E. A. Cooper

Physiological deadspace in passive ventilation

Much abdominal surgery is performed under general anasthesia and IPPR and evidence has been accumulating for several years that the excretion of carbon dioxide is then less efficient than might be expected from knowledge of normal respiratory physiology. Several workers have reported that the physiological deadspace is increased in such clinical practice or in comparable experiments on animals but there is still doubt of the quantitative effects upon it of changes of minute volume ; tidal volume ; frequency; respiratory wave form and of the possible effects of age and minor upsets of physical fitness. Work has been in progress in this department for several years and this is the first paper of a series. It is chiefly concerned with methods. Fifty subjects were studied. Pulmonary ventilation was carefully controlled to provide the desired tidal volume and frequency during steady periods, at the end of which expired and rebreathed gas samples were collected. The ventilation and carbon dioxide excretion per minute were thus measured and the arterial Pcoz was derived. The physiological deadspace was calculated by use of the Bohr equation. Subsequent periods were investigated at different respiratory activities, as required.

Indirect estimation of arterial pCO2

It is frequently desirable to know the arterial pC02 of a patient who is paralysed and on IPPR. Direct measurement can be made on a sample of arterial blood in certain circumstances, but frequently, either in an anasthetic emergency or in the routine care of patients on respirators, this is not practicable. Various indirect methods have, therefore, been used, usually dependent either on obtaining ‘arterialised’ blood from some other site, or on collecting a specimen of gas from the lungs whose pC02 bears a known relationship to the arterial. This paper is concerned with the value of four such techniques, using blood from the ear or the back of the hand and using gas obtained by a rebreathing technique, or end-tidal. Investigations are reported on thirty-three patients.

On the efficiency of intra‐gastric oxygen

The introduction of oxygen by catheter into the stomach and intestine of neonates1 is widely used in Britain in the treatment of anoxia. It is difficult to assess the value of the manceuvre since only a small proportion of babies are in need of resuscitation and many of these cannot be saved by any means. There is no published report of a series large enough for valid assessmentz. It is also difficult in clinical circumstances to discern the efficiency of oxygen uptake from stomach and intestines, since the procedure incidentally results in filling of the oroand nasopharynx with oxygen. Thus, any air which can come into effective contact with alveolar membrane is enriched in oxygen and apparent benefit may result from this, rather than from intestinal absorption. Furthermore, the procedure must cause sensory stimulation toward respiration. James et al have reported a case in which the arterial oxygen saturation fell catastrophically during administration of intragastric oxygen, but immediately rose to normal on administration via the trachea. The evidence of uptake of oxygen from the intestines of animals is limited. McIver et UP showed that in normally breathing adult cats, an uptake of about 0.2ml of oxygen per minute was possible from a 12in length of small intestine, but that stomach and colon absorbed at only half this rate. They drew attention to their finding and that of Boycott3 that a large proportion of the oxygen absorbed was utilised by the intestinal wall. No quantitative data has been found on the rate of passage from the lumen of the gut to the mesenteric blood under conditions of severe anoxia.

The estimation of minute volume.

Valid measurements cannot be made in a part of the circuit in which gases pass which may either not go to the patient or not come from him. Thus, in the normal use of a Magill semi-closed system the only valid site for measurement is between the patient and the expiratory valve. Here, there is to and fro gas flow and a device is, therefore, needed which is sensitive to flow in one direction only and has minimal dead space. Vane anemometers have these characteristics but at present they are not sensitive to very low flows and in consequence their calibration alters by several per cent in the desired range of minute volumes. It is however likely that a simple modification could largely eliminate this undesirable feature. The difficulty of measurement in a to and fro system can be overcome by the introduction of valves. This is simple on paper ( F I G . 1)

Postoperative Lung Dysfunction

out. With that assurance Dr Baskett and Dr Withnell had proceeded with the trial and, after publishing the initial results in the British Medical Journal (Baskett P J F & Withnell A, 1970, Brit. med. J. ii, 41), wrote to the Department of Health and Social Security, enclosing a reprint of the paper. The Department thanked them for their communication and said that they had noted its contents. Dr Baskett felt that was as far as he could comment on the legal aspects of the technique, but said it was perhaps pertinent to note that a further 23 local authorities had adopted the use of Entonox in their ambulance services. He thought it was now up to the Department of Health to comment further, but they might feel it was better that no definite ruling should be made on the matter yet.