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Estimation of size of pneumothorax under the new BTS guidelines
  1. S S Chan1
  1. 1Prince of Wales Hospital, Hong Kong; stewart_chanhotmail.com
  1. M Henry1,
  2. T Arnold2,
  3. J Harvey3
  1. 1Department of Respiratory Medicine, The General Infirmary at Leeds, Leeds LS1 3EX, UK; michael.henryleedsth.nhs.uk
  2. 2Medical Chest Unit, Castle Hill Hospital, Cottingham, North Humberside HU16 5QJ, UK
  3. 3Department of Respiratory Medicine, Southmead Hospital, Bristol BS10 5NB, UK

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I read with interest the new BTS guidelines for the management of spontaneous pneumothorax.1 Henry and colleagues acknowledged that the plain radiograph was a poor method of quantifying the size of a pneumothorax, yet then went on to use one radiographic method of assessment to estimate the degree of lung collapse.

Under the new guidelines, the size of a pneumothorax is divided into “small” or “large” depending on the presence of a visible rim of <2 cm or ⩾2 cm between the lung margin and the chest wall. The authors then explained in detail how these distances could be used to estimate the percentage of lung collapse. A schematic figure was even used to illustrate the calculations. However, the method used by the authors (the method of Axel),2 like most other methods, has been found to be unsatisfactory for determining pneumothorax size under clinical conditions.3

I do not see any evidence that the new classification is in any way better than the old one. The calculations based on the distance of the rim correlated poorly with the actual size of the pneumothorax.3 The “2 cm” used is an arbitrary figure. It is even more confusing to have the American guidelines use another arbitrary system of classification.4 In spontaneous pneumothorax, practitioners should at least agree on the same classification system of size before they continue to debate about what is the best option of treatment.

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Authors’ reply

We thank Dr Chan for his comments relating to the recently published guidelines for the diagnosis and management of spontaneous pneumothoraces.1 Dr Chan raises the contentious issue of estimation of the size of a pneumothorax from a plain chest radiograph. We have attempted to use a variation of the method of Axel based on the largest distance from the chest wall to the pleural line and using the assumption that, because the volume of the lung and the hemithorax are roughly proportional to the cube of their diameters, the volume of pneumothorax can be estimated by measuring an average diameter of the lung and the hemithorax, cubing these diameters, and finding the ratios.2

As Dr Chan rightly points out, this is not an exact science as the lungs have a propensity not to maintain a constant shape when they collapse. A CT scan of the thorax gives a more accurate estimate of the volume of the pneumothorax than a plain radiograph. However, while CT scanning may be the only way to obtain an exact estimate of pneumothorax volume and pattern of lung collapse, it is not often feasible in the emergency room. The correlation coefficient between CT scanning and plain radiography is 0.71 (p<0.01).3 Thus, while chest radiography is not as effective as CT scanning, it does still provide a useful and reasonably accurate estimate of pneumothorax size in most cases using the method outlined in the current guidelines. We suggest that the guideline is an improvement on the 1993 pneumothorax guidelines which tended to underestimate the size—and thus potentially the importance—of a pneumothorax. Choosing a distance of 2 cm above which the volume of pneumothorax is usually above 50% gives the emergency room physician a guideline which is easy to use and fairly reliable. It has been shown that secondary pneumothoraces above this volume are unlikely to respond to simple aspiration and this hopefully will provide useful guidance as to which patients to treat with intercostal tube drainage.4 This is supported by evidence and is now a clear and unambiguous guideline. We also hope that, by suggesting that pneumothoraces <2 cm in depth should not usually be aspirated, we may reduce the number of aspiration needle injuries to the lung parenchyma which would have a much greater approximation to the chest wall in primary spontaneous pneumothoraces of <2 cm depth.

As Dr Chan points out, the American College of Chest Physicians has proposed a different arbitrary system for estimating pneumothorax size. They suggest that “small” pneumothoraces should be defined by distances of <3 cm from the apex to cupula of the lung and “large” pneumothoraces by distances of >3 cm.5 This seems to have been arbitrarily defined and we are not provided with evidence to support these measurements. Several authors have suggested different distances ranging from 1 cm to 4 cm on the plain radiograph, or more complex equations depending on three separate distances between the pleural line and chest wall, or the routine use of CT scanning incorporating even more complex mathematics.6,7 Dr Chan comments on the lack of evidence regarding the classification of the chest radiograph; we have completed an analysis of the chest radiographic appearances in spontaneous pneumothorax, relating them to the various guidelines, and have presented it as an abstract at the winter meeting of the British Thoracic Society.7 Bearing in mind that the guidelines are primarily prepared for use by relatively inexperienced and non-specialist junior medical staff who often have to make management decisions in the middle of the night, we would suggest that the BTS guidelines have combined a fairly robust and accurate scientific approach with a guideline which is easy to interpret and implement to estimate and treat spontaneous pneumothoraces.

Finally, we would again take the opportunity to stress that, no matter what the size of a pneumothorax, the decision as to what constitutes appropriate treatment depends not just on the size of the pneumothorax on the chest radiograph but, more importantly, on the clinical status of the patient.

References

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