Practical strategies for airway clearance in patients with severe COVID-19

Background
Since it was first identified, COVID-19 has presented significant challenges to healthcare workers around the world. This has particularly been the case in critical care units, which have seen significant surges in admission numbers over the course of the pandemic. Due to the extreme severity of illness, patients admitted to an intensive care unit (ICU) with COVID-19 have often required prolonged periods of mechanical ventilation, coupled with prone positioning and paralysis to achieve adequate gas exchange and oxygenation (Chang et al., 2021, McWilliams et al., 2021). Whilst optimising ventilation, this strategy increases the risk of secretion retention and creates a number of challenges for respiratory physiotherapy.

The challenge of secretion clearance
In patients receiving invasive mechanical ventilation, normal secretion clearance mechanisms are significantly impaired. The delivery of gas under positive pressure inhibits cilia action and is associated with the embedding of secretions within the deeper airways (Konrad et al., 1994). The use of artificial airways bypasses the glottis and hinders the ability to generate an efficient cough (Gal, 1980). This bypassing of the upper airways also inhibits humidification and warming of inspired gases, causing damage to the airway epithelium and further promoting impairment of airway clearance (Kilgour et al., 2004).

For patients with severe COVID-19 infection, ventilation is challenging due to the heterogeneous lung pathology and associated severe hypoxaemia, often requiring high peak inspiratory and plateau pressures to achieve adequate oxygenation (National Institutes of Health, 2021). Prone positioning is also recommended and often utilised as a part of lung protective ventilation to optimise gas exchange (Alhazzani et al., 2020). Guidelines suggest for maximal effect patients should spend a minimum of 16 hours in the prone position, with a 30-degree reverse Trendelenburg tilt to prevent pressure damage or facial oedema. Whilst the prone position can aid in proximal secretion drainage (Scholten et al., 2017), this prevents use of traditional postural drainage positions during chest physiotherapy such as alternate side lying. In addition, neuromuscular blocking agents subsequently render the respiratory and abdominal muscles inactive meaning cough becomes absent.

Physiotherapy treatment
Respiratory physiotherapy is widely implemented in ICU to improve airway clearance, using techniques aiming to displace mucus from distal to proximal airways and enhance cough efficacy.

In patients who are mechanically ventilated, the challenge for secretion clearance becomes dependent on the mechanism of two-way gas liquid flow and the interplay between inspiratory and expiratory flow rates. As highlighted previously, the need for high ventilatory pressures, in combination with poor lung compliance and impaired or absent cough, result in an imbalance and subsequent inspiratory flow bias (see Fig. 1a). This further serves to restrict secretion clearance and has the potential to move secretions deeper into the lungs.