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End-tidal carbon dioxide in critically ill patients during changes in mechanical ventilation
R A Hoffman1, B P Krieger, M R Kramer
1Division of Pulmonary Disease, University of Miami School of Medicine, Mount Sinai Medical Center, Miami Beach, Florida.
The American Review of Respiratory Disease
|November 1, 1989
Summary
End-tidal CO2 (PETCO2) can approximate arterial CO2 (PaCO2) in stable patients. However, PETCO2 may be misleading for monitoring PaCO2 trends in critically ill patients on mechanical ventilation due to inconsistent gradients.
Area of Science:
- Critical Care Medicine
- Respiratory Physiology
- Pulmonary Medicine
Background:
- End-tidal CO2 (PETCO2) is often used to estimate arterial CO2 (PaCO2) in spontaneously breathing individuals and mechanically ventilated patients.
- Ventilation/perfusion (V/Q) inequalities, common in critically ill patients, may impact the reliability of PETCO2 as a surrogate for PaCO2.
Purpose of the Study:
- To evaluate the ability of PETCO2 to predict changes in PaCO2 (delta PaCO2) and minute ventilation (delta Ve) in critically ill patients receiving mechanical ventilatory support (MVS).
- To assess the consistency of the PaCO2-PETCO2 gradient in this patient population.
Main Methods:
- Twenty intubated patients with respiratory failure on MVS were studied.
- Mechanical ventilator settings (frequency, tidal volume) were adjusted, and simultaneous PaCO2 and PETCO2 measurements were taken after equilibration.
- Arterial CO2 (PaCO2), end-tidal CO2 (PETCO2), tidal volume (Vt), and minute ventilation (Ve) were recorded.
Main Results:
- A significant correlation was found between PETCO2 and PaCO2 (r = 0.78, p < 0.001).
- However, the correlation between changes in PETCO2 (delta PETCO2) and changes in PaCO2 (delta PaCO2) was weaker (r = 0.58, p < 0.001).
- In some patients, the trends of PETCO2 and PaCO2 moved in opposite directions during ventilation changes, indicating an inconstant gradient.
Conclusions:
- While PETCO2 correlates with PaCO2 in critically ill patients on MVS, it may not reliably track changes in PaCO2.
- The inconstant PaCO2-PETCO2 gradient in this population suggests that PETCO2 monitoring for PaCO2 trends can be misleading.
- Clinicians should exercise caution when using PETCO2 as a noninvasive substitute for PaCO2 trend monitoring in critically ill patients.