• Sinoatrial Node
• Heart Rhythm
• P Wave and Sinus Rhythm
• ECG and Sinus Rhythm
• 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

P Wave and Sinus Rhythm

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)

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 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)

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

Sinus Rhythm

• Frequency: 75/min.
• Physiological P wave
• Positive (I, II)
• Negative (aVR)
• Constant PQ interval (0.16s)
• Narrow QRS complex (<0.12s)

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)

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