Sinus Rhythms

All originate in the sinoatrial (SA) node.
ECG features common to all sinus rhythms:
- Upright P waves, all similar in appearance.
- Normal duration PR intervals ( $0.12$ to $0.20$ seconds).
- Normal duration QRS complexes ( $0.06$ – $0.12$ sec) if no ventricular conduction disturbances are present.

The SA node discharges and follows the normal conduction pathway
P Waves
- Uniform in appearance, positive (upright) in lead II, one and only one precedes each QRS complex
PR Interval
- $0.12$ to $0.20$ seconds and constant from cardiac cycle to cardiac cycle
QRS complex
- $0.06$ – $0.12$ sec
Rhythm
- Regular
Rate
- 60 to 100 beats/min

The SA node discharges slower than in NSR but follows the normal conduction pathway.

P Waves
- Uniform in appearance, positive (upright) in lead II, one and only one precedes each QRS complex.
PR Interval
- $0.12$ to $0.20$ seconds and constant from cardiac cycle to cardiac cycle
QRS Complex
- $0.06$ – $0.12$ sec
Rhythm
- Regular
Rate
- Less than 60 beats/min

Sinus bradycardia can be normal in athletes and during sleep, or in response to a vagal maneuver.
- Determine if the patient is symptomatic or not.

Nursing Interventions
- Monitor vital signs (including SpO2).
- Administer oxygen if hypoxic.
- Trendelenburg position (¯ HOB) for hypotension.
- Assess for angina.
- Assess for shock (cool clammy skin, ALOC - altered level of consciousness).
- Establish IV access.
- Identify and treat possible causes (hold negative chronotropic medications).
Pharmacologic Interventions
- If symptomatic (chest pain, shortness of breath, altered level of consciousness):
  - Atropine (initial dose is $1$ mg IVP q3 – 5 minutes for a max of $3$ mg); Isuprel
  - Stop offending drugs
Electrical Interventions
- Artificial pacing if patient is hemodynamically compromised
  - Transcutaneous / temporary pacing

SA node discharges more frequently than in NSR but follows the normal conduction pathway.

P Waves
- Uniform in appearance, positive (upright) in lead II, one and only one precedes each QRS complex; at very fast rates
PR Interval
- $0.12$ to $0.20$ second and constant from cardiac cycle to cardiac cycle
QRS Complex
- $0.06$ – $0.12$ sec
Rhythm
- Regular
Rate
- $101$ to $150$ beats/min

The clinical significance of this dysrhythmia depends on the underlying cause
Caused by vagal inhibition or sympathetic stimulation
Increased Circulating Catecholamines
Associated with physiologic (increased temperature) and psychologic stressors (anxiety)
Stimulants, exercise, fever, hypoxemia/hypoxia, & alterations in fluid status (hypovolemia)
Drugs can increase rate
Assess for symptoms of low cardiac output (cardiac failure/HF)
PE/hyperthyroidism
Response to pain

P Waves
- Uniform in appearance, positive (upright) in lead II, one and only one precedes each QRS complex
PR Interval
- $0.12$ to $.20$ second and constant from cardiac cycle to cardiac cycle
QRS:
- Narrow (<0.12 sec); sometimes wide
Rhythm
- Irregular
Rate
- 60 to 100 beats/min; frequently increases with inspiration & decreases with expiration; sometimes slower

SA node fails to discharges and then resumes when the SA node resets itself or when a lower pacemaker begins to discharge.
The pause (arrest) time interval is not a multiple of the normal R-R interval

P Waves
- normal in appearance, positive (upright & uniform), except in areas of pause (arrest)
PR Interval
- $0.12$ to $0.20$ second
QRS Complex
- $0.06$ – $0.12$ sec
Rhythm
- Irregular whenever a pause (arrest) occurs
Rate
- Normal to slow; determined by duration & frequency of sinus pause (arrest)

SA block occurs in some multiple of the R-R interval; after the dropped beat, cycles continue on time

P Waves
- normal in appearance, positive (upright & uniform), except in areas of dropped beats
PR Interval
- $0.12$ to $0.20$ second
QRS Complex
- $0.06$ – $0.12$ sec
Rhythm
- Irregular whenever an SA block occurs
Rate
- Normal to slow; determined by duration & frequency of SA block