The Sinus Family: Rhythm, Brady, Tachy, Arrhythmia
Flashcards covering the five criteria for normal sinus rhythm and how to distinguish sinus bradycardia, tachycardia, and arrhythmia.
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Questions Covered in This Set
12 cards to master
What single lead-II finding suggests the impulse started in the sinus node?
An upright P wave in lead II (with an inverted P in aVR), because sinus activation travels down and leftward toward lead II's positive electrode.
List the five criteria for normal sinus rhythm (NSR).
1) Rate 60–100 bpm; 2) regular R–R intervals; 3) a P wave before every QRS and QRS after every P; 4) P waves upright in II, III, aVF and inverted in aVR, all identical; 5) PR interval 120–200 ms and constant.
Does calling a rhythm 'normal sinus rhythm' mean the EKG is normal?
No — NSR says nothing about QRS width, ST segments, or axis. You can have a large anterior STEMI in perfect sinus rhythm; rhythm and morphology are separate reads.
Define sinus bradycardia and give the large-box shortcut.
All NSR criteria met but rate < 60 bpm; using 300-150-100-75-60-50, more than five large boxes between R waves means bradycardia.
Name physiologic/benign causes of sinus bradycardia.
Athletic conditioning (runners often in the 40s), sleep, and high vagal tone (vomiting, straining, carotid pressure).
Which drugs commonly cause sinus bradycardia?
Beta-blockers, non-dihydropyridine calcium channel blockers (diltiazem, verapamil), digoxin, amiodarone, and clonidine.
Which pathologic conditions cause sinus bradycardia?
Inferior MI (RCA feeds the SA node), raised intracranial pressure (Cushing reflex), hypothyroidism, hypothermia, hyperkalemia, and sick sinus syndrome.
How do you manage symptomatic bradycardia?
Treat the patient, not the number: if dizzy, hypotensive, confused, or having chest pain, give atropine 1 mg IV, then transcutaneous pacing / dopamine / epinephrine infusion if that fails.
How do you distinguish sinus tachycardia from AVNRT/SVT?
Sinus tach has gradual onset and warm-down with slight beat-to-beat variation; AVNRT/SVT starts and stops abruptly and is metronomically regular at 150–250 bpm.
Why is a regular narrow tachycardia at ~150 bpm suspicious?
Atrial flutter with 2:1 AV block parks near 150 bpm — always hunt for flutter waves.
Why should you not simply beta-block sinus tachycardia?
Sinus tach is almost always a symptom (pain, fever, hypovolemia/hemorrhage, anemia, hypoxia, PE, sepsis, hyperthyroidism, withdrawal, stimulants) — blocking it removes a compensatory mechanism. Fix the cause.
Define sinus arrhythmia.
Sinus rhythm with identical upright P waves and constant PR, but cyclic R–R variation greater than ~0.12 s (3 small boxes); in the respiratory form the rate speeds with inspiration and slows with expiration.
