Question 6

College Answer

Oxygen can be regarded as a ‘drug’ and the best answer will describe oxygen with such a
perspective in mind. A good answer would require good understanding and integration of
knowledge from different parts of the syllabus.
Most candidates mentioned the oxygen is an odourless and colourless gas and its effects on
pulmonary pressure and atelectasis. Common omissions included the clinical uses oxygen
other than reversing hypoxia, pharmaceutic properties of oxygen, pharmacodynamic response
of different body systems (CVS, CNS, RESP) to hyperoxia, and pharmacokinetics of oxygen
including distribution & transfer of oxygen between body systems and the metabolism of
oxygen.
Syllabus: B1f, B1h, B1i, B2a, C1f, O1.
Reference: Nunn’s applied respiratory physiology.

Discussion

Physico-chemical properties of oxygen

• Oxygen (O₂)is a diatomic gas with a density and viscosity slightly higher than that of air. 
• Conventional "wall" oxygen is delivered at approximately 4 atmospheres of pressure (415 kPa) and is close to 0°C at the wall outlet.

Administration:

• As an inhaled agent via a selection of fixed or variable performance delivery devices
• As intravenous or intra-arterial infusion of well-oxygenated blood (i.e. ECMO)
• Externally (as in hyperbaric oxygen therapy)

Absorption:

• Pulmonary absorption (250ml/min with 21% FiO₂, at rest)
• Cutaneous absorption (under 1ml/min at normal atmospheric pressure)
• Oral (and other) mucosal absorption (usually, nil)

Metabolism

• Metabolised in all tissues (mainly brain, heart and skeletal muscle)
• Mainly metabolised by cytochrome c mitochondrial enzymes (90%)
• Zero-order clearance kinetics, roughly 200ml/minute
• Main metabolites are CO₂ and H₂O, cleared via the lung and renally. 

Indications for use

• Supplementation (in hypoxemia)
• Prophylaxis (in pre-oxygenation for anaesthesia)
• As an antidote (carbon monoxide toxicity)
• Therapeutic uses:
  • As an antibiotic (hyperbaric oxygen for deep anaerobic infections)
  • To decrease the volume of air-filled body cavities by denitrogenation (eg. pneumothorax and pneumoencephalus)
  • In management of decompression sickness

Contraindications

• Bleomycin use (leads to pulmonary fibrosis)
• Paraquat toxicity (worsens ARDS)
• Aspiration of acid (worsens ARDS)

Complications

• Drying of mucous membranes and inspissation of secretions
• Inflammatory tracheobronchitis
• Decreased central respiratory drive (minimally)
• Hypecapnoea in "CO₂ retainers" mainly by virtue of V/Q mismatch and Haldane effect
• Absorption atelectasis
• Increased left-to-right shunting in ASDs
• Increased peripheral vascular resistance
• Cerebral and coronary vasoconstriction
• Euphoria
• Retrolental fibroplasia of the newborn
• Decreased erythropoiesis
• Immunosuppression

Toxicity

• Visual changes and seizures (hyperbaric) 
• Toxicity from free radicals (worsening ARDS)

Caveats

• Few uses are evidence-based
• Evidence does not support the use of supraphysological oxygen in most cases, i.e. hyperoxia is discouraged.

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