Effects of acute controlled changes in end‐tidal carbon dioxide on the diameter of the optic nerve sheath: a transorbital ultrasonographic study in healthy volunteers

• Published in: Anaesthesia Vol. 72; no. 5; pp. 618 - 623
• Main Authors: Dinsmore, M., Han, J. S., Fisher, J. A., Chan, V. W. S., Venkatraghavan, L.
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01-05-2017
• Subjects: Adult; Carbon dioxide; Carbon Dioxide - blood; cerebral vascular resistance: vasodilation; Female; Healthy Volunteers; Hemodynamics; Humans; hypercapnia; Hypercapnia - diagnostic imaging; Hypocapnia - diagnostic imaging; Intracranial Hypertension - diagnosis; Intracranial Pressure; Male; Optic nerve; Optic Nerve - diagnostic imaging; Point-of-Care Testing; Prospective Studies; Ultrasonography; ultrasonography: echogenicity; cerebral vascular resistance: vasodilation; intracranial pressure; ultrasonography: echogenicity; hypercapnia
• ISSN: 0003-2409; 1365-2044
• DOI: 10.1111/anae.13784

Summary
Transorbital ultrasonographic measurement of the diameter of the optic nerve sheath is a non‐invasive, bed‐side examination for detecting raised intracranial pressure. However, the ability of the optic nerve sheath diameter to predict acute changes in intracranial pressures remains unknown. The aim of this study was to examine the dynamic changes of the optic nerve sheath diameter in response to mild fluctuations in cerebral blood volume induced by changes in end‐tidal carbon dioxide. We studied 11 healthy volunteers. End‐tidal carbon dioxide was controlled by a model‐based prospective end‐tidal targeting system (RespirAct™). The volunteers' end‐tidal carbon dioxide was targeted and maintained for 10 min each at normocapnia (baseline); hypercapnia (6.5 kPa); normocapnia (baseline 1); hypocapnia (3.9 kPa) and on return to normocapnia (baseline 2). A single investigator repeatedly measured the optic nerve sheath diameter for 10 min at each level of carbon dioxide. With hypercapnia, there was a significant increase in optic nerve sheath diameter, with a mean (SD) increase from baseline 4.2 (0.7) mm to 4.8 (0.8) mm; p < 0.001. On return to normocapnia, the optic nerve sheath diameter rapidly reverted back to baseline values. This study confirms dynamic changes in the optic nerve sheath diameter with corresponding changes in carbon dioxide, and their reversibly with normocapnia.