The 12 lead ECG is one of the most common tests to diagnose cardiovascular abnormalities.

The heartbeat originates in the cardiac conduction system and spreads in this system to all parts of the myocardium.

It is estimated that the total yearly cost for computerized ECGs in the US is over $2 billion.

The ECG reflects the depolarization sequence of a reproducible pattern based on the anatomy and sequence of depolarization-repolarization of the various heart structures.

The conduction system includes with the sinoatrial node, the internodal atrial pathways , the atrioventricular node, the bundle of HIS and its branches, and the Purkinje system.

The various parts of the conduction system and parts of the myocardium are capable of spontaneous electrical discharge.

Six electrodes are placed on the chest: V1: fourth intercostal space at the right sternal border; V2 fourth intercostal  space at the left sternal border; V3: midway between V2 and V4; V4 fifth intercostal space in the midclavicular line; V5: in the horizontal plane of V4  at the anterior axillary line; V6: in the horizontal plane of V4 in the midaxillary line.
Incorrect placement of precordial leads may lead to force diagnosis of myocardial infarction.
Obesity limits the accuracy of locating the proper location for placement of V1 and V2 : due to the presence of fat on the precordium.
Incorrect placement V1 and V2 leads in the second or third interspace may be recognized by the presence of negative P waves in V2.
Studies suggest that 36% of precordial electrodes are positioned  more than 1.25 inches from the proper anatomic locations.
Swapped V leads can be identified by inconsistency in R wave progression from V1-V6.
Diagonal line lead rule for ECG interpretation is to consider all ECG with QRS duration greater than 120 ms, except for right right bundle branch block, to be at high risk or have specific abnormalities including electronic pacing, WPW syndrome, left BBB, and intraventricular conduction delays.

Diagonal line lead provides a pattern recognition scheme for identifying normal when over-reading ECGs.

Diagonal line lead rule is in the extension of the physiology and structure of cardiac depolarization-repolarization.

Diagonal line lead rule represents real electrical events in the normal heart and when violated it is associated with an abnormal heart.

In the above rule remaining ECG’s with QRS <120 ms and RBBB a diagonal line is drawn through III, aVL, and V1 to indicate that diagnostic Q waves and inverted P or T waves can be normal findings in these leads and abnormal in all other leads (Subbitt W).

In the above analysis a box drawn around aVR indicates that the P and T waves should be net negatively and a Q wave is normal in the lead, but a predominant R wave is abnormal.

Normally the SA node discharges the most rapidly to other regions of the electrical system, and is referred to as the cardiac pacemaker.

The SA node determines the cardiac heart rate by its discharge rate.

SA node impulses pass into the atrial pathways into the AV node, then into the bundle of HIS , and through the Purkinje system to the ventricular muscle.

The SA node is located at the junction of the SVC and the right atrium.

12-lead study in young athletes show repolarization abnormalities and increased R and S wave voltage suggestive of left ventricular hypertrophy.

Correlations of left ventricular mass and ECG voltage are weak in male athletes, so ECG is not useful in screening for LVH in athletes.

12 lead changes in young athletes generally felt to be of little concern and reflective of conditioning.

Low voltage is a nonspecific finding that can be associated with cor pulmonale, pericardial effusion and chest wall edema, infiltrative or fibrotic replacement of the myocardium.

Sinus rhythm is diagnosed based on P. wave morphology findings upright and lead 2 and inverted in lead aVR and normal AV nodal conduction, where P waves precede each QRS complex with the constant PR interval.

Atrial flutter can mimic an acute myocardial infarction pattern by deforming the ST segments, especially in the inferior leads.

Atrial flutter can resemble ST segment elevation or depression depending on where the flutter waves fall within the ST segment.

Criteria for anteroseptal Q wave MI require Q waves of 0.1 mV or deeper and at least 25% of the height of the R wave , in at least 2 contiguous leads for V1 through V3.

Transmural ischemia can cause Q waves, which can be transient and does not indicate a complete infarct.

J-point is the point at which the QRS complex meets the ST wave.

ST segment elevation with an upward convexity is usually benign, especially in healthy, asymptomatic individuals.

ST segment elevation with a downward concavity is more likely to be due to acute coronary syndrome.

Although ST elevation with an upward concavity and J-point notching often reflects a normal variant in the asymptomatic patient, and in a patient with chest pain is due to acute coronary syndrome until proven otherwise.

J-point elevation, ST-segment elevation, and T-wave changes are found frequently in athletes.

Routine ECG screening for individuals without heart disease is not recommended.

Right ventricular pathology is suggested by T wave inversions in V1-V 4 and the findings of right ventricular dilatation on EKG.

V1 is the only standard EKG lead that directly images the right ventricle.

T-wave inversions in leads III and VI suggest acute pulmonary embolism.

Incomplete right bundle branch block with the rSR morphology supports the diagnosis of pulmonary embolism.

Anterior or precordial T-wave inversions are among the most common EKG signs of pulmonary embolism and indicates severe pulmonary embolism, as most patients have accompanying pulmonary arterial hypertension.

Wide complex tachycardia differential includes: ventricular tachycardia, supra ventricular tachycardia with aberrancy, preexcited tachycardia, with antegrade conduction over an accessory pathway, ventricular pacing, drug or electrolyte induced QRS widening.

Q waves or T-wave inversions isolated to lead III, aVL, and V1 showed no association with increased cardiovascular risk.

In other leads, Q waves and T-wave inversions are associated with an increased risk for cardiovascular death.

Professor Chamberlains 10 rules of normal ECG:

RULE 1 PR interval should be 120 to 200 milliseconds or 3 to 5 little squares

RULE 2The width of the QRS complex should not exceed 110 ms, less than 3 little squares

RULE 3The QRS complex should be dominantly upright in leads I and II

RULE 4QRS and T waves tend to have the same general direction in the limb leads

RULE 5All waves are negative in lead aVR


The R wave must grow from V1 to at least V4The S wave must grow from V1 to at least V3 and disappear in V6

RULE 7The ST segment should start isoelectric except in V1 and V2 where it may be elevated.

RULE 8The P waves should be upright in I, II, and V2 to V6.

RULE 9There should be no Q wave or only a small q less than 0.04 seconds in width in I, II, V2 to V6

RULE 10The T wave must be upright in I, II, V2 to V6.

USPSTF recommends against screening with resting or exercise EKG to prevent cardiovascular disease events in asymptomatic adults at low risk of cardiovascular disease events.

USPSTF concluded current evidence is insufficient to assess the balance of benefits and harms of screening with resting or exercise EKG to prevent cardiovascular disease events in asymptomatic adults at intermediate or high risk of cardiovascular events.

Computerized EKG interpretation: The computer can review the full 10 seconds of each of the 12 leads where as a reader sees only 2 1/2 second samples of each of the 12 leads.

The reader still views the EKG as an analog recording with the device digitizes the incoming analog signal 500 times per second and, for the purpose of identifying pacemaker stimulus side effects, can you given over a sample at 1000 times per second.

The reader looks at the waves individually and mentally categorizes beat-beat difference as important diagnostic findings with variations that are minimal and negligible.

The computer composes the representative wave from each of the leads and analyzes an average wave separately rather than analyzing each week separately as the eye does.

The computer can temporarily align and simultaneously identify the earliest onser and latest offset of all the waves, an impossible task for the reader, who cannot recognize isoelectric onets or off sets of all leads at once.

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