Myocardial Action Potential - Cardiovascular

Snapshot

A 60-year-old man presents to his cardiologist for a follow-up of newly diagnosed diastolic heart failure. He has a history of asthma and chronic obstructive lung disease. He reports that his pulmonologist does not want him to take β-blockers. However, his cardiologist wants to start him on a medication to slow the heart rate. He reassures the patient that this drug acts not on β-receptors but on a Ca²⁺ channel. (Verapamil)

Introduction

Ions responsible for the action potential of atria, ventricles, and Purkinje fibers are the same
- upstroke
  - inward Na⁺ current
- plateau
  - slow inward Ca²⁺ current via L-type voltage-gated Ca²⁺ channels
  - a sustained period of depolarization
    - allows for ventricular filling

Ventricular Action Potential

Resting membrane potential
- -85 mV resting membrane potential is maintained by inward rectifier K⁺ channels
  - inward rectifier K⁺ channels open at rest and close with depolarization
Phase 0, upstroke
- rapid depolarization
  - caused by the opening of voltage-gated Na⁺ channels and inward Na⁺ current
    - Na⁺ current depolarizes cell membrane, which closes inward rectifier K⁺ channels
Phase 1, initial repolarization
- brief repolarization and net outward current
  - inactivation gates on Na⁺ channels close
  - Na⁺ current is decreased
  - outward K⁺ current down an electrochemical gradient
Phase 2, plateau
- stable, long period of depolarized membrane potential
- inward and outward current are equal, with no net current flow
  - inward Ca²⁺ current (slow inward current)
    - opened L-type Ca²⁺ channels
      - blocked by Ca²⁺ channel blockers
    - entry of Ca²⁺ also triggers the release of more Ca²⁺ from sarcoplasmic reticulum in the cell (Ca²⁺-induced Ca²⁺ release)
  - outward K⁺ current
    - driven by electrochemical driving force
Phase 3, repolarization
- net outward current
  - L-type voltage-gated Ca²⁺ channels close
    - a decrease in inward Ca²⁺ current
  - delayed rectifier K⁺ channels open completely and repolarize the membrane
    - increase in outward K⁺current
- inward rectifier K⁺ channels re-open
Phase 4, resting membrane potential
- inward and outward currents are equal
- delayed rectifier K⁺ channels close
- inward rectifier K⁺ channels are fully open, resetting resting membrane potential at -85 mV

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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(M1.CV.119) A 48-year-old female comes into the ER with chest pain. An electrocardiogram (EKG) shows a heart beat of this individual in Image A. The QR segment best correlates with what part of the action potential of the ventricular myocyte shown in Image B? Review Topic

QID: 100635

FIGURES:

Phase 0, which is primarily characterized by sodium influx

69%

(9/13)

Phase 0, which is primarily characterized by potassium efflux

(0/13)

Phase 1, which is primarily characterized by potassium and chloride efflux

(0/13)

Phase 1, which is primarily characterized by calcium efflux

(1/13)

Phase 3, which is primarily characterized by potassium efflux

15%

(2/13)

Select Answer to see Preferred Response

PREFERRED RESPONSE 1