INTRODUCTION TO ECGs

Electrocardiographs are typically interpreted by two methods. Most students (myself included) often "read" an ECG by comparing distinct characteristics. This is like reading handwriting and is dependent on familiarity. For example, the word "DOG" isn't read from left to right as much as it is simply recognised. Another example is reading an unfamiliar language - it takes longer and frequently words are misread or mispronounced due to their unrecognizability. This method of ECG interpretation is the most fallible and should be discouraged.

The preferred method of ECG interpretation is the usage and understanding of rules. For this reason, I won't use any actual strips to identify rhythms. Strips will be presented in the testing section.

NODES OF CONDUCTION

The heart is a muscle that contracts because of an electric current that flows through all of the specialized cells starting from the top and moving downwards. There are specialized nodes that initiate the conductive "spark". The first and foremost of the nodes is the Sinoatrial Node (SA Node). SA Node has an inherent firing rate of 60-100 bpm. It is the dominant pacemaker node and is represented by a P-wave in an ECG. The Atrioventricular (AV) Node is inferior to the SA Node and will take up the duty of primary pacemaker in the event of a failure of the SA Node. It has an inherent rate of 40-60 bpm. The Ventricular contraction begins at the apex of the heart (bottom, oddly enough) and moves upward along the Purkinje Fibers. The inherent rate of ventricular contraction is 20-40 bpm. Pacemaker nodes fire at different inherent rates to avoid problems. See, each node is like a timer. When the timer goes off it sets off the current and then resets itself. So the timers for each node is different. For example, if the SA Node fails then the AV Node will timeout and depolarize, setting off the conduction wave. It has to be slower so that the SA has time to reset but faster than the ventricular rate so that the Ventricular pacemaker cells don't timeout and depolarize. To be a little more clear, if the SA fires at 100bpm then they all fire at 100 bpm, but if left on their own they will fire at their inherent rate.

All cells of the heart have the unique ability to act as pacemakers. Their inherent rates depend on their location. When a cell depolarizes, it causes the neighboring cells to depolarize too (and so on and so on). When a cell depolarizes, it can not fire again until it "recharges" (repolarizes). This prevents the heart from just going insane in the membrane. Think of it like a domino effect. Start at the beginning and they all fall as they go. Start in the middle and the dominoes fall in 2 directions. The SA node is the starting point and when it depolarizes, all the cells follow from top to bottom. Its inherent rate is based on the fact that it can repolarize faster.

RULES OF INTERPRETATION

To begin, the structures of an ECG strip must be identified for the student to comprehend the 5 rules:

small squares = a time interval of 0.04 seconds for each individual 1mm by 1mm square, read horizontally.
large squares = five small squares that equal 0.20 seconds. There are five large squares in 1 second.
isoelectric line = the average horizontal line that is drawn from left to right across the paper (disregarding the deflections) that is neutral (no energy).

There are positive and negative deflections that deviate the line to indicate conduction through the heart. Positive, or up, indicates current moving from the base of the heart (top) to the apex (bottom), while a negative deflection indicates the current is moving back up (except when returning back to the isoelectric line). The easiest way to think of it is current moving down the centre of the heart and then dividing at the bottom and returning back up the sides...if that helps you.

1. RHYTHM

Determined by Atrial and Ventricular regularity. Atrial contractions may be measured as regular while ventricular contractions are all out of whack. Or the opposite can be true. Typically speaking, ventricular depolarization is the determinant factor of a Rhythm's regularity.

Regular = a rhythm that is almost or mostly in a consistent pattern with very little difference in time between depolarizations. The interpretor should be able to fold a strip in half and see the beats overlap each other when held up to a source of light, or at least be very close. If a beat occasionally falls out of rhythm but the next beat is in place, the rhythm can be considered regular.

Irregular = rhythm that cannot be counted with the popular methods (below). The time varies between beats and has no pattern.

2. RATE

The normal rate of a resting heartbeat is between 60-100 beats per minute (bpm).
However, athletes may have lower resting heart rates. For children 2-6y/o the normal rate may be 80-120. Toddlers 1-2y/o should be 80-130. Infants (7wks-1yr) are between 80-140. Newborn (0-6wks) are 120-160.
Counting rate is done by a few different methods. The two most popular are:

6-Second Count Method = is a popular starter's method. This is done by counting the number of QRS peaks within 6 seconds of strip and multiplying by 10 to get the number of beats per minute (60 seconds). Typically, ECGs are marked with elongated lines every 5 large squares at the bottom that indicate 1 second. There may even be an elongated line at the top of a strip every 3 seconds.
Example below shows 4 peaks in three seconds. This would suggest 8 beats in 6 seconds x 10, equal to 80 bpm.

Triplicate Method = quickest method. By memorizing 2 sets of numbers (300-150-100 and 75-60-50) it is possible to easily determine the heart rate per minute. To do this, mark a QRS peak on a regular rhythm that is close to a large square line. Then mark off the next QRS peak. Starting at the next square after the first peak, write 300. The next square is 150. The next is 100. If you have not reached the second peak, the next square is 75, the next is 60, and the next is 50. If the 2nd peak falls between, you can estimate a rate between the two numbers.
Example below indicates a rate of approximately 80 bpm..



If a rhythm is tachycardic and a more accurate count is required, there is also the R-R method. The R-R is done by counting the number of 1mm squares between 2 peaks and dividing that number into 1500. Example above would be 1500 / 18 small squares = 83 bpm.

3. P-WAVES

P-waves are small deflections that indicate that the SA node has fired (atrial depolarization), initiating the current that will cause the surrounding muscle to contract in response.
- Duration of the normal P-wave is 0.10 secconds, with an amplitude of 0.5-2.5 mm.
* Presence of a P-wave is important, but may not be visible (buried within or behind the QRS) or be inverted (meaning that the origin of conduction is at a point below the the SA Node).

4. PR-INTERVAL (PRI)

A measurement of time between the depolarization of the SA node (P-wave) to the instant of ventricular depolarization.
- The PRI is measured from the beginning off the P-wave to the beginning of the QRS complex.
- Absence of a visible P-wave makes PRI asssessment impossible.
* Duration of the normal PRI is 0.12-0.20 seconds.

5. QRS Complexes

Up-&-down deflections that represent ventricular conduction beginning at the Atrioventricular Node (AV Node).
- The first negative deflection (downward) after the P-wave is called the Q-wave (not always present).
- The second deflection (usually positive, but not necessarily) is the R-wave.
- The following wave returning to AND termiinates where beginning to flatten at, above, or below the baseline is the S-wave.
* Duration of QRS Complex is normally between 0.08-0.12 seconds or less.



P, PRI, QRS, T and J segments of a sinus rhythm

To distinguish a rhythm, determine the 5 rules: Rhythm? Rate? P-waves? PRI? QRS?

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