ECG criteria
- QRS complexes are clearly visible; select a stable fiducial point (usually the R peak if present)
- Measure the distance from the selected point of one QRS to the same point of the next QRS horizontally
- At a paper speed of 25 mm/s: each millimeter = 0.04 s; at 50 mm/s — each mm = 0.02 s
- Normal RR: 15–30 mm on a standard tracing (at 25 mm/s) corresponds to 0.6–1.2 s
- Sinus rhythm is assumed when there is sinus P wave morphology (usually positive in II, negative in aVR), typically 1:1 before each QRS, with a constant or physiologically slightly variable PR interval; RR regularity is assessed separately
- Repeated similar RR intervals → regular rhythm; significant variation → irregular rhythm
What it looks like on ECG
On the ECG strip, the RR interval is measured horizontally between corresponding points of two consecutive QRS complexes. Normally, during sinus rhythm, these distances are approximately equal. It is measured in any lead where QRS complexes are clearly distinguishable: most commonly standard lead II, or one of the chest leads V2–V5. When the R wave is clearly visible, its peak is used; for negative or small QRS complexes, another stable point of the complex or a different lead is chosen.
Standard reference points at 25 mm/s:
- 1 mm (width of a small square) = 0.04 s
- 5 mm (width of a large square) = 0.2 s
- 25 mm = 1 s
At a heart rate of 60 bpm, RR ≈ 25 mm; at 100 bpm — ≈ 15 mm; at 50 bpm — ≈ 30 mm. Minor distance fluctuations of 1–2 mm may reflect measurement error or slight physiological variability. Sinus respiratory arrhythmia is characterized by cyclic changes in RR related to respiration while maintaining sinus P waves.
How to measure
- Select the lead where consecutive QRS complexes are most clearly distinguishable (usually II or V2–V5).
- Identify a stable point of the QRS complex (most commonly the R peak, if R is present).
- Apply a ruler or calipers to identical points of two adjacent QRS complexes.
- Count the distance in millimeters or small squares.
- Convert to seconds: distance (mm) × 0.04 = time (s) at 25 mm/s.
- Repeat for several cycles: compare 3–5 consecutive RR intervals to assess regularity.
- Calculate heart rate: 60 / RR (in seconds) = beats per minute, or 1500 / RR in mm at 25 mm/s.
Normal Variants
Sinus rhythm with physiological variability: RR intervals may vary slightly, especially with respiration. In young people, cyclic changes in RR related to respiration (sinus respiratory arrhythmia) are normal.
Resting heart rate: a common reference range for a resting adult is 60–100 bpm. At 60 bpm, RR is about 1.0 s; at 100 bpm, it is about 0.6 s. A heart rate of 40–60 bpm can be physiological in trained individuals and during sleep due to increased vagal tone — this is not always pathological bradycardia.
During physical exertion: RR shortens proportionally to the increase in heart rate — this is normal.
During respiration: RR shortens during inspiration and lengthens during expiration — sinus respiratory arrhythmia, especially pronounced in children and young adults.
Pathological changes
Shortened RR (tachycardia): reference point — average or persistent heart rate > 100 bpm with a regular rhythm. In an irregular rhythm, a single short interval may be an extrasystole rather than a sign of true tachycardia; evaluate the average rate over several cycles. Causes of tachycardia: physical exertion, stress, fever, hyperthyroidism, hypoxia, MI, PE, COPD, anemia, sepsis, catecholamine intoxication.
Prolonged RR (bradycardia): reference point — average or persistent heart rate < 60 bpm with a regular rhythm. A single long interval may be a pause rather than true bradycardia. Causes: high vagal tone (athletes, sleep), hypothyroidism, hyperkalemia, inferior MI, sick sinus syndrome, AV blocks, beta-blockers, calcium channel blockers.
Irregular RR: may indicate atrial fibrillation (absolute irregularity, absence of organized P waves), atrial flutter with variable AV conduction, atrial or ventricular extrasystole (premature complex), sinoatrial block (regular pauses), sinus pause. The diagnosis requires analysis of P waves and the pattern of changes.
Absence of recorded ventricular complexes: requires immediate assessment of the patient, pulse, electrodes, and other leads — the cause may be an artifact, poor contact, dropped complex, or low voltage. Only confirmed absence of ventricular electrical activity in the appropriate clinical context allows for asystole to be diagnosed. In complete AV block, an independent ventricular escape rhythm is usually preserved, although asystole is possible if an escape pacemaker is absent.
Clinical significance and management
The RR interval is the clinician's primary tool for assessing a patient's condition:
In bradycardia (RR > 1.0 s, HR < 60): assess hemodynamics, rule out hypothyroidism, hyperkalemia, medication effects. In acute symptomatic bradycardia with hemodynamic instability, emergency therapy and temporary pacing are performed as indicated. Permanent pacing for sinus node dysfunction is considered in the presence of documented symptom correlation, without a universal minimum heart rate threshold.
In tachycardia (RR < 0.6 s, HR > 100): assess the context (fever, pain, hypoxia, MI); if regular narrow-complex tachycardia, differentiate sinus tachycardia, SVT, atrial flutter. Regular wide-complex tachycardia is treated as VT until proven otherwise; the rate alone does not establish a diagnosis.
In irregular RR: differentiate AF (absolute irregularity, absence of P waves), extrasystole — a premature complex of corresponding morphology, which may be followed by a compensatory or non-compensatory pause, and sometimes no pause (e.g., in interpolated extrasystoles) — SA block or pauses. AF requires individual assessment of stroke risk, bleeding risk, and anticoagulation indications according to current guidelines.
Prognostic assessment of heart rate variability (HRV) requires standardized recording and validated analysis methods; both short-term recordings (typically about 5 minutes) and long-term, e.g., 24-hour, monitoring are used depending on the study objective — visual assessment of a short ECG strip is not suitable for this.
Common interpretation errors
- Confusing R with other waves. An inexperienced practitioner may measure from the Q or S wave — this will result in an incorrect heart rate calculation. Always use identical fiducial points of consecutive complexes.
- Incorrect conversion at non-standard speed. At 50 mm/s, each mm = 0.02 s, and the formula for heart rate is 3000/RR in mm, not 1500/RR. An error in choosing the formula will give an incorrect result.
- Hasty conclusions from a single interval. A single shortened or prolonged RR among several normal ones requires checking the P and QRS morphology, prematurity of the complex, and recording quality — it may correspond to an extrasystole, artifact, pause, or onset of tachyarrhythmia rather than immediately indicating the mechanism. It is necessary to evaluate the average over 5–10 cycles or calculate it over a 6-second strip.
- Calculating heart rate from a single RR during arrhythmia. In AF, every interval is different — an average over 6 seconds is more reliable (count QRS complexes over a 6-second strip, multiply by 10).
- Missing a QRS complex. With low voltage, artifacts, or noise, the QRS may not be visible in a single lead — check other leads and recording quality, as well as the patient's condition, before concluding asystole or dropped complexes.
The distance between R and R — calculate the Rhythm and Rate per minute (HR)
1. On an ECG at 25 mm/s, the distance between two R waves is 20 mm. What is the heart rate?
RR = 20 mm × 0.04 s/mm = 0.8 s. HR = 60 / 0.8 = 75 bpm. Or directly: 1500 / 20 = 75 bpm.
2. RR intervals vary, P waves are visible before every QRS. What data are required to diagnose sinus respiratory arrhythmia?
The presence of P before every QRS is insufficient: it is necessary to assess the cyclic association with respiration and rule out extrasystoles, SA block, or pauses. The magnitude of RR variation is not a diagnostic criterion without evaluating other features.
3. A patient with sinus bradycardia, HR 45 bpm, complains of dizziness. What is the management?
There is no universal minimum HR threshold for pacing. The decision for temporary or permanent pacing is determined by the type of arrhythmia, hemodynamics, reversible causes, and proven correlation of symptoms with bradycardia. In acute symptomatic bradycardia with hemodynamic instability, urgent treatment and temporary pacing are indicated as appropriate.
Frequently asked questions
How to calculate heart rate from a single RR interval?
Divide 60 by the RR length in seconds. For example: RR = 0.8 s → HR = 60 / 0.8 = 75 bpm. Alternatively, at 25 mm/s: divide 1500 by the RR in millimeters: RR = 20 mm → HR = 1500 / 20 = 75 bpm. For a more accurate assessment, average 5–10 intervals or count the number of QRS complexes over a 6-second strip and multiply by 10.
What is sinus respiratory arrhythmia and is it dangerous?
Sinus respiratory arrhythmia is a cyclic variation in sinus RR intervals, typically related to respiration: RR shortens during inspiration and lengthens during expiration. It requires sinus P waves preceding each QRS. In children and young adults, it is frequently physiological and usually benign; in adults and with an unusual pattern, the clinical context is assessed and other causes of irregularity are ruled out.
RR intervals differ from each other — is this always an arrhythmia?
No. Minor RR variability is physiological, especially with respiration. The diagnosis is not established solely by the magnitude of RR variation. It is necessary to evaluate the morphology and timing of P/QRS, the relationship with respiration, and the presence of premature complexes and pauses. Significant variability may indicate AF, extrasystoles, SA block, or pauses.
Can the RR interval be completely identical during sinus rhythm?
Sinus rhythm can be nearly regular on a short tracing, especially with a fixed respiratory rate or in certain patients. High regularity does not rule out sinus rhythm. When pacing, VT, or another source is suspected, the P wave, QRS morphology, pacing artifacts, and clinical context are evaluated.
How to distinguish sinus bradycardia from AV block?
In sinus bradycardia, RR intervals may vary due to respiration, but a P wave always precedes the QRS with a constant PR interval. In Mobitz type I second-degree AV block, the PR interval progressively lengthens followed by a dropped QRS; in Mobitz type II, there is a sudden drop of QRS without progressive PR prolongation. In complete AV block, there is AV dissociation: independent atrial and ventricular rhythms.
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Orlov V. N. Electrocardiography Guide. St. Petersburg: SpetsLit, 2011
- Goldberger AL, Goldberger ZD, Shvilkin A. Goldberger's Clinical Electrocardiography. Philadelphia: Elsevier, 2018
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
- ESC Guidelines on cardiac pacing and cardiac resynchronization therapy, 2021
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.