Intraocular Pressure Pulse Parameters Decrease during Low-Level Hypercapnia

Abstract

Abstract: : Purpose: The inhalation of air mixed with CO₂ provides the opportunity to observe the eye under the stimulus of a known vasodilator. The aim of this study was to compare intraocular pressure (IOP) pulse measurements taken during low level hypercapnia to those taken whilst breathing ordinary room air. Methods: Twenty young healthy subjects with a mean age of 27.7 ± 4.9 yrs, were recruited to a randomised cross-over study. Investigations were undertaken under two breathing conditions: a) using inspired room air (normocapnic condition); and b) using room air mixed with additional CO₂ sufficient to raise their end-tidal CO₂ levels by 15% (hypercapnic condition). Pneumatonometric measurements of pulsatile ocular blood flow (POBF) and pulse amplitude (PA) were taken under each condition. Results: In order to achieve an increase in end-tidal CO₂ of 15%, an inhaled CO₂ level of 3.7 ± 0.7 % was required for the group. In comparison to ordinary room air, POBF and PA measures fell by 6.2% (p = 0.039) and 8.8% (p = 0.004) respectively under hypercapnia. Whereas IOP and systemic parameters of heart rate, arterial pulse pressure, and blood oxygen saturation remained stable, mean arterial blood pressure showed a significant rise of 4.0 mmHg (p < 0.001) under the hypercapnic condition. Conclusions: The findings of this study suggest that in contrast to previous reports using higher degrees of hypercapnia, mild hypercapnia results in a reduction of the IOP pulse parameters, POBF and PA. This may represent a shift from pulsatile to non-pulsatile flow under these conditions. Differences in or changes to POBF and PA measures taken in isolation should be treated with caution, and ideally interpreted in relation to other indices of ocular blood flow.

Keywords: blood supply • choroid • stress response