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How to use the expiratory time constant

Article

Author: Dr. med. Jean-Michel Arnal, Senior Intensivist, Hopital Sainte Musse, Toulon, France

Date of first publication: 25.04.2018

The expiratory time constant (RCexp) is a dynamic measurement of respiratory mechanics measured breath-by-breath on all Hamilton Medical ventilators.
How to use the expiratory time constant

Product of compliance and resistance

RCexp is reliable in both passive and spontaneously breathing patients, assuming expiration is passive. It may also be used during NIV, provided there are no unintentional leaks.

As RCexp is the product of compliance and resistance, this single variable can provide us with an assessment of the overall respiratory mechanics. It is very useful for diagnosing the lung condition and its severity, optimizing the ventilator settings, monitoring prone positioning, and understanding certain respiratory events.

Expiratory time constant in a normal lung

In a mechanically ventilated patient with a normal lung, RCexp is normally between 0.5 and 0.7 s (see Figure 1). However, it is important to check that compliance and resistance values are also within a normal range, because a mixed lung condition combining decreased compliance and increased resistance may result in a pseudo-normal RCexp.

Screenshot of display showing RCexp of 0.60
Figure 1: Typical respiratory mechanics for a normal-lung patient (green border indicates normal resistance and compliance referenced to height)
Screenshot of display showing RCexp of 0.60
Figure 1: Typical respiratory mechanics for a normal-lung patient (green border indicates normal resistance and compliance referenced to height)

A short expiratory time constant

An expiratory time constant shorter than 0.5 s indicates a decrease in compliance, either due to the lung or the chest wall (see Figure 2). In ARDS patients, RCexp is typically in the range of 0.4 to 0.6 s. It is shorter in patients with more severe ARDS, indicating low compliance and a small volume of aerated lung. In patients with lung fibrosis or chest-wall stiffness such as kyphoscoliosis, RCexp is usually very short and ranges from 0.15 to 0.25 s.

Screenshot of display showing RCexp of 0.41
Figure 2: Typical respiratory mechanics monitoring for an ARDS patient
Screenshot of display showing RCexp of 0.41
Figure 2: Typical respiratory mechanics monitoring for an ARDS patient

A long expiratory time constant

An expiratory time constant of longer than 0.7 s indicates increased resistance, which may be associated with increased compliance in the case of COPD patients with lung emphysema (see Figure 3). A long RCexp is typical in COPD and asthmatic patients. In patients with severe bronchospasm, RCexp can be as long as 3 s. If the patient is not COPD or asthmatic, a long RCexp may indicate incorrect positioning or kinking of the endotracheal tube.

Screenshot of display showing RCexp of 1.68
Figure 3: Typical respiratory mechanics monitoring for a COPD patient
Screenshot of display showing RCexp of 1.68
Figure 3: Typical respiratory mechanics monitoring for a COPD patient

RCexp for optimizing ventilator settings

Patients with a short RCexp are at risk of ventilator-induced lung injuries and should be closely monitored for tidal volume, driving pressure, and plateau pressure. In contrast, patients with a long RCexp are at risk of dynamic hyperinflation, so intrinsic PEEP should be measured regularly.

In pressure-support and ASV® modes, expiratory trigger sensitivity (ETS) is an important setting for optimizing patient-ventilator synchronization. ETS represents the percentage of the maximum inspiratory flow at which the mechanical breath ends. A high percentage results in a short mechanical breath and vice versa. This setting can be optimized according to the respiratory mechanics.

Adjusting ETS based on RCexp

As an initial approach, ETS may be adjusted based on RCexp as follows:

RCexp ETS
Normal 25%–40%
Short 5%–25%
Long 40%–70%

RCexp for monitoring prone positioning

The effect of prone positioning on respiratory mechanics can be assessed using the trend of RCexp and compliance. If prone positioning is associated with lung recruitment, it is indicated by an increase in compliance and RCexp. If RCexp increases with no change to compliance, the clinician should check for incorrect positioning or kinking of the endotracheal tube.

The image below displays an example of respiratory mechanics trends in the supine and prone position. The cursor indicates the start of the prone-position session. After prone positioning, RCexp and compliance both increase, indicating lung recruitment (Figure 4).

Screenshot showing increase in RCexp and compliance
Figure 4: Respiratory mechanics trends in the supine and prone position
Screenshot showing increase in RCexp and compliance
Figure 4: Respiratory mechanics trends in the supine and prone position

RCexp for understanding respiratory events

A sudden event leading to desaturation and/or an increase in airway pressure needs a rapid diagnosis. Looking at trends for RCexp will help us to understand whether the event is related to a rapid change in respiratory mechanics. An increase in RCexp indicates one of the following: an endotracheal tube obstruction or malpositioning, a patient biting the endotracheal tube, an excess of secretions, or a bronchospasm. Conversely, a decrease in RCexp indicates a pneumothorax, pleural effusion, or atelectasis. Sudden desaturation without a change in RCexp indicates a drop in cardiac output or a severe pulmonary embolism.

 

Full citations below: (Arnal JM, Garnero A, Saoli M, Chatburn RL. Parameters for Simulation of Adult Subjects During Mechanical Ventilation. Respir Care. 2018;63(2):158-168. doi:10.4187/respcare.057751​)

Parameters for Simulation of Adult Subjects During Mechanical Ventilation.

Arnal JM, Garnero A, Saoli M, Chatburn RL. Parameters for Simulation of Adult Subjects During Mechanical Ventilation. Respir Care. 2018;63(2):158-168. doi:10.4187/respcare.05775



BACKGROUND

Simulation studies are often used to examine ventilator performance. However, there are no standards for selecting simulation parameters. This study collected data in passively-ventilated adult human subjects and summarized the results as a set of parameters that can be used for simulation studies of intubated, passive, adult subjects with normal lungs, COPD, or ARDS.

METHODS

Consecutive adult patients admitted to the ICU were included if they were deeply sedated and mechanically ventilated for <48 h without any spontaneous breathing activity. Subjects were classified as having normal lungs, COPD, or ARDS. Respiratory mechanics variables were collected once per subject. Static compliance was calculated as the ratio between tidal volume and driving pressure. Inspiratory resistance was measured by the least-squares fitting method. The expiratory time constant was estimated by the tidal volume/flow ratio.

RESULTS

Of the 359 subjects included, 138 were classified as having normal lungs, 181 as ARDS, and 40 as COPD. Median (interquartile range) static compliance was significantly lower in ARDS subjects as compared with normal lung and COPD subjects (39 [32-50] mL/cm H2O vs 54 [44-64] and 59 [43-75] mL/cm H2O, respectively, P < .001). Inspiratory resistance was significantly higher in COPD subjects as compared with normal lung and ARDS subjects (22 [16-33] cm H2O/L/s vs 13 [10-15] and 12 [9-14] cm H2O/L/s, respectively, P < .001). The expiratory time constant was significantly different for each lung condition (0.60 [0.51-0.71], 1.07 [0.68-2.14], and 0.46 [0.40-0.55] s for normal lung, COPD, and ARDS subjects, respectively, P < .001). In the subgroup of subjects with ARDS, there were no significant differences in respiratory mechanics variables among mild, moderate, and severe ARDS.

CONCLUSIONS

This study provides educators, researchers, and manufacturers with a standard set of practical parameters for simulating the respiratory system's mechanical properties in passive conditions.