Studies have repeatedly shown that senior medical students miss 26% to 62% of acute myocardial infarctions. This inaccurate ECG interpretation can result in severe consequences for patients. This is why medical practitioners need more training in these areas of study.
Learning how to interpret ECG readings can help reduce diagnostic errors and the harm they cause. This skill set requires a lot of training and textbook reading to perfect, but for now, let's start from the basics. With that in mind, keep reading to get an overview of how to comprehend ECG readings.
What is an ECG?
An EKG/ECG records the electrical activity of the heart muscle. This activity varies over time, which is why it's printed on paper to help with interpretation. This shows providers a snapshot of a short period of time.
EKGs help identify heart attacks, electrical dysfunction, and other heart conditions. They are often used in conjunction with an echocardiogram to make the proper diagnosis.
Check the Patient First
Before analyzing an ECG reading, you must first assess the patient. There are several questions you have to consider to get an accurate overview of the situation. Answering these questions can increase the accuracy of your reading:
- Is the patient's skin dry and warm or wet and cold?
- What is their skin color?
- Are they experiencing chest or arm pain?
- What time of day does the discomfort often occur: morning, afternoon, or night?
- Would they characterize it as a mild pressure or a sharp, stabbing pain?
- Are there any tearing feelings?
- Do they smoke or have they ever smoked? If so, how many packs per day?
- Can you palpate their peripheral pulses?
- Is your patient conversing with you, or are they battling for air?
- What is their capillary refill time?
- Are there any existing cardiac conditions?
- What is their physical activity baseline?
- Have they ever received an EKG?
- Were they previously diagnosed with any cardiac diseases?
- Do they feel nauseous, dizzy, lightheaded, or exhausted?
A piece of paper with a trace does not give enough information on its own. For example, a well-trained athlete may have a normal resting heart rate of around 40 beats per minute, well below the usual adult resting range of 60-100 beats per minute.
If you combine symptoms from the patients and the reading, then that's another story. For example, chest discomfort and shortness of breath together with ST-segment elevation on the ECG point toward an acute myocardial infarction that needs urgent treatment. A very slow heart rate with P waves that have no relationship to the QRS complexes (third-degree heart block) in a patient who is dizzy or fainting may need emergency pacing. Our EKG/ECG Interpretation book covers conditions ranging from atrial fibrillation to myocardial infarction.
Interpret ECG Reading by Knowing What's Normal
A typical cardiac rhythm consists of P, QRS, and T waves. You must understand the normal amplitude, deflection, and length of each component.
Amplitude quantifies the voltage of a wave and is shown by its height or depth. Deflection is the direction of a wave: it points upward (positive) when the electrical impulse travels toward a lead's positive electrode and downward (negative) when it travels away from it. Duration is how long a wave or interval lasts; at the standard paper speed of 25 mm/s, each small box is 0.04 seconds and each large box is 0.20 seconds. In adults, a normal PR interval is 0.12-0.20 seconds, a normal QRS complex is narrower than 0.12 seconds, and the corrected QT interval (QTc) is normally no longer than about 450 ms in men and 460 ms in women. Here are other normal values to check out:
What Is a Typical Heart Rate for an Adult?
The normal resting heart rate for adults is between 60-100 bpm. If the patient's heart beats faster than 100 bpm, then this is known as tachycardia.
If their heart beats slower than 60 bpm, then it's bradycardia. Remember that athletes tend to have a lower heart rate, so you must assess for other symptoms.
Direction and Heart View
The twelve leads reveal the heart's electrical activity from various angles. Consider each lead a unique picture of the heart you attempt to decipher.
There are six limb leads (I, II, III, aVR, aVL, and aVF) and six precordial leads (V1 to V6). The limb leads view the heart in the vertical (frontal) plane, while the V leads show the horizontal plane. Here are more insights on the lead placements:
- V1: Fourth intercostal space (ICS), at the right sternal border
- V2: Fourth ICS, at the left sternal border
- V3: Halfway between V2 and V4
- V4: Fifth ICS, midclavicular line
- V5: Left anterior axillary line, at the same horizontal level as V4
- V6: Left midaxillary line, at the same horizontal level as V4 and V5
So why are these leads and directions important? This helps you analyze the images and determine which heart regions have the issue.
Utilize a Systematic Method
Always approach the analysis of a 12-lead EKG/ECG in a step-by-step, systematic manner. Here are the steps and factors to consider:
- Check if the patient is bradycardic, tachycardic, or normal
- Assess if their heart rhythm is regular or irregular
- If the rhythm is regular, count the number of large squares inside a single R-R interval
- Calculate heart rate by dividing 300 by this value (or 1,500 by the number of small squares)
- If the rhythm is irregular, determine the number of complexes on a 10-second rhythm strip
- Multiply the number of complexes by six
- Determine if the rhythm is quick or slow, erratic or regular
- Check that a P wave comes before each QRS complex, and measure the PR interval, QRS duration, and QT interval
- Determine the electrical axis (normally between -30° and +90°)
- Examine the ST segments for any elevations or depressions, and look at the T waves
You can hold the straight edge of a piece of paper along the baseline to judge whether the ST segments sit above or below it. Prepare to notify your team right away if you can see elevation or depression without using a piece of paper.
Understanding the Rhythm of the Heart
Rhythms that start above the ventricles (in the sinus node, the atria, or the AV junction) usually produce narrow QRS complexes. When such a rhythm is fast, consider atrial flutter, atrial fibrillation, or supraventricular tachycardia. Rhythms that start in the ventricles produce wide QRS complexes (0.12 seconds or more). Ventricular escape rhythms are slow (about 20-40 bpm) and more likely to be unstable, as can happen in complete (third-degree) heart block, while a fast wide-complex rhythm should be treated as ventricular tachycardia until proven otherwise.
Be aware of any medication your patient is taking. Many cardiac drugs, like beta-blockers, can slow down the heart rate. It's vital to assess whether a person's heart rhythm is:
- Regularly irregular (i.e. a recurrent pattern of irregularity)
- Irregularly irregular (i.e. no detectable pattern, as in atrial fibrillation)
Mark out multiple consecutive R-R intervals on paper, then move them down the rhythm strip. This is to test whether succeeding intervals are identical.
Are You Ready to Interpret Some ECG Readings?
An accurate ECG reading can save lives, but it requires some attention to detail. By understanding how to read them, we can catch potential heart problems before they become deadly. While this may seem daunting at first, with practice and some help, you can become confident in reading ECGs. Check out our EKG/ECG Interpretation book and our best-selling EKG/ECG Interpretation Workbook on Amazon.