RETINAL, CHOROIDAL AND TOTAL OCULAR BLOOD-FLOW RESPONSE TO HYPERCARBIA DURING SPONTANEOUS BREATHING AND MECHANICAL VENTILATION

The effect of hypercarbia on ocular blood flow was studied in the newborn piglet with the isotope-labeled microsphere method. Blood flow measurements were made during spontaneous breathing and during paralyzation (pancuronium) and mechanical ventilation. Retinal blood flow increased from 0.40 +/- 0.07 (mean +/- SEM) ml/min/g at baseline levels to 0.91 +/- 0.17 ml/min/g at a PaCO2 level of 11.0 kPa during spontaneous ventilation. A similar response was observed during paralyzation and mechanical ventilation (0.89 +/- 0.15 ml/min/g at a PaCO2 of 11.1 kPa). For choroidal blood flow, however, the increase caused by hypercarbia during spontaneous ventilation (16.14 +/- 3.69 to 29.15 +/- 3.22 ml/min/g) was significantly reduced when the animals were paralyzed and mechanically ventilated (15.99 +/- 2.99 to 23.51 +/- 3.41 ml/min/g). Since choroidal blood flow accounts for 60-80% of oxygen delivery to the retina, paralyzation and mechanical ventilation may significantly reduce oxygen delivery to the retina during hypercarbia.