The diving reflex: bradycardia during cold ocular irrigation.

R E P L Y I thank Drs. Mezon and Badner for their thoughts on the case, but I don't think diminished gas flow in the face o f increased minute volume explains the rapid hypercarbia. Firstly, within 15 min the PETCO 2 increased from 32 to 45 mmHg without alteration o f ventilation or fresh gas flow. Secondly, I have never seen a doubling o f PETCO 2 simply by increasing the minute volume without augmenting the fresh gas flow. To test Drs. Mezon and Badner's hypothesis, I conducted a trial simulation during an anaesthetic given to a stable 79yr-old woman undergoing mastectomy. She was given a general anaesthetic with tracheal intubation and mechanical ventilation delivering nitrous oxide, oxygen and Forane. After increasing her ventilator settings from respiratory rate 10~tidal volume 800 ml to RR 20/ VT 1000 ml and decreasing N20/ O2 from 4/2 L" rain t to 3/L5 L" rain -~ for 20 rain, her PETC02 only changed from 36 to 38 mmHg. This does not appear to be a large effect and does not explain the dramatic hypercarbia changes seen in the case l presented. However, I do agree that increasing fresh gas flow would assist in the elimination o f CO 2 and this may be more important when using the Bain circuit.