Atrioventricular (AV) Heart Block - Cardiovascular

Snapshot

A 20-year-old man presents to his primary care physician for an annual exam. He is currently feeling well. Medical history is unremarkable and he takes a daily multivitamin. He denies any alcohol or smoking history. He is a college student and part of the school's track team, which he continued since he was a student in high school. He is concerned because a classmate of his passed out during practice and ended up dying due to sudden cardiac death. He is requesting an electrocardiogram (ECG) to ensure his heart is healthy. An ECG demonstrates a PR interval of 0.25 sec and is otherwise unremarkable. (First-degree heart block)

Introduction

Clinical definition
- an impairment of the electrical transmission from the atria to the ventricles
Etiology
- idiopathic (most common)
  - e.g., fibrosis and sclerosis of the conduction system
- ischemic heart disease (2nd most common)
- ↑ vagal tone
- familial causes
- iatrogenic
  - medications
    - e.g., β-blockers, calcium channel blockers, adenosine, digoxin, and amiodarone
  - cardiac procedures
    - e.g., cardiac surgery and transcatheter aortic valve implantation

Atrioventricular (AV) Block

AV Blocks
Types	Electrocardiogram Findings	Treatment	Comments
First degree AV block	↑ PR interval (> 0.2 secs)	No treatment is required	Typically seen in young patients with a ↑ vagal tone Well-trained athletes
Second degree AV block (Mobitz type I)	Progressive ↑ PR interval until a P wave is not followed by a QRS complex (Wenckebach phenomenon) the sequence then repeats QRS complex is narrow	Treatment is unnecessary unless the patient is symptomatic use atropine or a temporary pacemaker in symptomatic cases	Patients are typically asymptomatic Can be seen in patients with drug intoxication (e.g., β-blockers and digitalis) ↑ vagal tone
Second degree AV block (Mobitz type II)	Fixed PR intervals with occasional dropped QRS complexes QRS complex is typically wide	Permanent pacemaker	May progress to a third-degree heart block
Third degree (complete) AV block	Atria and ventricles depolarize independently P waves and QRS complexes are not rhythmically synchronized	Permanent pacemaker unless the cause is reversible (such as medications)	Can be a complication of Lyme disease

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

(M1.CV.8) A 29-year-old computer programmer presents to the general medicine clinic complaining of dizziness, fatigue, and shortness of breath. One month ago, the patient went on a camping trip in northern Connecticut. On assessment, vital signs are temperature 36.9°C, blood pressure 100/65, heart rate 41, and respiratory rate 16 with an oxygen saturation of 99%. A print of the EKG strip is shown below (Figure A). What is the most likely diagnosis? Review Topic

QID: 100524

FIGURES:

Wolff-Parkinson-White syndrome

(12/154)

Second degree Mobitz type I AV block

12%

(19/154)

Second degree Mobitz type II AV block

14%

(21/154)

First degree AV block

10%