Respiratory Support

There are several options for supporting a patient’s respiratory system. These can be escalated as required. From least to most invasive, the options are:

• Oxygen therapy
• High-flow nasal cannula
• Non-invasive ventilation
• Intubation and mechanical ventilation
• Extracorporeal membrane oxygenation (ECMO)

Additionally, chest physiotherapy and suction can be used to help clear secretions and improve respiratory function.

Respiratory support does not fix the underlying problem. It buys time while the underlying problem is managed.

Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome occurs due to a severe inflammatory reaction in the lungs, often secondary to sepsis or trauma. There is an acute onset of:

• Collapse of the alveoli and lung tissue (atelectasis)
• Pulmonary oedema (not related to heart failure or fluid overload)
• Decreased lung compliance (how much the lungs inflate when ventilated with a given pressure)
• Fibrosis of the lung tissue (typically after 10 days or more)

Clinically there is:

• Acute respiratory distress
• Hypoxia with an inadequate response to oxygen therapy
• Bilateral infiltrates on a chest x-ray

Management of ARDS is supportive. This can include:

• Respiratory support
• Prone positioning (lying on their front)
• Careful fluid management to avoid excess fluid collecting in the lungs

In ARDS, only a small portion of the total lung volume is aerated and has functional alveoli. The remainder of the lungs are collapsed and non-aerated. During mechanical ventilation, low volumes and pressures are used to avoid over-inflating the small functional portion of the lungs (lung protective ventilation). Positive end-expiratory pressure (PEEP) is used to prevent the lungs from collapsing further (see below for more on PEEP).

Prone positioning has several benefits:

• Reducing compression of the lungs by other organs
• Improving blood flow to the lungs, particularly the well-ventilated areas
• Improving clearance of secretions
• Improving overall oxygenation
• Reducing the required assistance from mechanical ventilation

Oxygen Therapy

Oxygen can be delivered by several methods. The FiO₂ (concentration of oxygen) will depend on the oxygen flow rate:

• Nasal cannula: 24 – 44% oxygen
• Simple face mask: 40 – 60% oxygen
• Venturi masks: 24 – 60% oxygen
• Face mask with reservoir (non-rebreather mask): 60 – 95% oxygen

Venturi masks can be used to deliver exact concentrations of oxygen. The most common use for these is in patients with COPD who are at risk of retaining carbon dioxide if the FiO₂ is too high:

Positive End-Expiratory Pressure

Positive end-expiratory pressure (PEEP) is an important term you will likely come across while working in a respiratory ward or intensive care.

End-expiratory pressure refers to the pressure that remains in the airways at the end of exhalation. 

Additional pressure in the airways at the end of exhalation stops the airways from collapsing. Forms of respiratory support that add positive end-expiratory pressure help keep the airways from collapsing and improve ventilation. It reduces atelectasis, improves ventilation of the alveoli, opens more areas for gas exchange and decreases the effort of breathing.

Positive end-expiratory pressure is added by:

• High-flow nasal cannula
• Non-invasive ventilation
• Mechanical ventilation

High-Flow Nasal Cannula

Using a high-flow nasal cannula allows for carefully controlled flow rates of up to 60 L/min of humidified and warmed oxygen. 

Having a high flow rate reduces the amount of room air that the patient inhales alongside the supplementary oxygen, increasing the concentration of oxygen inhaled with each breath. 

It also adds some positive end-expiratory pressure to help prevent the airways from collapsing at the end of exhalation (although this effect is reduced if the patient opens their mouth). 

Finally, having a high flow of oxygen into the airways provides dead space washout. The physiological dead space is the air that does not contribute to gas exchange because it never reaches the alveoli. Dead space air remains in airways and oropharynx, not adding anything to respiration and collecting carbon dioxide. High-flow oxygen effectively clears this and replaces it with oxygen, improving patient oxygenation. 

Continuous Positive Airway Pressure (CPAP)

CPAP (continuous positive airway pressure) involves a constant pressure added to the lungs to keep the airways expanded. It is used to maintain the patient’s airways in conditions where they are likely to collapse (adding positive end-expiratory pressure), for example, in obstructive sleep apnoea.

CPAP does not technically involve “ventilation”, as it provides constant pressure and the job of ventilation is still dependent on the respiratory muscles. Therefore, CPAP is not technically classed as non-invasive ventilation (NIV).

Non-Invasive Ventilation

Non-invasive ventilation (NIV) involves using a full face mask, hood (covering the entire head) or a tight-fitting nasal mask to blow air forcefully into the lungs and ventilate them. It is not pleasant for the patient but is much less invasive than intubation and ventilation. It is a valuable middle-point between basic oxygen therapy and mechanical ventilation.

BiPAP is a specific machine that provides NIV. BiPAP stands for Bilevel Positive Airway Pressure. Generally, the term NIV is used instead of BiPAP, as BiPAP refers to a specific machine rather than the therapy.

NIV involves a cycle of high and low pressure to correspond to the patient’s inspiration and expiration:

• IPAP (inspiratory positive airway pressure) is the pressure during inspiration – where air is forced into the lungs
• EPAP (expiratory positive airway pressure) is the pressure during expiration – stopping the airways from collapsing

Mechanical Ventilation

Mechanical ventilation is used where other forms of respiratory support (e.g., oxygen and NIV) are inadequate or contraindicated. A ventilator machine is used to move air into and out of the lungs. Patients generally require a level of sedation whilst on a ventilator, as it can be uncomfortable and distressing. Mechanical ventilation has several adverse effects and is only used for the shortest time necessary.

An endotracheal tube (ETT) or tracheostomy is required to connect the ventilator to the lungs. There should be no leaks in the circuit to enable the ventilator to deliver controlled pressures and volumes into the lungs.

The basic settings used for mechanical ventilation are:

• FiO₂ (concentration of oxygen) 
• Respiratory rate (breaths per minute)
• Tidal volume (volume of air pushed in per breath)
• Inspiratory:expiratory ratio (the ratio of time spent in inspiration and expiration)
• Peak flow rate (the maximum rate of air flow during inspiration)
• Peak inspiratory pressure (the maximum pressure during inspiration
• Positive end-expiratory pressure (the positive pressure applied at the end of expiration to prevent airway collapse)

The modes of mechanical ventilation can be quite complicated. Some key modes to be aware of are:

• Volume controlled ventilation (VC) – the machine is set to deliver a specific tidal volume per breath
• Pressure controlled ventilation (PC) – the machine is set to deliver a specific pressure per breath
• Assist control (AC) – breaths are triggered by the patient (or by the machine if there is no respiratory effort)
• Continuous positive airway pressure (CPAP) – the patient breathes while the machine adds constant pressure

Extracorporeal Membrane Oxygenation

Extracorporeal membrane oxygenation (ECMO) is the most extreme form of respiratory support and is very rarely used. It is used where respiratory failure is not adequately managed by intubation and ventilation. 

Blood is removed from the body, passed through a machine where oxygen is added and carbon dioxide is removed, then pumped back into the body. The process is similar to haemodialysis but for respiratory support rather than renal support. 

ECMO is only used short-term, where there is a potentially reversible cause of respiratory failure. It is not a long-term treatment. It is only provided in specialist ECMO centres and is not available in most intensive care units. Patients need to be transferred to a specialist centre for ECMO.

Last updated August 2021