This chapter is related to the aims of Section F11(i) from the 2017 CICM Primary Syllabus, which expects the exam-going trainees to be able to "describe the pharmacology of anti-asthma drugs".  Unfortunately, though the examiners seem to love asthma, their love is expressed in the form of vague and haphazard gestures. Often, the questions will refer loosely to "drug groups commonly used to treat acute severe asthma", without specific boundaries, as if to test the candidates' capacity for self-control. Come on, we dare you: describe all the mechanisms.  This sort of stem is highly unhelpful, as asthma is a complex disease and drugs which are used to treat it come from a whole variety of classes. Taken completely literally, this syllabus item would also cover steroids, magnesium sulphate, ketamine, helium-oxygen mixtures and the halogenated anaesthetic ethers. 

Of course, a normal person would conclude that this would be insane, and that surely the college examiners expect a narrow focus; i.e. the question really asks "discuss the pharmacodynamics of bronchodilators". But in fact that is not the case. From the fragmented debris of past papers, one is able to reconstruct the incomplete skeletons of marking criteria, and this reveals that the examiners expected everything:

  • β-agonists and their secondary messenger pathways
  • Antimuscarinic agents
  • Corticosteroids
  • Methylxanthines
  • Magnesium sulphate
  • Ketamine
  • Volatile anaesthetics
  • Helium-oxygen mixtures
  • Phosphodiesterase inhibitors

Desirable elements in the answers included:

  • "A discussion of efficacy versus toxicity"
  • "more comprehensive answers"  including esoterica such as Heliox
  • A tabulated or "structured" approach was apparently favoured, at minimum featuring "details about drug class, mechanism of action and example(s)"

We also know they did not want:

  • long term inhaled steroids
  • leukotriene antagonists
  • anything relevant to the chronic management of asthma

Anyway. Given the vagueness and breadth of the SAQ, it is remarkable that in most cases the trainees do very well in answering it. Historically, the pass rates are in the order of 70%. Past paper questions which involved this subject were:

  • Question 4 from the second paper of 2016 ("categorise the drugs")
  • Question 11 from the second paper of 2014 ("mechanisms of action")
  • Question 9 from the first paper of 2010 ("mechanisms of action")
  • Question 11(p.2) from the second paper of 2007 ("mechanism of action")

In the interest of preserving some attachment to reality (i.e. what can be reasonably expected from a ten minute answer) the table offered here produces a minimalist entry for each class of drug, as well as a couple of examples, rather than a full-scale pharmacological breakdown (absorption, distribution., elimination, etcetera). Where possible, a faint trail of  references is left for the trainee to follow, in case they need more details from peer-reviewed publications. 

Drugs used in the Treatment of Asthma
Mechanism of action Examples
β-agonists (Waldeck, 2002)
  • Bind to G-protein coupled receptors
  • Increase the cAMP concentration in bronchial smooth muscle cells
  • cAMP activates Protein Kinase A
  • Active PKA inactivate myosin light-chain kinase and activates myosin light-chain phosphatase, leading to smooth muscle relaxation
  • High potency and efficiacy, but also high toxicity
  • Salbutamol
  • Adrenaline
  • R-enantiomer is usually the more effective one
Antimuscarinic agents (Soler & Ramsdell, 2014)
  • Muscarinic acetylcholine receptors are G-protein coupled receptors
  • Activation of muscarinic (M3) receptors results in a rise in cyclic GMP, increasing the availability of intracellular calcium
  • This leads to clinical effects (for M3 receptors in the lung, bronchoconstriction and increased bronchial secretion)
  • Antimuscarinic drugs act as competitive antagonists of the acetylcholine receptor, and prevent these clinical effects
  • High potency and efficacy, low toxicity
  • Ipratropium bromide
  • Tiotropium
  • Atropine
Corticosteroids (PJ Barnes, 1996)
  • Corticosteroids bind to cytoplasmic glucocorticoid receptors
  • These receptors, when activated, become dimers and are transported to the nucleus, where they regulate gene transcription
  • This downregulates the syhtesis of proinflammatory cytokines and enzymes involved in the synthesis of inflammatory mediators such as cyclooxygenase and phospholipase
  • High potency and efficacy, high long term toxicity
  • Hydrocortisone
  • Prednisolone
  • Methylprednisolone
  • Budesonide
  • Ciclesonide
Methylxanthines (Tilley, 2011)
  • Methylxanithines are nonselective adenosine receptor antagonists, but their main mechanism of action in asthma is by their nonselective inhibition of phosphodiesterase
  • By inhibiting phosphodiesterase, these drugs increase the intracellular concentration of cyclic AMP in airway smooth muscle cells
  • cAMP activates Protein Kinase A
  • Active PKA inactivate myosin light-chain kinase and activates myosin light-chain phosphatase, leading to smooth muscle relaxation
  • Low potency and efficiacy, high toxicity
  • Theophylline
  • Aminophylline
Magnesium sulphate (Noppen, 1990; Irazuzta et al, 2017)
  • Antagonists of calcium at the NMDA receptor-gated calcium channels, which produces smooth muscle relaxation
  • Also inhibits acetylcholine and histamine release
  • Low potency, low efficacy, low toxicity
  • Magnesium sulphate
Ketamine (Goyal & Agrawal, 2013; Sato et al, 1998)
  • NMDA receptor antagonist; blockade of these receptors reduces availability of intracellular calcium
  • Howeverm, ketamine seems to produce bronchodilation by a mechanism which is independent of the NMDA receptor
  • Instead it appears to interfere with a calcium-dependent step in histamine-induced bronchoconstriction
  • Low potency and efficiacy, potentially high toxicity
  • Ketamine
Volatile anaesthetics (Mondoñedo et al, 2015; Yamakage, 2002)
  • Decrease intracellular calcium concentration by an unknown mechanism, probably by inhibition of IP3- induced calcium release
  • Thought to be also due to decreased calcium sensitivity and inhibition of Protein Kinase C activity
  • High potency, low toxicity
  • Isoflurane
  • Sevoflurane
  • Enflurane
Helium-oxygen mixtures
  • Decrease the density of inspired gases
  • This decreases the Reynolds number, i.e. decreases the likelihood of turbulent flow through narrow airways
  • As laminar flow  is usually associated with lower resistance than turbulent flow at any given flow rate, the use of helium decreases the respiratory resistance in bronchospasm
  • This improves gas exchange and the distal delivery of nebulised medications
  • Low potency, nil toxicity
  • Helium

References

Zdanowicz, Martin M. "Pharmacotherapy of asthma." American journal of pharmaceutical education 71.5 (2007).

Waldeck, Bertil. "β-Adrenoceptor agonists and asthma—100 years of development." European journal of pharmacology 445.1-2 (2002): 1-12.

Soler, Xavier, and Joe Ramsdell. "Anticholinergics/antimuscarinic drugs in asthma." Current allergy and asthma reports 14.12 (2014): 484.

Barnes, Peter J. "Molecular mechanisms of steroid action in asthma." Journal of allergy and clinical immunology 97.1 (1996): 159-168.

Tilley, Stephen L. "Methylxanthines in asthma." Methylxanthines. Springer, Berlin, Heidelberg, 2011. 439-456.

Noppen, Marc, et al. "Bronchodilating effect of intravenous magnesium sulfate in acute severe bronchial asthma." Chest 97.2 (1990): 373-376.

Irazuzta, Jose Enrique, and Nicolas Chiriboga. "Magnesium sulfate infusion for acute asthma in the emergency department." Jornal de pediatria 93 (2017): 19-25.

Goyal, Shweta, and Amit Agrawal. "Ketamine in status asthmaticus: a review." Indian journal of critical care medicine: peer-reviewed, official publication of Indian Society of Critical Care Medicine 17.3 (2013): 154.

Sato, Tetsumi, et al. "The role of the N-methyl-D-aspartic acid receptor in the relaxant effect of ketamine on tracheal smooth muscle." Anesthesia & Analgesia 87.6 (1998): 1383-1388.

Khan, Khurram Saleem, Ivan Hayes, and Donal J. Buggy. "Pharmacology of anaesthetic agents II: inhalation anaesthetic agents." Continuing Education in Anaesthesia, Critical Care & Pain 14.3 (2014): 106-111.

Mondoñedo, Jarred R., et al. "Volatile anesthetics and the treatment of severe bronchospasm: a concept of targeted delivery." Drug Discovery Today: Disease Models 15 (2015): 43-50.

Yamakage, M. "Editorial II: Effects of anaesthetic agents on airway smooth muscles." (2002): 624-627.