RELATIONSHIP BETWEEN ARTERIAL CARBON-DIOXIDE AND END-TIDAL CARBON-DIOXIDE IN MECHANICALLY VENTILATED ADULTS WITH SEVERE HEAD TRAUMA

Objective: To examine the agreement and association of a noninvasive m ethod of measuring CO2 (using end-tidal Pco(2)) with Paco(2) in mechan ically ventilated adults with severe head trauma, Design: A prospectiv e, quasi-experimental, repeated-measures study was used to compare end -tidal Pco(2) and Paco(2) at two time points: before and after a stand ardized endotracheal suctioning procedure, Interventions: Controlled i ntervention of endotracheal suctioning. Setting: The study was conduct ed at two intensive care units designated as Level 1 trauma centers, P atients: A consecutive sample of 35 severe head injured patients with a Glasgow Coma Scale score of less than or equal to 8. Measurements an d Main Results: End-tidal Pco(2) and Paco(2) values were simultaneousl y obtained and compared, End-tidal Pco(2) was measured, using a sidest ream sensor placed in line of the ventilator circuit's deadspace, Arte rial gases were drawn from an indwelling arterial catheter, No relatio nship was found between arterial and end tidal measures (range r(2) = .09 to r(2) = .11), Using the Bland-Altman technique, agreement decrea sed as the amount of positive end-expiratory pressure increased. When a subset of patients (mechanically ventilated, with positive end-expir atory pressures of <5 cm H2O, paralyzed, and sedated) were examined (n = 12), the correlation between the CO2 measures improved (r(2) = .77) , Conclusions: This study indicated that end-tidal Pco(2), monitoring correlates well with Paco(2) in patients without respiratory complicat ions or without spontaneous breathing, resulting in rebreathing of gas es. However, its clinical validity is questionable in patients who hav e the greatest need for end-tidal Pco(2) monitoring (i.e., patients wh o have respiratory distress or who are breathing spontaneously and ove rriding the ventilator).