BIPN 100 - M1 Prelecture 23

The EKG records electrical activity in the heart, specifically focusing on contractile cells.
The vast majority of the heart is composed of contractile cells; hence, their activity dominates the EKG recording.
EKG recordings are extracellular, using two electrodes outside the cells, unlike intracellular recordings which have one electrode inside the cell.
- Extracellular recording involves a negative (reference) and a positive electrode.
Each stage of the cardiac cycle has electrical activity and contraction.

Electrical activity can occur without contraction, resulting in a flat line on the EKG.
Atrial systole and ventricular systole manifest different components of electrical activity.
The goal is to understand the relationship between the cardiac cycle, corresponding electrical activity, pressure changes, and volume changes in the heart.
Each contraction phase is linked to a specific wave on the EKG.

The EKG records electrical activity from a chain of cells connected via gap junctions.
Depolarization spreads from one cell to the next in the chain.
Depolarization starts at the base of the heart (SA node activation, atrium activation) and moves to the apex via conduction fibers, then upwards in the ventricles.
Repolarization starts in the upper parts of the ventricles (superior parts closest to the base/atrium) and spreads back down to the apex.
- Cells in the upper ventricles repolarize faster due to more potassium channels.
Depolarization starts in one cell, spreads to the next, until all cells are depolarized.
The calcium plateau in contractile cells (lasting 300-500ms) causes the cells to remain depolarized for a prolonged period.
- During the calcium plateau, the EKG recording is flat (zero) because there is no voltage difference between the recording points.
As repolarization begins in the last cells to depolarize, a wave of repolarization spreads back down to the apex.
The ventricles relax and open back up as repolarization spreads.

Inside of cells is negative (around $-90$ mV) relative to the outside, which is positive.
During depolarization, the inside becomes more positive, and the outside becomes more negative.
During repolarization, potassium exits, causing the inside to become more negative and the outside more positive.

EKG leads are recording electrodes on the skin.
One lead is positive, and the other is negative (reference/ground).
The EKG measures the electrical activity at the positive electrode (b) minus the electrical activity at the negative electrode (a).
- If there's no difference (b - a = 0), the recording is zero, indicating no voltage difference.
During depolarization, the outside becomes more negative.
- The spread of negativity is what's primarily being recorded by the EKG.
- If there are more positive charges on the side of electrode B than A, a positive voltage is recorded.
- The maximum peak activity occurs when half the cells are negative and half are positive, resulting in the largest difference between B and A.
As cells repolarize, the voltage difference decreases, moving back towards zero.
During the calcium plateau, the voltage is zero since the entire outside is negative, resulting in no difference between the electrodes.

Depolarization wave spreading towards the positive electrode results in a positive peak.
A negative wave going back towards the negative electrode will also give a positive peak.
A negative wave of repolarization towards the positive electrode would result in a negative peak.
A positive wave of depolarization towards the negative electrode would cause a negative peak.
In the heart, the last cells to depolarize are the first to repolarize, therefore, the negative repolarization wave goes towards the negative electrode, creating a positive peak.

The depolarization wave is a positive bump, and the calcium plateau is flat (zero).
The repolarization wave is a positive peak. Therefore:
- Before depolarization: zero.
- Depolarization: positive peak.
- Calcium plateau: flat (zero).
- Repolarization: positive peak.
- After repolarization: zero.

Phase 0: Depolarization.
Phase 2: Calcium plateau (flat on EKG because $V = IR$ and there's no current change).
Phase 3: Repolarization, potassium starts to win.
Phase 4: Resting phase (flat on EKG).
Only contractile cells are being measured, so phase 4 is flat until nodal or conductive cells fire.
Potassium causes repolarization starting in the last cell to depolarize, and this creates the positive peak of repolarization.

Multiple leads on arms and legs provide the correct angle relative to the heart.
The heart sits in the chest pointing to the left.
Limb lead two captures much of the electrical activity as it spreads down along the axis.

P wave: Atrial depolarization (atrium contracting after the electrical activity).
QRS complex: Ventricular depolarization (hiding atrial repolarization).
T wave: Ventricular repolarization.
The flat part between QRS and T wave is the ventricular calcium plateau (real contraction).
Some electrical activity goes in the opposite direction (e.g., from apex back up).
Septal depolarization and superior parts of the ventricle contribute to opposite waves.
- Positive electrical activity (depolarization) going towards a negative electrode gives a negative wave.