Normal Sinus Rhythm (NSR)

Normal Sinus Rhythm (NSR) is the expected rhythm on an EKG in patients without pathophysiology. It is the normal electrical cycle of the heart which was discovered once a tool capable of detecting it was invented; the electrocardiogram (EKG/ECG).

EKG monitor simulation of normal sinus rhythm at a rate of 68bpm.
Normal Sinus Rhythm / Regular Sinus Rhythm (NSR / RSR) EKG / ECG monitor capture (software gen.)
💬
EKG Vocabulary:
Normal Sinus Rhythm (NSR) and Regular Sinus Rhythm (RSR) are synonymous. In a clinical setting you can use and will hear both interchangeably, with Normal Sinus Rhythm being more common.
1:00 Normal Sinus Rhythm monitor capture.

Many concepts in EKG interpretation are defined in terms of the way a rhythm or wave differs from how it is found in NSR. Understanding why an EKG tracing is not a Normal Sinus Rhythm gets you most of the way to identifying what dysrhythmia a tracing is.

💬
EKG Vocabulary:
Clinically, the terms dysrhythmia and arrhythmia are synonymous and refer to all cardiac rhythms EXCEPT Normal Sinus Rhythm.

NSR is a type of Sinus Rhythm and the quintessential example of it. Sinus Rhythms, as a category, include all rhythms which originate (start) from the Sino-Atrial Node (SA Node) and follow the normal electrophysiologic path through the heart. The hallmark of sinus originating rhythms are P Waves at the beginning of the repeating organized pattern which are followed by a QRS-T pattern (described below).

The normal in Normal Sinus Rhythm refers to the expected "normal" rate of the SA Node. In the average healthy heart the SA Node will create an electrical action potential between 60 and 99 times a minuet, therefore a NSR should have a Heart Rate (HR) between 60 and 99.

Cardiac monitor capture of Regular Sinus Rhythm at 65bpm.
Regular Sinus Rhythm with a HR of 65.

When an EKG tracing meets these two criteria it can be called Normal Sinus Rhythm. Normal Sinus Rhythm means:

An EKG rhythm starting at the SA Node with a rate between 60 and 99 which follows the expected healthy path through the heart.

Defining Characteristics* of NSR in Lead II

Software generated Normal Sinus Rhythm EKG tracing with a HR of 76.
A typical NSR EKG tracing.
  • Rate: 60-99 | R-R Intervals greater than 0.60sec (15mm) [3 large boxes] and less than 1sec (25mm) [5 large boxes].
  • Rhythm: Regularly Regular | Equal R-R Intervals.
  • P Waves: Less than 0.12sec (3mm) [3 small boxes] in duration & less than 2.5mm (0.25mV) [2.5 small boxes] in amplitude | First deflection of organized complexes, upright, & uniform.
  • P-R Intervals: Greater than 0.12sec (3mm) [3 small boxes] and less than 0.20sec (5mm) [1 large box].
  • QRS Complexes: Maximum 0.12sec (3mm) [3 small boxes] | Present after every P Wave.
  • Q-T Intervals†: Approx. 0.42sec (10.5mm) [10.5 small boxes].
  • T Waves: Amplitude less than 5mm (0.5mV) [5 small boxes] | Present after every QRS Complex, upright, & uniform.

* Changes in some features may not categorize a rhythm from NSR to something else and instead be mentioned after, such as: NSR with a First Degree Heart Block or NSR with a Left Bundle Branch Block. Specific cases are noted below.
Q-T Intervals are variable with HR, age, and sex.

Software generated Normal Sinus Rhythm EKG monitor capture with HR of 94.
A typical RSR EKG monitor capture.

Rate of a Normal Sinus Rhythm

The Heart Rate (HR) of Normal Sinus Rhythm is between 60 and 99 beats per minute  (bpm).

Software generated EKG tracing of a normal sinus rhythm at 60bpm with R-R Interval of 25mm / 1sec marked .
Slowest rate of Normal Sinus Rhythm, 60bpm with an R-R Interval of 1sec (25mm) [5 large boxes] marked on the green bar.

In the above example you can see the maximum R-R Interval that a Regular Sinus Rhythm can have, 25mm (1sec) [5 large boxes]. A longer R-R Interval would result in a bradycardic HR of less than 60bpm.

Software generated EKG tracing of a Normal Sinus Rhythm at 99bpm with R-R Interval of 15mm / 0.60sec marked.
Fastest rate of Normal Sinus Rhythm, 99bpm with an R-R Interval of 0.60sec (15mm) [3 large boxes] marked on the green bar.

Here you can see the minimum R-R Interval that a Regular Sinus Rhythm can have, 15mm (0.60sec) [3 large boxes]. A shorter R-R Interval would result in a tachycardic HR of 100bpm or more.

A Normal Sinus Rhythm MUST have an R-R Interval between the two above examples (three large boxes and five large boxes.)

⚕️
Clinical Pearl:
Don't let the word "normal" in NSR fool you into thinking that all rates of NSR are clinically normal in a patient. While a HR of 90 may be called "Normal" Sinus Rhythm, whether it is clinically relevant depends on the status of the patient. A HR of 90 could be irrelevant, a sign of anxiety or stress, or a sign of underlying pathophysiology. "Normal" in this case is just part of the name of this ECG rhythm.

Expected Rhythm of NSR

Normal Sinus Rhythm is a Regularly Regular rhythm. This means that it has a regular beat to it like a metronome. Every beat comes at a fixed time interval, with no pattern or variation.

Software generated EKG tracing of a Normal Sinus Rhythm at 84bpm with R-R Interval of 18mm / 0.72sec marked.
Normal Sinus Rhythm, 84bpm with an R-R Interval of 18mm (0.72sec) marked on green line.

The R-R Intervals of a Regular Sinus Rhythm should be equal to each other in any given tracing. In the above ECG the R-R Interval is 18mm and throughout the tracing remains the same. The below tracing has an R-R Interval of 21mm which is also equal throughout. The defining factor of a Regularly Regular rhythm is that the R-R Intervals are constant.

ECG tracing of a Normal Sinus Rhythm with 21mm equal R-R Intervals highlighted.
Normal Sinus Rhythm, 72bpm with an R-R Interval of 21mm (0.84sec) marked on green line.
⚕️
Clinical Pearl:
In practice, no rhythm is perfectly regular. When interpreting an EKG as Normal Sinus Rhythm it's appropriate to ignore minor irregularity of +/- less than 1mm.

P Waves of NSR

P Waves in Regular Sinus Rhythm will be the first feature of the repeating pattern of organized electrical activity. P Waves represent the movement of the atrial action potential from its origin (SA Node) to the Atrio-Ventricular Node and throughout the atria. They are upright, uniform, and always followed by a QRS Complex. In the following example P Waves are highlighted in grey. The P Waves have positive deflections above baseline (upright), are identical from one complex to the next (uniform), and all precede QRS Complexes.

P Waves in NSR should be less than 0.12sec (3mm) [3 small boxes] in duration and less than 2.5mm (0.25mV) [2.5 small boxes] in amplitude.

EKG monitor screen capture of Regular Sinus Rhythm with highlighted P waves and a HR of 65.
Regular Sinus Rhythm, 88bpm with P Waves marked in grey.

The three primary criteria for P Waves in Normal Sinus Rhythm (upright, uniform, preceding QRS complexes) are all expected for specific reasons:

Upright

Upright P Waves indicate that atrial depolarization is moving generally from the patients upper right (SA Node in the right atrium) to the lower left (Atrio-Ventricular Junction). The positive deflection (in Lead II) from baseline hints at the sinus origin of Regular Sinus Rhythm.

Uniform

Sinus P Waves should be uniform throughout a tracing. In the below monitor capture P Waves have been marked in grey, notice how the morphology of the P Wave does not change from one to the next. P Waves being uniform mean that all the electrical cycles are originating from the same place and following the same path.

Preceding QRS Complexes

Regular Sinus Rhythm should always have a QRS Complexe after every P Wave. If any P waves are present WITHOUT a following QRS complex the tracing is not Normal Sinus Rhythm. (Most likely this would be some type of Second Degree Atrio-Ventricular Heart Block.)

ECG monitor screen capture of Regular Sinus Rhythm with highlighted P waves and a HR of 65.
Regular Sinus Rhythm, 65bpm with P Waves marked in grey.

P-R Interval in Regular Sinus Rhythms

💬
EKG Vocabulary:
The term P-R Interval is a convention. Measuring the P-R Interval starts at the beginning of the P Wave and ends at the first deflection of the QRS Complex, not the R Wave. The morphology of the QRS Complex and the presence of Q Waves do not alter that measurement. The P-R Interval ends at the first deflection (onset of the QRS) after the P Wave.
ECG tracing of a Normal Sinus Rhythm with a HR of 66 and P-R Intervals marked.
Normal Sinus Rhythm at 66 bpm with P-R Intervals marked in green.

The P-R Interval in NSR should be greater than or equal to 0.12sec (3mm) [3 small boxes] and less than or equal to 0.20sec (5mm) [1 large box]. This duration represents both atrial depolarization (P Wave) and the AV Node pause (P-R Segment). Like the P Wave, the P-R Interval should be fixed (the same in all complexes of the rhythm.)

⚕️
Clinical Pearl:
Changes in the P-R Interval don't necessarily change the name of a tracing which otherwise meets NSR criteria. A P-R Interval shorter than described (less than 3mm) would be called Normal Sinus Rhythm with [ventricular] Pre-Excitation. While RSR with a longer than listed (greater than 5mm) P-R Interval is Normal Sinus Rhythm with a First Degree Heart Block.
ECG tracing of a normal sinus rhythm with a HR of 94 and P-R Intervals marked.
ECG Tracing of Normal Sinus Rhythm with P-R Intervals marked in green.

The QRS Complex in RSR

💬
EKG Vocabulary:
The name QRS Complex (like P-R Interval) is a convention. A QRS complex does not need to contain Q, R, & S waves. QRS Complex refers to whatever morphology ventricular depolarization creates on a tracing.

The key factor of the QRS Complex in Normal Sinus Rhythm is that they are present after every P Wave. The QRS Complex begins with the first deflection (positive or negative) after the P Wave and should be a maximum of 0.12sec (3mm) [3 small boxes].

ECG monitor screen capture of Regular Sinus Rhythm with highlighted QRS Complexes and a HR of 63.
NSR with a HR of 63bpm and QRS Complexes marked in grey.
⚕️
Clinical Pearl:
Like the P-R Interval, the duration of the QRS Complex exceeding the 3mm normal does not change a Normal Sinus Rhythm interpretation. Instead, depending on the specific morphology of the QRS additional information is included after. (I.E. Normal Sinus Rhythm with a Left / Right Bundle Branch Block.) Being less specific, such a rhythm (with a QRS complex wider than 3mm) could be called a Normal Sinus Rhythm with a Ventricular Conduction Delay.

Q-T Interval of Normal Sinus Rhythm

The Q-T Interval should be approx. 0.42sec (10.5mm) [10.5 small boxes] in NSR. The specific duration of any given Q-T Interval depends on the patients age, sex, and HR. Exact measurements should be made on a 12 Lead EKG not a rhythm strip or monitor screen.

EKG tracing of regular sinus rhythm with highlighted Q-T Intervals.
EKG monitor capture of RSR with Q-T Intervals marked in gray.
⚕️
Clinical Pearl:
Q-T Interval abnormalities are another feature that can be described after the title NSR rather than indicating a different rhythm. I.E. an extended Q-T Interval (calculated based on the sex, age, and HR of the patient and measured on a 12 Lead) on a tracing which otherwise meets NSR criteria would be described as Normal Sinus Rhythm with a Long Q-T.

NSR's T Waves

ECG tracing of a normal sinus rhythm with T Waves marked.
Normal Sinus Rhythm with a HR of 75bpm with T Waves marked in green.

The T Wave is the final feature of the repeating pattern in NSR. T Waves are expected to be less than 5mm (0.5mV) [5 small boxes], present after every QRS Complex, upright, and uniform. They represent the repolarization wave of the ventricles.

⚕️
Clinical Pearl:
As we've seen in several other features, abnormalities of the T Wave are mentioned after Normal Sinus Rhythm and don't change a tracings primary name. I.E. Normal Sinus Rhythm with Inverted T Waves.

Regular Sinus Rhythm's Repeating Pattern

After the T Wave there will be a section of baseline followed by the P Wave of the next complex. This repeating pattern of organized electrical activity (P-QRS-T) meeting the criteria discussed above is the hallmark of a Normal Sinus Rhythm.

Below are several more examples of Regular Sinus Rhythms presented as monitor captures as well as on EGK graphs.

Normal Sinus Rhythm EKG example, split printout / monitor display.
Normal Sinus Rhythm practice EKG tracing.
Regular Sinus Rhythm EKG example, split printout / monitor display.
Regular Sinus Rhythm practice EKG tracing.
NSR / RSR example EKG / ECG, split printout / monitor display.
NSR / RSR practice EKG / ECG tracing.

After becoming confident in identifying EKG's as Normal Sinus Rhythm or not, you can continue by learning about specific dysrhythmia such as: Sinus Tachycardia, Sinus Bradycardia, or Sinus Arrhythmia. Or head back to our EKG Rhythm Index to find information on another ECG. Otherwise practice interpreting novel EKG's with our EKG Generator:

Basic EKG App

Our Basic EKG Generator is free with an email signup and covers Normal Sinus Rhythm along with common arrhythmia.

Pro EKG App

Our Pro EKG Generator covers over 40 different rhythm categories, multiple display options, has Quiz and Simulation modes, and more! Try it out for just $5 for a month.