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Ophthalmic artery blood flow velocity increases during hypocapnia

[La vitesse circulatoire de l’artère ophtalmique augmente pendant l’hypocapnie]

Lorri A. Lee, MD^*, Monica S. Vavilala, MD^*,, Arthur M. Lam, MD FRCPC^*,, Colleen Douville, BA RVT, Anne Moore, RVT, Elizabeth Visco, CRNA^* and David W. Newell, MD

^* From the Departments of Anesthesiology,
Pediatrics, and
Neurological Surgery, Harborview Medical Center, University of Washington, Seattle, Washington, USA.

Address correspondence to: Dr. Lorri A. Lee, Department of Anesthesiology, Harborview Medical Center, Box 359724, 325 Ninth Avenue, Seattle, Washington 98104-2499, USA. Phone: 206-731-3059; Fax: 206-731-8009; E-mail: lorlee{at}u.washington.edu

Purpose: The effects of anesthetic management on blood flow to the optic nerve have not been well-studied. The ophthalmic artery provides the majority of the blood supply to the optic nerve via several smaller branches. Retinal blood flow has been shown to react to carbon dioxide (CO₂) similar to intracranial vessels, but insufficient data exist for the ophthalmic artery. The purpose of this study is to examine the CO₂-reactivity of the ophthalmic artery.

Methods: Eight healthy awake subjects aged 28 to 50 yr were tested for CO₂-reactivity in the ophthalmic artery using transcranial Doppler (TCD) insonation of blood flow velocity (V_op), while simultaneously recording the V_op of the middle cerebral artery (V_mca) as an internal control. V_op and V_mca recordings were made under hypocapnic, normocapnic and hypercapnic conditions.

Results: The CO₂-reactivity slope of V_mca was 3.27% per mmHg PaCO₂. From normocapnia to hypercapnia, V_op did not change significantly (mean ± SD, 18 ± 4 cm•sec^–1 to 18 ± 6 cm•sec^–1), (end-tidal CO₂, etCO₂, = 43 ± 5 mmHg to 53 ± 4 mmHg, respectively). In contrast, V_op increased significantly under hypocapnic conditions (etCO₂ = 26 ± 4 mmHg) to 25 ± 5 cm•sec^–1 (P < 0.05). The CO₂-reactivity slope of V_op from normocapnia to hypocapnia was 2.57% per mmHg.

Conclusions: This study demonstrates that V_op increases with hypocapnia, but is unaffected by hypercapnia. The anastomoses of the ophthalmic artery with the external carotid artery, which displays a relatively fixed resistance, may account for these findings.