Automatic ventilation during bronchoscopy.

Sir,—The recent paper (Bennetts, 1977) describing the use of the Bird Mark 2 Ventilator and the jet injection technique for bronchoscopy states, ".. . there is dilution with air by as much as 50%". In his original paper describing the injection technique, Sanders (1967) demonstrated entrainment volumes in the region of 5 : 1 with adult bronchoscopes. Recent measurements in our laboratory using a Sanders Venturi and a 7 x 40 bronchoscope reveal similar entrainment volumes. Both calculation and actual measurement showed that 100% oxygen as the driving gas delivers 33% oxygen at the end of the bronchoscope, 60% delivers 26%, and, as suggested by Dr Bennetts, 40% delivers 23%. Further calculations show that even 75% nitrous oxide in the driving gas will lead to only 12% nitrous oxide being delivered to the patient. Such nitrous oxide concentrations are of little value. The amount of entrainment will vary with the size of the injection orifice and it may be that, with the bronchoscope used by Bennetts (1977), which is not available to us, only 50% dilution is produced. However, we would caution against the use of any technique involving air entrainment by a nitrous oxide in oxygen mixture unless hard data concerning oxygen delivery are available. Sir,—Many thanks for allowing me an opportunity of responding to the comments of Dr B. Wolfson and Dr L. Shete. Sanders' paper showed not only that entrainment of air of the order of 5 : 1 or 6 : 1 occurred when the outflow from the bronchoscope is unrestricted, but also that the situation changes dramatically when the gas flow is impeded. Oxygen (the driving gas) concentration increased and entrained gas volumes decreased strikingly at a point distal to the constriction in the simulated clinical situation. My own (unreported) experiments with a model tracheobronchial tree and lung confirmed this effect and demonstrated substantial reductions in the volume of entrained air under conditions which, I thought, bore a close relationship to actual bronchoscopy. During the virtual "wedging" of the bronchoscope in the main bronchi, the lung under examination is ventilated with the relatively undiluted driving gas mixture, but a greater degree of air entrainment occurs while the bronchoscope is in the trachea. Flows of the order of 100 litre min~ l may be obtained in the laboratory when there is no resistance at the distal end of the bronchoscope, but lung ventilation of this order …