A sinus rhythm occurs when the SA node acts as the primary pacemaker of the heart.

Normal Sinus Rhythm

• Rate: 60-100 bpm
• Rhythm: Regular
• P Waves: Identical in shape and before each QRS complex
• PR Intervals: Normal and equal
• R-R Intervals: Equal
• QRS Complexes: Normal and after each P wave

Sinus Arrhythmia

• Rate: 60-100 bpm
• Rhythm: Irregular
• P Waves: Identical in shape and before each QRS complex
• PR Intervals: Normal and equal
• R-R Intervals: Unequal
• QRS Complexes: Normal and after each P wave

Sinus Bradycardia

• Rate: < 60 bpm
• Rhythm: Regular
• P Waves: Identical in shape and before each QRS complex
• PR Intervals: Normal and equal
• R-R Intervals: Equal
• QRS Complexes: Normal and after each P wave

Sinus Tachycardia

• Rate: 100-150 bpm
• Rhythm: Regular
• P Waves: Identical in shape and before each QRS complex
• PR Intervals: Normal and equal
• R-R Intervals: Equal
• QRS Complexes: Normal and after each P wave

Sinus Arrest

• Rate: 60-100 bpm
• Rhythm: Regular, except during a pause
• P Waves: Identical in shape and before each QRS complex, but are absent during a pause
• PR Intervals: Normal and equal, but are not measurable during a pause
• R-R Intervals: Equal, except during a pause
• QRS Complexes: Normal and after each P wave, but are absent during a pause