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Arterial to end-tidal carbon dioxide pressure difference and correlation of the difference with end-expiratory pressure in critically ill patients with severe lung injury

Setting

The general ICU of Hyllel Yaffe Medical Center, Israel.

Objective

End tidal carbon dioxide (ETCO2) monitoring is a non-invasive way to estimate blood carbon dioxide (PCO2). The purpose of this study was to determine whether ETCO2 measurement reliably indicates PCO2 in mechanically ventilated patients and analyses the effects of positive end-expiratory pressure (PEEP) on volumetric capnography and respiratory system mechanics in mechanically ventilated patients.

Methods

Twenty normal subjects (control group) and 20 patients with acute respiratory distress syndrome (ARDS group) were studied. Respiratory system mechanics – compliance and Bohr's dead space (physiologic dead space to tidal volume ratio [VD/VT(Bohr)]) – at different levels of PEEP were measured. ETCO2 and carbon dioxide in arterial blood (PaCO2) measurements were recorded. ETCO2 was measured using a mainstream ETCO2 monitor by Mannen Medical. All patients were ventilated with a Dreger Evita2 ventilator. Demographic data and primary diagnosis were recorded. Linear regression was used to analyze ETCO2/PCO2 pairs. Statistical significance was considered P < 0.05.

Results

See Table 1. The mean dead-space fraction was markedly elevated (0.58 ± 0.01) at PEEP 11 cmH2O and 0.66 ± 0.02 at PEEP 15 cmH2O early in the course of ARDS. Large differences were found between PaCO2 and mixed expired carbon dioxide (PETCO2) in ARDS patients. The difference between arterial and end-tidal PCO2 correlated closely with VD/VT.

Table 1 Volumetric capnographic indices at different PEEP levels in control and ARDS patients

Conclusion

Our studies confirm that PETCO2 is a poor estimate of PaCO2 in patients with respiratory failure in PEEP values greater than 11 cmH2O. Furthermore, the PaCO2-PETCO2 gradient is not stable over time and cannot predict variations of PaCO2. The use of PETCO2 instead of PaCO2 could be deleterious in patients in whom strict control of PaCO2 values is required.

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Isakovich, B., Mahtais, B. & Litmanovitch, M. Arterial to end-tidal carbon dioxide pressure difference and correlation of the difference with end-expiratory pressure in critically ill patients with severe lung injury. Crit Care 10, P350 (2006). https://doi.org/10.1186/cc4697

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Keywords

  • Dead Space
  • Carbon Dioxide Pressure
  • Physiologic Dead Space
  • Severe Lung Injury
  • Respiratory System Mechanic