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Expiratory time constant for determinations of plateau pressure, respiratory system compliance, and total resistance

Nawar Al-Rawas1, Michael J Banner1*, Neil R Euliano2, Carl G Tams2, Jeff Brown1, A Daniel Martin13 and Andrea Gabrielli14

Author Affiliations

1 Department of Anesthesiology, College of Medicine, University of Florida, PO Box 100254, Gainesville, FL 32610-0254, USA

2 Convergent Engineering, 107 SW 140th Terrace, #1, Newberry, FL 32669, USA

3 Department of Physical Therapy, College of Public Health and Health Professions, University of Florida, PO Box 100154, Gainesville, FL 32610-0154, USA

4 Department of Surgery, College of Medicine, University of Florida, PO Box 100254, Gainesville, FL, USA

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Critical Care 2013, 17:R23  doi:10.1186/cc12500

See related letters by Barbini, and Becher et al.,

Published: 5 February 2013



We hypothesized the expiratory time constant (ƬE) may be used to provide real time determinations of inspiratory plateau pressure (Pplt), respiratory system compliance (Crs), and total resistance (respiratory system resistance plus series resistance of endotracheal tube) (Rtot) of patients with respiratory failure using various modes of ventilatory support.


Adults (n = 92) with acute respiratory failure were categorized into four groups depending on the mode of ventilatory support ordered by attending physicians, i.e., volume controlled-continuous mandatory ventilation (VC-CMV), volume controlled-synchronized intermittent mandatory ventilation (VC-SIMV), volume control plus (VC+), and pressure support ventilation (PSV). Positive end expiratory pressure as ordered was combined with all aforementioned modes. Pplt, determined by the traditional end inspiratory pause (EIP) method, was combined in equations to determine Crs and Rtot. Following that, the ƬE method was employed, ƬE was estimated from point-by-point measurements of exhaled tidal volume and flow rate, it was then combined in equations to determine Pplt, Crs, and Rtot. Both methods were compared using regression analysis.


ƬE, ranging from mean values of 0.54 sec to 0.66 sec, was not significantly different among ventilatory modes. The ƬE method was an excellent predictor of Pplt, Crs, and Rtot for various ventilatory modes; r2 values for the relationships of ƬE and EIP methods ranged from 0.94 to 0.99 for Pplt, 0.90 to 0.99 for Crs, and 0.88 to 0.94 for Rtot (P <0.001). Bias and precision values were negligible.


We found the ƬE method was just as good as the EIP method for determining Pplt, Crs, and Rtot for various modes of ventilatory support for patients with acute respiratory failure. It is unclear if the ƬE method can be generalized to patients with chronic obstructive lung disease. ƬE is determined during passive deflation of the lungs without the need for changing the ventilatory mode and disrupting a patient's breathing. The ƬE method obviates the need to apply an EIP, allows for continuous and automatic surveillance of inspiratory Pplt so it can be maintained ≤ 30 cm H2O for lung protection and patient safety, and permits real time assessments of pulmonary mechanics.