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Lead systems allow you to look at the heart from different angles.  Each different angle is called a lead.  The different leads can be compared to radiographs taken from different angles. 

Each lead has a positive and negative pole attached to the surface of the skin, which can then be used to measure the spread of electrical activity within the heart.

Upward deflection on the ECG- is produced when electrical impulses travel towards a positive electrode.

Downward deflection on the ECG- is produced when electrical impulses travel towards a negative electrode.

Flat line (isoelectric line)- is produced when there is no electrical spread through the heart, or if the electrical forces are equal.

ECG Lead deflects

 

The standard ECG has 3 standard limb leads:

Lead I- right front leg (-) compared to left front leg (+)

 

Lead II- right front leg (-) compared to left hind leg (+)

 

Lead III- left front leg (-) compared to left hind leg (+)

 

aVR- right front leg (+) compared to a point halfway between the left front leg and left hind leg (-)

 

aVL- left front leg (+) compared to a point halfway between the right front leg and left hind leg (-)

 

aVF- left hind leg (+) compared to a point halfway between left front leg and right front leg (-)

 

 

 

The good news is we only need to use Lead II to assess arrhythmias

 

In Lead II, the RFL is negative and the LHL is positive.  Therefore, normally in Lead II, the P-wave is positive and the QRS is predominantly positive (R-wave).  The ECG typically pictured (as in the picture below) is a Lead II ECG.

Lead II ECG

 

 

*Above information courtesy of ECG Electrocardiography for the Small Animal Practitioner*

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