Six methods to determine expiratory time constants in mechanically ventilated patients: a prospective observational physiology study

Filip Depta; Caitlyn M Chiofolo; Nicolas W Chbat; Neil R Euliano; Michael A Gentile; Dušan Rybár; Viliam Donič; Marko Zdravkovic

doi:10.1186/s40635-024-00612-z

Six methods to determine expiratory time constants in mechanically ventilated patients: a prospective observational physiology study

Intensive Care Med Exp. 2024 Mar 7;12(1):25. doi: 10.1186/s40635-024-00612-z.

Authors

Filip Depta^{1

2}, Caitlyn M Chiofolo³, Nicolas W Chbat³, Neil R Euliano⁴, Michael A Gentile⁵, Dušan Rybár^{6

7}, Viliam Donič⁸, Marko Zdravkovic^{9

10}

Affiliations

¹ Department of Critical Care, East Slovak Institute for Cardiovascular Diseases, Ondavská 8, Košice, 040 01, Slovakia. fdepta@protonmail.com.
² Faculty of Medicine, Pavol Jozef Šafarik University, Košice, Slovakia. fdepta@protonmail.com.
³ Quadrus Medical Technologies, Inc, White Plains, NY, USA.
⁴ Convergent Engineering, Gainesville, FL, USA.
⁵ Department of Anesthesia, Duke University Medical Center, Durham, NC, USA.
⁶ Department of Critical Care, East Slovak Institute for Cardiovascular Diseases, Ondavská 8, Košice, 040 01, Slovakia.
⁷ Faculty of Medicine, Pavol Jozef Šafarik University, Košice, Slovakia.
⁸ Department of Physiology, Pavol Jozef Šafarik University, Košice, Slovakia.
⁹ Faculty of Medicine, University of Ljubljana, Ljubljana, Slovenia.
¹⁰ Department of Anaesthesiology, Intensive Care and Pain Management, University Medical Centre Maribor, Maribor, Slovenia.

Abstract

Background: Expiratory time constant (τ) objectively assesses the speed of exhalation and can guide adjustments of the respiratory rate and the I:E ratio with the goal of achieving complete exhalation. Multiple methods of obtaining τ are available, but they have not been compared. The purpose of this study was to compare six different methods to obtain τ and to test if the exponentially decaying flow corresponds to the measured time constants.

Methods: In this prospective study, pressure, flow, and volume waveforms of 30 postoperative patients undergoing volume (VCV) and pressure-controlled ventilation (PCV) were obtained using a data acquisition device and analyzed. τ was measured as the first 63% of the exhaled tidal volume (V_T) and compared to the calculated τ as the product of expiratory resistance (R_E) and respiratory system compliance (C_RS), or τ derived from passive flow/volume waveforms using previously published equations as proposed by Aerts, Brunner, Guttmann, and Lourens. We tested if the duration of exponentially decaying flow during exhalation corresponded to the duration of the predicted second and third τ, based on multiples of the first measured τ.

Results: Mean (95% CI) measured τ was 0.59 (0.57-0.62) s and 0.60 (0.58-0.63) s for PCV and VCV (p = 0.45), respectively. Aerts method showed the shortest values of all methods for both modes: 0.57 (0.54-0.59) s for PCV and 0.58 (0.55-0.61) s for VCV. Calculated (C_RS * R_E) and Brunner's τ were identical with mean τ of 0.64 (0.61-0.67) s for PCV and 0.66 (0.63-069) s for VCV. Mean Guttmann's τ was 0.64 (0.61-0.68) in PCV and 0.65 (0.62-0.69) in VCV. Comparison of each τ method between PCV and VCV was not significant. Predicted time to exhale 95% of the V_T (i.e., 3*τ) was 1.77 (1.70-1.84) s for PCV and 1.80 (1.73-1.88) s for VCV, which was significantly longer than measured values: 1.27 (1.22-1.32) for PCV and 1.30 (1.25-1.35) s for VCV (p < 0.0001). The first, the second and the third measured τ were progressively shorter: 0.6, 0.4 and 0.3 s, in both ventilation modes (p < 0.0001).

Conclusion: All six methods to determine τ show similar values and are feasible in postoperative mechanically ventilated patients in both PCV and VCV modes.

Keywords: Exhalation; Expiratory time constant; Mechanical ventilation; Pressure-controlled ventilation; Volume-controlled ventilation.