Overview
- Deceleration of normal sinus rhythm <60 BPM in adults (or below normal age-adjusted range in children)
- U waves can be seen (often precordial leads)
Rhythm Characteristics
- Regularity: Regular
- Rate: <60 BPM (adults; age-dependent in children)
- P wave: present with constant morphology
- Axis (frontal plane): 0° to +75° (leftward and inferior)
- Configuration: upright in I, II, V4-V6; inverted in aVR; upright, inverted, or biphasic (positive-negative) in V1-V2; variable in III and aVL (related to respiratory cycles)
- P wave:QRS ratio = 1:1
- PR interval: constant, normal (120-200 ms), unless underlying AV conduction defect or aberrancy
- QRS interval: normal (70-110 ms), unless underlying intraventricular conduction defect or aberrancy
- Grouping: none, but may be present
- Dropped beats: none, but may be present
Normal Age-adjusted Heart Rates
- Newborn: 90-155 BPM
- 1-20 months: 100-170 BPM
- 1-3 years: 90-150 BPM
- 3-5 years: 70-140 BPM
- 5-12 years: 60-130 BPM
- 12-16 years: 60-120 BPM
- 16+ years (adults): 60-100 BPM
Causes
- Cardiac: inferior myocardial infarction, myocarditis, sinus node disease
- Other: sleep (often normal), vagal stimulation (eg, pain), increased vagal tone (eg, athletes) hypothyroidism, hypothermia, anorexia nervosa, brain stem herniation (Cushing reflex)
- Electrolyte abnormalities: hyperkalemia, hypermagnesemia
- Medications: beta-blockers (eg, metoprolol), calcium-channel blockers (eg, verapamil, diltiazem), digoxin, opiates, central alpha-2 agonists (eg, dexmedetomidine, clonidine), GABAergic agents (eg, benzodiazepines, barbiturates, baclofen, gamma hydroxybutyrate)
ECG Example
Patient: 26-year-old healthy female athlete
Interpretation: Sinus bradycardia, low anterior forces, otherwise normal
– Ventricular rate: 55 BPM
– PR interval: 142 ms
– QRS duration: 90 ms
– QT/QTc interval: 450/430 ms
– P-R-T axes: 26 -18 41
Key Points from Example ECG
- Regular rhythm at 55 BPM
- Normal P wave morphology and axis (upright in I and II, inverted in aVR)
- Narrow QRS complexes (<110 ms wide)
- Each P wave is followed by a QRS complex (ie, P wave:QRS ratio = 1:1)
- Constant PR interval
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