Pressure-volume curve–derived recruitability indices are protocol-dependent in lavage-induced lung injury

Abstract

Background

Bedside assessments of lung recruitability using the static pressure-volume (PV) curve are crucial in managing acute respiratory distress syndrome (ARDS). We hypothesized that quantitative recruitability indices, such as the hysteresis ratio (HR) and normalized maximal distance (NMD), are inherently influenced by the peak airway pressure used during the measurement maneuver.

Methods

We conducted a secondary analysis of data from 17 sheep with lavage-induced ARDS. Static PV curves were generated using the super-syringe method at peak airway pressures of 40 and 60 cm H₂O. HR and NMD were calculated and compared between the two pressure conditions using a two-tailed paired t-test.

Results

Both indices were significantly higher at a peak pressure of 60 cm H₂O compared to 40 cm H₂O. The mean HR increased from 0.18 ± 0.05 at 40 cm H₂O to 0.22 ± 0.06 at 60 cm H₂O (P = .01). Similarly, the mean NMD increased significantly from 0.33 ± 0.08 at 40 cm H₂O to 0.39 ± 0.07 at 60 cm H₂O (P = .008).

Conclusions

Lung recruitability quantified by HR and NMD increased significantly when higher airway pressures were used during the assessment. Clinicians must cautiously interpret PV curve-based assessments, recognizing that assessments performed at lower pressures may underestimate the lung’s true recruitment potential.

Quick look

Current knowledge

The static pressure-volume (PV) curve is a well-established tool for assessing lung mechanics and recruitable lung volume in acute respiratory distress syndrome (ARDS). While the overall shape of the PV curve is known to be altered by the applied airway pressure, its specific impact on quantitative recruitability indices such as the hysteresis ratio and normalized maximal distance is not fully understood.

What this paper contributes to our knowledge

In an animal ARDS model, increasing the peak airway pressure from 40 to 60 cm H₂O during a static PV maneuver significantly increased the hysteresis ratio and normalized maximal distance. These findings indicated that quantitative recruitability indices were intrinsically dependent on the maximum pressure used during the measurement, suggesting that lower assessment pressures may underestimate the lung’s true recruitment potential.

Introduction

Bedside assessments of lung recruitability, such as the static pressure-volume (PV) curve, are crucial in personalizing mechanical ventilation for patients with acute respiratory distress syndrome (ARDS). ¹ Previous physiological studies have established that the overall shape of the PV curve is altered by the peak pressure applied during the maneuver.^2,3 Recently, quantitative indices derived from the PV curve, such as the hysteresis ratio (HR) and normalized maximal distance (NMD), have been increasingly utilized as noninvasive surrogates to estimate lung recruitability without the need for a computed tomography scan^4,5 (Figure 1).

Figure 1.

Hysteresis ratio (HR) and normalized maximal distance (NMD), and representative pressure-volume (PV) curves. The left panel shows a schematic diagram of the calculation methods for HR and NMD, while the right panel displays a representative PV curve.

However, it remains unclear how the choice of peak airway pressure specifically affects these contemporary indices of recruitability. This is clinically relevant because predefined airway pressures used during PV maneuvers at the bedside may differ from those required to fully open collapsed alveoli. If the measurement itself is fundamentally influenced by the applied pressures, assessments performed at relatively lower, safer pressures might not accurately reflect the lung’s true recruitment potential. Therefore, we hypothesized that lung recruitability, as assessed quantitatively by HR and NMD, is intrinsically influenced by the peak pressure used during measurement.

Methods

We conducted a secondary analysis of data from two previous studies involving 17 female Dorset sheep (28.9 ± 2.7 kg) with lavage-induced ARDS.^2,3 In both studies, a surfactant-depletion lung injury model was established using a consistent bilateral lung lavage protocol. Specifically, the procedure involved the instillation of 30 mL/kg of warmed normal saline (39° C) into the lungs. This process was repeated until severe and stable lung injury was confirmed, defined as P_aO2 of less than 150 mm Hg that remained stable (less than 10% change) for at least 30 min. These studies were approved by the Subcommittee on Research Animal Care of the Massachusetts General Hospital. The ARDS model showed a P_aO2/F_IO2 of 67.4 ± 21.5 mm Hg at a PEEP of 5 cm H₂O and a respiratory system compliance of 20.3 ± 4.7 mL/cm H₂O.

Static PV curves were generated for each animal using the super-syringe method at peak airway pressure values of 40 cm H₂O and 60 cm H₂O (Figure 1). We calculated two indices known to correlate with lung recruitability: HR and NMD. HR was defined as the hysteresis area divided by the product of peak pressure and maximal volume, while NMD was defined as the maximal distance between the inflation and deflation limbs divided by the maximal volume.

Statistical Analysis Data are expressed as mean ± standard deviation. HR and NMD were compared between the two peak pressure conditions (40 cm H₂O and 60 cm H₂O) using a two-tailed paired t-test. A P value of <.05 was considered statistically significant. A formal sample size calculation was not performed because this was a secondary analysis of previously collected data. Statistical analyses were performed using R software (R 4.5.1; R Foundation for Statistical Computing, Vienna, Austria).

Results

Both HR and NMD were significantly higher at a peak pressure of 60 cm H₂O than at a peak pressure of 40 cm H₂O. The mean HR increased from 0.18 ± 0.05 at 40 cm H₂O to 0.22 ± 0.06 at 60 cm H₂O (P = .01). Similarly, the mean NMD increased significantly from 0.33 ± 0.08 at 40 cm H₂O to 0.39 ± 0.07 at 60 cm H₂O (P = .008) (Figure 2).

Figure 2.

Comparison of hysteresis ratio (HR) and normalized maximal distance (NMD) between the 40 cm H₂O and 60 cm H₂O conditions.

Discussion

In this study, we evaluated the impact of peak airway pressure on contemporary indices of lung recruitability derived from the static PV curve. Our findings confirmed that both HR and NMD were significantly higher at a peak pressure of 60 cm H₂O than at 40 cm H₂O.

While previous studies have well established that peak pressure alters the overall shape and compliance of the PV curve, our findings provide specific, quantitative evidence that modern indices of recruitability (HR and NMD) depend on the maximum pressure applied. Clinically, this means that if a PV curve is generated using a relatively low airway pressure—often chosen to prioritize patient safety and prevent overdistension—the calculated HR or NMD may significantly underestimate the lung’s true recruitment potential. Alveolar recruitment continues as airway pressure is progressively increased, and total lung recruitment requires high and sustained airway pressures. Therefore, if the pressure used to generate the PV curve differs from the peak pressure during actual ventilation, the recruitability potential specific to the clinical setting may not be accurately estimated. Standardizing the peak pressure during the PV maneuver is essential to ensure consistent interpretation of recruitability across different patients and studies.

This study has several limitations. First, this is a secondary analysis of animal data, and a formal sample size calculation was not performed. Second, the super-syringe method and the specific supra-physiologic peak pressures evaluated (up to 60 cm H₂O) are rarely used in routine clinical practice today due to safety concerns and the risk of barotrauma. However, utilizing these high pressures in this experimental model was necessary to demonstrate the maximum physiological recruitment potential and effectively highlight the pressure-dependency of these indices. In addition, no direct measurements of inflammatory burden, such as cytokines or histopathology, were available in the original studies; however, the present analysis used paired within-animal comparisons, which reduces the likelihood that between-subject differences in injury severity fully account for the observed findings.

Conclusions

Lung recruitability indices derived from the PV curve, specifically the HR and NMD, are significantly influenced by the peak pressure used during the measurement. Clinicians should cautiously interpret PV curve-based assessments, recognizing that lower assessment pressures may lead to an underestimation of true lung recruitability.