The PFT Lab I am associated with has been making a point of having the technicians re-measure patient height with each visit. Part of the reason for this is that several years ago the medical assistants in the pulmonary outpatient clinic were tasked with obtaining patient heights, weights and blood pressures. For a period of time the technicians used these heights when entering the patient demographic information but it was soon noticed that patient heights often changed by several inches from visit to visit. For this reason we have asked the technicians to re-measure patient height instead.

One possible cause for the fluctuation in heights was that the medical assistants were measuring patient height using the height rod attached to the scale while also taking their weight. The PFT Lab has wall-mounted stadiometers in or near all of the lab’s testing rooms so that patient height can be taken with their back against the wall rather than free-standing.

Another reason to regularly re-measure patient height is that the lab’s population has a significant number of patients that have routinely been seen by the lab and the pulmonary physicians for years. The lab’s patient and results database now goes back over twenty years and patients that were seen 15 and 20 years ago have been referred again for pulmonary function testing and their height has changed in the meantime.

My feeling has been that it is important to base the patient’s predicted values on their current height, not on whatever their original height was. This is because the population studies that generate reference equations are based on the current height of their subjects, not their original height.

Recently I have seen several patients whose height was re-measured and are now one or two inches shorter than what was previously measured. This has caused enough of a shift in their predicted values that although their test results are essentially unchanged they are now within normal limits when before they were classified as obstructed or restricted. This makes it clear that accurate heights are essential to accurate interpretation of results.

Coincidentally or not I also recently ran across an article where the authors advocate the use of arm span over height when calculating predicted values for patients with COPD. Their contention is that because of disease processes and the effects of medications patients with COPD are more prone to an accelerated height loss and for this reason the severity of their lung disease is being underestimated. This is an interesting point and other investigators have noted that arm span most closely matches actual standing height when people are young adults and that as people age height is lost because of degenerative changes in the vertebrae and disks of the vertebral column but arm span tends to remain constant (although there are very few longitudinal studies so this is not as clear as it should be). For this reason the arm span to height ratio tends to increase with age. Other researchers have noted that an increased arm span/height ratio is associated with a reduced FVC and increased dyspnea as well as increasing age.

So arm span may very well be a more accurate representation of a patient’s original height but a reasonable response may be, so what? Since much of any individual’s height loss is from the vertebral column this has a direct effect on the volume of the thoracic cage and therefore the lungs. For this reason standing height may still be a better indicator of lung function than arm span and this is what the reference equations use. A counterpoint would be that arm span is and should be used to estimate height in patients with scoliosis or other skeletal deformities and that in these cases comparison of current results with what they “should” have been can be very informative.

I think the core question is that when we are calculating predicted values what are we trying to accomplish? For example, say that with MRI or other imaging of the thorax and airways it was possible to accurately determine what a patient’s actual lung volume and flow rates “should” be. If this was possible (and it already is to a reasonable extent with CAT scans), it would likely produce predicted values that were skewed to the patient’s disease process and a patient with advanced obstructive or restrictive lung disease could be “normal” when compared to what was “expected” for them. This would not be particularly informative so I think that the case for comparing patient test results with their theoretically normal values is fairly clear.

I am not convinced however, that arm span should routinely be used in place of standing height. There is a lot of variance in arm span/height ratios and I would like to see more research that categorizes and explains these differences. A much bigger and more important concern is a lack of standardization in how arm span is measured. There are stadiometers for measuring standing height but there is no equivalent for arm span and from personal experience I can say that a lot of inaccuracy is possible when using a tape measure.

I am also not convinced that using a patient’s original (greatest) height to calculate their predicted values is a correct approach but this is mostly because reference equations are created from their study population’s current height, not from any prior height. I can see that patient height loss is a factor that can affect lung function but I am not sure there is an organized approach for doing this.

I think there is enough evidence concerning the utility of arm span measurements that it may well be reasonable for a PFT Lab to add this as part of their routine practice. But regardless of whether standing height or arm span is used the first step has to be an accurate measurement.

References:

Aggarwal AN, Gupta D, Jindal SK. Interpreting spirometric data. Impact of substitution of arm span for standing height in adults from North India. Chest 1999; 115: 557-562.

Allen SC. The relation between height, armspan and forced expiratory volume in elderly women. Age & Ageing, 1989, 18: 113-+116.

Ansari K, Keaney N, Price M, Munby J, Kay A, Taylor I, King K. Precision in diagnosiing and classifying COPD: comparison of historical height with current height and arm span to predict FEV1. The Open Respiratory Medicine Journal 2012; 6:54-58.

Parker JM, Dillard TA, Phillips YY. Arm span-height relationships in patients referred for spirometry. Am J Respir Crit Care Med 1996; 154: 533-536

Tan MP, Wynn NN, Umerov M, Henderson A, Gillham A, Junejo S, Bansal SK. Arm span to height ration is related to severity of dyspnea, reduced spirometry volumes and right heart strain. Chest 2009; 135: 448-454.

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