Reading an EKG starts with understanding the grid paper, recognizing the basic waveforms, and then working through a systematic checklist every time. The process is the same whether you’re looking at a 12-lead printout or a single rhythm strip: you measure time left to right, voltage up and down, and look for patterns that fall inside or outside normal ranges. Once you know what each wave represents and how to measure it, an EKG becomes surprisingly readable.
Understanding the Grid Paper
EKG paper runs at a standard speed of 25 millimeters per second. Each small square on the grid is 1 millimeter wide and represents 0.04 seconds of time. Five small squares make up one large square, which equals 0.2 seconds. That means five large squares across equal exactly one second, and 30 large squares span a six-second strip. Vertically, each small square represents 0.1 millivolts of electrical voltage, so one large square (five small squares tall) equals 0.5 millivolts.
These measurements matter because every interval you’ll check on an EKG is defined in seconds, and you’ll count squares to get there. A quick reference: 3 small squares wide = 0.12 seconds, 5 small squares = 0.2 seconds. You’ll use these numbers constantly.
The Three Main Waveforms
Each heartbeat produces a predictable series of waves on the EKG. The first bump is the P wave, which represents the upper chambers (atria) firing electrically. Next comes the QRS complex, the tall, sharp spike that represents the lower chambers (ventricles) firing. This is the most prominent feature on any EKG. Finally, the T wave is a rounded bump that follows the QRS, representing the ventricles resetting their electrical charge to prepare for the next beat.
Between these waves are segments and intervals with specific names. The PR interval runs from the start of the P wave to the start of the QRS complex, representing the time it takes for the electrical signal to travel from the upper chambers down to the lower chambers. The ST segment sits between the end of the QRS and the beginning of the T wave. The QT interval spans from the start of the QRS to the end of the T wave, covering the entire cycle of the ventricles firing and recovering.
Step 1: Calculate the Heart Rate
The simplest method is the six-second strip approach. Count the number of QRS complexes (the tall spikes) in 30 large squares and multiply by 10. That gives you the ventricular rate in beats per minute. You can do the same with P waves to get the atrial rate. If these two numbers don’t match, that’s a flag worth noting.
For a more precise rate on a regular rhythm, use the “300 method.” Count the number of large squares between two consecutive R waves (the peak of each QRS complex) and divide 300 by that number. Three large squares between beats means a rate of 100. Four large squares means 75. Five means 60. For even greater precision, count small squares between R waves and divide 1,500 by that number. For example, 22 small squares between two R waves gives you 1,500 รท 22 = roughly 68 beats per minute.
Step 2: Check the Rhythm
Look at the spacing between R waves across the entire strip. You can use a ruler, calipers, or even mark two R waves on a piece of paper and slide it along the strip to compare. There are four patterns to recognize:
- Regular: The R-to-R distances are equal throughout the strip.
- Irregularly irregular: The R-to-R distances are never the same, with no discernible pattern. This is the hallmark of atrial fibrillation.
- Regularly irregular: The spacing is unequal, but there’s a repeating pattern to the irregularity.
- Occasionally irregular: The spacing is equal except for an occasional out-of-place beat, like a premature contraction.
Step 3: Evaluate the P Waves
Look for a small, rounded bump before each QRS complex. Ask yourself four questions: Are P waves present? Do they all look the same? Do they occur at a regular rate? Is there exactly one P wave before every QRS complex? If you answer yes to all four, the rhythm is likely originating from the heart’s normal pacemaker. Missing P waves, extra P waves, or P waves with inconsistent shapes point to different types of rhythm problems.
Step 4: Measure the PR Interval
Count the small squares from the beginning of the P wave to the beginning of the QRS complex. The normal range is 0.12 to 0.20 seconds, or 3 to 5 small squares. A PR interval shorter than 3 small squares suggests the electrical signal is taking a shortcut between the upper and lower chambers. A PR interval longer than 5 small squares means the signal is being delayed, which can indicate a heart block.
Step 5: Measure the QRS Duration
Count from the beginning to the end of the QRS complex. Normal QRS duration is 0.04 to 0.12 seconds, or 1 to 3 small squares. A QRS wider than 3 small squares means the ventricles are taking longer than normal to conduct electricity, which happens with bundle branch blocks or when the heartbeat originates from an abnormal location in the ventricles.
Step 6: Examine the ST Segment and T Waves
The ST segment should be flat and level with the baseline (the flat line between the T wave and the next P wave, also called the isoelectric line). Any ST segment that rises or dips more than 1 millimeter (one small square) from the baseline at the point where the QRS ends is considered abnormal. ST elevation is the classic sign of an active heart attack, while ST depression can indicate reduced blood flow to the heart muscle.
T waves should be upright and gently rounded in most leads, and their height is typically between one-eighth and two-thirds the size of the R wave. Flattened, inverted, or unusually tall and peaked T waves can signal problems ranging from low blood flow to electrolyte imbalances.
Step 7: Check the QT Interval
The QT interval covers the full electrical cycle of the ventricles, from the start of the QRS to the end of the T wave. Normal QT is generally less than 0.44 seconds (about 11 small squares). The American Heart Association considers a prolonged QT to be 460 milliseconds or longer in women and longer than 450 milliseconds in men. A QT shorter than 390 milliseconds is considered abnormally short. Both extremes increase the risk of dangerous heart rhythms.
Recognizing Artifacts
Not everything on an EKG comes from the heart. Muscle tremors, shivering, patient movement, and even talking can produce squiggly, irregular signals on the tracing that mimic real cardiac problems. The most common artifact is a jittery, irregular baseline caused by muscle tremor, which can look strikingly similar to atrial fibrillation. The key to telling them apart is looking carefully for P waves hiding in the noise. If you can spot consistent P waves in any lead, the chaotic baseline is likely artifact, not a true arrhythmia.
A wandering baseline, where the entire tracing drifts up and down in slow waves, usually comes from the patient breathing deeply or from electrodes that aren’t sticking well. These artifacts don’t change the shape or timing of the waveforms themselves, just the position of the whole complex on the page. When something on a strip looks unusual, always consider whether the patient was moving, shivering, or anxious before jumping to a cardiac diagnosis.
Putting It All Together
Every time you look at an EKG, work through the same sequence: rate, rhythm, P waves, PR interval, QRS duration, ST segment, T waves, QT interval. Skipping steps is how abnormalities get missed. With practice, the normal pattern becomes so familiar that deviations jump out immediately. A normal sinus rhythm has a rate between 60 and 100, regular R-to-R spacing, one upright P wave before each narrow QRS complex, a PR interval of 3 to 5 small squares, a QRS of 1 to 3 small squares, a flat ST segment, upright rounded T waves, and a QT interval under 11 small squares. Anything outside those parameters tells you where to look next.