Patient self-induced lung injury risk in severe COVID-19.

To the editor,

We read with interest the recent comment by J Schmitt et al. on our study [1], [2] and thank them for their interest and valuable insights.

The authors pointed out the phenomenon of patient self-induced lung injury (P-SILI), which is described in non-COVID-19 patients with acute respiratory distress syndrome. P-SILI is characterised by shift of brain homeostasis towards a lower PaCO2 threshold in response to hypoxaemia resulting in increased patient respiratory effort (increased minute ventilation) and hypocapnia. Patients with P-SILI are at increased risk of progressive lung injury due to abnormal increase in transpulmonary pressure leading to baro- and volutrauma, and possibly negative pressure lung oedema. Hence, the authors suggest that early hypocapnia would be a sensitive predictor of the need for ventilatory support and/or poor patient outcomes.

In our current report concerning diaphragmatic excursion in patients with severe COVID-19 [2], we did not encounter any patient with exaggerated diaphragmatic function and the maximum diaphragmatic excursion was 45- and 37 mm in the right and left sides, respectively. In our earlier report, we evaluated diaphragmatic excursion at weaning from invasive mechanical ventilation; however, we did not assess the patients at hospital admission [3].

Unfortunately, we did not record PaCO2 values during our two studies [2], [4]; however, previous literature showed that patients with severe COVID-19 have lower inspiratory effort in comparison to matched non-COVID-19 cohort with respiratory failure, suggesting lower risk of P-SILI in patients with COVID-19 [4]. Furthermore, data from COVID-19 patients showed that PaCO2 was not significantly lower in patients who later needed mechanical ventilation, and that PaCO2 showed low accuracy in predicting the need for mechanical ventilation and/or death with area under receiver operating characteristic (95% confidence interval) of 0.57 (0.51 to 0.68) [5]. Thus, we suggest that the level of PaCO2 on hospital admission is unlikely a predictor of severity of the disease. However, we agree that direct comparison between the diaphragmatic excursion and PaCO2 level as risk predictors warrants further research.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflict of interests

The authors have no competing interest to declare.