OXYGEN THERAPY

General principles

The most uncomplicated mode of O2 administration should be used

The lowest FiO2 (typically 40-60%) that can maintain SaO2above an adequate 90% should be administered

ABGs should be monitored after oxygen is given and a PaO2of at least 8kPa (60 mmHg) is ideal as it achieves SaO2 of 90%

NICE guidelines: pre-hospital COPD patients with exacerbations should start at 40% O2titrated up if SaO2<90% and titrated down if patient is drowsy, SaO2>93%, or type 2 RF had occurred before

Known type 2 RF patients should start at a low 24% oxygen just in case hypoxia stimulates their breathing and to avoid worsening their respiratory acidosis (hypercapnia)

High concentrations of oxygen can be toxic since it can exacerbate hypercapnia and damage alveolar membranes, but never dismiss the use of oxygen for the fear of its toxicity, as the result of hypoxic damage is likely to be more severe

After oxygen therapy, if patient still has:

  • PaO2< 8kPa
  • PaCO2rises >1.5 kPa of their original hypercapnic value

Or they become fatigued; then consider assisted ventilation or a

respiratory stimulant (e.g. doxapram) if hypercapnic.

Methods of oxygen delivery

For a patient breathing spontaneously (i.e. not intubated), there are two simple ways to deliver oxygen:

1)Nasal cannula

2)Face masks

Nasal cannula:

Flexible plastic tube that fits in the patient’s nostrils

Constant flow-rate of O2 delivered into the nostrils and hence FiO2varies with ventilation

Lower flow rates ( 4 L/min) prevent the drying of the nasal mucous membranes, but higher rates require humidification

Can eat and drink food without removal, and O2 is still inhaled with mouth breathing

Can typically provide FiO2 of 24-40% at 1-6L/min

Types of face masks

Low-flow (simple) mask:

Fits over nose and mouth

Oxygen is delivered at a certain flow-rate into the mask

Side-ports draw in room air that supplements the O2 in the mask, and removes exhaled CO2

The O2flow rate should be at least 5L/min to prevent re-breathing of exhaled CO2that can get trapped in the mask

The overall FiO2concentration varies depending on the patient’s ventilation rate that in turn determines the supplemented air flow rate

Precise FiO2can therefore not be achieved

Disadvantages of face masks include difficulty eating and speaking, and generally uncomfortable

Can typically provide FiO2 of 35-50% at 5-10 L/min

High-flow Venturi mask:

This mask is comprised of a normal mask with a venturi adapter attached

O2 is delivered as a jet through the valve

The valve also entrains (traps) a fixed amount of air to create a precise O2 concentration

The total flow of gas from the valve to the mask exceeds the peak inspiratory flow of the patient

As a result, FiO2 concentration is fixed i.e. the patient’s ventilation will not alter its value

Ports in the mask allows expired air to leave

Suited for COPD patients as it provides a lower FiO2 to avoid CO2 retention

The different valves typically range from a fixed FiO2 value of 24%-40% at flow rates specified by the manufacturer

Non-rebreather masks

Similar to a simple mask, but the side ports only enables exhaled air to leave and prevents room air entering the mask

The attached reservoir bag has a one-way valve to prevent exhaled air from entering, and hence much higher O2 concentrations can be collected in the reservoir bag for inhalation

They are used in anaesthetics, and conditions such as acute asthma, suspected acute myocardial infarction and trauma

Non-rebreather masks can typically provide FiO2 >60% at 10 L/min minimum, and for acute illness management 80-90% FiO2 at 15 L/min may be used

Summary

Delivery method / Typical flow rate (L/min) / Typical FiO2 delivered
Nasal cannula / 1-6 / 24-40%
Venturi mask / specified by manufacturer / 24-40%
Simple mask / 5-10 / 35-50%
Non rebreather / 10 minimum
15 for acute illness / >60%
80-90%