Dear Editor,
Authors of the PRoVENT-COVID study reported that hypercapnic patients developed more acute respiratory distress syndrome (ARDS), incurred more venous thromboembolic events, with higher ventilation ratios and longer hospital stays but without having a significant impact on the morbidity or mortality [1]. We would like to offer the following points for further consideration.
Firstly, lung protective ventilation using lower tidal volume and higher respiratory rate is the cornerstone of the management of patients with severe ARDS, including that attributable to Covid-19, with the potential consequence of hypercapnia. The latter has significant pathophysiological effects and may itself increase hospital mortality [2]. On the contrary, there is also ample evidence to suggest that hypercapnia may have a therapeutic potential in ARDS [3]. Excess carbon dioxide (CO2), generated by the catabolic processes of ARDS, can lead to better survival [4]. Also, as a paradox, obesity has been proved to be protective in ARDS [5]. Notably, most obese patients in this study developed hypercapnia. We would like to know whether authors have thought both these sides of hypercapnia during interpretation of their data or not.
Secondly, ventilatory ratio, defined as [minute ventilation (ml/min) × partial pressure of arterial CO2 tension (mm Hg)]/(predicted body weight × 100 × 37.5), is a simple bedside index of assessing the efficiency of mechanical ventilation [6]. It correlates well with the fraction of dead space ventilation (VD) in ARDS. A higher value, being indicative of increased VD and inadequate minute ventilation, leads to higher mortality [6]. In this study, although the ventilatory ratio was calculated, but its correlation with inevitable hypercapnia was not highlighted.
Finally, hypoxemia, quantified by the ratio of partial pressure arterial oxygen tension and fraction of inspired oxygen concentration (PaO2/FiO2), does not necessarily assess the alveolar ventilation and physiological dead space which reflect the overall efficacy of the lungs to eliminate the CO2 load. Although lower PaO2/FiO2 may indicate severity of ARDS as per the Berlin criteria, but it may not gauge the adequacy of alveolar ventilation which is the most important determinant of overall outcome [6].
We congratulate the authors for their excellent thought-provoking study. However, to evaluate the overall outcomes of this study, we would like authors' response on these points.
Dipasri Bhattacharya,
Pradipta Bhakta,
Brian O'Brien,
Habib Md Reazaul Karim,
Antonio M. Esquinas.
Authors and their individual contribution
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1.
Dr. Dipasri Bhattacharya: Was involved in analysis of the article, writing, and editing the letter.
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2.
Dr. Pradipta Bhakta: Was involved in analysis of the article, writing, and editing the letter.
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3.
Dr. Brian O'Brien: Was involved in analysis of the article, writing, and editing the letter.
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4.
Dr. Habib Md Reazaul Karim: Was involved in analysis of the article, writing, and editing the letter.
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5.
Dr. Antonio M. Esquinas: Was involved in analysis of the article, writing, and editing the letter.
Financial support
No external funding supported the preparation of this manuscript. The authors have no financial and/or personal relationships with other people or organizations that might inappropriately influence our work.
Declaration of Competing Interest
The authors report no conflicts of interest.
References
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