What do we do when we face an 

ECG (electrocardiogram)?

This is a common situation that can be a little disorienting and disorganized at first.

But throughout our dynamic and direct course, we'll emphasize the importance of systematization and you'll learn how to evaluate and understand the pathophysiology behind electrocardiographic tracings.

To do this, we'll use the

7 steps

RRAHABI method

What is  RRAHABI method?

It's a mnemonic method covering the

7 steps of interpreting an ECG

Don't worry about memorizing it now; by the end of the course, you won't be able to forget it.

What is the ECG device?

It's a voltmeter that records the heart's electrical activity using 12 electrodes on the surface of the body.

. 

How is this possible?

Being composed of water and electrolytes, our body is able to conduct the electrical stimuli generated in the heart to the skin, where they are captured by the ECG device's electrodes!

How amazing is that?

Potential difference

What are the main intracellular and extracellular ions?

The extracellular ion is Sodium (Na) + 142 mEq/L

The intracellular ion is Potassium (K) + 140 mEq/L

Cellular Depolarization:

In the "resting" cell (before cell depolarization) there are more positive charges outside the cell, so there is an internal relative negativity.

Remember that there are many other cations and anions apart from sodium and potassium.

Cellular Depolarization: resting potential

The potential difference between the intra- and extracellular media is known as the resting potential.

For ventricular muscle cells, the resting potential is approximately - 90mV

What makes depolarization important?

1.     Cellular depolarization is the phenomenon that generates cardiac contraction to let calcium enter myocardial cells.

2.     It is also responsible for forming the electric current observed on the ECG.

The cell depolarization cycle consists of 5 phases.

Phase 0 to phase 4

Phase 0: depolarization

 The sodium (Na+) channels open.

Sodium is more prevalent outside the cell (extracellular)

This different concentration causes Na+ cations to passively (and quickly) enter the cell.

Gaining a positive charge, the cell becomes less negative.

Observe:

Phase 0

Also in phase 0, note that as positive charges (sodium) enter, the cell becomes more positive. (This animation makes it easy to understand! Right?)

Phase 1

In phase 1:

The sodium channels close.

The potassium (K+) channels open.

Chlorine (Cl-) enters.

With the entry of negative charges (Cl-) and the exit of positive charges (K+), there's a small change in electronegativity.

Phase 2: contraction

The entry of Ca++ leads to cardiac contraction!

When positive charges enter, why doesn't the cell become more positive?

It's due to the outflow of K+ (positive).

Observe:

Phase 2:

It's easy to understand why cardiac contraction occurs during this phase and why the membrane potential is stable!

Note the cardiac contraction with calcium inflow and potassium outflow:

Phase 3: repolarization

New potassium (K+) channels open.

Potassium leaves the cell, making it more negative (repolarization).

The cardiac muscle relaxes (this is the beginning of ventricular diastole, which will only end when another contraction begins).

Losing its positive charge, the cell becomes more negative.

Observe:

Phase 3

With the outflow of potassium, the cell becomes negative again.

Phase 4

During the previous phases, we noticed a K+ outflow and an Na+ inflow. How is this exchange reversed?

Through the sodium-potassium ATPase pump which, with energy expenditure, takes Na+ out of the cell and brings K+ in

Note that in the previous phases there is no energy expenditure when the channels open

Observe:

Cardiac depolarization

After depolarization is initiated in a given group of cells, a depolarization stimulus is created which causes the neighboring cells to depolarize.

Myocardial depolarization

Below we can see the ventricular depolarization through the bundles of the His-Purkinje system

Now imagine what would happen if we could take a close look at the conduction system.

The cardiac conduction system

And this next animation will help us understand what we'd see if we had a close enough view of the myocardial conduction system.

Take a deep breath and watch (this animation is really cool):

Why is the resting cell negative?

There are two answers to this question.

First:

Due to the smaller size of the hydrated K+ compared with the hydrated Na+, making it easier to cross the plasma membrane (which is 50 times more permeable to K+)

The K+ (positive) will exit the cell leaving it more negative, i.e. polarized.

Second:

To understand the second answer, let's review how the sodium-potassium ATPase pump works.

3 sodium ions are linked to the sodium-potassium pump:

How the sodium-potassium pump works

ATP is linked to the sodium-potassium pump:

How the sodium-potassium pump works

The ATP is broken down into adenine diphosphate (ADP) and phosphorus (which remains linked to the sodium-potassium pump)

How the sodium-potassium pump works

The 3 sodium ions are taken out of the cell:

Observe the complete cycle:

there's an outflow of 3 positive charges and an inflow of 2 positive charges, so there's a greater outflow of positive charges... This efflux of positive charges contributes to making the cell negative at rest.