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.