Sinus Rhythm ECG Interpretation

• Chapter 7 focuses on rhythms originating in the sinus node.
• Upon reading this chapter, one should be able to state the criteria, adverse effects, and possible treatments for each of the sinus rhythms.
• One should be able to correctly interpret a variety of sinus rhythms on single- and double-lead strips using the five steps.

Introduction

• Sinus rhythms start in the sinus node, travel through the atria to depolarize them, and then proceed down the normal conduction pathway to depolarize the ventricles.
• The sinus node is the normal pacemaker of the heart.
• When the sinus node fires its impulse, it travels throughout the atria, causing atrial depolarization and writing the P wave on the EKG.
• The impulse then heads down through the AV node to the ventricle. Ventricular depolarization results in the writing of the QRS.

The Word on Sinus Rhythms

• The sinus node is the primary pacemaker of the conduction system.
• The sinus node relinquishes its throne in only two ways:
  • Illness or death of the sinus node, requiring a lower pacemaker to step in for it (escape).
  • Being overthrown by a lower pacemaker (usurpation/irritability).
• Sinus rhythms are mostly regular, like the ticking of a clock.
• The inherent rate of the sinus node is 60 to 100, but it can go higher or lower if the sinus node is acted on by the sympathetic or parasympathetic nervous system.
• Tolerance of these rhythms depends largely on the heart rate.
• Fast or slow heart rates can cause decreased cardiac output, leading to symptoms like pallor, dizziness, chest pain, shortness of breath, confusion, decreased urine output, hypotension, and diaphoresis.
• Treatment is only needed if symptoms arise, with the goal of returning the heart rate to normal.
• Sinus rhythms are the standard for comparison with all other rhythms.
• Thorough comprehension is needed because most rhythms will be sinus rhythms.
• To be considered sinus in origin the following criteria must be met:
  • Upright matching P waves in Lead II followed by a QRS.
  • Consistent PR intervals.
  • Heart rate less than or equal to 160 at rest.
• All matching upright P waves in Lead II are considered sinus P waves until proven otherwise.
• The width and deflection of the QRS complex is irrelevant in determining whether a rhythm originates in the sinus node.
• The QRS is normally narrow (<0.12 seconds) in sinus rhythms, the QRS can be wide (≥0.12 secs) if conduction through the bundle branches is altered.
• The deflection of the QRS depends on the lead in which the patient is being monitored.
  • The QRS in Lead II should be upright but in V₁ should be inverted.

Sinus Rhythm

• Sinus rhythm is the normal rhythm.
• The impulse originates in the sinus node and travels down the conduction pathway to the ventricle.
• Each P wave is followed by a QRS complex, and the heart rate is normal (60 to 100).
• The QRS complex can be positive, negative, or isoelectric depending on the lead being monitored.

Key Characteristics

• Rate: 60-100
• Regularity: Regular
• P waves: Upright in most leads, may be inverted in V₁, one P to each QRS, all P waves have the same shape, P-P interval is regular
• Intervals: PR is 0.12 to 0.20 secs, constant from beat to beat, QRS is <0.12 secs
• Cause: Normal
• Adverse effects: None (unless the heart rate is a drastic change from previously-always look at the trend)
• Treatment: None

Sinus Bradycardia

• Sinus bradycardia is a slower-than-normal rhythm from the sinus node.
• The impulse starts in the sinus node and travels the conduction system normally.
• The QRS complex can be positive, negative, or isoelectric depending on the lead being monitored.

Key Characteristics

• Rate: Less than 60
• Regularity: Regular
• P waves: Upright in most leads, may be inverted in V₁, one P to each QRS, all P waves have the same shape, P-P interval regular
• Intervals: PR is 0.12 to 0.20 secs, constant from beat to beat, QRS is <0.12 secs
• Cause: Vagal stimulation, MI, hypoxia, digitalis toxicity and other medication side effects
• Adverse effects: Too slow a heart rate can cause decreased cardiac output.
• Treatment: None unless the patient is symptomatic.
  • Atropine can be used to speed up the heart rate by increasing impulse propagation and conduction through the AV node.
  • If unsuccessful, use an electronic pacemaker or medications (epinephrine, dopamine).
  • Consider starting oxygen because conduction system cells become ischemic (oxygen starved) if the heart doesn't receive adequate oxygen, these cells may respond with firing rates above or below their norm, supplemental oxygen can help these stricken cells to return to normal functioning

Sinus Tachycardia

• Sinus tachycardia is a rhythm in which the sinus node fires at a faster rate than normal.
• The impulse originates in the sinus node and travels down the conduction pathway normally.
• The QRS complex can be positive, negative, or isoelectric depending on the lead being monitored.

Key Characteristics

• Rate: 101 to 160
• Regularity: Regular
• P waves: Upright in most leads, may be inverted in V₁, one P to each QRS, P waves shaped the same, P-P interval regular
• Intervals: PR is 0.12 to 0.20 secs, constant from beat to beat, QRS is <0.12 secs
• Cause: Medications, emotional upset, pulmonary embolus, MI, CHF, fever, inhibition of the vagus nerve, hypoxia, and thyrotoxicosis
• Adverse effects: Increased heart rate which causes an increased cardiac workload, especially true in the patient with an acute Ml
• Treatment: Treat the cause, if the patient has a fever, give medications to decrease the fever, if the tachycardia is caused by anxiety, consider sedation, for cardiac patients with persistent sinus tachycardia, a class of medications called beta-blockers may be used to slow the heart rate, consider starting oxygen to decrease the heart's workload.
  • Every one degree increase in body temperature causes the heart rate to rise by about 10 beats per minute.

Sinus Arrhythmia

• Sinus arrhythmia is the only irregular rhythm from the sinus node, and it has a pattern that is cyclic and usually corresponds with the breathing pattern.
• The QRS complex can be positive, negative, or isoelectric depending on the lead being monitored.

Key Characteristics

• Rate: Varies with respiratory pattern—faster with inspiration, slower with expiration.
• Regularity: Irregular in a repetitive pattern; longest R-R cycle exceeds the shortest by ≥0.16 secs
• P waves: Upright in most leads, may be inverted in V₁, one P to each QRS, P waves shaped the same, P-P interval is irregular
• Intervals: PR is 0.12 to 0.20 secs, constant from beat to beat, QRS is <0.12 secs
• Cause: Usually caused by the breathing pattern, but can be caused by heart disease
• Adverse effects: Usually no ill effects
• Treatment: Usually none required

Sinus Arrest

• Sinus arrest is a pause that occurs when the regularly firing sinus node suddenly stops firing for a brief period.
• One or more P-QRS-T sequences will be missing.
• An escape beat from a lower pacemaker may then take over for one or more beats.
• The sinus node may resume functioning after missing one or more beats, or the lower pacemaker may continue as the pacemaker, creating a new escape rhythm.
• The pause is not a multiple of the previous R-R intervals.
• The escape beat or rhythm resumes whenever it can.
• The QRS complex can be positive, negative, or isoelectric depending on the lead being monitored.

Key Characteristics

• Rate: Can occur at any heart rate
• Regularity: Regular but (interrupted) by a pause, always measure the length of the pause in seconds
• P waves: Normal sinus P waves before the pause, normal or different-shaped Ps on the beat ending the pause, P-P interval is usually regular before the pause and may vary after the pause, depending on whether the sinus node regains pacemaking control
• Intervals: PR 0.12 to 0.20 secs before the pause, may be shorter or absent after the pause, QRS on the sinus beats will be <0.12 secs, on the escape beat(s), the QRS may be narrow or wide depending on which pacemaker of the heart resumes following the pause
• Cause: Sinus node ischemia, hypoxia, digitalis toxicity, excessive vagal tone, other medication side effects
• Adverse effects: Frequent or very long sinus arrests can cause decreased cardiac output
• Treatment: Occasional sinus arrests may not cause a problem, frequent sinus arrests may require that the medication causing it be stopped and can require atropine and/or a pacemaker to speed up the heart rate, consider starting oxygen

Sinus Block (Also Called Sinus Exit Block)

• Sinus block is a pause that occurs when the sinus node fires its impulse on time, but the impulse's exit from the sinus node to the atrial tissue is blocked, in other words, the beat that the sinus node propagated is not conducted anywhere.
• Results in one or more P-QRS-T sequences being missing, creating a pause, the length of which will depend on how many sinus beats are blocked.
• When conduction of the regularly firing sinus impulses resumes, the sinus beats return on time at the end of the pause.
• The pause will be a multiple of the previous R-R intervals, exactly 2 or more R-R cycles will fit into the pause.
• The QRS complex can be positive, negative, or isoelectric depending on the lead being monitored.

Key Characteristics

• Rate: Can occur at any heart rate
• Regularity: Regular but interrupted (by a pause)
• P waves: Normal sinus Ps both before and after the pause; P waves shaped the same
• Intervals: PR is 0.12 to 0.20 secs, QRS is <0.12 secs.
• Cause: Medication side effects, hypoxia, or strong vagal stimulation
• Adverse effects: Same as sinus arrest
• Treatment: Same as sinus arrest

Test your knowledge of sinus rhythm interpretation on ECGs. Questions cover P wave and QRS complex relationships and normal heart rate ranges. This quiz helps to reinforce understanding of basic ECG analysis and rhythm identification.