Correlation between the hysteresis of the PV curve and R/I ratio

In mechanically ventilated COVID-19 patients, what is the relationship between lung recruitability assessments based on the low-flow quasi-static pressure-volume (PV) curve and the recruitment-to-inflation (R/I) ratio?

• Despite the theoretical benefits of lung recruitment and PEEP titration, there is ongoing debate about their use and the optimal ventilator settings for patients with ARDS. These strategies are particularly relevant for patients with moderate to severe ARDS, as indicated by a low PaO2/FiO2 ratio (P/F ratio) of ≤ 200 mmHg.
• The response to lung recruitment varies greatly among patients; therefore, lung recruitability should be assessed before performing a lung recruitment maneuver (LRM).
• The pressure–volume curve (PV curve) and recruitment-to-inflation ratio (R/I ratio) can be used at the bedside for evaluating lung recruitability and individualizing PEEP.
• The correlation between the R/I ratio and CT evaluation has not been validated, whereas hysteresis parameters in PV curves show some correlation with lung-tissue recruitment on CT.
   

• Retrospective, single-center cohort study (Nakayama R, Bunya N, Katayama S, et al. Correlation between the hysteresis of the pressure-volume curve and the recruitment-to-inflation ratio in patients with coronavirus disease 2019. Ann Intensive Care. 2022;12(1):106. Published 2022 Nov 12. doi:10.1186/s13613-022-01081-x1​)
• Patients with COVID-19 who underwent invasive mechanical ventilation in the intensive care unit of the Department of Emergency Medicine, Sapporo Medical University, Sapporo, Hokkaido, Japan, between January 1, 2021, and September 30, 2021
   

• Diagnosed with COVID-19
• Treated with invasive mechanical ventilation using a HAMILTON-C6 ventilator
• Unable to maintain an SpO2 of 93% with noninvasive oxygen therapy (ventilation then initiated following Japanese COVID-19 practice guidelines)
   

• There is a significantly higher correlation between NMD and the R/I ratio than between other parameters like distance at 20 cm H2O and the R/I ratio. 
• Neither NMD nor the R/I ratio correlated with Crs. Instead, they showed a correlation with the Crs ratio (higher/lower).
• The above findings indicate that NMD and R/I ratios may be more informative than other parameters for assessing lung recruitability.

There is a strong correlation between NMD of the quasi-static PV curve and the R/I ratio for recruitability assessment. In addition, NMD and the R/I ratio correlate with the Crs ratio (higher/lower). Therefore, NMD and the R/I ratio could be potential indicators of recruitability that can be performed at the bedside.

• The study had a relatively small sample size, which may limit the generalizability of its findings to a broader population.
• The single-center, retrospective design of the study could introduce bias and also limit the generalizability of the results.
• The study acknowledged that it lacked CT data for assessing lung recruitability, which could have provided a more robust reference for validating the findings.
• The study highlighted the heterogeneous nature of acute respiratory distress syndrome (ARDS), which may affect the consistency of compliance as an indicator of recruitability.
• The study used different methods than previous studies (AOP was identified as the lower inflection point in the PV curve, and plateau pressure was measured at least 2 min later), which may introduce variability in the results.

• The response to PEEP or LRMs in ARDS is difficult to predict, as it is dependent on the individual lung’s recruitability and therefore varies greatly between patients. Increasing PEEP or conducting an LRM might be detrimental, especially in patients with low recruitability. Excessive PEEP may cause lung overdistention, resulting in left and/or right cardiac dysfunction.
• The strong correlation between NMD and the R/I ratio in patients with airway pressure below 20 cmH2O at maximal distance on the PV curve suggests that NMD is a valuable marker of lung recruitability within a safe range of PEEP. This understanding can help healthcare providers make more informed decisions about PEEP settings in specific patients, ensuring optimal lung recruitment and minimizing the risk of overdistension.
• NMD obtained using the P/V Tool technology on Hamilton Medical ventilators has previously been validated in a CT-scan study (Chiumello D, Arnal JM, Umbrello M, et al. Hysteresis and Lung Recruitment in Acute Respiratory Distress Syndrome Patients: A CT Scan Study [published correction appears in Crit Care Med. 2022 Mar 1;50(3):e339]. Crit Care Med. 2020;48(10):1494-1502. doi:10.1097/CCM.00000000000045182​), unlike the R/I ratio that has not been directly validated against reference methods like CT scans or hysteresis.   
• Evidence has shown the P/V Tool to be an adaptable, reliable, and effective means of measuring lung recruitment. It requires no modification to the existing mode, enabling smooth integration into clinical practice.

Hamilton Medical offers an extensive range of educational material on use of the P/V Tool for assessing recruitability, performing recruitment maneuvers, and setting PEEP. Follow the link below to access articles, videos, e-learning modules, and more.