Guide — Labs & Diagnostics
ECG Rhythm Recognition
The monitor is always on — in the ICU, at a code, on transport, during procedural sedation. This guide builds a repeatable way to read a rhythm strip, then walks the rhythms the boards and the bedside demand, with the one question that drives every RT action: is this patient stable, and is this a code rhythm?
12 min read · Labs & Diagnostics
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
ECG interpretation shows up under four separate task lines on the 2027 NBRC Detailed Content Outline, and for good reason — respiratory therapists read the monitor constantly: in the ICU, at a code, during transport, through procedural sedation, and across exercise or stress testing.
The RT is not the primary interpreter of a 12-lead, but must recognize a rhythm fast enough to act. Three questions matter in the moment: Is the patient stable or unstable? Is this a code (pulseless) rhythm? Does it change my airway, oxygenation, or ventilation priorities?
A systematic approach beats pattern-matching — read every strip the same way, in the same order, every time. And never forget the orienting rule: treat the patient, not the monitor— always correlate the rhythm with pulse, blood pressure, and mental status.
Key Concepts
Read every strip in the same order: rate → rhythm (regularity) → P waves → PR interval → QRS width → (axis / QT) → interpret in context.
- Rate.Standard paper speed is 25 mm/s: each small box is 0.04 s (40 ms), each large box is 0.20 s (200 ms). Use the rate ruler by counting large boxes between R waves: 1 box = 300, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50. For irregular rhythms, count the QRS complexes in a 6-second strip and multiply by 10. Normal is 60–100; under 60 is bradycardia, over 100 is tachycardia.
- Rhythm / regularity. Are the R-R intervals regular, regularly irregular, or irregularly irregular?
- P waves. Present? One before every QRS? Uniform shape? Upright in lead II?
- PR interval.Normal is 0.12–0.20 s (120–200 ms, or 3–5 small boxes). A long PR interval signals an AV conduction delay or block.
- QRS duration. Normal is under 0.12 s (under 120 ms). A wide QRS points to ventricular origin, a bundle-branch block, or aberrant conduction.
- Axis (quick screen) and QT.Normal QRS axis is −30° to +90°. The fast two-lead screen: lead I and aVF both upright means a normal axis. (Full axis interpretation lives in the 12-Lead ECG reference — use it when a full axis workup is needed.) QT is rate-dependent; a very long QT sets up torsades.
Assessment & Findings
The board-tested rhythm set groups naturally by where the impulse originates. The table below is a scan reference — rate, regularity, P waves, QRS width, and the one clue or RT note that separates each rhythm from its look-alikes.
| Rhythm | Rate | Regularity | P waves | QRS | Key clue / RT note |
|---|---|---|---|---|---|
| Sinus tachycardia | Rate >100 | Regular | Normal, 1:1 | Narrow | A response (fever, pain, hypoxemia, hypovolemia, anxiety); treat the cause, not the rate. |
| Sinus bradycardia | Rate <60 | Regular | Normal, 1:1 | Narrow | Benign in athletes/sleep; ominous if from late hypoxemia; treat only if symptomatic. |
| Atrial fibrillation | Rate varies | Irregularly irregular | Absent (fibrillatory waves) | Narrow | Loss of atrial kick; embolic/stroke risk; rate control + anticoagulation. |
| Atrial flutter | Atrial ~250–350, ventricular often ~150 | Regular (often 2:1) | Sawtooth flutter waves | Narrow | A regular narrow tachycardia at ~150 is 2:1 flutter until proven otherwise. |
| SVT (paroxysmal / AVNRT) | 150–250 | Regular | Hidden/retrograde | Narrow | Abrupt onset and offset; vagal maneuvers, then adenosine. |
| PACs | Underlying rate | Irregular (early beats) | Early, abnormal P | Narrow | Early atrial beat with a non-compensatory pause; usually benign. |
| PVCs | Underlying rate | Irregular (early beats) | None before the ectopic | Wide (≥0.12 s) | Wide bizarre early beat, T opposite the QRS; watch bigeminy, couplets, and R-on-T. |
| Ventricular tachycardia (VT) | 100–250 | Regular | AV dissociation | Wide | Three or more PVCs in a row (nonsustained if <30 s, sustained if ≥30 s); assume VT for any wide-complex tachycardia. Pulseless VT is shockable. |
| Ventricular fibrillation (VF) | None | Chaotic | None | No organized QRS | No pulse; shockable — defibrillate. |
| Asystole | None | Flat | None | None | No pulse; NOT shockable; confirm in a second lead, check leads/gain. |
| PEA | Any organized rate | Organized on monitor | May be present | Any | Organized rhythm but NO pulse; NOT shockable; find and treat the cause. |
| 1st-degree AV block | Underlying | Regular | Normal, 1:1 | Narrow | PR >0.20 s, constant; every P conducts; usually benign. |
| 2nd-degree Mobitz I (Wenckebach) | Atrial > ventricular | Regularly irregular (grouped) | More Ps than QRS | Narrow | PR lengthens progressively until a QRS drops, then repeats; usually benign nodal. |
| 2nd-degree Mobitz II | Atrial > ventricular | Regular or irregular | More Ps than QRS | Often wide | Constant PR with sudden non-conducted Ps; infranodal; can progress to complete block — pacing. |
| 3rd-degree (complete) AV block | Atrial > ventricular | Both regular but dissociated | Present, unrelated to QRS | Narrow (junctional escape 40–60) or wide (ventricular escape 20–40) | Full AV dissociation; needs pacing. |
| Paced rhythm | Set rate | Regular | Pacer spike before P and/or QRS | Wide if ventricular-paced | Spike then capture (P or wide QRS); watch failure to capture/sense. |
Group them by origin to keep the set manageable: supraventricular rhythms (sinus tachycardia/bradycardia, atrial fibrillation, atrial flutter, SVT, PACs) are narrow unless aberrantly conducted; ventricular rhythms (PVCs, VT, VF) are wide; the pulseless arrest set— VF, pulseless VT, asystole, and PEA — is covered stop-by-stop in the Cardiac Arrest Rhythms chart; conduction blocks run from 1st-degree through Mobitz I, Mobitz II, and 3rd-degree (complete) block; and paced rhythms show a pacer spike driving capture.
RT Priorities / Interventions
- Correlate the rhythm with the patient.A rhythm is never read in isolation — check pulse, blood pressure, mental status, and symptoms before you act.
- Know the RT-relevant drivers. Hypoxemia and hypercapnia drive sinus tachycardia; severe or late hypoxemia and hyperkalemia drive bradycardia and blocks. A new arrhythmia on the ventilator or during sedation should send you to check oxygenation and ventilation first.
- Own your role in the code.Learn the split cold: shockable (VF and pulseless VT) get immediate defibrillation; non-shockable (asystole and PEA) get high-quality CPR, epinephrine, and a hunt for the reversible cause. The RT keeps uninterrupted ventilation, confirms the tube with waveform capnography, and uses ETCO₂ to gauge CPR quality and catch ROSC (AHA 2025).
- Unstable with a pulse. An unstable tachyarrhythmia with a pulse calls for synchronized cardioversion; unstable bradycardia calls for atropine, then transcutaneous pacing and an epinephrine or dopamine infusion (team/physician-led).
- Rule out artifact before you call a lethal rhythm.A disconnected lead mimics asystole; shivering, tremor, or a brushing motion mimics VF/VT. Confirm the patient and check a second lead first — never defibrillate artifact.
Common Pitfalls
- Treating the number instead of the patient — shocking a stable patient, or dismissing an unstable one because the rate “looks okay.”
- Calling artifact a lethal rhythm. Always confirm asystole or VF in a second lead and check for a pulse. Asystole is never shocked.
- Missing 2:1 atrial flutter. A regular narrow tachycardia at about 150 is flutter until proven otherwise — look for buried flutter waves; vagal maneuvers or adenosine can unmask them.
- Confusing SVT with sinus tachycardia. Sinus tach has visible P waves and an obvious cause; SVT starts and stops abruptly, is usually faster, and hides its P waves.
- Mixing up Mobitz I and II. Mobitz I lengthens the PR interval then drops a beat (benign, nodal); Mobitz II keeps the PR interval constant then drops suddenly (dangerous, infranodal). Do not call complete block while P waves and QRS complexes still relate to each other.
- Reading PEA off the monitor. PEA is defined by the pulse check, not the tracing — an organized rhythm is not perfusion.
Board Exam Pearls
- Rate ruler: 300-150-100-75-60-50 by large boxes; 6-second strip × 10 for irregular rhythms.
- Irregularly irregular with no P waves = atrial fibrillation. Sawtooth = atrial flutter. Regular, narrow, ~150 = suspect 2:1 flutter.
- Wide + regular + fast + AV dissociation = VT. Treat any wide-complex tachycardia as VT until proven otherwise, especially with a cardiac history.
- Shockable = VF and pulseless VT only. Asystole and PEA are never shocked — CPR, epinephrine, and reverse the cause.
- AV block memory line: “longer, longer, longer, drop = Wenckebach (Mobitz I); some Ps just don’t get through = Mobitz II; Ps and Qs don’t agree = complete (3rd degree).”
- Third-degree block: atrial rate faster than ventricular rate, both regular but fully dissociated; a narrow escape (~40–60) is junctional, a wide escape (~20–40) is ventricular.
- Torsades de pointes = polymorphic VT on a long QT — think magnesium and stop the QT-prolonging drugs.
FAQ
What is the fastest way to estimate heart rate on an ECG?
Use the 300-150-100-75-60-50 rule — count the large boxes between two R waves (one box is 300, two is 150, and so on down to six boxes for 50). For an irregular rhythm, count the QRS complexes in a 6-second strip and multiply by 10.
How do I tell ventricular tachycardia from SVT with aberrancy?
A wide-complex tachycardia is VT until proven otherwise, especially in a patient with structural heart disease or a prior MI. AV dissociation, capture or fusion beats, and a very wide QRS all favor VT. When the patient is unstable and you are unsure, treat it as VT.
Which cardiac arrest rhythms are shockable?
Only ventricular fibrillation and pulseless ventricular tachycardia. Asystole and PEA are not shocked — they get high-quality CPR, epinephrine, and a search for the reversible cause (the H's and T's).
What is the difference between Mobitz I and Mobitz II second-degree block?
Mobitz I (Wenckebach) shows a progressively lengthening PR interval until a beat is dropped, then the cycle repeats; it is usually a benign AV-nodal problem. Mobitz II keeps a constant PR interval and then drops a beat suddenly; it is infranodal, more dangerous, and can progress to complete heart block, so it warrants pacing.
Why does a respiratory therapist need to read the rhythm at all?
Rhythm changes track oxygenation, ventilation, and hemodynamics — hypoxemia drives tachycardia early and bradycardia late — and the RT owns the airway, ventilation, and capnography in every code. You also watch the monitor during transport, procedural sedation, and weaning, where a new rhythm is often the first sign something has changed.
In a code
When the rhythm turns lethal, the split between shockable and non-shockable drives everything that happens next — see it mapped stop-by-stop, or go deeper on the 12-lead for axis and ischemia patterns.
See the Cardiac Arrest Rhythms chart →Open the 12-Lead ECG reference →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.
- American Heart Association. 2025 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation. 2025;152.
- National Board for Respiratory Care. Detailed Content Outline for the Respiratory Therapy Examination. NBRC.