An Observational Study of the Relationship Between Pressure-volume Curves and Recruitability of the Lung in Mechanically Ventilated Critically Ill Patients With Respiratory Failure

Critical care units provide supportive therapies to help patients survive a period of life-threatening illness. Among such treatments, mechanical ventilation constitutes an essential asset to assist in patient recovery from respiratory failure and to provide airway protection in neurologically compromised patients. The need for mechanical ventilation is more often driven by respiratory failure than a need for neuroprotection. In excess of 100,000 adult patients are mechanically ventilated in UK ICUs annually, and this number has risen considerably recently due to the emergence of the COVID-19 pandemic.

Respiratory failure may be triggered by pulmonary (e.g., pneumonia, acute respiratory distress syndrome, interstitial fibrosis) or extra-pulmonary (e.g., sepsis, shock) disturbances. The abnormal function of the cardiorespiratory system in these critical conditions results in carrying degrees of hypoxaemia and hypercapnia, leading to the need for respiratory support by mechanical ventilation.

In patients undergoing mechanical ventilation for respiratory failure, the lung is characterized by a much-enhanced tendency to collapse. This collapse worsens hypoxaemia, and increases the stress and strain applied to those regions of the lung that remain aerated, leading to ventilator-induced lung injury (VILI). Re-aeration of non-aerated lung (recruitment) improves oxygenation and prevents VILI. For this reason, some clinicians employ recruitment manoeuvres following intubation, or subsequently during their ICU stay. However, it remains unclear which patients benefit from this intervention, at what time point(s) it is most beneficial, and the underlying mechanisms.

It is recognised that the volume of lung that is potentially recruitable (recruitability) varies widely from patient to patient, being influenced by: (i) the underlying disease precipitating respiratory failure (pulmonary versus extrapulmonary injury); (ii) the distribution of lung injury (lobar versus non lobar); and (iii) the time from initiation of lung injury.

An ability to assess recruitability is a pre-requisite for a rational recruitment strategy and selecting parameters for mechanical ventilation. The gold standard to assess recruitability involves performing a CT scan at two different levels of inspiratory pressure and assessing the mass of lung tissue (grams) that transitions from a non-aerated to an aerated state. This is not feasible in everyday clinical practice since it is time consuming, requires transfer of a critically ill patient to the radiology suite, and involves a significant exposure to ionising radiation. An alternative is to use some measure of the change in lung volume between the different pressure levels, but this is not the same as a change in non-aerated lung mass and indeed the two measures may not even be correlated.