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CDHB

Oxygen Therapy

Oxygen is a drug and must be prescribed on the drug administration chart indicating the flow rate, the device to be used, and the target oxygen saturations.

Aim - to prevent tissue hypoxia and thereby reduce morbidity and mortality. There is virtually no evidence based data on the therapeutic use of oxygen in most acute clinical situations.

Note: Some individuals with chronic respiratory disease may tolerate moderate hypoxaemia for significant periods of time without acute symptoms. If the patient is well and asymptomatic, acute oxygen therapy may not be required (see section on Long Term Oxygen Therapy, Domiciliary Oxygen).

Background - Oxygen Therapy

Tissue oxygenation depends on two factors:

• Tissue perfusion - affected by cardiac output and peripheral vascular resistance.
• Arterial oxygen content - this is determined by the haemoglobin content and haemoglobin oxygen saturation.

  The latter is the only factor affected by oxygen administration.

Indications for Oxygen Therapy

• PaO₂ less than 60 mm Hg or sat.O₂ <90%.
• Conditions such as myocardial infarction, CO poisoning, acute/severe anaemia where marginal increases in arterial oxygen content may be beneficial.
• At risk of hypoxia such as post-op, LVF etc.

Pulse Oximetry

• This is very useful for determining haemoglobin oxygen saturation (sat.O₂) i.e., oxygenation. However, it does not assess haemoglobin level, ventilation (CO₂) problems, cardiac output or tissue perfusion. It's useful for monitoring but is not a substitute for arterial blood gases.

  Oxygen therapy is indicated primarily to relieve hypoxia not dyspnoea.

Administration of Oxygen

• Do not withhold oxygen in severely hypoxaemic patients merely to get a "baseline blood gas estimation."
• Monitor oxygen administration carefully according to the clinical circumstances.
• Nasal cannulae: 0.5-4 L/min, provide an inspired oxygen concentration of 24% to 40% depending on the flow. Remember that this is uncontrolled oxygen therapy and it is not possible to accurately predict the inspired oxygen concentration (FiO₂). Most patients can be treated with oxygen using nasal cannulae. This mode is most comfortable for the patient and in the absence of profound gas exchange problems, will provide more than adequate oxygen saturation levels. They allow oral intake, communication and the easy use of nebulizers. They do not cause the sense of suffocation some patients have with a face mask. For a flow rate of 0.5 L/min you will need a low flow oxygen meter.
• Standard mask (Hudson)
  This is also uncontrolled oxygen therapy. 6-10 L/min, provides about 50% oxygen depending on the patient's ventilation levels. The initial method of choice in acutely hypoxic patients i.e., acute asthma, pneumonia, LVF and pulmonary embolism. Don't use these at flow rates less than 6 L/min as CO₂ retention can occur through rebreathing. A reservoir bag can further increase the percentage oxygen.
• Variable concentration mask (24-50%) (Venturi mask)
  • Use initially in COPD patients during the acute phase.
  • Use 24% initially when there is a possibility of CO₂ retention (check previous case notes).
• High flow humidified oxygen (e.g., via a Fisher and Paykel)
  Used for longer term therapy where drying of the bronchial secretions needs to be avoided. It is only indicated in special circumstances but can provide more accurate inspired oxygen concentrations than other methods. Contact ICU or Respiratory Ward 25.

Adjusting the Oxygen Dose

• Does the ABG show evidence of chronic CO₂ retention, i.e., a compensated respiratory acidosis (elevated HCO₃ level), together with chronic hypoxaemia? If so, take care to avoid making CO₂ retention worse.
• Using a pulse oximeter as a monitor, adjust the flow rates:
  • For nasal cannulae in 0.5 - 1 L/min steps.
  • For variable concentration masks in percentage increments.
  • For standard masks in 2 L/min steps.
• Get the haemoglobin oxygen saturation to about 90%, wait about five minutes at each step for those with COPD.
• Once stable, if there is any risk of CO₂ retention, check the blood gases about 30 minutes later.

Note: The predicted oxygen percentages supplied by masks and nasal cannulae are not precise - they will depend on the patient's respiratory minute volume, i.e., the degree of "dilution" by room air.

Monitoring Oxygen Therapy

• Pulse oximetry provides an estimate of capillary haemoglobin oxygen saturation. It does not assess the adequacy of ventilation nor the gas exchange status.
• ABG analysis must be performed on admission and in many cases at regular intervals to assess response to treatment.
• Hyperoxia can, in some cases, induce hypercapnia by a combination of worsening ventilation perfusion mismatch and to a lesser extent depression of respiratory drive. It is unpredictable and emphasizes the importance of ABG monitoring. If the patient is at risk, monitor ABG every 30 minutes until stable. Sometimes, following the initiation of oxygen therapy, the PaCO₂ may rise by 10-15% then stabilize. This may be the cost of adequate oxygenation and is acceptable as long as there are no adverse clinical events.

Note: There is limited availability of transcutaneous CO₂ monitoring on the Respiratory Ward/Sleep Service.

Topic Code: 1636