Rapid evaluation and management of intracranial pressure (ICP) can help to early detection of increased ICP and improve postoperative outcomes in neurocritically-ill patients. Sonographic measurement of optic nerve sheath diameter (ONSD) is a non-invasive method of evaluating increased intracranial pressure at the bedside. In the present study, we hypothesized that sonographic ONSD, as a surrogate of ICP change, can be dynamically changed in response to carbon dioxide change using short-term hyperventilation.

Fourteen patients were enrolled. During general anesthesia, end-tidal carbon dioxide concentration (ETCO₂) was decreased from 40 mmHg to 30 mmHg within 10 minutes. ONSD, which was monitored continuously in the single sonographic plane, was repeatedly measured at 1 and 5 minutes with ETCO₂ 40 mmHg (time-point 1 and 2) and measured again at 1 and 5 minutes with ETCO₂ 30 mmHg (time-point 3 and 4).

The mean ± standard deviation of ONSD sequentially measured at four time-points were 5.0 ± 0.5, 5.0 ± 0.4, 3.8 ± 0.6, and 4.0 ± 0.4 mm, respectively. ONSD was significantly decreased at time-point 3 and 4, compared with 1 and 2 (P < 0.001).

The ONSD was rapidly changed in response to ETCO₂. This finding may support that ONSD may be beneficial to close ICP monitoring in response to CO₂ change.