Types of oxygen delivery devices

1. Airway.
2. Breathing.
3. Circulation.
4.  Disability.
5. Exposure.

Effective ventilation requires a patent airway, movement of oxygen and carbon dioxide in and out of the airways into the lungs and gas exchange between the alveoli and pulmonary circulation. This is the ultimate aim of Airway and Breathing.

In this chapter, we will discuss the various methods of effectively managing and supporting the airway and breathing in critical care, including their merits and limitations. Some of these methods will be familiar to you and are commonplace in any acute inpatient ward. However, some methods can only be safely delivered and monitored in a level 2 or 3 environment. The choice of oxygen delivery device will depend upon the amount of oxygen required, additional respiratory support needed and patient compliance.

Your ABCDE assessment of the patient has identified that the patient is hypoxic, but maintaining their own airway, breathing spontaneously and not demonstrating any features of respiratory distress. In this situation, supplemental oxygen is all that is required in the first instance. Depending on the percentage of oxygen that you wish to deliver, there is a choice of devices available.

The amount of oxygen that is being provided is expressed as a percentage (%) or fraction of inspired oxygen (FiO2), with 100% O2 being equivalent to an FiO2 of 1.0. Flow rates are expressed in litres per minute (L/min). Room air delivers 21% oxygen (FiO2 0.21).

NASAL CANNULAE Types of oxygen delivery devices

These are clear prongs that fit into each nostril, with tubing secured behind the ears and under the chin. They can deliver between 24%–35% oxygen with flow rates of 2–4 L/min. A higher FiO2 can be achieved with flow rates up to 6 L/min if tolerated. The advantages and disadvantages of nasal cannulae are summarised below:

SIMPLE (HUDSON) FACE MASK Types of oxygen delivery devices

Air is mixed with oxygen within a clear mask that fits around the mouth and nose, and breathed in by the patient. A minimum of 5 L/min oxygen flow is required to ensure expired carbon dioxide is removed from the mask chamber, and not rebreathed. With flow rates of 5–10 L/min, it can deliver between 35% and 60% oxygen. However, with air and oxygen being mixed in varying ratios, it is not possible to determine exactly how much oxygen is being inspired.

• The patient can eat, drink and talk normally.
•  Higher flow rates can cause discomfort and bleeding.
• Cannot reliably determine how much oxygen the patient is receiving.
• Only suitable for patients with lower oxygen requirements.

This form of oxygen delivery is more disrupting to the patient. The mask requires removal to eat and drink and the higher flow rates can rapidly dry out the upper airways, potentially reducing its tolerance. If this level of supplemental oxygen is expected to be required for a prolonged period of time, humidified oxygen delivery should be considered. This will keep the airways and any secretions moist, facilitating expectoration and becoming potentially more tolerable to the patient.

These masks are used to deliver a fixed percentage of oxygen – the only method available to do so via a simple face mask. A Venturi mask consists of a face mask attached to a valve that will entrain a fixed amount of oxygen through the valve. At a set flow rate, this air and oxygen mix will deliver a specific percentage of oxygen into the mask for inspiration.

There are five different valves available, each colour-coded with a specified oxygen flow rate in L/min and the percentage of oxygen that the given flow rate will deliver written on the side of the mask. The percentages available are 24%, 28%, 35%, 40% and 60% oxygen.

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You may have seen these masks used where controlled oxygen therapy is required, typically in COPD patients, where the level of hypoxia to be corrected has to be carefully balanced against their respiratory (hypoxic) drive.

FACE MASK WITH RESERVOIR BAG Types of oxygen delivery devices

Also known as a non-rebreathe mask, this is a clear mask with a one-way valve and a reservoir bag beneath it. Once the reservoir bag is inflated, 100% oxygen contained within the bag is inspired through the one-way valve. As this oxygen passes through the face mask into the patient, it will inevitably mix with room air and expired carbon dioxide contained within the mask. However, it still provides 80%–98% inspired oxygen when provided with a flow rate of 15 L/min and is the highest oxygen-delivery method of all the supplemental oxygen devices listed above.

Once any of these methods of oxygen delivery have been administered, remember to reassess your patient using the ABCDE approach, ensuring the method chosen is sufficient for the patient’s needs. A regular re-review is required to escalate or de-escalate the oxygen therapy as needed.

OXYGEN DELIVERY WITH RESPIRATORY SUPPORT

Your ABCDE assessment may indicate that the patient requires more than just supplemental oxygen in supporting breathing, particularly if the following features are present:

• The need for high-flow oxygen therapy (FiO2 of 60% or more), titrated against the target SpO2 or PaO2 on the ABG.
•  A type 2 respiratory failure on the ABG (acutely elevated CO2 with acidosis).
•  Evidence of respiratory distress – dyspnoeic and tachypnoeic, use of accessory muscles of respiration, unable to complete full sentences in one breath.

In these situations, extra-respiratory support is likely to be required. First, you need to decide if the patient requires respiratory support in the form of continuous positive airway pressure (CPAP), providing positive end-expiratory pressure (PEEP), or if they require ventilatory support in the form of non-invasive positive pressure ventilation (NIPPV), also referred to as BiPAP (bi-level positive airway pressure) or NIV (non-invasive ventilation).

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The distinction between respiratory and ventilatory support is important, as it ensures the correct method of support if chosen in the correct circumstance.

HIGH-FLOW NASAL CANNULA (HFNC)

This form of oxygen therapy consists of wide-bore nasal cannulae attached to an air–oxygen mixer, providing heated and humidified oxygen up to 70 L/min flow. The flow of oxygen is much greater than the 15 L/min, which can be provided via standard face masks, and is substantially greater than that which can be provided via conventional nasal cannulae due to much wider tubing. This high flow rate can deliver PEEP up to 3 cmH2O. The PEEP prevents alveolar collapse at the end of expiration, facilitating greater gas exchange within the alveolae and reducing the work of breathing required to keep these alveolae open. In conjunction with a fixed percentage oxygen delivery up to FiO2 70%, HFNC can be used to improve oxygenation and reduce work of breathing.

Much like the standard nasal cannulae, HFNC has the advantage of being less disruptive than face masks and generally well tolerated compared to other forms of respiratory support.

However, whilst the percentage oxygen and flow rate can be altered, the amount of PEEP generated cannot be measured and for patients who require greater PEEP for respiratory support, escalation to CPAP may be required.

CONTINUOUS POSITIVE AIRWAY PRESSURE (CPAP)

CPAP can be delivered either via a tight-fitting mask that is strapped around the head, and fits over the mouth and nose or via a hood that encloses the entire head and fits tightly around the neck. It is possible to alter the PEEP, oxygen percentage and oxygen flow rate. CPAP aims to improve oxygenation and reduce work of breathing. The continuous airway pressure will provide a peak end-expiratory pressure (PEEP) that prevents alveolar collapse at the end of expiration, facilitating greater gas exchange within the alveolae and reducing the work of breathing required to keep these alveolae open.

As with HFNC, the flow of oxygen delivered can be much greater than the rate of 15 L/min due to sider tubing, improving overall oxygenation, but with the additional benefit of a greater, measurable and titratable PEEP.

However, CPAP can be claustrophobic and the sensation of the continuous positive pressure can be difficult to tolerate. The mask requires removal or the hood deflating via a side window to allow the patient to eat and drink. In these circumstances, it can be useful to temporarily de-escalate the respiratory support to HFNC, preventing all support provided by the CPAP from being lost.

NON-INVASIVE POSITIVE PRESSURE VENTILATION (NIPPV/BIPAP/NIV)

This uses the same tight-fitting mask as with CPAP. NIPPV provides ventilatory support, as both inspiration and expiration are supported via two alternating levels of pressure, both of which can be separately adjusted. The higher pressure provides inspiratory pressure support, to reduce the work of breathing during inspiration, and the lower pressure provides the PEEP, preventing alveolar collapse with expiration. The inspiratory pressure effectively forces air into the patient’s lungs as they breathe in, and this sensation may not be tolerated by some patients, often to a greater extent than with CPAP.

NIPPV can be used for a number of reasons, but is particularly efficacious in patients with type 2 respiratory failure, for the purpose of both improving oxygenation and carbon dioxide reduction, and in patients who require ventilatory support but might not be suitable for invasive ventilation.

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For all forms of non-invasive respiratory support, there should be a well documented plan regarding the management steps if it should fail to improve the patient’s breathing, and whether progression to invasive ventilation would be suitable. Ideally, this should be discussed with the patient prior to commencement of respiratory support, and their wishes should be sorted to guide decision making where appropriate.

OXYGEN DELIVERING WITH AIRWAY SUPPORT

This section introduces the two most common forms of airway support in the critical care setting; an endotracheal tube (ETT) and a tracheostomy. These devices are adjuncts to secure a patent airway in a patient who requires invasive ventilation or who has a low Glasgow Coma Scale (GCS) and is unable to protect their airway independently as a consequence. These devices are positioned in the airway to keep it patent and can be attached to a ventilator or CPAP machine to provide both airway and respiratory support.

ENDOTRACHEAL TUBE (ETT)

An ETT is a clear flexible tube with length markings (in cm) along the side and a surrounding inflatable cuff at the distal end. It is inserted through the mouth, past the vocal cords and into the trachea, viewed via a laryngoscope, while the patient is sedated. The cuff is then inflated from a port at the proximal end to occlude the airway around the tube. The tube is secured in place with either tape or a mask attached to the tube and fitted to the face. If this insertion method proves difficult, there are other advanced techniques that are not covered in this chapter. The insertion of an ETT should always beperformed by a skilled airway-trained practitioner.

ETTs are available in a range of diameters, but typically size 7.0 for females and size 8.0 for males is used.

The main risks associated with the use of an ETT are increased risk of pneumonia, due to the inability of the patient to occlude the vocal cords, creating an open passage from the outside air directly to the lungs, and pooling of secretions of the inflated cuff, which can trickle down into the lungs, leading to infection.

The inflated cuff can cause tracheal stenosis if it is too inflated or sited for too long. If longer-term support is needed, a tracheostomy may be required.

TRACHEOSTOMY

A tracheostomy is inserted when a patient is likely to require longer-term airway support. A tracheostomy is a curved tube with an inflatable cuff that sits in the trachea, below the vocal cords and is secured by tape strapped around the neck. As with an ET tube, both mechanical ventilation and spontaneous breathing are possible via a tracheostomy, including CPAP.

Tracheostomy insertion can be performed percutaneously on the critical care unit, or surgically in theatre. Anticipated complications due to patient anatomy and bleeding risk are often the deciding factors as to which method is preferred.

As the tracheostomy does not pass through the vocal cords, it can be fitted with a speaking (or Passy Muir) valve. When the cuff surrounding the tracheostomy is deflated, this one-way valve allows the patient to breathe in through the tube and out past the tracheostomy and vocal cords through their nose and mouth, generating speech as they do so.