- Understand differences in provided respiratory support between commonly used oxygen delivery devices
Anesthesiologists think about oxygen delivery devices in terms of effective reservoir size, variable or fixed FiO2 devices and low versus high flow systems. As internists, it may be easier to characterize oxygen delivery devices as their approximate effective FiO2. Both ways of thinking about devices are detailed below.
Oxygen Delivery Devices
Oxygenation is affected by inspired FiO2 and positive end expiratory pressure (or PEEP). Oxygen delivery devices used on acute care floors are largely limited to adjustments in FiO2. Non-invasive positive pressure ventilation (NIPPV), such as CPAP or Bilevel, provide PEEP to further support oxygenation (discussed in additional learning section).
In addition to PEEP and FiO2, oxygen flow rate (as compared to a patient’s inspiratory flow rate) can have an indirect effect on a patient’s oxygenation. It is important to recognize that the most commonly used oxygen devices on acute care floors (nasal cannula, simple face mask) deliver 100% FiO2 oxygen at an adjustable flow rate. This oxygen intermixes with ambient air (21% FiO2) which dilutes the oxygen to produce an “effective FiO2.” With respiratory distress, a patient’s inspiratory flow rate can range from 30 – 120 L/min which far exceeds the flow rate provided by some oxygen delivery devices.
Only more advanced oxygen delivery systems (Venturi mask, high flow nasal cannula) allow fine tune control over the delivered FiO2.
Most common oxygen delivery device used on acute care floors. It is convenient and allows easy access to the face so the patient can easily talk and eat. It is most effective in nose breathing patients with mild hypoxia.
A form of nasal cannula that contains a small external reservoir, which collects a small volume of 100% FiO2 oxygen that is delivered immediately upon inhalation. The reservoir and larger bore nasal prongs allow for more effective oxygen delivery (higher effective FiO2 per flow rate compared to nasal cannula).
Simple Face Mask
Commonly used in patients who are mouth breathers or do not tolerate nasal cannula at higher flow rates. However, the mask is less convenient than the nasal cannula and can be claustrophobic.
The Venturi device delivers a consistent FiO2 at higher flow rates (achieved by intentionally entraining ambient air through the device). It can be useful if a patient has variable or higher work of breathing (and resultant increased inspiratory flow rate) or a consistent delivery of an exact FiO2 is desired.
Non-rebreather Mask (NRB)
This is a rescue method of oxygen delivery and can provide ~ 90% FiO2 at maximum flow rate of 15 L/min. A NRB additionally can be used to assess whether a patient will have a significant response to oxygen (e.g. presence of a large shunt).
High flow nasal cannula (HFNC)
HFNC is a newer form of oxygen delivery device that allows for delivery of high flow rates that can more closely match a patient’s inspiratory flow rate and high FiO2‘s. HFNC has been shown to not only improve oxygenation, but also to improves work of breathing and minute ventilation (VE). This is also the only form of delivery device that delivers a small amount of PEEP, though it is not readily measurable or adjustable.
In the past few years, a number of studies have shown the benefit of HFNC compared to traditional oxygen therapy in patients with acute hypoxemic respiratory failure.
Summary of oxygen delivery devices:
Why don’t we just use as much oxygen as possible? (~1-2 min)
Too much oxygen, or hyperoxia, has been associated with increased mortality except in rare situations such as CO poisoning. Hyperoxia reduces blood flow to vital organs and can result in myocardial or cerebrovascular ischemia. In the lungs, hyperoxia can paradoxically worsen hypoxia. High concentrations of oxygen in poorly ventilated areas decrease hypoxic vasoconstriction and worsen V/Q mismatch. Oxygen is reabsorbed more quickly from the alveoli and can cause alveolar collapse.
TAKE HOME POINTS:
- The effective FiO2 of nasal cannula, oxymizer, and simple face mask are variable and subject to a patient’s respiratory effort.
- Venti-mask and high flow nasal cannula are less affected by respiratory pattern and deliver consistent FiO2‘s at higher flow rates.
- High flow nasal cannula reduces respiratory effort and has mortality benefit in patients with hypoxemic respiratory failure.
Non-invasive Positive Pressure Ventilation (~ 10 minutes)
Non-invasive positive pressure ventilation (NIPPV) – which includes continuous positive airway pressure (CPAP) and bilevel – are other forms of respiratory support short of intubation and mechanical ventilation. These modes of ventilation allow for titration of both FiO2 and positive end expiratory pressure, or PEEP to support oxygenation.
CPAP delivers a continuous airway pressure that acts similarly to PEEP and stents open airways/ alveoli during end expiration. This recruits alveoli to participate in gas exchange that would otherwise be collapsed during expiration. In order to keep a constant pressure, high inspiratory flow rates (in excess of 60 L/min) may be necessary to maintain this pressure in patients with high work of breathing.
Bilevel (more commonly referred to as BiPAP)
Bilevel allows you to adjust both an inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP). EPAP, acts in the same way as PEEP, to stent open airways at end expiration and recruit collapsed alveoli to participate in gas exchange. IPAP additionally recruits alveoli during inspiration, effectively increasing tidal volume and consequentially, minute ventilation.
Minute ventilation = respiratory rate x tidal volume
Because Bilevel affects minute ventilation, it can be used in ventilatory or hypercarbic respiratory failure in addition to hypoxemic respiratory failure. The effect of Bilevel on minute ventilation is proportional to the amount of pressure support (or the difference between IPAP and EPAP).
What are the indications and contraindications for NIPPV? (~5 min)
Use in immunocompromised patients is driven by the rationale that endotracheal intubation and presence of indwelling tube increases the risk of infection. While there is some evidence to support NIPPV use in hypoxemic respiratory failure (not from cardiogenic pulmonary edema), there are conflicting studies depending on the specific cause of hypoxemic failure10. Use of NIPPV has not been well studied and is not recommended in hypoxemic respiratory failure from ARDS (acute respiratory distress syndrome)10. In pneumonia, there is stronger evidence to support the use of NIPPV in patients with concurrent COPD10.
- Anything that prevents the patient’s ability to remove the mask – significant AMS, paralysis
- Anything “coming out of the patient” – active nausea/vomiting, increased secretions
- Inability to wear the mask – facial trauma
- Recent esophageal/tracheal surgery (surgical anastomosis/healing at risk from increased pressure
How does NIPPV improve outcomes in cardiogenic pulmonary edema? (~3 min)
Now you’re ready for some practice cases!
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- Fernandez, R, et al. 2017. High-flow nasal cannula to prevent postextubation respriatory failure in high-risk non-hypercapneic patients: a randomnized multicenter trial. Annals of Intensive Care. 7(47): 1-7.
- Hill NS. Chapter 18. Noninvasive Positive-Pressure Ventilation. In: Tobin MJ. eds. Principles and Practice of Mechanical Ventilation, 3e New York, NY: McGraw-Hill; 2013. http://accessmedicine.mhmedical.com.offcampus.lib.washington.edu/content.aspx?bookid=520§ionid=41692258. Accessed September 06, 2017.
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