Except AVR and V1-2:R-wave progression across the precordium.
T-wave in V1 should be small, flat or flipped.
- Except AVR and V1-2:R-wave progression across the precordium.
- T-wave in V1 should be small, flat or flipped.

# The P wave Morphology
The P wave represents atrial depolarization (stimulation). At either slow or normal heart rates, the small, rounded P wave is clearly visible just before the taller, more peaked QRS complex. At more rapid rates, however, the P wave may merge with the preceding T wave and become difficult to identify. represents depolarization of the atrial myocardium. Sinus node depolarization is too small in amplitude to be recorded from the body surface so it is not seen.
P waves have four steps to be identified;
A. Examination of the P wave contour. The P wave contour is normally smooth, and is either entirely positive or entirely negative wave (monophasic wave) in all leads except V1.
B. Measurement of the P wave duration. The P wave duration is normally less than 0.12 s.
C. Measurement of the maximal P wave amplitude. The maximal P-wave amplitude is normally no more than 0.2 mV in the frontal plane leads and no more than 0.1 mV in the transverse plane leads.
D. Estimation of the P wave axis. The P wave normally appears entirely upright on leftward and inferiorly oriented leads such as I, II, aVF, and V4 to V6. It is negative in aVR because of the rightward orientation of that lead, and it is variable in the other standard leads.
The direction of the P wave, or its axis in the frontal plane, should be determined on the morphology of the QRS complex. The normal limits of the P wave axis are 0 degrees and +75 degrees

# The PR Interval

## Definition
The PR interval measures the time required for an electrical impulse to travel from the atrial myocardium adjacent to the sinoatrial (SA) node to the ventricular myocardium adjacent to the fibers of the Purkinje network.
This duration is normally from 0.10 to 0.21 s. A major portion of the PR interval reflects the slow conduction of an impulse through the AV node, which is controlled by the balance between the sympathetic and parasympathetic divisions of the autonomic nervous system. Therefore, the PR interval varies with the heart rate, being shorter at faster rates when the sympathetic component predominates, and vice versa. The PR interval tends to increase with age  ;
- In childhood:   0.10 - 0.20 sec
- In adolescence: 0.12 - 0.16 sec
- In adulthood:   0.14 - 0.21 sec

## Background
- Shortens up to a rate of 140 to 150 beats per minute (bpm) through a withdrawal of parasympathetic tone
- PR may increase with increasing rate in the presence of digoxin or if the conducting system is diseased.
- If the atria are artificially paced the PR increases as the paced rate increases.
- Children have shorter PR intervals (0.11 at 1 year).
- Prolongation can be a normal variant: 6700 healthy airmen studied and 0.52% found to have a prolonged PR. 80% of the PR prolongations ranged from 0.21 to 0.24. In a second study 59 of 19,000 (0.31%) airmen had a PR of 0.24 or more.
- In healthy middle aged men, a prolongation of the PR in the presence of a normal QRS does not affect prognosis and is not related to ischemic heart disease.
- PR prolongation often signifies a delay in the AV node but may reflect intra atrial or His Purkinje disease.

## Differential Diagnosis of the Shortened PR Interval:
- AV junctional and low atrial rhythms.
- Wolff Parkinson White syndrome.
- Lown Ganong Levine syndrome.
- Glycogen storage disease.
- Hypertension.
- Normal variant.
- Fabry's disease.
- Pheochromocytoma.

## Differential Diagnosis of a Prolonged PR Interval:
- AV block due to coronary artery disease, rheumatic disease.
- Hyperthyroidism.
- Normal variant.

## Differential Diagnosis of the Depressed PR Interval:
- Acute pericarditis.
- Atrial infarction.

# The QRS Interval
The QRS interval represents ventricular depolarization (stimulation).

## Duration
- The precordial leads are approximately 0.01 or 0.02 seconds longer than the standard leads.

## Amplitude
- Definitions of low voltage: 
If the total amplitude above and below the isoelectric line is < 5 mm in all 3 standard leads. 
An average voltage in the limb leads of < 5 mm with an average of < 10 mm in the chest leads. 
Should be at least 5 mm in V1 and V6 , 7 mm in V2 and V5 and 9 mm in V3 and V4.
- If the total amplitude above and below the isoelectric line is < 5 mm in all 3 standard leads.
- An average voltage in the limb leads of < 5 mm with an average of < 10 mm in the chest leads.
- Should be at least 5 mm in V1 and V6 , 7 mm in V2 and V5 and 9 mm in V3 and V4.
- Differential diagnosis of low voltage: 
Diffuse Coronary artery disease. 
Congestive heart failure. 
Pericardial effusion (See also Pericarditis electrocardiogram). 
Myxedema 
Amyloidosis 
Emphysema 
Obesity
- Diffuse Coronary artery disease.
- Congestive heart failure.
- Pericardial effusion (See also Pericarditis electrocardiogram).
- Myxedema
- Amyloidosis
- Emphysema
- Obesity
- Maximal acceptable voltage: can be up to 20 to 30 mm in lead 2 and can be up to 25 to 30 mmm in the precordial leads.