At this stage things are progressing nicely. In the previous two articles we learned about The Anatomy & Electrical System and then The Electrocardiograph. In the last blog we used the analagy of the mugshot to describe how it is important to view the heart from different perspectives to give us a better understanding as to whats actually happening. We also mentioned that the mugshot pictures were always taken against a height chart. This helps put things in perspective for us.
Imagine we had no perespective or scale, then we might well assume that the picture above was somebody actually grabbing hold of the Eiffel Tower. However, as we are familiar with the size of the Eiffel Tower, we know that this must just be a trick...or somebody with a very large arm! We need to put things in perspective. In photography we do this by having an object of known scale or size in the picture. While doing the ECG it is no different.
The ECG Grid
Let's take a look at how the ECG is given a bit of scale and perspective. Looking at the picture below we notice that the paper is laid out in such as way as to somewhat resemble a maths or handwriting copy. The paper is essentially laid out with large boxes filled with smaller boxes. If you click on the picture below you will get a larger picture which should be easier to see. The small box is 1mm x 1mm. It represents 1mV or 1/1000th of a volt. If we took our 9 volt square smoke alarm battery, we would need 9000 small boxes to show its electrical representation.
In the picture above we can see a small yellow box in the upper left corner of the image. This represents a "small box" on the ECG reading. The small box also represents a given time, usually 0.04 seconds, or 40 ms (milliseconds). The large shaded green box is a grouping of 5 x 5 small boxes. As a result the "large box" represents a time of 0.20 seconds or 200ms. OK taking it one step further, if 1 large box is equal to 0.20 seconds. Then 1.00 second divided by 0.20 is 5, therefore 5 large boxes represent 1 second of electrical activity indicated by the yellow line above. You follow? Well there is one caveat, that is that it only holds true where the paper is printing at 25mm/s. This is usually written somewhere on the ECG strip as seen below.
One other factor that we need to pay attention to is the amplitude or ECG gain. Usually, 10mm represents 1mV with standard calibration. With certain monitors we can increase the gain, or magnification so that if the waves or complexes are very small we can get a closer look at the finer details. You should check the exact specifications of the machine you have available to see what gain settings it has.
While we haven't discussed the formation of the waves or complexes we will do some simple calculations to determine the rate using a couple of different methods. The only thing we need to be able to see in order to determine the rate is large spike, we will cover the proper name for it in the next blog. For the moment we will assume that the monitor doesn't have the capacity to display heart rate, so we must do it manually.
1. The Triplicate Method
This method is quote simple, it involves finding a spike that falls close to the edge of a large box. Count the number of lines until the next spike. A scale can then be overlayed and the rate estimated.
2. Small Box Method
With this method we count the small boxes between the spikes. This method can be quite difficult to do, with some paper it is more difficult to count the small boxes than others. You will also need to hold the strip quite still as you squint to count the small boxes, so probably not ideal in the back of a moving ambulance.
In this example we count 21 small boxes. What we do then is divide 1500 / 21. This gives us an answer of 71.42 or simply a heart rate of 71 beats per minute.
3. Large Box Method
Similar to the small box method, but as the name suggest we count the large boxes instead. There is slightly more than 4 large boxes, 4.2 large boxes to be 100% accurate. To calculate the heart rate we divide 300 / 4.2 to get an answer of 71.42 or simply 71 bpm as above.
4.Six Second Method
This method is slightly different, as it relies on the printer paper that your machine uses. We can see 2 vertical lines towards the top margin of the paper. We have circled those lines in yellow. The space between those lines represents 3 seconds of electrical activity. In order to fully utilise this method we have to print off a strip with at least three of those lines present. We have only included two in our sample. For this example, what we do is count the number of spikes between the two lines, and then multiply by 2 to give us a six second count. We then multiply this by 10 to give us an approximate heart beat value. In this sample we count 3 beats between the two lines, multiple by two to give us a six second count, multiple by 10 that gives us a heart rate of 60. Luckily for us, we know this to be accurate as we can see the calculated rate by the monitor in the upper left corner.
While we have covered some very useful techniques for determining the heart rate from an ECG strip there are some other issues we should consider. Sometimes ,for various reasons, there can be early beats or beats can be dropped completely. It is important that when we are calculating the heart beat that we make sure the rhythm is regular. We don't want to calculate the heart beat on the ECG strip where a beat has been dropped, as this will give us an artificially low result. Similarly if we calculate it between a normal beat and an early beat it will give a false high reading. If the rhythm is not steady, it is important to determine a time-averaged rate over a longer interval.
In the next blog we will start to get into the meat of the subject, the structure of the QRS complex. We will also introduce a simplified approach to interpreting a rhythm strip and how to break it down into it's fundamental parts.