Several studies comparing retrobulbar block (RB) and general anaesthesia (GA) for cataract surgery in the elderly have been published. Most of them were retrospective. Our prospective study was designed in order to determine the benefits or disadvantages using RB or GA. Arterial blood gases (ABG) and cardiovascular stability were explored in high-risk patients undergoing elective unilateral cataract extraction. METHODS. Forty patients over 65 years of age and with other co-existing diseases (ASA III-IV) were allocated randomly to receive either GA or RB. No premedication was given to either group of patients. On arrival in the anaesthetic room, a radial artery was cannulated for collection of blood samples and direct monitoring of the blood pressure. Pulse oximetry and ECG were continuously monitored in all patients, the end-expiratory CO2 (F(eexCO2)) only in the GA group. GA was induced with vecuronium 0.1 mg/kg and thiopentone 5 mg/kg; the lungs were ventilated with 100% oxygen. After intubation of the trachea controlled mechanical ventilation was continued with N2O/O2 (55:45) and enflurane 1%-2%. Only enflurane concentrations were varied to correct changes in mean arterial pressure (MAP) if these exceeded +/- 20%. Respiratory frequency and tidal volume were kept constant until completion of surgery. The patients were extubated when they were able to ventilate more than 5 1/min (pressure support 10 cmH2O; PEEP 5 cmH2O). After extubation no O2 was given. In the RB group neural block was undertaken with prilocaine 2% (3 ml) as a retrobulbar injection and prilocaine 1% (5 ml) to block the facial innervation of the orbicularis muscle (Van Lint, O'Brien). Oxygen 3 1/min was administered by nasal tube during the operation. Nine arterial samples for blood gas analysis were collected: (1) control; (2) before operation; (3) 5 min after beginning the operation; (4) 15 min after beginning the operation and before i.v. administration of 500 mg acetazolamide over 5 min; (5) after acetazolamide; (6 and 7) 10 and 20 min after acetazolamide; and (8 and 9) 15 and 30 min after operation (RB) or extubation (GA). RESULTS. The patient demography, including duration of anaesthesia and operation, was similar in both groups (Table 1). Four patients in the GA group (2 needed O2 after extubation because of hypoxaemia) and 2 in the RB group were excluded. No significant differences were seen in base excess (BE) and standard bicarbonate (SHCO3). Arterial O2 tension, arterial O2 saturation, and pulse-oximetric O2 saturation were higher in the RB group intra- and postoperatively (Figs. 1, 3, 4). Arterial CO2 tension (PaCO2) was significantly higher in the GA group during the pre- and postoperative period (Fig. 2), but not during the operation. The PaCO2- F(eexCO2) gradient ranged between 5 and 9 mmHg. Administration of acetazolamide did not influence this gradient by regressive analysis. The postoperative outcome of the patients was comparable in both groups. Nausea or vomiting did not occur. MAP was significantly higher in the RB group during the operation. No significant differences were seen in the pre- and postoperative period. Heart rate in the GA group was higher only after extubation, but was within physiological limits. DISCUSSION. Despite the differing results between the two groups, our study showed no important advantage related to either RB or GA. Changes in ABG, MAP, and heart rate during the investigation period were within physiological limits in elderly patients. Intravenous acetazolamide did not influence ABG in a significant manner. With regard to the preference of each patient, we recommend both RB and GA for cataract surgery in high-risk patients on the assumption of sufficient preoperative treatment of co-existing diseases. In conclusion, cardiovascular and ABG stability were maintained during both anaesthetic techniques.