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).
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.
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.
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.
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
- 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.
Rate of a Normal Sinus Rhythm
The Heart Rate (HR) of Normal Sinus Rhythm is between 60 and 99 beats per minute (bpm).
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.
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.)
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.
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.
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.
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.)
P-R Interval in Regular Sinus Rhythms
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.
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.)
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.
The QRS Complex in RSR
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].
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.
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
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.
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.
After becoming confident in identifying EKGs 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 EKGs with our EKG Generator:
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