Chapter 9 ECG Cardiovascular System: Function and Exercise Responses

Heart Action potential –The transient depolarization of a cell as a result of activity of ion channels –Depolarization conducted throughout functional (electrical) syncytium Ions in body fluids have electrical charges –Conduct electricity –Electrical signal measured at body surface- ECG

Heart Polarization-opposite effects –Cells are negatively charged on the inside –Cells are positively charged on the outside Electrical potential difference

ECG Equilateral (Einthoven’s) triangle –Heart provides the current (center) –Triangle points – limb leads Right arm Left arm Left leg –Potential difference between two leads Size and direction of deflection Muscle mass and direction of current flow

ECG 12 lead ECG (10 electrodes) –Bipolar leads-3 views of heart Leads I, II, III –Separated by 60 degrees Lead I – RA-LA Lead II – RA-LL Lead III – LA-LL –Unipolar leads Augmented (amplitude of deflections increased 50%) –aV R –aV L –aV F

ECG Chest Leads (unipolar) –V1=4 th intercostal space, right side of sternum –V2=4 th intercostal space, left side of sternum –V3=5 th intercostal space, between V2 and V4 –V4=5 th intercostal space, mid clavicular line –V5=5 th intercostal space, anterior axillary line –V6=5 th intercostal space, mid-axillary line

Electrical Axis Deflection –Muscle mass –Direction of current flow Maximal potential occurs in line with the maximal current

Mean Electrical Axis Leads may have positive or negative deflection Current flow-base to apex SUM-mean electrical axis –Predominant current flow –Positive and downward direction

Mean Electrical Axis MEA –Primary direction of current flow during depolarization –Normal heart = 59 degrees

MEA Normal range -30 to +110 degrees Abnormal right axis deviation +110 to +180 Abnormal left axis deviation -30 to -90

Vectors