Comments on the use of expiratory time constant for determinations of plateau pressure, respiratory system compliance, and total resistance

Critical Care201317:421

DOI: 10.1186/cc12561

Published: 27 March 2013

In the previous issue of Critical Care, Al-Rawas and colleagues [1] presented a method for assessing elastic and resistive properties of the respiratory system in patients with respiratory failure. The method assumes a first-order resistance-compliance (RC) model of breathing mechanics.

Although the procedure for estimating expiratory time constant (τE) is correct and efficient, some clarifications need to be made about the use of equations 3 to 5 in mechanically ventilated patients, in whom an intrinsic positive end-expiratory pressure (PEEPi) can be present [2, 3].

Equations 3 to 5 are correct only if the term indicated with the acronym PEEP accounts not only for external PEEP but also for PEEPi. Neglecting PEEPi can lead to significant errors, as demonstrated by the following model-to-model analysis, in which data were generated by using a first-order RC model. Table 1 shows the true model values and their corresponding estimates, which were obtained with the method of Al-Rawas and colleagues and which neglect PEEPi.
Table 1

True model values and corresponding estimates

Patient

Parameter or variable

True value

Estimated value

Percentage error

A

τE

0.25 s

0.25 s

0%

 

Rtot

5 cm H2O × s/L

5 cm H2O × s/L

0%

 

Crs

0.05 L/cm H2O

0.05 L/cm H2O

0%

 

Pplt

13 cm H2O

13 cm H2O

0%

B

τE

1.995 s

1.995 s

0%

 

Rtot

21 cm H2O × s/L

24.6 cm H2O × s/L

17%

 

Crs

0.095 L/cm H2O

0.0812 L/cm H2O

−14.5%

 

Pplt

9.78 cm H2O

8.00 cm H2O

−18.2%

A volume-controlled continuous mandatory ventilation was simulated with a constant inspiratory flow, a tidal volume of 0.65 L, a breathing frequency of 15 breaths per minute, an inspiration time of 1.3 seconds, a post-expiratory pause of 0.3 seconds, and an expiration time of 2.4 seconds. No external positive end-expiratory pressure was simulated. For each patient, 10 breathing cycles were simulated to allow the steady state to be reached. Data from the last simulated cycle were used to estimate expiratory time constant (τE), plateau pressure (Pplt), respiratory system compliance (Crs), and total resistance (Rtot).

The estimate of τE is correct for both patients because, in the first-order RC model, the relationship between lung volume changes and expiratory flow does not depend on PEEP (intrinsic or external or both). The estimates of total resistance (Rtot), respiratory system compliance (Crs), and plateau pressure (Pplt) for patient A (in whom PEEPi is 0) are identical to the true values. For patient B (in whom PEEPi is equal to about 3 cm H2O), the percentages of error of the estimations of Rtot, Crs, and Pplt are 17%, −14.5%, and −18.2%, respectively. This confirms that these estimates necessitate the measurement of PEEPi and this requires, for example, the end-expiratory occlusion technique [4, 5].

Authors' response

Nawar Al-Rawas, Michael J Banner, Neil R Euliano, A Daniel Martin, Carl Tams and Andrea Gabrielli

We thank the editor for the opportunity to respond to this letter. At this time, it is unclear whether PEEPi or some critical level of PEEPi may confound determinations of Pplt, Crs, and Rtot when the τE method is used as we described [1]. A proper and scientific way to address this matter is through a carefully conducted, follow-up clinical study of patients with acute or chronic forms of respiratory failure (such as chronic obstructive pulmonary disease), in which total PEEP (including its component parts of PEEPi and applied PEEP) is correctly determined for an appropriate number of patients by using our τE method.

Abbreviations

Crs

respiratory system compliance

PEEP: 

positive end-expiratory pressure

PEEPi: 

intrinsic positive end-expiratory pressure

Pplt

plateau pressure

RC: 

resistance-compliance

Rtot

total resistance

τE

expiratory time constant

Declarations

Authors’ Affiliations

(1)
Department of Medical Biotechnology, University of Siena

References

  1. Al-Rawas N, Banner MJ, Euliano NR, Tams CG, Brown J, Martin AD, Gabrielli A: Expiratory time constant for determinations of plateau pressure, respiratory system compliance, and total resistance. Crit Care 2013, 17:R23.PubMedView Article
  2. Appendini L: About the relevance of dynamic intrinsic PEEP (PEEPi, dyn) measurement. Intensive Care Med 1999, 25:252–254.PubMedView Article
  3. Leith DE, Brown R: Human lung volumes and the mechanisms that set them. Eur Respir J 1999, 13:468–472.PubMedView Article
  4. Mughal MM, Culver DA, Minai OA, Arroliga AC: Auto-positive end-expiratory pressure: mechanisms and treatment. Cleve Clin J Med 2005, 72:801–809.PubMedView Article
  5. Pepe PE, Marini JJ: Occult positive end-expiratory pressure in mechanically ventilated patients with airflow obstruction. Am Rev Respir Dis 1982, 126:166–170.PubMed

Copyright

© BioMed Central Ltd 2013

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