Cardiac Monitoring
3-Lead System
3-Lead ECG Monitoring
This method makes use of 3 leads: RA(Right Arm – White), LA (Left Arm – Black), and LL (Left Limb – Red). Because we are only using 3 leads, these leads are best placed as indicated in the diagram below, where they are equidistant from the heart. RA is placed at 2nd intercostal space, around the right mid-clavicular line. LA is placed at 2nd intercostal space, around the left mid-clavicular line. LL placed on the left side, below pectoral muscles, lower edge of left rib cage
Using these three bipolar leads, you can see the ECG tracing from lead positions I, II, and III. Most 3-lead ECG systems will show the tracing from lead II as a default setting. While a 3-lead system is fine for monitoring changes to rhythm, heart rate, and the presence of lethal arrhythmias, a 12-lead ECG should always be performed to allow for diagnosis.
5-Lead System
This method makes use of 5 leads: RA(Right Arm – White), RL(Right Limb – Green), V(Voltage – Brown), LA (Left Arm – Black), and LL (Left Limb – Red). In this configuration the RA, LA, and LL are placed the same as in 3-Lead, to be equidistant from the heart. The V-lead is placed at the 4th intercostal space, just to the right of the sternal border (in a 12-lead, this is the V1 position). RL is placed on the right side in the mirror position to LL.
Using 5 lead monitoring you will be able to see tracing from lead positions I, II, III, & V1. You can also move the V-lead to any of the V-lead positions (V1-6) and see that lead position (please see 12-lead placement for placement guidelines of V1-6). Like the 3-lead ECG systems, a 5-Lead system will show the tracing from lead II as a default setting. While a 5-lead system is fine for monitoring changes to rhythm, heart rate, and the presence of lethal arrhythmias, a 12-lead ECG should always be performed to allow for diagnosis.
ECG Waveform
The complete cardiac cycle is composed of the following: The P-wave, the P-R segment, the QRS complex, the ST segment, and the T-wave. Occasionally, a U-wave may also be present.
Image courtesy of the McGill University Physiology Virtual Lab.
5 Steps to Rhythm Interpretation
- Heart Rate
We can get our heart rate several ways. If you have a standard 6-second strip, then simply count the number of complexes and multiply by 10. This gives you the beats-per-minute or BPM.
You can also count the number of smallest squares between the R-waves on your rhythm strip. This is the most accurate way to measure heart rate. Take this number and divide 1500 by it.
Example: I have 15 small blocks between each R-wave. 1500 divided by 15 equals 100. My heart rate is 100 bpm.
- Heart Rhythm
Determine if rhythm is regular or irregular. The easiest way to do this is with a piece of paper that has a flat edge. Take the paper and place it along the peaks of the R-waves. Then mark the peak on the paper for at least 3 consecutive R-waves. Move the paper along the strip so that your marks line up with the other R-waves on the strip. If they line up, the rhythm is regular. If they don’t line up, the rhythm is irregular.
- P wave
Are there P-waves present for every QRS complex? Is the shape of the P-waves similar?
- P-R interval
Measure the number of blocks between the start of the P-wave and the start of the QRS complex. Multiply the number of blocks by 0.04 to get the PR interval (each block represents 0.04 seconds)
- QRS complex
Measure the number of blocks between the start of the QRS complex and the end of the QRS complex. Multiply the number of blocks by 0.04 to get the QRS size (each block represents 0.04 seconds)
References
1. 3-lead and 5-lead lead placements courtesy of Lifeinthefastlane. https://litfl.com/ecg-lead-positioning/
2. ECG Waveform image courtesy of McGill University. http://www.medicine.mcgill.ca/physio/vlab/cardio/introECG.htm