Heart pacemakers (SA, AV, Perkinje fibers, His bundle) fire automatically thus creating a depolarization wave that travels through the surrounding myocytes. This depolarization wave spreads in three dimensions (like an expanding balloon) until it reaches body surface where it can be recorded at different locations on the skin by an ECG machine.
APs of the SA node differ from APs of all the other heart cells by having a slow phase 4. How does the SA overcome all the other heart pacemakers? It has a steeper phase 4. The autonomic nervous system influences the SA firing by changing the configuration of the phase 4 (making it less or more steep).
By putting 10 electrodes on a person (6 on the chest, and 4 on the extremities (the one on the right leg used for ground)) it is possible to calculate 12 different depolarization vectors. These 12 vectors are used to analyze the depolarization of the atria and ventricles.
Depolarization towards a pole will yield an upward deflection on the ECG.
Depolarization away from a pole will yield an downward deflection on the ECG.
Depolarization perpendicular to a pole will yield no deflection on the ECG.
(1) Unipolar Limb Leads: aVR, aVF, aVL
Unipolar = there is no single (-) pole, rather, all of the other electrodes are averaged together to create a composite (-) reference.
aVR
= is a unipolar lead where the right arm electrode is the (+) pole.
Electrical forces traveling towards the right arm deflect up on the ECG.
Electrical forces traveling 180o away from the right arm deflect down.
aVF
= is a unipolar lead where the left leg electrode is the (+) pole.
aVL
= is a unipolar lead where the left arm electrode is the (+) pole.
(2) Bipolar Limb Leads: I, II, III
Bipolar = is when one limb electrode is the (+) pole and another single electrode is the (-) reference. The ECG machine inscribes an upward deflection if the electrical forces are heading toward the (+) electrode, and records a downward deflection if the forces are heading toward the (-) electrode.
I
= Connects left arm to the right arm.
II
= Connects the right arm to the left leg.
III
= Connects the left arm to the left leg.
Mnemonic: the number of L’s correlates to the lead name.
(3) Chest (
or Precordial) Leads
These are six unipolar leads located on the chest immediately by the heart. Used to record the perpendicular plane.
Labeled V1, V2, V3, V4, V5, and V6.
ECG Paper
looks like graph paper. Has 5mm x 5mm large boxes which contain 1mm x 1 mm small boxes.
X-axis is time, Y-axis is voltage
each little box = 40 msec, one big box = 200 msec
Cardiac Activation and the ECG
P wave
= summation of all vectors of atrial depolarization (normal P wave is 100 msec = <2.5 small boxes)
aVF – will be positive because depolarization travels down towards the legs (has a right component as well)
II – will have the biggest positive p wave because II points down and right
aVR – activity goes in opposite direction of aVR so get a negative depolarization
(note: just because see p wave doesn’t mean that it necessarily comes from sinus node, could come from auxiliary pacemaker right by it)
PR segment
= flat line represents depolarization being slowed down in the AV node.
QRS complex
= ventricular depolarization. Very fast. (Normal QRS is < 100 msec = 2.5 small boxes).
ST segment
and T wave = Ventricular Repolarization. (don’t see atrial repolarization because the atrial myocardium mass is too small). T wave abnormalities could be a sign of ischemia.
