Normal Electrocardiography (ECG) Intervals: Normal Electrocardiography Intervals (2024)

Electrocardiography (ECG) is one of the most vital and readily used screening tools in clinical medicine. It is inexpensive and easily obtained in both the inpatient and outpatient setting. The ECG is used to diagnose numerous cardiac conditions, including prior infarction and active cardiac ischemia, as well as conduction abnormalities such as atrial fibrillation and life-threatening tachycardias. The information provided by ECGs also is used in determining which type of implantable cardiac defibrillator should be used for the management of advanced heart failure. Numerous noncardiac conditions, including electrolyte abnormalities and medication side effects, also are detectable on ECG owing to their distinct effect on conduction patterns. [1, 2]

A well-planned approach to 12-lead ECG interpretation will prevent the interpreter from missing crucial information. Key aspects in the interpretation of the 12-lead ECG include the heart rate, the heart rhythm (both atrial and ventricular), the electrical axis (both the P-wave axis and the QRS axis), and knowledge of the normal intervals. Next, determine the relationship of P waves to QRS complexes. Finally, analyze the QRS morphology and ST and T-wave segments.

ECG paper commonly moves at 25 mm/second; thus, each small box (1 mm) is equivalent to 0.04 seconds (40 milliseconds), and each large box (5 mm) is equivalent to 0.2 seconds (200 milliseconds). At the beginning of an ECG, make note of the standardization square, normally 10 mm high by 5 mm wide. This will alert you to the correct paper speed and standard amplification of P, QRS, and T-wave complexes.

Normal ECG values for waves and intervals are as follows:

  • RR interval: 0.6-1.2 seconds

  • P wave: 80 milliseconds

  • PR interval: 120-200 milliseconds

  • PR segment: 50-120 milliseconds

  • QRS complex: 80-100 milliseconds

  • ST segment: 80-120 milliseconds

  • T wave: 160 milliseconds

  • QT interval: 420 milliseconds or less if heart rate is 60 beats per minute (bpm)

Basic Physiology of the Cardiac Conduction System

Physiologically, ECG tracing represents the conduction pathway through the heart. The normal conduction pathway originates in the sinoatrial (SA) node, which initiates sinus impulses, and a wave of depolarization spreads out over the right and left atria, forming the P wave. At the level of the atrioventricular (AV) node, the beat is conducted to the ventricles over the His bundle to the right and left bundle branches and the Purkinje system. The resulting atrial repolarization and early ventricular depolarization result in the QRS complex. Ventricular depolarization and subsequent repolarization lead to the completion of the cycle, forming the T-wave. The periods between each wave and complex are made up of intervals and segments. The PR, QT, and RR intervals represent the duration of conduction through the AV node, the duration of ventricular depolarization to repolarization, and the duration between each cardiac cycle, respectively. The PR and ST segments represent the isoelectric interval between depolarization and repolarization of the atria and ventricles.

Anatomy Corresponding to the Cardiac Conduction System

The right coronary artery (RCA) typically supplies blood to the SA node, right atrium, right ventricle, and right bundle branch; it may also supply the left posterior fascicle. When the posterior descending artery (PDA) arises from the RCA (“right dominance”), it usually supplies blood to the AV node. The left main coronary artery typically is 1-2 cm in length and gives rise to the left anterior descending coronary artery (LAD) and the left circumflex artery (LCx). The LAD typically gives off perpendicular branches (septal perforators) that supply the AV node and the left anterior and posterior fascicles. The posterior fascicle also receives blood from the RCA and thus has a dual blood supply. Other branches, called diagonal branches, supply areas of the left ventricle. The LCx supplies blood to the back of the heart, and its branches are called obtuse marginals (OM). A PDA that arises from the LCx are is described as “left dominance.” This explains why patients with proximal RCA infarcts often present with complete heart block or sinus arrest.

Cardiac Action Potential

At the molecular level, the complex phenomenon surrounding depolarization and repolarization of the cardiac action potential results from the movement of ions—mainly sodium, calcium, and potassium—across the cell membrane. [3]

The cardiac action potential cycle comprises five phases. The rapid upstroke of the ventricular myocyte action potential in phase 0 is caused by the rapid influx of sodium ions into the cell, generating a depolarizing (positive) current. When net intracellular charge reaches a well-defined threshold, cellular depolarization occurs. During the next 4 phases, the cardiac cell enters repolarization, which is the electrical reset allowing for the next beat.

Phase 1 results from inactivation of the inward sodium current and activation of a short-lived outward current. Phase 2 represents the plateau phase and consists of inward, depolarizing calcium currents and outward, repolarizing potassium currents. As the calcium currents decay, the potassium currents increase, ending the plateau phase. Phase 3 includes more rapid repolarizing currents and is generated by a family of potassium channels. The two main currents are described by their kinetics (slow and fast), and these channels are the targets for many class-III antiarrhythmic drugs. Phase 4 represents the resting state or electric diastole.

Cardiac arrhythmias are believed to result from abnormalities of impulse formation, impulse propagation, or repolarization. Tachycardias that result from impulse formation are termed automatic. Tachycardias that result from impulse propagation are considered reentrant. Tachycardias generated from abnormal repolarization result from genetic defects in ion channels (so-called channelopathies) and can be lethal. In addition, catecholamines, ischemia, cellular ion concentrations (potassium), and cardioactive medicines all influence the development of cardiac arrhythmias.

Normal Electrocardiography (ECG) Intervals: Normal Electrocardiography Intervals (2024)


Normal Electrocardiography (ECG) Intervals: Normal Electrocardiography Intervals? ›

Normal ECG values for waves and intervals are as follows: RR interval: 0.6-1.2 seconds. P wave: 80 milliseconds. PR interval

PR interval
In electrocardiography, the PR interval is the period, measured in milliseconds, that extends from the beginning of the P wave (the onset of atrial depolarization) until the beginning of the QRS complex (the onset of ventricular depolarization); it is normally between 120 and 200 ms in duration. › wiki › PR_interval
: 120-200 milliseconds.

What is normal range ECG normal value? ›

Frequently Asked Questions:
HEART RATE49 to 100 BPM55 to 108 BPM
P WAVELENGTH81 to 130 ms84 to 130 ms
PR INTERVAL119 to 210 ms120 to 202 ms
QRS DURATION74 to 110 ms78–88 ms

How to read ECG results numbers? ›

ECG paper is marked with a grid of small and large squares. Each small square represents 40 milliseconds (ms) in time along the horizontal axis and each larger square contains 5 small squares, thus representing 200 ms. Standard paper speeds and square markings allow easy measurement of cardiac timing intervals.

What are the time intervals on ECG paper? ›

The ECG paper speed is ordinarily 25 mm/sec. As a result, each 1 mm (small) horizontal box corresponds to 0.04 sec (40 ms), with heavier lines forming larger boxes that include five small boxes and hence represent 0.20 sec (200 ms) intervals.

What causes irregular RR intervals? ›

Respirations lead to vagal stimuli resulting in R-R interval variations. Typically its presence is an indicator of good cardiovascular health. Loss of sinus arrhythmia may indicate underlying heart failure or structural heart disease.

What is an unhealthy ECG? ›

Many different heart conditions can show up on an ECG, including a fast, slow, or abnormal heart rhythm, a heart defect, coronary artery disease, heart valve disease, or an enlarged heart. An abnormal ECG may also be a sign that you've had a heart attack in the past, or that you're at risk for one in the near future.

What are the intervals of the ECG? ›

ECG intervals are major elements to look at when reading an ECG strip. These include the PR interval, the QRS complex itself, the QT, and the RR intervals. The PR interval is the time from the beginning of the P wave to the beginning of the QRS complex.

What is a good reading on an ECG? ›

Normal duration of ECG segments: PR interval: 0.12 – 0.2 secs (3-5 small squares) QRS: <0.12 secs (3 small squares) QTc: 0.38 – 0.42 secs.

What should a normal ECG look like? ›

Normal ECG obtained from ECG heart monitor looks like a smooth curve. The distance between each spike is almost constant. Each spike represents one whole heartbeat, the distance between spikes represents your heart rate.

What is a normal QT interval on an ECG? ›

The normal QT interval is controversial, and multiple normal durations have been reported. In general, the normal QT interval is below 400 to 440 milliseconds (ms), or 0.4 to 0.44 seconds. Women have a longer QT interval than men. Lower heart rates also result in a longer QT interval.

What is the normal PR and QRS interval? ›

The P-R Interval

This measurement should be 0.12-0.20 seconds, or 3-5 small squares in duration. The second measurement is the width of the QRS which should be less than 3 small squares, or less than 0.12 seconds in duration.

How do you read ECG time? ›

How to Read an EKG Strip. ECG paper is a grid where time is measured along the horizontal axis. Each small square is 1 mm in length and represents 0.04 seconds. Each larger square is 5 mm in length and represents 0.2 seconds.

What is the time scale on an ECG? ›

The standard paper speed is 25mm/sec:

1 SMALL square (1mm) = 0.04 sec (40ms) 5 SMALL squares (5mm) = 1 LARGE square = 0.2 sec (200ms) 5 LARGE squares = 1 second.

What is a normal RR interval? ›

Normal ECG values for waves and intervals are as follows: RR interval: 0.6-1.2 seconds. P wave: 80 milliseconds. PR interval: 120-200 milliseconds.

What is an abnormal RR? ›

Norms - Respiratory Rates

Recent evidence suggests that an adult with a respiratory rate of over 20 breaths/minute is probably unwell, and an adult with a respiratory rate of over 24 breaths/minute is likely to be critically ill.

How to check if an ECG report is normal? ›

  1. Heart Rate: 60 - 90 bpm.
  2. PR Interval: 0.12 - 0.20 sec.
  3. QRS Duration: 0.06 - 0.10 sec.
  4. QT Interval (QTc ≤ 0.40 sec) Bazett's Formula: QTc = (QT)/SqRoot RR (in seconds) ...
  5. Frontal Plane QRS Axis: +90° to -30° (in the adult)

What is a normal heart rhythm on an ECG? ›

By convention, the term "normal sinus rhythm" is taken to imply that not only are the P waves (reflecting activity of the sinus node itself) normal in morphology but that all other ECG measurements are also normal. Criteria therefore include: Normal heart rate (classically 60 to 100 beats per minute for an adult).

What is a normal QTc level on an ECG? ›

The normal QT interval is controversial, and multiple normal durations have been reported. In general, the normal QT interval is below 400 to 440 milliseconds (ms), or 0.4 to 0.44 seconds.

What is a normal ventricular rate on an ECG? ›

The ventricular rate depends on the degree of atrioventricular conduction, and with normal conduction it varies between 100 and 180 beats/min. Slower rates suggest a higher degree of atrioventricular block or the patient may be taking medication such as digoxin.

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