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 (ETCO2) are often used to optimize ICP and improve oxygenation in TBI patients. This study evaluated the effects of PEEP and ETCO2 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 cmH(2)O and in ETCO2 from 40 to 30mmHg on both pathologic and nonpathologic sides. Results: Increasing PEEP and reducing ETCO2 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 cmH(2)O:ETCO2 40mmHg combination and lowest with PEEP of 5 cmH(2)O:ETCO2 30mmHg (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 ETCO2 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 cmH(2)O: ETCO2 40mmHg and PEEP of 5 cmH(2)O:ETCO2 30mmHg combinations, respectively (ONSD: 4.93 +/- 0.46 vs. 4.02 +/- 0.40mm, P < 0.001; rScO(2): 74.77 +/- 8.30% vs. 70.69 +/- 8.12%, P < 0.001). ETCO2 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 ETCO2 significantly decreased ONSD and rScO(2).