4.1 Normal sinus rhythm, sinus bradycardia & sinus tachycardia

The Sinus Node: Origin of Normal Rhythm

The sinoatrial (SA) node, located in the upper right atrium near the junction of the superior vena cava, is the heart's dominant pacemaker. It normally discharges 60-100 times per minute, and every impulse it generates spreads through both atria to produce an upright P wave in lead II before conducting to the ventricles. On the CRAT exam, you are expected to recognize that all three rhythms in this section originate in the SA node and travel the identical conduction pathway; they differ only in rate. Mastering the criteria for normal sinus rhythm gives you the template against which every other rhythm is measured.

Normal Sinus Rhythm (NSR)

Normal sinus rhythm is the baseline "healthy" tracing and the standard by which arrhythmias are judged. Memorize its five defining criteria:

• Rate: 60-100 beats per minute
• Regularity: Regular - R-R intervals are constant
• P waves: One upright, uniform P wave precedes every QRS complex in lead II
• PR interval: 0.12-0.20 seconds and constant
• QRS duration: Less than 0.12 seconds (narrow)

Because the impulse begins in the SA node and follows the normal conduction system, atrial and ventricular depolarization occur in their proper sequence. Each P wave has a matching QRS, the P-to-P and R-to-R intervals are equal, and the complexes are uniform across the strip. If any single criterion is violated - an absent P wave, an irregular rhythm, a prolonged PR, or a wide QRS - the rhythm is, by definition, no longer normal sinus rhythm. NSR itself requires no treatment; it is the goal state.

Sinus Bradycardia

Sinus bradycardia meets every criterion of NSR except one: the rate falls below 60 beats per minute. P waves remain upright and uniform, each is followed by a QRS, the PR interval stays 0.12-0.20 seconds, and the rhythm is regular. Only the rate distinguishes it from NSR.

Common causes include:

• Increased vagal (parasympathetic) tone - vomiting, Valsalva, carotid sinus pressure, or sleep
• Well-conditioned athletes, whose efficient hearts maintain output at low rates
• Medications - beta-blockers, calcium channel blockers, digoxin
• Inferior wall myocardial infarction, hypothyroidism, hypothermia, and increased intracranial pressure

Sinus bradycardia is frequently benign, particularly in athletes and during sleep, where it may represent normal physiology. It becomes clinically significant when the slow rate reduces cardiac output enough to cause symptoms: dizziness, syncope, hypotension, confusion, chest pain, or shortness of breath. Symptomatic bradycardia is treated per ACLS guidelines with atropine, and if refractory, with transcutaneous pacing or a dopamine/epinephrine infusion. The technician's role is to recognize the rate, confirm the P-QRS relationship, and note whether the patient is symptomatic.

Sinus Tachycardia

Sinus tachycardia is the mirror image of bradycardia: every NSR criterion is met except the rate, which exceeds 100 beats per minute, typically ranging up to about 150-160. P waves stay upright and uniform and precede each QRS, though at very fast rates a P wave may hide within the preceding T wave, making it harder to identify. The rhythm remains regular and the QRS narrow.

Sinus tachycardia is almost always a response to an underlying demand or stressor rather than a primary electrical problem. Typical causes include:

Because the fast rate is compensatory, treatment targets the cause - fluids for hypovolemia, antipyretics for fever, analgesia for pain, oxygen for hypoxia - rather than the rhythm itself. Slowing the heart without addressing the trigger can be harmful. Distinguishing sinus tachycardia from other narrow-complex tachycardias (such as SVT or atrial flutter) rests on identifying normal, upright sinus P waves before each QRS and a rate that rarely exceeds about 160; when P waves disappear and the rate climbs above 150-180, suspect a re-entrant tachycardia instead.

Calculating the Rate at the Bedside

Accurately classifying these three rhythms depends first on measuring the rate correctly, a core CRAT skill. Two methods are common. The 6-second method works for any rhythm: count the number of R waves within a 6-second strip (ECG paper is marked at 3-second intervals) and multiply by 10. The large-box (300) method works only when the rhythm is regular: count the number of large boxes between two consecutive R waves and divide 300 by that number - for example, four large boxes yields a rate of 75 bpm, while six large boxes yields 50 bpm. Because sinus bradycardia and sinus tachycardia are both regular, either method applies; for a subtly irregular tracing, favor the 6-second method. Always confirm your rate against the P-QRS relationship: if the atrial (P-P) and ventricular (R-R) rates match and each P wave is followed by a QRS, the impulse is sinus in origin, and the rate alone tells you whether you are looking at bradycardia, normal sinus rhythm, or tachycardia.

Putting the Three Together

The unifying concept is simple and heavily tested: NSR, sinus bradycardia, and sinus tachycardia are the same rhythm at three different rates. All three show upright P waves, a 1:1 P-to-QRS ratio, a normal constant PR interval, and a narrow QRS. Count the rate, verify the sinus P wave before every QRS, and you can classify all three instantly.