When you’re first being introduced to EKGs it can be confusing. Learning the electrical activity of the heart and how it’s translated into a rhythm can seem overwhelming at first! However, understanding EKGs does not have to be as bad as you think. Reading and understanding EKGs is completely doable and I’ll show you how:
First, you must understand the conduction system of the heart in order to recognize the process of a normal heartbeat. There are specific cardiac cells that are all connected and are responsible for conducting signals to cause the heart to beat. The first signal is the SA node which is known as the natural pacemaker of the heart. It is located in the right atrium and has an inherent rate of 60-100 beats per minute (bpm) which is considered the normal rate of the heart. The SA node then travels to the AV node which is found at the border between the right atrium and the right ventricle and has an inherent rate of 40-60 bpm. AV node rate will kick in only if the SA node rate fails. From the AV node, the conduction goes through a single structure known as the bundle of his where the inherent rate is also 40-60 bpm. After the bundle of his, the conduction divides between the right and left bundle branches. If the bundle of his fails, the rate in the right and left bundle branches will kick in at 20-40 bpm. Lastly, the conduction travels to the Purkinje fibers located on the apex of the heart. The inherent rate of the Purkinje fibers is 20-40 bpm.
Now it's time to learn about the EKG waves and what each represents. One full set of EKG waves represents a complete heart contraction cycle. Reading the cycle from left to right, the first wave is the P wave. The P wave represents atrial depolarization meaning it's the period in which both of the atria are contracting. The QRS complex is the spike that takes place in the middle. This complex represents the period of the ventricular depolarization. The last wave in the cycle of the T-wave which signals the ventricular repolarization or relaxing. Also, note that the atria repolarization does occur however it is so small that it isn't individually visible and is typically buried in the QRS complex during the ventricular depolarization. The main intervals that are used when reading and EKG are the PR Interval and the QRS complex.
ECG paper is used to measure heart activity. It consists of a grid of boxes with specific measurements using the small square and large square.
Small square Length: 1 mm
Small square time: 0.04 sec
Large Square length: 5 mm
Large Square time: 0.2 sec
Voltage is measured along the vertical axis. The large box consists of 0.5mV. Note that 10 mm is equal to 1mV.
A. Identify and examine the P waves. The normal p wave is present and upright. If the p wave is absent or inverted it could mean that a dysrhythmia is taking place such as a junctional rhythm.
B. Measure the PR Interval. This is the distance between the beginning of the P wave and the beginning of the QRS complex. In order to get this measurement, you must count the number of small boxes in between then multiply it by 0.04 sec. The goal number for the PR interval is 0.12-0.20 seconds. Anything that exceeds 0.20 seconds is considered a heart block.
C. Measure the QRS Complex. In order to do this, you must count the small boxes in between the QRS complex. Multiply this by 0.04 seconds. The goal number for the QRS complex is 0.06-0.12 seconds. Anything greater than 0.12 could be a Preventricular Contraction (PVC)
D. Identify the Rhythm. With this, you are trying to identify if the rhythm is regular or irregular. You do this by measuring the distance between R and R. If they are the same distance the rhythm is regular but if not it is irregular.
E. Determine the Heart Rate. You can do this one of two ways. The first can be done only when using an EKG that is on a 6-second strip. You can count the Rs and multiply by 10. The second method is dividing 300 by the number of big boxes between two Rs. So first counting the number of big boxes between the two consecutive Rs. Take 300 and divide it by the number you got which should give you the heart rate.
F. Finally, Interpret the Strip.