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Pharm 101: Theophylline

Class

Methylxanthine

Pharmacodynamics
  • Theophylline is a methylxanthine. It’s exact mechanism of action is not established but there are several propositions:
    • Inhibition of PDE enzyme family, thereby increasing intracellular cAMP and cGMP. This causes stimulation of cardiac function and relaxation of smooth muscle
    • Inhibition of cell surface receptors for adenosine. Adenosine causes airway smooth muscle contraction and release of histamine from airway mast cells
  • Multiple organ system effects, including;
  • Central nervous system:
    • Mild cortical arousal with increased alertness and deferral of fatigue
    • Tremor
    • Anxiety
  • Cardiovascular:
    • Positive chronotropic and inotropic effects
    • Decreases blood viscosity
  • Gastrointestinal:
    • Secretion of gastric acid and digestive enzymes
  • Renal:
    • Weak diuretic effect
    • Increased glomerular filtration rate, reduced tubular Na absorption
  • Smooth muscle:
    • Bronchodilation
    • Tolerance does not develop
  • Skeletal muscle:
    • Improved contractility
Pharmacokinetics
  • Bioavailability 96%
  • Volume of distribution 0.5L/kg
  • Narrow therapeutic window. Therapeutic and toxic effects are related to its blood level. Improvement in pulmonary function is seen at blood levels of 5-20mg/L
Clinical uses
  • Previously used in the treatment of asthma, the toxicity and need for blood concentration monitoring has made theophylline almost obsolete in asthma treatment
    • Acute asthma: bronchodilator effect relieves airflow obstruction
    • Chronic asthma: reduces severity of symptoms
Adverse effects
  • Adverse effects include sinus tachycardia, tremor and vomiting
  • Toxic effects are related to its blood level:
    • > 20mg/L: vomiting, abdominal discomfort, headache
    • > 40mg/L: seizures or arrhythmias
    • Hypotension, hypokalaemia and hyperglycaemia can occur due to beta-2 adrenergic activation (cause unknown)
  • Theophylline is an adenosine receptor blocker and reduces the effectiveness of adenosine used in the treatment of SVT
Further Reading

Pharmacology 101

Top 200 drugs

Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Co-creator of the LITFL ECG Library. Twitter: @rob_buttner

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