Relationship between arterial carbon dioxide and end-tidal carbon dioxide in mechanically ventilated adults with severe head trauma

M E Kerr, J Zempsky, S Sereika, P Orndoff, E B Rudy
Critical Care Medicine 1996, 24 (5): 785-90

OBJECTIVE: To examine the agreement and association of a noninvasive method of measuring CO2 (using end-tidal PCO2) with PaCO2 in mechanically ventilated adults with severe head trauma.

DESIGN: A prospective, quasi-experimental, repeated-measures study was used to compare end-tidal PCO2 and PaCO2 at two time points: before and after a standardized endotracheal suctioning procedure.

INTERVENTIONS: Controlled intervention of endotracheal suctioning.

SETTING: The study was conducted at two intensive care units designated as Level 1 trauma centers.

PATIENTS: A consecutive sample of 35 severe head-injured patients with a Glasgow Coma Scale score of < or = 8.

MEASUREMENTS AND MAIN RESULTS: End-tidal PCO2 and PaC02 values were simultaneously obtained and compared. End-tidal PCO2 was measured, using a sidestream sensor placed in line of the ventilator circuit's deadspace. Arterial gases were drawn from an indwelling arterial catheter. No relationship was found between arterial and end-tidal measures (range r2 = .09 to r2 = .11). Using the Bland-Altman technique, agreement decreased as the amount of positive end-expiratory pressure increased. When a subset of patients (mechanically ventilated, with positive end-expiratory pressures of < 5 cm H2O, paralyzed, and sedated) were examined (n = 12), the correlation between the CO2 measures improved (r2 = .77).

CONCLUSIONS: This study indicated that end-tidal PCO2 monitoring correlated well with PaCO2 in patients without respiratory complications or without spontaneous breathing, resulting in rebreathing of gases. However, its clinical validity is questionable in patients who have the greatest need for end-tidal PCO2 monitoring (i.e., patients who have respiratory distress or who are breathing spontaneously and overriding the ventilator.

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