EKG Technician — Complete Study Guide (2026)

Rhythm interpretation is the heart of the job and the heart of the test. It is also the part of the work where recognition speed genuinely matters to a patient: the difference between motion artifact and ventricular fibrillation is the difference between charting a note and calling a code. This chapter builds the skill in the order the exam tests it — first the waveforms and the numbers that define normal, then the three ways to calculate a heart rate, then a rhythm-by-rhythm reference, and finally the two judgment calls a technician makes every shift: telling artifact from a real rhythm, and knowing which rhythms need someone at the bedside immediately.

The single most important habit to build is a consistent five-step method applied to every strip, in the same order, every time: rate, regularity, P waves, PR interval, QRS duration. Reading a strip is not pattern-guessing; it is running that checklist and letting the answer fall out of it. Most wrong answers on the exam — and most wrong calls at the bedside — come from skipping a step because the strip "looks like" something familiar.

1.1 The Waveforms and the Numbers That Define Normal

Every deflection on a tracing corresponds to an electrical event in the heart. The P wave is atrial depolarization — the electrical signal spreading across the atria that triggers them to contract. The QRS complex is ventricular depolarization, the much larger signal spreading through the ventricles. The T wave is ventricular repolarization, the electrical recovery of the ventricles back to their resting state. Atrial repolarization happens too, but it is buried inside the QRS and is not seen as its own wave.

The intervals between these waves are where the numbers live, and the exam expects them cold. The PR interval is measured from the beginning of the P wave to the beginning of the QRS. It represents the time from the start of atrial depolarization through the deliberate pause at the AV node, and it is the single interval that tells you how cleanly the signal is crossing from atria to ventricles. The QRS duration is measured from the first deflection off the baseline to the point where the complex returns to baseline; it tells you whether the ventricles were activated through the normal fast conduction pathway or by a slower, abnormal route. The QT interval runs from the start of the QRS to the end of the T wave and represents the entire ventricular cycle of depolarization plus recovery.

All of these are measured by counting boxes, and the boxes only mean what they mean because of the paper speed. EKG paper runs at a standard 25 millimeters per second. At that speed, one small box (1 mm) represents 0.04 second, and one large box (5 mm, five small boxes) represents 0.20 second. Five large boxes make one full second, and 1,500 small boxes pass in a minute. Every interval measurement in this book assumes 25 mm/sec; if the speed is changed, every number changes with it, which is why a speed change must always be documented on the tracing.

Two normal-interval limits anchor almost every rhythm question. The PR interval is normally 0.12 to 0.20 second — three to five small boxes. A PR that is constant and longer than 0.20 second is first-degree AV block; a PR shorter than 0.12 second suggests the beat did not start in the sinus node but lower down, at the AV junction. The QRS duration is normally less than 0.12 second — under three small boxes. A QRS of 0.12 second or wider means the impulse did not travel the normal fast pathway, which points either to a bundle branch block or to a beat that started in the ventricles themselves. The QT interval varies with heart rate, but a useful bedside check is that it should be less than half the R-R interval; a long QT raises the risk of dangerous ventricular rhythms.

Finally, regularity is measured R wave to R wave. March the R-R intervals across the strip with calipers or the marked edge of a piece of paper. When you suspect an AV block, do the same thing P wave to P wave to measure the atrial rhythm separately — comparing the P-P march against the R-R march is exactly how blocks are unmasked.

1.2 The Three Ways to Calculate Heart Rate

The exam expects all three rate-calculation methods, and it expects you to know when each one is valid. Choosing the wrong method for the rhythm in front of you is one of the most common traps on the test.

The 300 method is the fastest. Divide 300 by the number of large boxes between two R waves. Because the numbers repeat, most technicians simply memorize the sequence: one large box between R waves is 300, then 300, 150, 100, 75, 60, 50 for one through six large boxes. It is an estimate, and it is only valid for a regular rhythm.

The 1500 method is the most precise. Divide 1500 by the number of small boxes between two R waves. The 1500 comes from the 1,500 small boxes that pass in one minute at 25 mm/sec. It is more accurate than the 300 method because you are counting in finer units, but like the 300 method it assumes the rhythm is regular — it uses a single R-R interval to stand in for the whole minute.

The six-second method is the one to reach for when the rhythm is irregular. Count the number of QRS complexes in a six-second strip and multiply by 10. EKG paper is marked with a tick at the top every three seconds, so six seconds spans two of those marked intervals, or 30 large boxes. This is the only correct method for an irregular rhythm such as atrial fibrillation, because the box methods assume every R-R interval is identical, and in atrial fibrillation no two are.

When the atrial and ventricular rates are not the same — as in atrial flutter with a conduction ratio, or in complete heart block — you must calculate both rates and report both. Use the P-P interval for the atrial rate and the R-R interval for the ventricular rate. Reporting a single number in those rhythms would misrepresent the strip.

1.3 A Rhythm-by-Rhythm Reference

Rhythms are named by two things: where the impulse starts and how fast it goes. The origin determines what the P wave and QRS look like; the rate finishes the name. Run your five-step method and the criteria below like a checklist, matching one feature at a time.

Sinus rhythms start in the SA node, so there is one upright P wave before every QRS. Normal sinus rhythm is a rate of 60 to 100, regular, with a normal PR (0.12–0.20 s) and a narrow QRS (<0.12 s). Sinus bradycardia meets every sinus criterion except the rate is under 60; sinus tachycardia meets every criterion except the rate is over 100 — usually a normal response to fever, pain, exertion, anxiety, blood loss, or dehydration. Sinus arrhythmia meets every criterion except regularity: the R-R shortens with inspiration and lengthens with expiration. It is common and benign in children and young adults.

Atrial rhythms start somewhere in the atria other than the SA node, so the P wave changes — it looks different, disappears, or becomes a repetitive wave — while the QRS stays narrow because the ventricles are still activated normally. Atrial fibrillation is irregularly irregular with no identifiable P waves and a chaotic, wavy baseline. Atrial flutter shows uniform sawtooth "flutter" waves at roughly 250 to 350 per minute, conducted to the ventricles in ratios such as 2:1, 3:1, or 4:1.

Junctional rhythms start at the AV junction, so the atria are depolarized backward. The P wave is therefore inverted, hidden inside the QRS, or seen just after the QRS, and when a P wave is visible before the QRS the PR is short (under 0.12 s). The QRS stays narrow. By rate: junctional escape is 40 to 60, accelerated junctional is 60 to 100, and junctional tachycardia is over 100.

Ventricular rhythms start below the junction, so there is no related P wave and the QRS is wide and bizarre (0.12 s or greater). By rate: idioventricular rhythm is 20 to 40, accelerated idioventricular is 40 to 100, and ventricular tachycardia (VT) is over 100, defined as three or more consecutive wide beats. Ventricular fibrillation (VF) is the exception to all measurement — it has no measurable rate, no regularity, no P wave, and no true QRS, just a chaotic quivering baseline.

1.4 Ectopic Beats and the AV Blocks

Premature beats interrupt an underlying rhythm and are named for where they start. A premature atrial contraction (PAC) is early, with an abnormally shaped but still upright P wave and a narrow QRS. A premature junctional contraction (PJC) is early with an inverted or absent P wave and a short PR. A premature ventricular contraction (PVC) is early, wide, and bizarre, with no preceding P wave and a T wave that points opposite the QRS.

PVCs are described further by pattern and by origin. By pattern: bigeminy is a PVC every other beat, trigeminy every third beat, quadrigeminy every fourth; a couplet is two PVCs in a row. By origin: unifocal PVCs all look alike (one irritable site), while multifocal PVCs vary in shape (several irritable sites) and are more concerning. The most dangerous PVC of all is the R-on-T beat: a PVC that lands on the preceding T wave falls in the vulnerable period of repolarization and can trigger VT or VF. Report it promptly.

The AV blocks are a separate family, defined entirely by the relationship between the P waves and the QRS complexes. Working them out is a matter of measuring every PR interval and checking whether every P wave is followed by a QRS.

• First-degree AV block: every P wave conducts, nothing is dropped, but the PR is constant and longer than 0.20 second. It is a delay, not a true block.
• Second-degree, Mobitz I (Wenckebach): the PR interval lengthens progressively beat to beat until one P wave fails to conduct and a QRS is dropped, then the cycle repeats. The ventricular rhythm is irregular in a repeating pattern.
• Second-degree, Mobitz II: the conducted beats have a constant PR interval, but QRS complexes are dropped suddenly without warning. It is more dangerous than Mobitz I because it can progress to complete block.
• Third-degree (complete) AV block: no impulses cross the AV node. The P-P is regular and the R-R is regular, but they are completely independent of each other, so the PR interval constantly changes — the atria and ventricles are dissociated. The ventricles are kept going by an escape pacemaker: about 40–60 if junctional, 20–40 if ventricular.

1.5 Paced Rhythms, Artifact, and the Lethal Rhythms

A pacemaker impulse appears on the strip as a thin, sharp vertical spike. A spike followed by a P wave is atrial pacing; a spike followed by a wide QRS is ventricular pacing; a dual-chamber device may show both. Two malfunctions are tested: failure to capture is a spike with no waveform after it (the impulse fired but the heart did not respond), and failure to sense is a spike that appears at the wrong time in the cycle (the device did not "see" the heart's own beat).

Artifact is any deflection on the tracing that the heart did not generate, and the skill the exam rewards most is refusing to treat the monitor instead of the patient. Chaotic-looking noise in a patient who is awake, talking, or brushing their teeth is motion artifact — and very often you can see the real, regular complexes marching straight through the noise. The lethal rhythms are the mirror image: they require you to leave the machine and go to the patient at once.

• Always check the patient before the monitor. A patient who is alert and conversing is not in ventricular fibrillation. Confirm any alarming rhythm at the bedside before acting on it — or before dismissing it.
• Confirm asystole in more than one lead. A flat line in a single lead is far more often a disconnected wire than true asystole. Verify in a second lead and at the bedside.
• The lethal rhythms — ventricular fibrillation, pulseless ventricular tachycardia, and asystole — produce no effective cardiac output. Call for help, activate the emergency response, and assist with CPR within the limits of your training.
• Report promptly (but not as an arrest): new-onset atrial fibrillation, symptomatic bradycardia, Mobitz II, third-degree block, frequent or multifocal PVCs, and R-on-T beats all need the nurse or physician now, even though they are not codes.
• A technician recognizes; a physician diagnoses. Naming a rhythm on the strip for the chart is part of the job. Telling the patient what it means medically is not.

### Key Numbers & Facts — Chapter 1 - Five-step method: rate → regularity → P waves → PR interval → QRS duration. Same order, every strip. - Small box = 0.04 s; large box = 0.20 s at 25 mm/sec. Five large boxes = 1 second; 1,500 small boxes = 1 minute. - PR interval normal: 0.12–0.20 s (3–5 small boxes). Long + constant = first-degree block; short (<0.12 s) = junctional origin. - QRS normal: < 0.12 s (< 3 small boxes). ≥ 0.12 s = wide = bundle branch block or ventricular origin. - QT: should be less than half the R-R interval; long QT is dangerous. - 300 method: 300 ÷ large boxes between R waves (memorize 300-150-100-75-60-50). Regular rhythms only. - 1500 method: 1500 ÷ small boxes between R waves. Most precise; regular rhythms only. - Six-second method: QRS complexes in 6 seconds × 10. Required for irregular rhythms (e.g., a-fib). 6 s = 30 large boxes = two 3-second tick marks. - Rates: normal sinus 60–100; bradycardia < 60; tachycardia > 100. - Atrial flutter: sawtooth waves at ~250–350/min. Atrial fibrillation: irregularly irregular, no P waves. - Junctional: escape 40–60, accelerated 60–100, tachycardia > 100; P inverted/absent/after QRS, PR < 0.12 s, narrow QRS. - Ventricular: idioventricular 20–40, accelerated 40–100, VT > 100 (≥ 3 wide beats); QRS ≥ 0.12 s, no related P. VF has no measurable rate/rhythm/P/QRS. - First-degree block: PR > 0.20 s, constant, nothing dropped. Mobitz I: PR lengthens → drop → repeat. Mobitz II: PR constant → sudden drop. Third-degree: P-P regular, R-R regular, PR ever-changing (dissociation). - Lethal (go to patient): VF, pulseless VT, asystole. R-on-T PVC can trigger VF.

### Strip Walkthrough — Chapter 1 The strip: a rhythm strip shows QRS complexes that are wide and bizarre, with no P waves anywhere, at a regular rate. You count 4 large boxes between each R wave. Step through the method. Rate: using the 300 method, 300 ÷ 4 large boxes = 75/min (this rhythm is regular, so the box method is valid). Regularity: regular. P waves: none are present or related to the QRS. PR interval: not measurable — there are no P waves to start it. QRS: wide, 0.12 s or greater. Name it. No P waves + wide QRS = a ventricular origin. A regular ventricular rhythm at a rate of 75 falls in the 40–100 band, which is an accelerated idioventricular rhythm. It is not VT (that would be over 100) and not an idioventricular escape (that would be 20–40). The lesson: the rate alone did not name this rhythm — the combination of "no related P + wide QRS" told you it was ventricular, and only then did the rate of 75 pick the specific name. Run the whole checklist before you commit.

### Exam Traps — Chapter 1 - Using a box method on an irregular rhythm. If the R-R intervals are not identical (atrial fibrillation is the classic case), the 300 and 1500 methods are invalid — you must use the six-second method. - Confusing "no P waves" with "hidden/inverted P waves." Atrial fibrillation has no P waves and a chaotic baseline; junctional rhythms have P waves that are inverted, buried, or after the QRS. Look before you conclude. - Calling a long-but-constant PR a "block that drops beats." First-degree AV block drops nothing — every P conducts. Only the second-degree blocks drop QRS complexes. - Mobitz I vs. Mobitz II. The tell is the PR interval of the conducted beats: lengthening before the drop = Mobitz I; constant before a sudden drop = Mobitz II. - Treating the monitor, not the patient. Chaotic tracing in an awake, talking patient is artifact. A flat line in one lead is usually a loose wire. Always verify at the bedside and in a second lead. - Reporting one rate when there are two. In atrial flutter with a conduction ratio and in third-degree block, report the atrial rate (P-P) and the ventricular rate (R-R) separately.