Effect of Respiratory Physiological Changes on Optic Nerve Sheath Diameter and Cerebral Oxygen Saturation in Patients With Acute Traumatic Brain Injury

Background: Severe traumatic brain injury (TBI) results in raised intracranial pressure (ICP). Ultrasonographic measurement of the optic nerve sheath diameter (ONSD) is a noninvasive method for the assessment of raised ICP. Manipulation of positive end-expiratory pressure (PEEP) and end-tidal carbon dioxide (ETCO 2 ) are often used to optimize ICP and improve oxygenation in TBI patients. This study evaluated the effects of PEEP and ETCO 2 on ONSD and regional cerebral oxygen saturation (rScO 2 ) in patients with acute TBI. Methods: A total of 14 patients (11 males) aged older than 18 years with acute severe TBI were included in this study. ONSD and rScO 2 were assessed before and after changes in PEEP from 5 to 10 cm H 2 O and in ETCO 2 from 40 to 30 mm Hg on both pathologic and nonpathologic sides. Results: Increasing PEEP and reducing ETCO 2 resulted in changes in ONSD and rScO 2 on both pathologic and nonpathologic sides. On the pathologic side, ONSD and rScO 2 were highest with a PEEP of 10 cm H 2 O:ETCO 2 40 mm Hg combination and lowest with PEEP of 5 cm H 2 O:ETCO 2 30 mm Hg (ONSD 5.24±0.49 vs. 4.27±0.36 mm, P <0.001; rScO 2 70.7±9.91% vs. 66.3±9.75%, P <0.001); both PEEP and ETCO 2 had significant effects on ONSD and rScO 2 ( P <0.001). On the nonpathologic side, ONSD and rScO 2 were highest and lowest with PEEP of 10 cm H 2 O:ETCO 2 40 mm Hg and PEEP of 5 cm H 2 O:ETCO 2 30 mm Hg combinations, respectively (ONSD: 4.93±0.46 vs. 4.02±0.40 mm, P <0.001; rScO 2 : 74.77±8.30% vs. 70.69±8.12%, P <0.001). ETCO 2 had a significant effect on rScO 2 ( P <0.001), but the impact of PEEP on rScO 2 was not statistically significant ( P =0.05). Conclusion: Increasing PEEP resulted in significant increases in ONSD and rScO 2 , whereas reducing ETCO 2 significantly decreased ONSD and rScO 2 .