OXYGEN

     
   

 

     
     

How do I administer Oxygen?

  • Oxygen is given thru fixed and variable performance devices.

  • Fixed performance devices deliver a flow of oxygen equal to or in excess of peak inspiratory flow

  • Variable performance devices use the deadspace of the nasopharynx or face masks as a reservoir of oxygen. They cannot deliver high inspired concentrations of oxygen.

Oxygen can be delivered to the upper airway by a variety of devices, in this section we will address only the non invasive methods. There are two types of devices – variable performance devices and fixed perfomance devices. The differentiation is based on the difference between the delivered concentration of oxygen FDO2 and the actual inspired concentration FiO2. Performance is based on matching the flow rate of gas leaving the device with the inspiratory flow rate entering the patient.

Take a deep breath in: you have probably just inspired 1 liter of air in about 1 second. Your inspiratory flow rate is thus approximately 60 liters per minute during this deep breath. Every breath you take varies in depth and volume, but if you were in respiratory failure you may well require flow rates of this magnitude (or more). To be guaranteed a FiO2 appropriate to your flow demand, a fixed performance flow-generating device must be placed at your airway with a flow rate of 60 or so liters of oxygen-air (mixed as required) to satisfy demand. To be a fixed performance device, the gas flow must exceed the patient’s peak inspiratory flow. Machines with this type of performance are expensive, and are usually only located in intensive care or high dependency units. Moreover, not all patients requiring supplemental oxygen require facemasks, which are unpleasant to wear.

Variable performance devices fit into two categories, nasal cannula and facemasks. The premise behind nasal cannula is to use the dead space of the nasopharynx as a reservoir for oxygen. When the patient inspires, entrained air mixes with the reservoir air and the inspired gas is enriched. Obviously, the FIO2 depends on the magnitude of flow of oxygen, the patient’s minute ventilation and peak flow. For most patients, each addition 1litre per minute of O2 flow with nasal cannula represents an increase in the FIO2 by 4%. So 1 liter is 24%, 2 liters is 28% and so on. At 6 liters (44%), it is not possible to raise the FIO2 further, due to turbulence, in the tubing and in the airway.

There are a couple of problems with nasal cannula: if they are not positioned at the nares, they are useless. Disorientated patients appear to be remarkably successful at dislodging cannula. Secondly, the effectiveness may be disrupted by the pattern of breathing: there appears to be little difference whether the patient is a mouth or a nose breather, but it is preferable if the patient exhales through his/her mouth rather than nose, so the reservoir is maintained.

The big advantage of nasal cannula is comfort for the patient – they can eat and speak easily while receiving oxygen.

Standard oxygen masks provide a reservoir for oxygen, but the FIO2 is difficult to calculate unless calibrated Venturi devices are attached. With Venturis, there are slits in the oxygen delivery system which become smaller or larger depending whether a high or low FIO2 is required. The rate of delivery of oxygen is calibrated for the size of the Venturi and amount of mixing therein. For example, a 60% oxygen Venturi requires 15L/min fresh gas flow. Standard masks struggle to provide an FIO2 of greater than 60%. A non rebreather reservoir bag can be attached to the facemask, to provide a larger reservoir (the bag fills when the patient is not actively inspiring), the two liter capacity should, in theory at least, allow the patient to inspire 100% oxygen.

       
   

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