Anatomy of a Heartbeat

When you experience an irregular heartbeat for the very first time, it’d be hard to think of it as anything but terrifying. Thankfully, for most of us the arrhythmias we suffer from are not harmful or life threatening.

Even so, for many of us, getting the pat on the head from our family doctor and told “You’re fine, run along and play” simply isn’t enough reassurance that we’re not in mortal danger. So we go to the internet to learn as much as we can about heart rhythms, which arrhythmias are dangerous and which aren’t, and what tests we should ask for.

So it would make sense that one of your first stops in your cardiological education would be to learn the lingo of an EKG (or ECG)/rhythm strip.

But First: How Does an EKG/ECG Work?

It’s actually not all that complicated. The electrical impulses generated inside your heart cells are called “action potential.” You don’t really need to know much about the term itself, other than to know that it refers to the process of your body’s electrolytes (such as sodium or potassium) sending electrical signals over and through cells. ^[1][2] And to make this easier to understand, let’s just call it an AP.

When electrical leads are placed on your body for an EKG, they are positively and negatively charged and are sending electrical current from one lead to the other. The voltage being sent between the leads is uniform. However, when that voltage from the EKG leads travels through your body, it’s altered by the APs that your heart is firing when it’s pumping. It’s the difference in expected voltage that the EKG machine is reading, and each of those differences shows up in real-time as a spike or electrical deflection away from the baseline voltage (also called a ‘wave’). The change in value of that electrical voltage (either positive or negative from the “baseline”) shows up on the EKG machines as either a spike upward or downward. Because EKGs often use multiple leads with different placements on your body, the conduction and deflections of voltage between the different leads varies, resulting in EKG readings that look very different from each other (see image above, left).

Now that we know how the EKG works, let’s take a look at the spikes on what’s probably the most familiar looking EKG pattern and define each of the deflections as well as what’s happening in the heart during each deflection.

The graphic to the right shows the different morphologies (shapes) of a normal EKG, depending on which set of leads are being read. (click to enlarge)

The inventor of the EKG machine, Willem Einthoven, labeled each of the deflections or waves seen on an EKG strip back in 1901 (P,Q,R,S and T), and those labels continue to be used today:^[4]

P wave - the very beginning of a heartbeat. A small, low-voltage electrochemical signal is directed from the SA (sinoatrial) node to the AV (atrioventricular) node, and spreads from the right atrium to the left atrium. The upward deflection (wave) away from the EKG baseline shows the depolarization* of the atria immediately prior to the actual physical contraction of atria muscle.

Of note, the length and shape of the P wave can indicate atrial enlargement. While the absence of a P wave on an EKG may indicate atrial fibrillation, sinus arrest or sinoatrial exit block, a P wave with saw tooth morphology is indicative of atrial flutter.^[6][7][8][9]

The length of the PR segment (the distance (or time) between the end of the P wave and the start of the QRS complex) and the PR interval (the distance between the start of the P wave and the start of the QRS complex) often give clues to the identity of some heart problems like AV (atrioventricular) block, pericarditis and atrial infarction^[5].

QRS Complex – the greatest deflection seen on the EKG, the QRS complex together is the electrical depolarization of the ventricles just prior to their contraction. The repolarization of the atria occur during QRS, but because the amplitude of QRS is so large (meaning that the voltage change measured through the EKG leads is so great) that the relatively small electrical change resulting from atria repolarization can’t be seen via EKG.

Individually, the Q wave, the initial downward deflection from the EKG baseline, represents the depolarization of the interventricular septum. A significant Q wave amplitude can be indicative of a myocardial infarction.^[10]

The R wave , the sharp upward deflection from baseline on the EKG represents early depolarization of the ventricles. The morphology of this wave is more ‘spiked’ because conduction of this wave of depolarization occurs much more quickly than the others (thanks to the His/Purkinje system). The spike is of greater amplitude than the other waves because the change in voltage in this electrochemical wave is greater.

The final depolarization of the ventricles happens with the S wave. Changes in the amplitude of the S wave relative to the R wave can be a sign of heart injury.

T wave – the repolarization of the ventricles. The morphology of the T wave, whether it is symmetric on either side of it’s peak, whether the peak is sharp (“tented”) or rounded, or is inverted are all important clues in diagnosing heart problems.^[11]

If you look at the pressure curves for the cardiac cycle (click to enlarge), it’s interesting to note that the P, QRS and T waves are not indicative of simultaneous heart muscle contraction. In fact, you see that the greatest pressure in the ventricles occurs long after their complete depolarization. This means the muscular walls of the ventricles don’t begin to squeeze the volume of blood in them until depolarization has occurred. The ventricles have already repolarized before they’ve even finished contracting! Once the aortic and pulmonary valves have opened to release their blood payload, pressure in the ventricles begins to drop.

The correlation of the EKG to the actual electrical activity in the heart can be seen in the animated graphic to the left.

There are several other diagnostically significant measures on an EKG, including the PR/PQ interval, ST segment, QT interval and RR interval, but they’re beyond the scope of this article and will be covered in future articles.

* What is depolarization?

Every muscle cell in your body has what’s called a resting potential. That means that inside that cell there is an electrical charge just waiting to be used (like a battery). Proteins that are sensitive to slight variances in voltages undergo a change when the variance in voltage reaches a certain threshold. That change makes the protein permeable by sodium (Na+), one of your body’s electrolytes. The sodium immediately rushes into the cell, drawn in part by the difference in charge (positive attracts to negative, just like magnets), and depolarizes the cell (makes the cell less negatively charged). This depolarization causes the release of calcium ions, causing another chemical reaction that results in muscle contraction. All of this happens in a few milliseconds.^[3]

As a side note, the function of calcium channel blockers in the treatment of certain arrhythmias is now made more clear: by slowing or partially blocking the release of calcium ions, the heart muscle is less able to generate spasmodic contractions or fibrillation.

References