Time constant to determine PEEP - A feasible approach?


Autor: Aakash Soni

Fecha: 04.04.2024

The authors' point? Using the expiratory time constant instead of inspiratory parameters holds promise as an approach for personalized PEEP titration in patients with ARDS.

Time constant to determine PEEP - A feasible approach?

Clinical question

What is the clinical utility of assessing optimal positive end-expiratory pressure (PEEP) levels using the expiratory time constant (RCexp) as compared to traditional methods based on respiratory system compliance (CRS) in terms of balancing alveolar recruitment and preventing overdistension in patients with nonhomogeneous lungs?

Clinical background

  • Mechanical ventilation can be a crucial intervention, but can also lead to ventilator-induced lung injury (VILI), especially in ARDS.
  • Selection of the optimal PEEP level to balance lung recruitment and avoid overdistension remains a subject of debate.
  • Traditional methods assess inspiratory parameters, whereas the current study (Depta F, Euliano NR, Zdravkovic M, Török P, Gentile MA. Time constant to determine PEEP levels in mechanically ventilated COVID-19 ARDS: a feasibility study. BMC Anesthesiol. 2022;22(1):387. Published 2022 Dec 13. doi:10.1186/s12871-022-01935-81​) explores the use of RCexp, an expiratory parameter, to titrate PEEP.

Design and setting

Prospective observational investigation conducted in a tertiary teaching hospital, involving 16 COVID-19 ARDS patients who were assessed for moderate-to-severe ARDS as per the Berlin definition. The research aimed to determine lung recruitability patterns by analysing RCexp measurements during PEEP titrations in both the supine and prone positions


Sixteen consecutive patients, who had been admitted to the ICU between March and April 2021, and were diagnosed with COVID-19 pneumonia confirmed by polymerase chain reaction (PCR) testing

Inclusion criteria Patients with moderate or severe ARDS according to the Berlin definition (PaO2:FiO2 ratio < 200 with PEEP > 5 cmH2O)
Exclusion criteria Patients with spontaneous breathing efforts


Ventilation strategy Ventilation mode: Pressure-controlled ventilation
₋ RR: 18
₋ I:E ratio: 1:1.5
₋ Maximal inspiratory flow: 60 l/min
Flow and pressure were measured using a sensor at the end of the tracheal tube
Patients were pre-oxygenated with 100% oxygen for 5 minutes before measurements were taken
PEEP levels were adjusted in escalating order: 0, 5, 8, 10, 12, 15, and 18 cmH2O, with an inspiratory pressure of 14 cmH2O applied on top of each PEEP level
RCexp, CRS, and other parameters were recorded once stabilized during the last 10 of 15 consecutive breaths at each PEEP level
CRS was calculated as CRS = VTexp / (PIP – PEEP), where PIP is peak inspiratory pressure, and VTexp is expiratory tidal volume
RCexp measurement The mechanical ventilator measured the time required to exhale 63% of the delivered tidal volume (Vt) from the expiratory flow curve of the previous breath. The measured time constant was recorded as an average of the last 10 breaths

Definitions and patterns

Definitions Optimal PEEP: The level where the maximum RCexp (RCexpMAX) was observed on the plot PEEP versus Time constant
Optimal PEEP range: The range that correlates with a 5% variation from RCexpMAX
The optimal PEEP and optimal PEEP range determined by RCexp were compared with those determined by CRS
PEEP levels in the prone position were compared with PEEP in the supine position to assess any potential differences in recruitability patterns on the PEEP versus RCexp plots
Recruitability patterns RCexp was used to assess recruitability patterns. The data was divided into two groups: recruitable and non-recruitable.
Recruitable group: Defined as having either a greater than 10% increase in RCexp as PEEP increased or an RCexp remaining almost constant with increasing PEEP
Non-recruitable group: Defined as having similar RCexp values at low PEEP levels (0–8 cmH2O) and then a substantial decrease (> 10%) in RCexp as PEEP levels continued to increase

Main results

Aspect Findings
PEEP titrations and patient characteristics 53 PEEP titrations conducted (29 prone, 24 supine); 75% of patients had a PaO2:FiO2 ratio below 100
Recruitability 75% categorized as “recruitable” (40 out of 53), 25% categorized as “non-recruitable” (13 out of 53)
Optimal PEEP and PEEP range Optimal PEEP values significantly higher (p < 0.001) in the recruitable group
Optimal PEEP range significantly wider with the CRS method vs. RCexp method in the recruitable group (in both prone and supine position: prone p = 0.016, supine p = 0.02)
Comparison of supine and prone positions (recruitable patterns) Optimal PEEP higher in the supine vs. prone position (p < 0.001)
No significant difference in the optimal PEEP range between positions (p = 0.09)
Comparison of supine and prone positions (non-recruitable patterns) No significant differences in optimal PEEP (p = 0.47), PEEP range (p = 0.82), or VTexp (p = 0.48) between supine and prone positions


  • Feasibility of RCexp: The study suggests that RCexp may be a feasible method for determining optimal PEEP levels in COVID-19 ARDS patients.
  • Distinguishing recruitability: RCexp patterns effectively distinguished between recruitable and non-recruitable lungs during PEEP titration, thus helping to individualize ventilation strategies.
  • Positional differences: PEEP levels and RCexp patterns differed between the supine and prone positions, with higher optimal PEEP observed in the supine position for the recruitable group.
  • Comparison with CRS: In the recruitable group, RCexp-based PEEP titration showed a narrower PEEP range compared to CRS, suggesting a potential advantage in avoiding overdistension.


Measuring RCexp is a feasible method for assessing physiologic responses to changes in PEEP. It also holds promise as an approach for personalized PEEP titration in patients with ARDS.

Clinical highlights

  • RCexp measurements offer the potential for individualized mechanical ventilation strategies. By assessing real-time RCexp values, healthcare providers can tailor ventilator settings to suit each patient's unique respiratory needs.
  • The study highlights the importance of repeated RCexp measurements. Regular and frequent assessments of RCexp are probably beneficial, especially during the initial stages of ARDS. This approach ensures that ventilation is optimized for the individual patient, contributing to a more protective ventilation strategy.
  • The study also highlights the need for additional clinical research to further evaluate the utility and validity of RCexp in ARDS due to different underlying causes. More studies are required to determine the broader applicability of RCexp in diverse clinical contexts.

Strengths and considerations

Strengths The study addresses an important clinical question related to PEEP titration in COVID-19 ARDS patients.
It provides detailed methods and measurements, which enhance the transparency and replicability of the research.
Considerations The sample size of 16 patients is relatively small, which may limit the generalizability of the findings.
The practical implications and limitations should be explored further in a more comprehensive study.
The study acknowledges the high individual variation in assessing optimal PEEP, which should be considered when interpreting the results.

How to incorporate these findings into my daily work with Hamilton Medical technology?

Hamilton Medical ventilators measure the RCexp breath‑by‑breath. Results are shown on the monitoring panel and trends for all variables of respiratory mechanics can be displayed. The table below shows the potential practical application of RCexp.

Recommendation / finding Details
Continuous monitoring for ARDS optimization Continuous monitoring of RCexp and PEEP levels is recommended, especially during the early stages of ARDS, to optimize ventilation and minimize VILI.
Use of RCexp Patients with an RCexp < 0.5 sec are at risk of VILI.
Close monitoring of tidal volume, driving pressure, and plateau pressure is crucial.
Identifying recruitability using RCexp An increase in RCexp as PEEP levels rise suggests recruitable lungs.

Non-recruitable patterns show a relatively constant or decreasing RCexp with higher PEEP.
Determining optimal PEEP with RCexp The study shows the high individual variation in assessing optimal PEEP using RCexp and optimal PEEP using compliance, which should be considered when interpreting the results.
The P/V Tool available on selected Hamilton Medical ventilators provides a more effective and verified way of assessing lung recruitability and customizing PEEP levels.
RCexp as an index for monitoring lung recruitment may be a practical alternative where the P/V Tool is not available.
Techniques should be modified depending on ventilator features to ensure a comprehensive and flexible approach to managing patients.
Effect of prone positioning assessment Assess the impact of prone positioning on respiratory mechanics.
An increase in compliance and RCexp indicates lung recruitment due to prone positioning.


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.

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Time constant to determine PEEP levels in mechanically ventilated COVID-19 ARDS: a feasibility study.

Depta F, Euliano NR, Zdravkovic M, Török P, Gentile MA. Time constant to determine PEEP levels in mechanically ventilated COVID-19 ARDS: a feasibility study. BMC Anesthesiol. 2022;22(1):387. Published 2022 Dec 13. doi:10.1186/s12871-022-01935-8


We hypothesized that the measured expiratory time constant (TauE) could be a bedside parameter for the evaluation of positive end-expiratory pressure (PEEP) settings in mechanically ventilated COVID-19 patients during pressure-controlled ventilation (PCV).


A prospective study was conducted including consecutively admitted adults (n = 16) with COVID-19-related ARDS requiring mechanical ventilation. A PEEP titration using PCV with a fixed driving pressure of 14 cmH2O was performed and TauE recorded at each PEEP level (0 to 18 cmH2O) in prone (n = 29) or supine (n = 24) positions. The PEEP setting with the highest TauE (TauEMAX) was considered to represent the best tradeoff between recruitment and overdistention.


Two groups of patterns were observed in the TauE plots: recruitable (R) (75%) and nonrecruitable (NR) (25%). In the R group, the optimal PEEP and PEEP ranges were 8 ± 3 cmH2O and 6-10 cmH2O for the prone position and 9 ± 3 cmH2O and 7-12 cmH2O for the supine position. In the NR group, the optimal PEEP and PEEP ranges were 4 ± 4 cmH2O and 1-8 cmH2O for the prone position and 5 ± 3 cmH2O and 1-7 cmH2O for the supine position, respectively. The R group showed significantly higher optimal PEEP (p < 0.004) and PEEP ranges (p < 0.001) than the NR group. Forty-five percent of measurements resulted in the most optimal PEEP being significantly different between the positions (p < 0.01). Moderate positive correlation has been found between TauE vs CRS at all PEEP levels (r2 = 0.43, p < 0.001).


TauE may be a novel method to assess PEEP levels. There was wide variation in patient responses to PEEP, which indicates the need for personalized evaluation.