Question 14

Describe the mechanism of action, and adverse effects, of pulmonary vasodilators that are administered via the inhalational route.

[Click here to toggle visibility of the answers]

College Answer

Many candidates neglected to include oxygen which is also a drug with significant
pulmonary vasodilating properties. Accurate detail concerning the receptor and
second messenger effects of drugs was expected. The importance of V/Q matching
and reduction in systemic effects via inhalational administration needed to be stated.
Better answers included discussion of serious adverse effects such as methaemoglobinaemia,
acute lung injury, systemic hypotension, rebound phenomena
and heart failure.
Syllabus: B2a, 2b,c,d,e
Recommended sources: Basic and Clinical Pharmacology, Katzung, Chp 18, 19

Discussion

In summary:

  • Inhaled nitrix oxide and inhaled epoprostenol are the most widely available inhaled pulmonary vasodilators
  • Oxygen can be viewed as a vasodilator, as it prevents or reverses hypoxic pulmonary vasoconstriction, i.e it does not have a very potent vasodilator function for patients whose oxygenation is normal

Rationale for use

  • In heterogenous lung disease, inhaled pulmonary vasodilators will be deposited in lung regions in proportion to their regional ventilation
  • Thus, they will improve flow (Q) into regions of the lung which have good ventilation (V), thereby improving the V/Q ratio 
  • This may result in an improvement of oxygenation
  • The decrease in pulmonary vascular resistance will also decrease right ventricular afterload and improve right heart function

The properties of these drugs in detail:

Inhaled Pulmonary Vasodilators
Name Nitric oxide Epoprostenol
Class Inhaled pulmonary vasodilator Inhaled pulmonary vasodilator
Chemistry A free radical with the formula NO Synthetic analogue of the naturally occurring eicosanoid prostacyclin (prostaglandin I2 or PGI2)
Routes of administration Administered as part of inspired gas mixture, usually as an admixture fraction measured in tens of ppm, via a proprietary system (INOMax) Can be intravenous, but usually nebulised as a part of a solution with a glycine buffer, using a continuous ultrasonic nebuliser
Absorption Absorbs rapidly into the pulmonary circulation via the lungs Absorbs rapidly into the pulmonary circulation via the lungs
Solubility As it dissociates in water, nitric oxide produces nitric acid (HNO3) which has a pKa of -1.3 Natural pKa is 4.4; requires a diluent which contains glycine and sodium hydroxide. The pH of the reconstituted drug mixture has a pH of around 12, because the drug tends to spontaneously hydrolyse in aqueous solution at a normal pH
Distribution VOD is impossible to measure, but is potentially very large. NO reacts with oxygen and water to produce nitrogen dioxide and nitrites, which then bind to haemoglobin and produce either nitrosylhaemoglobin or methaemoglobin, i.e. it can be described as "highly protein bound". 0.357L/kg
Target receptor Soluble guanylyl cyclase (which is induced by NO) Activates G protein-coupled PGE receptors on platelets and endothelial cells, which activates adenlyl cyclase and increases cAMP
Mechanism of action Inhibits vasoconstriction by increasing the amount of cyclic GMP (cGMP) in the cytosol, thus decreasing the amount of cytosolic calcium ions available to sustain smooth muscle contraction Increased cyclic AMP leads to decreased platelet activation and activates PKA, which phosphorylates and inhibits myosin light-chain kinase which leads to smooth muscle relaxation and vasodilation
Metabolism One way or another, nitric oxide ends up as methaemoglobin and nitrate. Either it reacts with lung water, becoming nitrite (which reacts with oxyhemoglobin and generates methaemoglobin and nitrate) or it combines directly with oxyhaemoglobin, with the same results. If it encounters hypoxic blood, it can combine with deoxyhaemoglobin to create nitrosyl-haemoglobin, which then rapidly becomes methaemoglobin when it contacts oxygen. Degrades spontaneously as well as enzymatically into about sixteen major and minor metabolites
Elimination Nitrates are eliminated mainly in urine whereas methaemoglobin is metabolised in several hours into
haemoglobin by endogenic reductases. The nitrates excreted in urine represent over 70% of the inhaled NO
dose.
Half-life is about six minutes
Time course of action Onset of effect is seen within seconds Platelet inhibition effects last up to 2 hrs; smooth muscle vasodilation is very shortlived (comparable with half-life)
Clinical effects Apart from pulmonary vasodilation, there is methemoglobinaemia, hypotension (maybe some of it does leak into the systemic circulation, or maybe this the effect of depressed LV function, rebound hypoxia after abrupt withdrawal, thrombocytopenia (in as many as 10% of patients) and increased susceptibility to pulmonary infections probably due to NO2 formation and associated lung injury. Vasodilation (pulmonary as well as systemic); inhibition of platelet aggregation; facial flushing, tachycardia, bronchodilation, inhibition of gastric acid secretion, and decreased gastric
emptying
Single best reference for further information TGA (AusPAR) product information Flolan PI by GlaxoSmithCline

References

Roberts, David H., et al. "Oxygen therapy improves cardiac index and pulmonary vascular resistance in patients with pulmonary hypertension." Chest 120.5 (2001): 1547-1555.

Siobal, Mark S. "Pulmonary vasodilators." Respiratory care 52.7 (2007): 885-899