Willem Einthoven (In • doe • ven) was born in the golden era of electricity and science. He designed a non-invasive way to measure the electrical activity of the heart using buckets of saline, huge electromagnets, a tiny conductive wire and photosensitive paper. Modern ECGs have replaced the buckets with electrodes and the rest with electrical amplifiers and a CPU, but the design remains practically the same.
Limb Leads and Such
Einthoven discovered that by placing different limbs in the buckets he could get slightly different traces. This was because he was measuring the activity across a different axis, or from a different viewpoint. These views were the different ECG leads.
A "lead" refers to the relationship between one electrode and one or more other. The bipolar leads are leads I, II and III and they relate to the relationship between two electrodes. The unipolar leads are aVL, aVF and aVR and they relate to the relationship between one lead and the average of the two others.
Lead
|
+ Electrode
|
- Electrode
|
I
|
Left Arm
|
Right Arm
|
II
|
Left Leg
|
Right Arm
|
III
|
Left Leg
|
Left Arm
|
aVL
|
Left Arm
|
(Right Arm + Left Leg)/2
|
aVF
|
Left Leg
|
(Right Arm + Left Arm)/2
|
aVR
|
Right Arm
|
(Left Arm + Left Leg)/2
|
These leads were chosen for the axis or "viewpoint" they measure. All together they create a 360º coronal view of the heart in 30º increments.
Next up:
How the Heart's Wired
Next up:
How the Heart's Wired
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