Sir,

I would like to congratulate doctors Inan et al1 on yet another well-conducted study into the extensively documented problem of Carbon dioxide (CO2) retention under surgical drapes.2 Their technique appears to be efficacious; however, the clinical need for such a system for cataract patients is questionable. In the recent surveys conducted by the Royal College of Ophthalmology, it was noted that there had been a drop in the rate of sedation for cataract surgery under local anaesthetic block from 45% in 1991 to 8% in 1996. This rate has probably reduced further since Katz demonstrated an improved safety record for unsedated patients.3, 4 Where sedation with midazolam is required, a dose of 0.02 mg/kg or 1–1.5 mg is generally effective for the average cataract patient compared with the 3 mg used in the present study. In elderly patients, 3 mg may lead to hypoventilation, CO2 retention and reduced cerebral responsiveness to CO2.5 This may also explain the reduced oxygen saturations observed even in the treatment group.

The time for which the patients are kept under the surgical drapes was alluded to in the study. Periods greater than 60 min resulted in exclusion. The average draping time for surgery in both arms of the study was prolonged, the average time in our institution being approximately 10 min. In the study by Schlager into CO2 retention, the time averaged about 20 min.6 The longer a patient remains below the drapes, the greater the likelihood of CO2 retention and the resultant adverse physiological effects.

It has been demonstrated that a flow of between 5–10 l/m oxygen below surgical drapes will prevent rebreathing without any special equipment.7 In conclusion, in answer to the question posed by the authors, there is probably no clinical need for any aspiration system for cataract patients performed under local anaesthetic block provided excessive sedation and prolonged draping time are avoided and adequate flows of oxygen are maintained. The system may, however have a place in other forms of prolonged head and neck surgery under local anaesthetic with sedation.