Basic ECG Interpretation Made Easy

“I stare at these strips filled with connecting lines of ups and downs. I look back to that time when I thought that one of the coolest things a nurse could do is interpret these complicated drawings and stuff. Well, that and of course the emergency and adrenaline rush-inducing scenarios I see on TV.

“Once, I thought it would just be easy. Like, by just taking a swift look at these records of heart activity and voila! I know exactly what’s wrong with my patient. But some things aren’t just what they seem to be. And reading ECG tracings are nowhere near simple,” Martha says to herself as she stares at the ECG tracings mounted and clipped inside the patient’s chart. “What should I do?,” she thinks for herself. “I don’t even understand any of this. Well, maybe I’ll just leave it to the doctors. After all, trying to interpret these simple lines doesn’t at all matter.”

ECGInterpreting basic ECG

Yup, we are all familiar with ECG also known as the representation of the electrical activity of the heart muscle as it changes with time. While, we may derive some advantages from it especially in taking a peek at a cardiac patient’s current status, ECG interpretation could be one of the most annoying and difficult things nurses face in the clinical setting. Some would think it requires real hard work to understand those tracings and they need not add it to the long list of to do things they already have, while there are others who want to learn how to interpret stuff like those yet find it difficult to do so. Here are some pointers to learn how to interpret ECG tracings like a boss.

Usually, ECGs are ordered for cases of Chest pain, Myocardial infarction; Shock; Heart failure; Palpitations; History of syncope and during cardiopulmonary resuscitation (CPR).

So, you placed all the electrodes and leads in place, careful of the correct placements and colors. Those leads you placed at the limbs are leads I, II, III, AVR, AVL and AVF and are used to determine the electrical axis, which is usually measured only in the coronal plane, while those leads you place at the best are V1, V2, V3, V4, V5 and V6 which are used to record the ECG in the transverse or horizontal plane. Then after ensuring that they are all placed accurately, you press start and print the recorded electrical activity of your patient’s heart muscle. What now?

ECG machines all run at a standard rate (25 mm per second) and use paper with standard-sized squares.  Each of these small squares (1 mm) represents 40 ms (0.04 seconds), while each large square (5 mm) represents 200 ms (0.2 seconds). On the y axis, each small square represents 0.1 mV.

You need to identify the conduction pathways.

> P wave signifies atrial depolarization, its presence indicate a normal heart rhythm or what you call sinus rhythm;

> PR Interval = impulse from atria to ventricles. This should be between 120–200 ms, or less than 5 mm on the ECG paper. A prolonged or shortened PR interval can indicate certain disease states. If the PR interval is > 200 ms, first degree heart block is said to be present, while if it is < 120 ms, it may suggest pre-excitation or the presence of an accessory pathway between the atria and ventricles, or AV nodal (junctional) rhythm.

> QRS complex = ventricular depolarization. A Q wave is any negative deflection at the beginning of a QRS complex. Small Q waves in some leads may be normal. Large Q waves (> 2 mm) may be abnormal. An R wave is the first positive deflection, and an S wave is the negative deflection which immediately follows an R wave. The QRS complex should be less than 120 ms (3 mm) as its width is useful in determining the origin of each QRS complex (e.g. sinus, atrial, junctional or ventricular).

For example, narrow complexes (QRS < 100 ms) are supraventricular in origin, while broad complexes (QRS > 100 ms) may be either ventricular in origin, or may be due to aberrant conduction of supraventricular complexes (e.g. due to bundle branch block, hyperkalaemia or sodium-channel blockade).

> ST segment = isoelectric – part of re-polarization; Isoelectric means being in the same level as the part between the T wave and the next P wave. The ST segment should not be elevated or depressed. Any change from baseline may indicated cardiac disease.

> T wave = usually same direction as QRS – ventricular re-polarization;

> QT Interval = spans the onset of depolarization to the completion of re-polarization of the ventricles. Normal range up to 440 ms though it may vary with heart rate and slightly longer in females. QT interval prolongation can be very serious.


Estimating Heart Rate

You can estimate the patient’s heart rate with a standard ECG paper filled with ECG recording. One second equals 5 large squares or 250 mm along the horizontal axis. Thus, if you have five large squares between the QRS complex, then you have 60 beats per minute HR. If 3, the HR is 100 per minute and if 2, the HR is 150 per minute.

Not always will your tracings have sinus rhythm, at times you also get to encounter patients with abnormal ECG tracings.

> Atrial fibrillation Rhythm:

Irregular Rate A: 350 – 650; V: varies

 P: poorly defined

P-R: N/A

 QRS: narrow complex

 S-T: normal

 T: normal

Q-T: normal

> Atrial flutter Rhythm:

Regular / Irregular Rate: A: 220 – 430; V: <300 (2:1, 3:1 or sometimes 4:1)

P: Saw toothed appearance

 P-R: N/A

 QRS: narrow complex

S-T: normal

T: normal

Q-T: normal

> Supraventricular tachycardia (SVT) Rhythm:

Regular Rate: >100

P: not visible

P-R: not defined

QRS: narrow complex

S-T: depression (sometimes)

 T: normal

 Q-T: prolonged (sometimes)

Like other things, nursing isn’t always what it seems. A noble profession, it is jam-packed with numbers of things that are hard to comprehend. Things that take perseverance, a critical mind and a willing heart to understand. While some try their best to dip their head into that sea of complications, there are also those who choose to trudge the path of ignorance and leave the work to the doctors. However, what others do not know is that those little complicated stuff can give us so much insight on the real status of our patients.


Liane Clores, RN MAN

Currently an Intensive Care Unit nurse, pursuing a degree in Master of Arts in Nursing Major in Nursing Service Administration. Has been a contributor of Student Nurses Quarterly, Vox Populi, The Hillside Echo and the Voice of Nightingale publications. Other experience include: Medical-Surgical, Pediatric, Obstetric, Emergency and Recovery Room Nursing.

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