Volumetric analysis of aeration in the lungs during general anaesthesia.

Sir,—I read with interest the article by Reber and colleagues on volumetric analysis of aeration in the lungs during general anaesthesia.1 This is a study of radiographic density of the lung, using data consisting of picture elements (pixels) sized 0.9 0.9 4 mm. The density of the tissue in the pixel varies with the amount of air it contains, and the authors interpret these changes in two ways. First, they use density to assess the degree of expansion of the lung tissue in terms of aeration. Second, they use changes in density during the breathing cycle to estimate regional ventilation. I would agree that the less dense the lung, the better aerated it is likely to be, because air causes less attenuation than lung tissue (or blood, which here seems to have been assumed to remain a constant fraction of the tissue). Air causes attenuation, in the scale used, of 1000 units, whereas collapsed atelectatic lung causes a much greater attenuation of 100 to 100 units. The authors argue that the distribution of pixel density indicates the distribution of poorly aerated and atelectatic lung. They suggest that the amount of lung tissue in an atelectatic area is four to five times greater than in a normally expanded part of the lung. This implies that by expansion with four times that volume of air, one atelectatic pixel would become five pixels with a normal radiographic density, as the radiographic density of air is very small compared with that of lung tissue. We are left uncertain as to how much air needs to be added to “poorly expanded” lung tissue to convert it into tissue with “reduced aeration” or “normal aeration”, although these quantities would be useful for relating the current measures to previous data obtained with other methods. This knowledge is of particular importance when it comes to drawing conclusions on “distribution of ventilation”, as the authors have done in their subjects both during spontaneous breathing and mechanical ventilation. Here it is useful to remember how earlier studies of regional ventilation were performed, as this may help to elucidate some of the findings in the study of Reber and co-workers. Regional ventilation, measured with radioactive xenon, is usually expressed as ventilation in proportion to the volume of that region at total lung capacity (TLC);2 this is done because at TLC the alveoli are nearly equal in size throughout the lung, and therefore expression of regional volumes and regional ventilation as a proportion of volume at TLC becomes equivalent to expressing these values in terms of “per alveolus”. This is more useful than expressing ventilation or lung volume in terms of lung tissue at other states of inflation. If inflation of each part is unequal, then the amount of lung tissue present in each unit volume (in the present case, pixel volume) depends on the amount of distension of the lung that is being considered. Indeed, it is intuitively obvious that erroneous conclusions could result if regional ventilation were based on any other lung volume than the volume at TLC. Consider 100 ml of collapsed lung tissue which is distended by 100 ml of air. It would then have a volume of 200 ml, and if its regional ventilation were expressed as a percentage of the original tissue volume, it would have received 100% ventilation. However, this volume of atelectatic tissue can probably be fully expanded to a volume of approximately 1000 ml and hence if regional ventilation is expressed as a proportion of the volume of tissue at TLC, this ventilation would be 10%. There may be an analogous effect, of an inappropriate denominator, when radiographic density is used as a measure of aeration. This may explain some of the results when Reber and co-workers have “assumed a gas distribution in proportion to the attenuation changes” that they observed between inspiration and expiration. There may be a relationship but it is most unlikely to be a direct linear proportion between density change and regional ventilation. They report that the radiographic density of the gravitationally upper parts of the lung is less than the density of the lower parts, in subjects at functional residual capacity. Setting aside the distinct possibility that the gradient could also be affected by a gravitationally determined gradient of pulmonary capillary distension, this observation would be expected from many previous measurements made with the isotope technique (for a succinct review, see Milic-Emili2). This gradient of density means that the lower pixels contain more alveoli than the upper ones. The article also reports that the lower pixels show a greater change in radiographic density than the upper ones, with a tidal volume inspiration. From an assumption that gas distribution is proportional to attenuation, the authors concluded that “distribution of ventilation” to the dependent regions is two times greater than that to the upper regions. I think that this conclusion is likely to be incorrect. First, as argued above, a change in density from 780 to 840 (approximate changes in the topmost slice) could well represent just as much, or even more, air/alveolus, as a change in density from 570 to 720. The proportional change could perhaps be better expressed as the change relative to the density difference from fully distended lung (say 900 units). If this were considered, then the topmost density change would be from ( 900) ( 780) at FRC to ( 900) ( 840) at end-inspiration. This gives a fractional change of density of ( 60)/( 120) in the topmost level and ( 180)/( 330) for the lowest level, and would lead to the conclusion that ventilation was relatively equal from top to bottom. However, this argument depends on the density value taken to represent fully distended tissue and ignores blood contribution to density. Second, the inference of a vertically uniform distribution of ventilation agrees with findings in subjects breathing spontaneously with normal inspiratory flow rates, where the vertical distribution of ventilation in the lung depends not only on regional compliance but also on differences in airway resistance.3 4 A vertical discrepancy in ventilation distribution, such as Reber and colleagues conclude from their data, is only found in sitting subjects in very artificial circumstances when slow manoeuvres are undertaken so that airway resistance differences are minimized. Reber and co-workers did not report tidal volumes, inspiratory duration or ventilatory frequencies of their subjects so the mean inspiratory flow rates cannot be estimated, although these are clearly relevant. A similar argument should probably be applied to the data obtained in subjects undergoing artificial ventilation which would result in the conclusion that ventilation is distributed preferentially to the upper regions. However, comparison of the data in subjects breathing spontaneously with those whose lungs were ventilated artificially indicates a further difficulty in the use of radiographic density to indicate aeration alone. The tidal volumes of the patients breathing spontaneously are unlikely to have been greater than those of the patients undergoing ventilation with volumes of 8 ml kg 1. What is remarkable is that the change in mean lung density caused by mechanical ventilation (54 units) was smaller than the density change during spontaneous breathing (131 units), although tidal volume was probably less with spontaneous breathing. A factor that could explain this difference is a change in the volume of blood in the lungs during the 10-s breath-hold at end-inspiration necessary for the scan. This change in blood volume in the tissue may well not be uniform from the top to the bottom of the lung. This underlines the fact that factors other than ventilation can affect radiographic density, and this could affect the comparison of measurements made during spontaneous breathing with those made during mechanical ventilation. Until these questions and uncertainties regarding the proportional relationship of radiographic density and aeration are addressed, I suggest that the inferences from CT studies should be limited to qualitative assessment of aeration, and that the expression “ventilation distribution” is not used.