Model-Based Approach to Estimate dFRC in the ICU Using Measured Lung Dynamics

Abstract

Acute Respiratory Distress Syndrome (ARDS) is characterized by inflammation and filling of the lung with fluid. Mechanical ventilation (MV) is used to treat ARDS/ALI using positive end expiratory pressure (PEEP) to recruit and retain lung units, thus increasing pulmonary volume and dynamic functional residual capacity (dFRC) at the end of expiration. However, simple methods to measure dFRC at the bedside currently do not exist and other methods are too invasive and impractical to carry out on a regular basis.Stress-strain theory is used to estimate ∆dFRC, which represents the extra pulmonary volume due to PEEP, utilizing readily available patient data from a single breath. The model uses commonly controlled or measured parameters (lung compliance, plateau airway pressure, PV data) to identify a parameter as a function of PEEP and tidal volume. A median value is calculated for each PEEP level over a cohort and is hypothesised as a constant throughout the population for the particular PEEP. Estimated ∆dFRC values are then compared to measured values to assess accuracy of the model. The model-based approach offers a simple and non-invasive method which does not require interruption of MV to estimate dFRC. The clinical accuracy of the model is limited but was able to track the impact of changes in PEEP and tidal volume on dFRC, on a breath-by-breath basis for each PEEP.

Analysis of different model-based approaches for estimating dFRC for real-time application

“…Mishra et al [16] proposed a model to estimate ∆dFRC using only data from a single PEEP level. Once again, combining Eqs.…”

“…The value of β for a single value of PEEP is assumed constant across all patients. Calculated β values were normalized by tidal volume as dFRC can vary with the applied tidal volume [16]. …”

“…Initially, when data at only one PEEP level is available, the model relies on the SSSB analysis [16]. As additional higher PEEP settings are introduced during the course of care, the model converts from SSSB to SSMB analysis [15].…”

“…Additional PV loops at a wide range of PEEP levels are required for higher accuracy thus requiring an interruption to ongoing MV treatment. The second method, proposed by Mishra et al [16] is an extension to [15] and only requires PV data from a single breath at one PEEP level to estimate dFRC for a given patient eliminating interruption. The third method is based on a single compartment lung model and also requires PV data from a single breath at one PEEP level.…”

“…The third method is based on a single compartment lung model and also requires PV data from a single breath at one PEEP level. The final method is a combination of the methods proposed by [15] and [16] with the aim of providing both accuracy and clinical functionality as data becomes available. All four methods are tested on two separate clinical data cohorts where both PV data and dFRC were measured directly.…”