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Sinus Rhythm

Sinus rhythm, Normal sinus rhythm

Sinoatrial Node
Heart Rhythm
P Wave and Sinus Rhythm
ECG and Sinus Rhythm
Sinus Rhythm and EKG Curve
Frequency and Sinus Rhythm

Sinoatrial Node

Automaticity

Every part of the conduction system and the working myocardium
Has the ability to spontaneously generate impulses

Overdrive Suppression

Impulses spread through the conduction system from the focus

which generates impulses at the highest frequency

A focus with the highest frequency deactivates foci

with a lower frequency (overdrive suppression)

Sinoatrial Node (SA node)

Spontaneously generates impulses at a frequency of 60 - 100/min.
Other foci generate impulses with a lower frequency

Pacemaker sinoatrial node (SA node) and conduction system

Heart Rhythm

sinus rhythm, junctional rhythm, ventricular rhythm

Basic Heart Rhythms

Heart rhythm is always determined by the focus (foci) that generates impulses with the highest frequency (overdrive suppression)
There are 3 basic heart rhythms

Sinus Rhythm (Impulses are generated by the SA node)
Junctional Rhythm (Impulses are generated by the AV junction)
Ventricular Rhythm (Impulses are generated by Purkinje fibers in the ventricles)

Sinus Rhythm is the physiological heart rhythm

Because the sinoatrial (SA) node generates impulses with the highest frequency (overdrive suppression)
The SA node is referred to as the primary pacemaker (the tempo setter)

P Wave and Sinus Rhythm

P wave electrical vector limb leads aVR, II

P Wave and Limb Leads

In sinus rhythm, impulses originate in the sinoatrial (SA) node
Each impulse propagates from the SA node towards the atria toward the AV node

This creates an atrial vector, which appears on the ECG as a P wave

The P wave is the primary characteristic of sinus rhythm
Physiological P wave

It is negative in lead aVR (because the vector is directed away from lead aVR)
It is positive in leads I, II (because the vector is directed towards leads I, II)

ECG and Sinus Rhythm

P waves (are the primary characteristic of sinus rhythm)

Impulses from the SA node create the P wave. The P wave is:

positive in leads I, II
negative in lead aVR

Heart rate 60-100/min.

Children have physiologically sinus tachycardia (rate > 100/min.)

Heart action is regular (PP and RR intervals do not change)

Respiratory arrhythmia - is an irregular sinus rhythm due to breathing

Impulses then travel to the ventricles through the AV junction

This results in narrow QRS complexes (< 0.12s), if there is no aberrant conduction

Sinoatrial SA node and sinus rhythm

ECG normal sinus rhythm

Sinus Rhythm

P wave: Normal size and shape, appears before each QRS complex
Heart rate: 77/min.
Heart action is regular: PP interval is constant
PQ interval: constant (0.12s)
QRS complex is narrow (< 0.12s)

Sinus Rhythm and EKG Curve

The video shows the Lead II during sinus rhythm
Review the summary heart vector and how the EKG curve is generated
Slow down and pause the video in the top left corner
Red arrow indicates depolarization

of the atria - P wave
of the ventricles - QRS complex

Blue arrow indicates repolarization of the ventricles (T wave)

Repolarization of the atria (Ta wave) is not visible on the EKG

Frequency and Sinus Rhythm

SA node is partially controlled by the nervous system, which adjusts the frequency of the SA node in the range of 60-100/min

Sympathetic nervous system - increases the frequency of the SA node
Parasympathetic nervous system - decreases the frequency of the SA node

Physiological variants of sinus rhythm:

Sinus tachycardia: f > 100/min. (in adults, or above the upper limit of normal in children)
Sinus bradycardia: f < 60/min. (in adults, or below the lower limit of normal in children)
Sinus arrhythmia: irregular sinus rhythm, a normal variant in young, trained individuals

Physiological frequency of the SA node by age:

Newborns: 130/min.
2-year-old child: 120/min.
4-year-old child: 110/min.
6-year-old child: 100/min.
10-year-old child: 90/min.
14-year-old child: 80/min.
18-year-old adult: 70/min.

ECG sinus tachycardia, electrical alternans

Sinus Tachycardia

Frequency: 150/min.
P waves are less discernible, merging with T waves
QRS alternans: changes in the amplitude of QRS complexes
Heart rate is regular (PP interval remains constant)

ECG sinus bradycardia

Sinus Bradycardia

Frequency: 35/min.
P waves are present before each QRS complex
Heart rate is regular (PP interval remains constant)

ECG sinus respiratory arrhythmia

Sinus Respiratory Arrhythmia

Average frequency: 90/min.
Heart rate is irregular (PP interval changes)
PQ interval is constant
P waves are present and are consistent, not indicative of Atrial Fibrillation
Sinus arrhythmia is a variant of normal, especially in young athletes, with SA node frequency changing:

During inhalation (sympathetic dominance) - SA node frequency increases
During exhalation (parasympathetic dominance) - SA node frequency decreases

It is a sinus rhythm (with respiratory arrhythmia), as impulses originate in the SA node

ECG sinus rhythm, P wave morphology and axis (upright in I and II, inverted in aVR), Narrow QRS complexes, constant PR interval, regular rhythm

Sinus Rhythm

Frequency: 60/min.
Physiological P wave

Positive (I, II)
Negative (aVR)

Constant PQ interval (0.18s)
Narrow QRS complex (<0.12s)

SA node sinus rhythm

ECG sinus rhythm, narrow QRS, constant PQ PR interval, positive P wave II, negative P wave aVR

Sinus Rhythm

Frequency: 75/min.
Physiological P wave

Positive (I, II)
Negative (aVR)

Constant PQ interval (0.16s)
Narrow QRS complex (<0.12s)

Sinoatrial node sinus rhythm

ECG sinus rhythm, regular rhythm 75bpm, normal P waves, narrow QRS, constant PQ PR interval

Sinus Rhythm

Frequency: 75/min.
Physiological P wave

Positive (I, II)
Negative (aVR)

Constant PQ interval (0.12s)
Narrow QRS complex (<0.12s)

Sinoatrial node sinus rhythm

ECG respiratory sinus arrhytmia, irregular respiratory rhythm

Sinus Respiratory Arrhythmia

Average Frequency: 80/min.
Physiological P Wave

Positive (I, II)
Negative (aVR)

Heart Rhythm is Irregular (changes in PP interval and RR interval)

Irregular rhythm is best seen in the continuous lead (II)

It is a sinus rhythm (with respiratory arrhythmia)

because impulses originate in the SA node

Constant PQ Interval (0.16s)
Narrow QRS Complex (<0.12s)

sinoatrial node sinus rhythm

ECG atrial focal tachycardia

Focal Atrial Tachycardia

P wave has an abnormal shape

Negative (II, III, aVF)
Positive (V1)
Normal P wave is not negative (II, III, aVF) and positive (V1)

Frequency: 120/min.
This EKG does not show a sinus rhythm

because impulses do not originate in the SA node
it is Focal Atrial Tachycardia

focal atrial tachycardia, ectopic atrial focus

ECG irregular rhythm atrial fibrillation

Atrial Fibrillation

Frequency 80/min.
Heart rhythm is irregularly irregular (various RR intervals)
P waves are not visible, because impulses do not originate in the SA node
On the EKG, there is atrial fibrillation (the most common cardiac arrhythmia)

In the atria, there are multiple ectopic foci (micro re-entry)
Each focus generates its own vector and activates the atria
The atria have a frequency of 350-600/min., hence P waves are not visible
QRS complexes are irregularly irregular (various RR intervals)

Because impulses from the atria (350-600/min.)

are irregularly filtered through the AV node

atrial fibrillation, micro reentry

Sources

ECG from Basics to Essentials Step by Step
litfl.com
ecgwaves.com
metealpaslan.com
medmastery.com
uptodate.com
ecgpedia.org
wikipedia.org
Strong Medicine
Understanding Pacemakers