Core pharmacology

Class Group

MEDICAL GAS

Legacy Cicm Level

Level 1

Introduction

Oxygen is a colourless, odourless, tasteless gas present in the atmosphere at a concentration of approximately 21%.

Indications Uses

1. Treatment and prevention of hypoxia
2. Treatment of carbon monoxide poisoning
3. Pneumotosis coli
4. Anaerobic infection
5. Cluster headaches
6. Resoprtion of pneumothoraxes

Chemical Pharmaceutics

Medical oxygen is produced via fractional distillation of atmospheric air or using an oxygen concentrator, which absorbs nitrogen.
• Critical pressure of 50bar
• Critical temperature of -119’C
• Boiling point -182’C
• MW of 32

Presentation

It is presented as a liquid or compressed gas (Clear, colourless, odourless)
• Stored in black cylinders with white shoulders at 137bar
• Variable size cylinders (from B which is 170L to G which is 8000L)
• Available as wall oxygen in hospitals

Main Action

participate in oxidative phosphorylation, which produces the ATP required for cellular function

Mechanism of action

It moves down the oxygen cascade from a partial pressure of 159mmHg in atmospheric gas to approximately 105mmHg in the alveoli (dependent on PACO2 as per the alveolar gas equation), then to the mitochondria, where the PO2 may be as low as 2-3mmHg. Note the Pasteur point (1-2mmHg) is the minimum mitochondrial PO2 required for oxidative phosphorylation to proceed.

Physiological effects

CVS
- If used for hypoxaemia, all cardiovascular parameters are expected to improve
- However, administration of 100% for prolonged periods of time will decrease HR (effects on chemoreceptors), decrease diastolic blood pressure, slightly reduce cardiac output and cause coronary vasoconstriction
- It causes a decrease in pulmonary vascular resistance
Resp
- 100% oxygen will lead to a mild respiratory depression
- Nitrogen is eliminated from the lungs within 2-3min, leading to atelectasis
CNS
- 100% O2 leads to cerebrovascular vasoconstriction and a decrease in CNS blood flow

Adverse Effects Toxicity

CNS –
visual changes and seizures may occur at 3 atmospheres
Ocular –
retrolental fibroplasia has been seen in premature babies treated with oxygen, possibly due to vasoconstriction of developing retinal vessels

CVS –
improvement in haemodynamics if oxygen is being used to correct hypoxaemia. Prolonged administration of 100% FiO2 may cause a reduction in heart rate and cardiac output, and coronary artery vasoconstriction.

Respiratory –
Potential decrease in respiratory drive (significant in patients who are CO2 retainers and rely on their hypoxic drive to breathe). Absorption atelectasis. Tracheobronchitis. ALI/ARDS due to the production of oxygen free radicals (superoxide, hydroxide, hydrogen peroxide), which cause parenchymal damage and diffuse lung injury.

Absorption

- Freely absorbed across the normal alveolar membrane
- Fick’s Law of diffusion

Distribution

Diffuses across the alveolar membrane as per Fick’s Law to enter the blood. Main mode of transportation is bound to haemoglobin, with a small fraction dissolved in blood. Under normal circumstances there is approximately 1.75L of oxygen stored within the body. This may be increased with preoxygenation in anaesthesia or with hyperbaric oxygenation.

Metabolism

Oxygen is consumed in oxidative phosphorylation, producing ATP, CO2 and H2O.

Excretion

Exhaled as CO2 and water

Special Points

Measurement:
1. Gas
- Mass spectrometer
- Paramagnetic analyzer
- Fuel cell
2. Blood
- Clarke electrode
- Pulse oximetry

Route And Dose

Oxygen can be delivered via variable intake devices (nasal prongs, Hudson masks), or in fixed amounts via venturi masks or endotracheal tubes.