Background & Aims Severe forms of COVID-19 are associated with systemic inflammation and hypercatabolism. We compared the time course of the size and quality of both rectus femoris and diaphragm muscles between critically-ill, COVID-19 survivors and non-survivors, and explored the correlation between the change in muscles size and quality with the amount of nutritional support delivered and the cumulative fluid balance. Methods Prospective observational study in the general ICU of a tertiary care hospital for COVID-19. The right rectus femoris cross-sectional area and the right diaphragm thickness, as well as their echodensities were assessed within 24 hours from ICU admission and on day 7. Anthropometric and biochemical data, respiratory mechanics and gas exchange, daily fluid balance and the amount of calories and proteins administered were recorded. Results 28 patients were analysed (age 65±10 years, 80% males, BMI 30.0±7.8). Rectus femoris and diaphragm sizes were significantly reduced at day 7 (-26.1 [-37.8;-15.2] and -29.2 [-37.8;-19.6]%, respectively) and this reduction was significantly higher in non-survivors. Both rectus femoris and diaphragm echodensity were significantly increased at day 7, with a significantly higher increase in non-survivors. The change in both rectus femoris and diaphragm size at day 7 was related to the cumulative protein deficit (R=0.664, p<0.001 and R=0.640, p<0.001, respectively), while the change in rectus femoris and diaphragm echodensity was related to the cumulative fluid balance (R=0.734, p<0.001 and R=0.646, p<0.001, respectively) Conclusions Early changes in muscle size and quality seem related to the outcome of critically-ill, COVID-19 patients, and be influenced by nutritional and fluid management strategies.

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The measurement of pleural (or intrathoracic) pressure is a key element for a proper setting of mechanical ventilator assistance as both under- and over-assistance may cause detrimental effects on both the lungs and the diaphragm. Esophageal pressure (Pes) is the gold standard tool for such measurements; however, it is invasive and seldom used in daily practice, and easier, bedside-available tools that allow for rapid and continuous monitoring are greatly needed. The tidal swing of central venous pressure (CVP) has long been proposed as a surrogate for pleural pressure (Ppl); however, despite the wide availability of central venous catheters, this variable is very often overlooked in critically ill patients. In the present narrative review, the physiological basis for the use of CVP waveforms to estimate Ppl is presented; the findings of previous and recent papers that addressed this topic are systematically reviewed, and the studies are divided into those reporting positive findings (i.e., CVP was found to be a reliable estimate of Pes or Ppl) and those reporting negative findings. Both the strength and pitfalls of this approach are highlighted, and the current knowledge gaps and direction for future research are delineated.