Effects of oxygen and carbogen breathing on choroidal hemodynamics in humans

Previous studies in humans have demonstrated that oxygen (O2) reduces retinal vessel caliber and blood flow, whereas carbon dioxide (CO2) usually has opposite effects. The influence of O2 and CO2 on choroidal circulation is not fully understood, however. This study was conducted to determine the effects of systemic hyperoxia and hypercapnia on global choroidal hemodynamics, as evaluated by pulsatile ocular blood flow (POBF) tonography.

In this experiment, 16 and 22 healthy volunteers breathed 100% O2 and carbogen, respectively. POBF and intraocular pressure (IOP) were measured twice with a UK OBF tonograph for each of the following conditions: in ambient room air breathing, after breathing pure O2 or carbogen through a face mask, and in ambient room air 10 minutes after mask removal. Heart rate (HR), hemoglobin oxygen saturation level (SaO2), and systemic arterial blood pressure (BP) were monitored throughout testing. The end-tidal CO2 (EtCO2) level and respiratory rate (RR) were also recorded during carbogen breathing.

Results revealed that HR was reduced (P<0.004) and SaO2 was increased (P = 0.0001) by both oxygen and carbogen breathing. Systemic arterial BP remained stable throughout the experiment. EtCO2 was increased during carbogen breathing (P = 0.0001), whereas RR was reduced (P = 0.0175). IOP was significantly decreased during both phases of the experiment (P = 0.0001). Finally, POBF was not altered by pure O2 breathing, but it increased on average by 7.7% during carbogen breathing (P = 0.0222).

The data obtained with POBF tonography indicate that the choroid reacts to increased blood CO2 concentration, but not to systemic hyperoxia, in a manner similar to that in retinal and brain vessels.