ECGs

■ The most commonly used clinical ECG system is the 12-lead ECG. It consists of the following leads: I, II, III, aVR, aVL, aVF, Vi, V2, V3, V4, V5, and Vs. Both limb and chest electrodes are used to record 12-lead ECGs.

■ Measurements are central to 12-lead ECG analysis. The height and depth of waves can be important in the diagnosis of certain conditions, including MI or ventricular hypertrophy.

■ The direction of ventricular depolarization is an important factor in determining the axis of the heart.

■ In the case of MI, multiple leads are necessary to recognize its presence and determine its location. If large areas of the heart are affected, the patient can develop cardiogenic shock.

■ ECG signs of an MI are best seen in the reflecting leads — those facing the affected surface of the heart. Reciprocal leads are in the same plane but opposite the area of the MI; they show a "mirror image" of the electrical complex.

■ Prehospital EMS systems may use 12-lead ECGs to discover signs of acute myocardial infarction, such as ST segment elevation, in preparation for in-hospital administration of thrombolytic drugs.

■ Once a 12-lead ECG is performed, a 15-lead, or right-sided, ECG may be used for an even more comprehensive view if it appears that the right ventricle or posterior portion of the heart has been affected.

y Clinical Tip: Always compare the patient's current 12-lead ECG with the previous one.

Copyright2005 F A Davis

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| R Wave Progression

■ Normal ventricular depolarization in the heart progresses from right to left and from front to back.

■ In a normal heart the R wave becomes taller and the S wave becomes smaller as electrical activity moves across the heart from right to left. This phenomenon is called R wave progression.

■ Alteration in the normal progression of the R wave may be seen in left ventricular hypertrophy, COPD, left bundle branch block, or anteroseptal MI.

Wave Progression Chest Leads
Normal R wave progression in chest leads Vi-Ve-

The electrical axis is the sum total of all electrical currents generated by the ventricular myocardium during depolarization. Analysis of the axis may help to determine the location and extent of cardiac injury, such as ventricular hypertrophy, bundle branch block, or changes in the position of the heart in the chest (from, e.g., pregnancy or ascites).

The direction of the QRS complex in leads I and aVF determines the axis quadrant in relation to the heart.

y Clinical Tip: Extreme right axis deviation is also called indeterminate, "no man's land," and "northwest."

lopyright 'E' 2005 F A Davis

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1 Ischemia, Injury, or Infarction in

1 Relation to the Heart

- Mk

WT^ Anterior wall^fe^

Septal walllnferior wall

Anterior view Anterior view Posterior view

Anterior view Anterior view Posterior view

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