Capnodynamic monitoring of lung volume and blood flow in response to increased positive end-expiratory pressure in moderate to severe COVID-19 pneumonia: an observational study

Abstract

Background The optimal level of positive end-expiratory pressure (PEEP) during mechanical ventilation for COVID-19 pneumonia remains debated and should ideally be guided by responses in both lung volume and perfusion. Capnodynamic monitoring allows both end-expiratory lung volume ($${\text{EELV}}_{{{\text{CO}}_{2} }}$$ EELV CO 2 ) and effective pulmonary blood flow (EPBF) to be determined at the bedside with ongoing ventilation. Methods Patients with COVID-19-related moderate to severe respiratory failure underwent capnodynamic monitoring of $${\text{EELV}}_{{{\text{CO}}_{2} }}$$ EELV CO 2 and EPBF during a step increase in PEEP by 50% above the baseline (PEEPlow to PEEPhigh). The primary outcome was a > 20 mm Hg increase in arterial oxygen tension to inspired fraction of oxygen (P/F) ratio to define responders versus non-responders. Secondary outcomes included changes in physiological dead space and correlations with independently determined recruited lung volume and the recruitment-to-inflation ratio at an instantaneous, single breath decrease in PEEP. Mixed factor ANOVA for group mean differences and correlations by Pearson’s correlation coefficient are reported including their 95% confidence intervals. Results Of 27 patients studied, 15 responders increased the P/F ratio by 55 [24–86] mm Hg compared to 12 non-responders (p < 0.01) as PEEPlow (11 ± 2.7 cm H2O) was increased to PEEPhigh (18 ± 3.0 cm H2O). The $${\text{EELV}}_{{{\text{CO}}_{2} }}$$ EELV CO 2 was 461 [82–839] ml less in responders at PEEPlow (p = 0.02) but not statistically different between groups at PEEPhigh. Responders increased both $${\text{EELV}}_{{{\text{CO}}_{2} }}$$ EELV CO 2 and EPBF at PEEPhigh (r = 0.56 [0.18–0.83], p = 0.03). In contrast, non-responders demonstrated a negative correlation (r = − 0.65 [− 0.12 to − 0.89], p = 0.02) with increased lung volume associated with decreased pulmonary perfusion. Decreased (− 0.06 [− 0.02 to − 0.09] %, p < 0.01) dead space was observed in responders. The change in $${\text{EELV}}_{{{\text{CO}}_{2} }}$$ EELV CO 2 correlated with both the recruited lung volume (r = 0.85 [0.69–0.93], p < 0.01) and the recruitment-to-inflation ratio (r = 0.87 [0.74–0.94], p < 0.01). Conclusions In mechanically ventilated patients with moderate to severe COVID-19 respiratory failure, improved oxygenation in response to increased PEEP was associated with increased end-expiratory lung volume and pulmonary perfusion. The change in end-expiratory lung volume was positively correlated with the lung volume recruited and the recruitment-to-inflation ratio. This study demonstrates the feasibility of capnodynamic monitoring to assess physiological responses to PEEP at the bedside to facilitate an individualised setting of PEEP. Trial registration: NCT05082168 (18th October 2021).

Capnodynamic end‐expiratory lung volume assessment in anesthetized healthy children

“…Even then, the results should be carefully evaluated since most, if not all, of these studies do not use reference methods either but rather rely on experimental validation of the tested method for comparison. From this perspective, it is reassuring that EELV‐CO 2 has been validated against both inert tracer gas washin/washout and CT methods with good agreement in experimental studies as mentioned above 6–9 . The agreement depends however on recruitment status and applied PEEP, and this should be taken into consideration when applying EELV‐CO 2 in the clinical context 7 .…”

“…From this perspective, it is reassuring that EELV-CO 2 has been validated against both inert tracer gas washin/washout and CT methods with good agreement in experimental studies as mentioned above. [6][7][8][9] The agreement depends however on recruitment status and applied PEEP, and this should be taken into consideration when applying EELV-CO 2 in the clinical context. 7 In addition, factors such as remaining Bronchodilatory effects of sevoflurane induction were not strictly investigated, even if this impact most likely would be small after the 10-min stabilization washout period since relatively high sevoflurane concentrations may be needed to induce bronchodilatation in healthy lungs.…”

“…As such, EELV-CO 2 could potentially be used as a proxy for aerated EELV and serve as a tool for adjusting ventilation to ensure open lung conditions, prevent atelectasis and respiratory complications associated with mechanical ventilation in children. [4][5][6][7][8][9] Furthermore, in combination with tidal volume, it can be used to continuously assess dynamic lung strain, a factor correlated to lung injury that is currently not possible to assess without using cumbersome technical equipment. 10 Thus, the primary aim of this study was to establish normal values for EELV-CO 2 in healthy children undergoing anesthesia and to compare these values to previously published values obtained with alternative EELV methods.…”

“…EELV‐CO 2 is thus a lung volume that is very similar, but not identical, to aerated end‐expiratory lung volume. As such, EELV‐CO 2 could potentially be used as a proxy for aerated EELV and serve as a tool for adjusting ventilation to ensure open lung conditions, prevent atelectasis and respiratory complications associated with mechanical ventilation in children 4–9 . Furthermore, in combination with tidal volume, it can be used to continuously assess dynamic lung strain, a factor correlated to lung injury that is currently not possible to assess without using cumbersome technical equipment 10 …”

Capnodynamics – noninvasive cardiac output and mixed venous oxygen saturation monitoring in children

“…This relation is currently investigated in clinical studies. The same is true for the capnodynamic in the intensive care setting so far only one adult study exists (36). It remains to be investigated how EPBF performs in these situations where conditions for open lung ventilation may be more challenging to obtain than in the operating room.…”