Oxygen Delivery
Nasal Cannula
Used for mild hypoxia, typically in non-acute situations.
It can deliver 1-4 L/min (24-30% O2 i.e. a bit more than normal air).
Potential issues:
If the flow is too high, it will dry and irritate the nostrils
Doesn’t allow close control of FiO2
Hudson Mask
Simple face masks used for mild to moderate hypoxia.
It can deliver 5-10 L/min (30-40% O2).
A potential issue is that it doesn’t allow close control of FiO2.
Non-rebreather mask (NRB)
Used for moderate to severe hypoxia.
It can deliver 10-15 L/min (40-70% O2).
The benefit of this mask is that it can deliver high FiO2 concentrations as the O2 is inhaled from both the reservoir bag and a direct oxygen source.
A potential issue is that the mask doesn’t have a true seal, so some of the surrounding air can enter, therefore isn’t a fixed performance device (i.e. doesn't constantly deliver the same amount of O2 with every breath).
Venturi mask
Used to deliver a constant FiO2 regardless of the patient's respiratory rate and flow pattern (i.e. a fixed-performance device).
It can deliver either:
2-4 L/min (24% FiO2)
4-6 L/min (28% FiO2)
8-10 L/min (35% FiO2)
10-12 L/min (40% FiO2)
12-15 L/min (60% FiO2)
The benefit of this mask is that it can deliver a specific FiO2 to patients with COPD due to the risk of T2RF.
A potential issue is that the flow rate has to be the specific amount for the mask, otherwise the specific FiO2 won’t be delivered.
NIV (BiPAP) and CPAP
These are forms of ventilatory support used in acute respiratory failure, where a patient remains hypoxic despite medical optimisation.
Recruitment is the process where bronchioles and alveoli, which would normally collapse at the end of expiration, are kept open (more lung volume is ‘recruited’). To re-expand these collapsed lung areas, the pressure needs to be overcome, therefore the phase of inspiration that requires the most energy. By recruiting alveoli and keeping them open at the end of expiration, gas exchange is more efficient as less energy is required to breathe, and there is more surface area for gas exchange to occur.
BiPAP/NIV is a form of ventilatory support that delivers 2 different positive pressures of air, and is used in respiratory failure. The iPAP allows for ventilation (increases gradient), whereas the ePAP recruits collapsed alveoli. This overall makes gas exchange more efficient.
It's typically used in T2RF with respiratory acidosis (e.g. in a COPD exacerbation)
CPAP delivers a constant fixed positive pressure throughout inspiration and expiration. It, therefore, is not a form of ventilation, but keeps the airways patent and allows for more oxygen to be inspired.
In the acute setting, it's used for T1RF in acute pulmonary oedema (recruiting collapsed alveoli)
In the chronic setting, it's used for OSA
