An electrocardiogram - an ECG - or the dutch and german version of the word - elektrokardiogram or EKG, is a tool used to visualize “gram” the electricity “electro” that flows through the heart “cardio”.
An ECG tracing specifically shows how the depolarization wave moves during each heartbeat - which is a wave of positive charge - looks from the perspective of different sets of electrodes.
This particular set of electrodes is called lead II, with one electrode on the right arm and the other on the left leg, so essentially when the wave’s moving toward the left leg electrode, you get a positive deflection, like this big positive deflection correspond to the wave moving down the septum.
To read an ECG there are a few key elements to keep in mind, one of them includes figuring out the rate and rhythm. There are a couple ways of figuring out the heart rate on an ECG.
The first one is called the “box method” because you count the number of boxes between heartbeats. Each small box represents 0.04 seconds, and each big box is five small boxes, so each big box is 0.2 seconds.
To do that, you can count the number of small boxes between R waves since R waves are tall and pointy and easy to see in lead 2 of an ECG strip.
You can find an R wave that has a peak that falls at the beginning of a box, and then count up how many boxes until the same point on the next R wave.
Let’s say that there are 4 big boxes and 1.5 small boxes between two R waves, meaning there are 4 x 5 + 1.5 = 21.5 small boxes, and that means there’s 0.04 seconds x 21.5 or .86 seconds between heartbeats.
Now, to get something a little more meaningful we can take the inverse which is 1 over 0.86 beats per second, or 1.16 beats per second.
Now there’re 60 seconds in a minute, so multiplying that by 60 we end up with 70 beats per minute—the heart rate!
Now, if the distance between two R waves is exactly 1 big box, then the heart rate would be 300 beats per minute—really fast. If R waves are two big boxes apart or 0.4 seconds apart, then the heart rate is 150 beats per minute.
Three big boxes apart is 100 beats per minute, four, five, and six big boxes apart is 75, 60, and 50 beats per minute respectively.
Remembering these numbers makes it easier to make a rough estimate. For example, if there are three to four large boxes between R waves, then the heart rate must be between 75 and 100 beats per minute.
Another method to determine the heart rate is to count the number of beats in ten seconds, which is the standard length of time on the rhythm strip portion of a 12-lead ECG.
So in this case we’ve got 15. All you’ve gotta do is multiply this by 6 to get the heart rate, which would be 90 beats per minute.
The reason this little trick works is that you’ve got 15 beats per 10 seconds, and again to convert to per minute you multiply by 60 seconds in a minute. Looking at this we see that 60 / 10 equals 6 per minute. So 6 times 15 is 90 beats per minute.
If the heart rate is too slow or too fast, it could be because something other than the SA node is pacing the heart rate.
For example, there could be atrial flutter which is when an ectopic focus in the atria - like an irritated atrial cell - starts to spontaneously fire between 250-350 depolarizations per minute, with only one out of every few atrial depolarizations passes through to the ventricles *B*.
To calculate the atrial rate, you can use the same methods as before, except you look at P waves instead of R waves.