Updated: 2/22/2020

Bradycardia

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Snapshot
  • A 22-year-old man presents to his primary care physician for an annual examination. He currently has no acute complaints but is worried he has "heart disease" since his father died from a "heart attack." He denies any episodes of lightheadedness or syncopal episodes. He exercises regularly as he is part of his university's basketball team. An electrocardiogram demonstrates sinus rhythm with a PR interval of 250 ms. He is provided reassurance that nothing needs to be done. (First-degree atrioventricular block)
Introduction
  • Overview
    • electrical activation of the heart usually is initiated by the sinatrial (SA) node
    • bradycardia results from impaired impulse initiation from the SA node or impaired impulse conduction from the SA node to the atria or ventricles
      • leads to a pulse < 60/min
    • common causes of bradycardia include
      • SA node dysfunction
      • atrioventricular (AV) node conduction block
  • Etiology
    • SA and AV node dysfunction
      • sick sinus syndrome (seen in SA node dysfunction)
      • carotid sinus hypersensitivity
      • inferior wall myocardial infarction (seen in SA node dysfunction and sometimes AV node dysfunction)
        • electrocardiogram demonstrate ST-segment elevation in II, III, and aVF
      • senile amyloidosis
      • medications (e.g., beta-blockers)
      • increased intracranial pressure
      • malignancy
      • radiation
      • infectious
Presentation
  • Symptoms
    • lightheadedness
    • fatigue
    • generalized weakness
    • presyncope or syncope
    • angina
  • Physical exam
    • hypotension
    • bradycardia
Studies
  • Electrocardiogram
    • diagnostic study of choice
      • sinus bradycardia 
        • P wave is upright in leads I, II, and aVL, and a negative P wave in lead aVR
      • first-degree AV block 
        • PR interval is > 200 ms
      • second-degree AV block
        • Mobitz type I 
          • progressive PR interval lengthening with a subsequent "dropped" QRS complex
        • Mobitz type II  
          • fixed prolonged PR interval (> 200 ms) with a "dropped" QRS complex
      • third-degree AV block (complete heart block) 
        • no supraventricular pulses are conducted to the ventricles due to an absence of AV node condution
          • the atria and the ventricles independently contract
Treatment
  • Overview
    • management of bradycardia depends on the patient's hemodynamic stability and type of bradycardia
      • hemodynamically unstable patients are managed with intravenous atropine or temporary transvenous pacing
        • once hemodynamic stability is achieved, patients are monitored with transcutaneous pacing pads
          • patients are also evaluated for underlying and reversible causes of their bradycardia
    • management of bradycardia depends on the underlying cause
      • e.g., bradycardia secondary to hypothyroidism is managed by treating the hypothyroidism
  • Conservative
    • observation
      • indication
        • asymptomatic first degree AV block
        • asymptomatic Mobitz type I
  • Medical
    • atropine
      • indication
        • hemodynamically unstable bradycardia
      • comments
        • 0.5 mg is given every 3-5 minutes if needed
        • maximum dose is 3 mg
  • Procedural
    • tranvenous pacing
      • indication
        • hemodynamically unstably bradycardia in patients unresponsive to intravenous atropine 
    • permanent pacemaker
      • indication
        • hemodynamically stable patients with Mobitz type II and third degree AV heart block with no identifiable reversible cause
Complications
  • Syncope
  • Cardiac death
 

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Questions (1)
Lab Values
Blood, Plasma, Serum Reference Range
ALT 8-20 U/L
Amylase, serum 25-125 U/L
AST 8-20 U/L
Bilirubin, serum (adult) Total // Direct 0.1-1.0 mg/dL // 0.0-0.3 mg/dL
Calcium, serum (Ca2+) 8.4-10.2 mg/dL
Cholesterol, serum Rec: < 200 mg/dL
Cortisol, serum 0800 h: 5-23 μg/dL //1600 h:
3-15 μg/dL
2000 h: ≤ 50% of 0800 h
Creatine kinase, serum Male: 25-90 U/L
Female: 10-70 U/L
Creatinine, serum 0.6-1.2 mg/dL
Electrolytes, serum  
Sodium (Na+) 136-145 mEq/L
Chloride (Cl-) 95-105 mEq/L
Potassium (K+) 3.5-5.0 mEq/L
Bicarbonate (HCO3-) 22-28 mEq/L
Magnesium (Mg2+) 1.5-2.0 mEq/L
Estriol, total, serum (in pregnancy)  
24-28 wks // 32-36 wks 30-170 ng/mL // 60-280 ng/mL
28-32 wk // 36-40 wks 40-220 ng/mL // 80-350 ng/mL
Ferritin, serum Male: 15-200 ng/mL
Female: 12-150 ng/mL
Follicle-stimulating hormone, serum/plasma Male: 4-25 mIU/mL
Female: premenopause: 4-30 mIU/mL
midcycle peak: 10-90 mIU/mL
postmenopause: 40-250
pH 7.35-7.45
PCO2 33-45 mmHg
PO2 75-105 mmHg
Glucose, serum Fasting: 70-110 mg/dL
2-h postprandial:<120 mg/dL
Growth hormone - arginine stimulation Fasting: <5 ng/mL
Provocative stimuli: > 7ng/mL
Immunoglobulins, serum  
IgA 76-390 mg/dL
IgE 0-380 IU/mL
IgG 650-1500 mg/dL
IgM 40-345 mg/dL
Iron 50-170 μg/dL
Lactate dehydrogenase, serum 45-90 U/L
Luteinizing hormone, serum/plasma Male: 6-23 mIU/mL
Female: follicular phase: 5-30 mIU/mL
midcycle: 75-150 mIU/mL
postmenopause 30-200 mIU/mL
Osmolality, serum 275-295 mOsmol/kd H2O
Parathyroid hormone, serume, N-terminal 230-630 pg/mL
Phosphatase (alkaline), serum (p-NPP at 30° C) 20-70 U/L
Phosphorus (inorganic), serum 3.0-4.5 mg/dL
Prolactin, serum (hPRL) < 20 ng/mL
Proteins, serum  
Total (recumbent) 6.0-7.8 g/dL
Albumin 3.5-5.5 g/dL
Globulin 2.3-3.5 g/dL
Thyroid-stimulating hormone, serum or plasma .5-5.0 μU/mL
Thyroidal iodine (123I) uptake 8%-30% of administered dose/24h
Thyroxine (T4), serum 5-12 μg/dL
Triglycerides, serum 35-160 mg/dL
Triiodothyronine (T3), serum (RIA) 115-190 ng/dL
Triiodothyronine (T3) resin uptake 25%-35%
Urea nitrogen, serum 7-18 mg/dL
Uric acid, serum 3.0-8.2 mg/dL
Hematologic Reference Range
Bleeding time 2-7 minutes
Erythrocyte count Male: 4.3-5.9 million/mm3
Female: 3.5-5.5 million mm3
Erythrocyte sedimentation rate (Westergren) Male: 0-15 mm/h
Female: 0-20 mm/h
Hematocrit Male: 41%-53%
Female: 36%-46%
Hemoglobin A1c ≤ 6 %
Hemoglobin, blood Male: 13.5-17.5 g/dL
Female: 12.0-16.0 g/dL
Hemoglobin, plasma 1-4 mg/dL
Leukocyte count and differential  
Leukocyte count 4,500-11,000/mm3
Segmented neutrophils 54%-62%
Bands 3%-5%
Eosinophils 1%-3%
Basophils 0%-0.75%
Lymphocytes 25%-33%
Monocytes 3%-7%
Mean corpuscular hemoglobin 25.4-34.6 pg/cell
Mean corpuscular hemoglobin concentration 31%-36% Hb/cell
Mean corpuscular volume 80-100 μm3
Partial thromboplastin time (activated) 25-40 seconds
Platelet count 150,000-400,000/mm3
Prothrombin time 11-15 seconds
Reticulocyte count 0.5%-1.5% of red cells
Thrombin time < 2 seconds deviation from control
Volume  
Plasma Male: 25-43 mL/kg
Female: 28-45 mL/kg
Red cell Male: 20-36 mL/kg
Female: 19-31 mL/kg
Cerebrospinal Fluid Reference Range
Cell count 0-5/mm3
Chloride 118-132 mEq/L
Gamma globulin 3%-12% total proteins
Glucose 40-70 mg/dL
Pressure 70-180 mm H2O
Proteins, total < 40 mg/dL
Sweat Reference Range
Chloride 0-35 mmol/L
Urine  
Calcium 100-300 mg/24 h
Chloride Varies with intake
Creatinine clearance Male: 97-137 mL/min
Female: 88-128 mL/min
Estriol, total (in pregnancy)  
30 wks 6-18 mg/24 h
35 wks 9-28 mg/24 h
40 wks 13-42 mg/24 h
17-Hydroxycorticosteroids Male: 3.0-10.0 mg/24 h
Female: 2.0-8.0 mg/24 h
17-Ketosteroids, total Male: 8-20 mg/24 h
Female: 6-15 mg/24 h
Osmolality 50-1400 mOsmol/kg H2O
Oxalate 8-40 μg/mL
Potassium Varies with diet
Proteins, total < 150 mg/24 h
Sodium Varies with diet
Uric acid Varies with diet
Body Mass Index (BMI) Adult: 19-25 kg/m2
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