EditorialEditorials

The Value of Expired Gas Analysis Is Never Expired

Hideaki Imanaka
Respiratory Care March 2011, 56 (3) 362-363; DOI: https://doi.org/10.4187/respcare.01237

Complications of mechanical ventilation increase with duration. Delayed weaning and premature weaning are both associated with increasing complications. In addition, adverse events and deterioration relating to the ventilator system could happen any time. Therefore, accurate monitoring of the patient's respiratory condition and adjustment of ventilator settings are essential during mechanical ventilation.1 End-tidal CO2 (PETCO2) monitoring is useful in mechanically ventilated patients in many situations, such as intensive care and anesthesia. Although PETCO2 is not a replacement or substitute for assessing the arterial CO2 (PaCO2), PETCO2 monitoring allows continuous evaluation of alveolar ventilation, severity of pulmonary disease, and response to therapy in mechanically ventilated patients.2,3

The closed-loop knowledge-based algorithm in the SmartCare automated ventilation and weaning system (Dräger Medical, Lübeck, Germany) was originally designed to continuously adapt the ventilatory assistance during pressure support ventilation to the patient's need, to manage a strategy of gradually decreasing the ventilatory assistance, and to indicate when the patient is able to breathe without assistance.4 As compared to a physician-controlled weaning process, the SmartCare computer-driven system reduced mechanical ventilation duration and intensive-care-unit stay but did not change the re-intubation rate.5 However, I still am not convinced that SmartCare is any better than skilled clinicians. When the SmartCare system was compared to weaning managed by experienced critical care nurses, with a 1:1 nurse-to-patient ratio, there was no substantial difference in weaning duration.6

SmartCare uses PETCO2 as a safety parameter in addition to respiratory rate and tidal volume to automatically control the pressure support level. When the respiratory rate is low, for example, PETCO2 can help differentiate between central hypoventilation leading to hypercapnia and hyperventilation with hypocapnia. In this issue of the Journal, Galia and coworkers7 extend the role of PETCO2 measurement in patients supported with the SmartCare system. Because a spontaneously breathing patient intermittently has higher individual PETCO2 values than the averaged PETCO2 value during prolonged expiration, they hypothesized that using the maximum PETCO2 value instead of the averaged PETCO2 value would improve PaCO2 estimation and would improve the classification of ventilatory diagnosis in the SmartCare system. Galia et al monitored breath-by-breath PETCO2 in 36 patients and compared the maximum PETCO2 values, averaged PETCO2 values, and PaCO2 values from arterial blood samples.

As they expected, the maximum PETCO2 was closer to PaCO2 than the averaged PETCO2. The averaged PETCO2 was smaller than PaCO2 by 10 ± 6 mm Hg, whereas that discrepancy decreased to 6 ± 6 mm Hg with the maximum PETCO2. They also observed that using the maximum PETCO2 value changed the SmartCare classification in 1.6% of the breaths. A great deal of interesting information was obtained in the study. They confirmed the role of PETCO2 in evaluating PaCO2 in mechanical ventilated patients. The maximum PETCO2 may lead to more accurate adjustment of automated ventilation systems.

However, after reading the report I was left with several questions. First, can the findings be extrapolated to other patients with different backgrounds and severity of illness? Second, is the maximum PETCO2 value also useful in other ventilation modes, such as assist control ventilation and intermittent mandatory ventilation? How do auto-PEEP and other cardiorespiratory variables affect the role of the maximum PETCO2? And finally, does use of the maximum PETCO2 value improve the weaning outcome in mechanically ventilated patients? The study by Galia et al7 was a simple observational study, with no randomization. To clarify the role of the maximum PETCO2 value in the weaning process with the SmartCare system, further study may be warranted, using a randomized design and a larger number of patients. Although PETCO2 monitoring is useful in many clinical situations, the challenge to clinicians is to objectively determine when this monitoring is indicated.

Footnotes

  • Correspondence: Hideaki Imanaka MD, Department of Emergency and Critical Care Medicine, Tokushima University Hospital, 3-18-15, Kuramoto, Tokushima 770-8503 Japan. E-mail: imanakah{at}clin.med.tokushima-u.ac.jp.

  • The author has disclosed no conflicts of interest.

  • See the Original Study on Page 278

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