Reading an EKG starts with understanding a simple principle: each wave, spike, and dip on the tracing represents a specific electrical event in the heart. Once you know what each part means and what “normal” looks like, you can spot the patterns that signal trouble. The skill is surprisingly learnable, even without a medical background.
How the Grid Works
EKG paper is a grid of small and large squares. The small squares are 1 mm across, and five of them make up one large square (5 mm). Time runs left to right, and voltage (which reflects the strength of the heart’s electrical signal) runs bottom to top.
At standard paper speed, each small square represents 0.04 seconds (40 milliseconds). Each large square represents 0.2 seconds. So five large squares equal exactly one second. On the vertical axis, each small square equals 0.1 millivolts of electrical activity, and each large square equals 0.5 millivolts. These measurements are the foundation for every calculation you’ll make on an EKG, from heart rate to the duration of specific waves.
The Waves and What They Mean
A single heartbeat produces a characteristic pattern of waves. Reading an EKG means recognizing these waves, measuring them, and checking whether they fall within normal ranges.
P Wave
The P wave is the first small, rounded bump in each cycle. It represents the electrical activation of both atria (the heart’s upper chambers), which causes them to contract and push blood into the ventricles below. A normal P wave is upright in most leads and relatively short. When P waves are absent, abnormally shaped, or replaced by chaotic activity, it points to an atrial rhythm problem like atrial fibrillation.
QRS Complex
The QRS complex is the tall, sharp spike that dominates each heartbeat on the tracing. It represents the simultaneous electrical activation of both ventricles (the heart’s lower chambers), the powerful contraction that pumps blood to the lungs and body. A normal QRS is narrow, typically lasting less than 120 milliseconds (three small squares). A wide QRS suggests the electrical signal is taking an abnormal path through the ventricles, which can indicate a bundle branch block or a ventricular rhythm.
T Wave
After the QRS, you’ll see a broader, gentler wave called the T wave. This represents the ventricles resetting their electrical charge in preparation for the next beat. The T wave is normally upright in most leads and roughly proportional to the QRS. Flattened, inverted, or unusually tall T waves can signal problems ranging from low potassium to reduced blood flow to the heart muscle.
The Segments Between
The PR interval runs from the start of the P wave to the start of the QRS. It reflects how long it takes the electrical signal to travel from the atria through the conduction system to the ventricles. Normal is 120 to 200 milliseconds (three to five small squares). A PR interval longer than 200 ms means the signal is being delayed, the hallmark of a heart block.
The ST segment sits between the end of the QRS and the beginning of the T wave. In a healthy heart, this segment is flat, sitting right at the baseline. Elevation or depression of the ST segment is one of the most clinically important findings on any EKG. ST elevation of more than 1 mm (one small square) in two or more related leads is the classic sign of an acute heart attack. ST depression of more than 1 mm, particularly when it’s flat or downsloping, suggests the heart muscle isn’t getting enough blood.
The QT interval stretches from the start of the QRS to the end of the T wave and represents the full cycle of ventricular contraction and recovery. Because it changes with heart rate, it’s usually corrected mathematically (called QTc). A prolonged QTc, defined as longer than 450 ms in men or 460 ms in women, increases the risk of dangerous heart rhythms. A QTc of 390 ms or shorter is considered abnormally short.
How to Calculate Heart Rate
There are two quick methods, and which one you use depends on whether the rhythm is regular.
The sequence method works best for regular rhythms. Find an R wave (the tallest peak in the QRS) that lands on or near a dark line. Then count the dark lines after it using this sequence: 300, 150, 100, 75, 60, 50. Where the next R wave falls tells you the heart rate. If the next R wave lands on the second dark line, the rate is 150. If it lands on the fourth, it’s 75. This method gives you a fast, reasonably precise answer.
The six-second method is better for irregular rhythms. Count out 30 large squares, which equals six seconds. Count the number of QRS complexes within that span, then multiply by 10. If you count seven QRS complexes in six seconds, the heart rate is about 70 beats per minute. It’s less precise than the sequence method but works when the spacing between beats varies.
Determining the Heart’s Electrical Axis
The axis tells you the general direction of the heart’s electrical activity. Shifts in axis can point to conditions like an enlarged heart chamber or a conduction problem. The fastest way to estimate it is to look at just two leads: Lead I and Lead aVF.
Check whether the QRS complex in each of these leads is mostly positive (pointing upward) or mostly negative (pointing downward):
- Both positive: Normal axis, between 0° and +90°.
- Lead I positive, aVF negative: Left axis deviation (less than -30°). This can indicate a block in part of the left ventricle’s conduction system or left ventricular enlargement.
- Lead I negative, aVF positive: Right axis deviation (greater than +90°). This may suggest right ventricular strain, a lung condition, or a conduction abnormality.
- Both negative: Extreme axis deviation, sometimes called “northwest axis.” This is uncommon and usually significant.
Recognizing Common Abnormal Rhythms
Atrial Fibrillation
Atrial fibrillation is one of the most common abnormal rhythms you’ll encounter. The key features: no identifiable P waves (the baseline looks chaotic and wavy instead), and the R-R intervals are completely irregular with no discernible pattern. The heart rate can be fast, slow, or normal, but the spacing between beats will be unpredictable.
Atrial Flutter
Atrial flutter looks different from fibrillation. Instead of chaotic atrial activity, you’ll see a very regular “sawtooth” pattern, especially in leads II, III, and aVF. The atria fire at roughly 300 beats per minute, but the ventricles don’t respond to every signal. The most common pattern is 2:1 conduction, meaning the ventricles beat at about 150 per minute. A key distinguishing feature from fibrillation: even when conduction varies, the R-R intervals in flutter tend to be multiples of each other (for example, some intervals are exactly twice as long as others) rather than completely random.
Ventricular Tachycardia
Ventricular tachycardia (VT) shows a fast rhythm of three or more consecutive beats originating from the ventricles at over 100 beats per minute. The QRS complexes are wide and abnormal-looking, and they come in a coordinated, regular pattern. VT can be sustained (lasting longer than 30 seconds) or nonsustained (resolving on its own within 30 seconds). Both forms are taken seriously because VT can deteriorate into the next rhythm on this list.
Ventricular Fibrillation
Ventricular fibrillation is the most immediately life-threatening rhythm. The tracing looks like a chaotic, disorganized squiggle with no identifiable waves, no QRS complexes, and no pattern at all. Unlike ventricular tachycardia, where the ventricles contract in a coordinated way (just too fast), in fibrillation the muscle fibers twitch randomly and no blood is pumped. This is a cardiac arrest rhythm.
A Practical Reading Sequence
When you sit down with an EKG, working through a consistent checklist prevents you from missing findings. A reliable approach covers these steps in order:
- Rate: Use the sequence method or six-second method. Normal resting rate is 60 to 100 beats per minute.
- Rhythm: Are the R-R intervals regular? Are P waves present before every QRS? Is every P wave followed by a QRS?
- Axis: Check Lead I and aVF to place the axis in the correct quadrant.
- Intervals: Measure the PR interval (normal: 120 to 200 ms), QRS duration (normal: less than 120 ms), and QTc (normal: under 450 ms for men, under 460 ms for women).
- ST segments and T waves: Look for elevation, depression, or T wave inversions across all leads. Group any changes by which leads are affected, since adjacent leads correspond to specific areas of the heart.
Running through this sequence every time builds the habit of systematic reading. Most missed findings happen not because the pattern was subtle, but because the reader skipped a step and never looked for it.