Precision in Diagnosing and Classifying COPD: Comparison of Historical Height with Current Height and Arm Span to Predict FEV₁

1. INTRODUCTION

The diagnosis of COPD, chronic obstructive pulmonary disease, currently depends on demonstrating limitations of airflow, using spirometry, the gold standard method for obstructive disease of the airways. A diagnosis of COPD is normally made if the ratio of the forced expiratory volume in the first second (FEV₁) to forced vital capacity (FVC) is below 0.70 (Global Initiative for Obstructive Lung Disease – GOLD, 2010) [1].

Impairment of FEV₁ has also been utilised to classify the severity of airflow obstruction for COPD, in a range of guidelines, including those recommended by the National Institute of Clinical Excellence (NICE) in the UK [2]. Impairment of FEV₁ is determined by making reference to predicted (normal) volumes. As spirometric values are dependant on the size of the lungs, which is in turn directly related to height, gender, age and ethnic origin [3] the predicted normal value for lung capacity is determined using measures for these variables. One regression equation that is commonly used is that proposed in the European Respiratory Society (ERS) guidelines [4].

For Males: PREDICTED FEV₁= (4.30 x height) – (0.029 x age - 2.49)

For Females: PREDICTED FEV₁= (3.95 x height) – (0.025 x age - 2.60)

However, some of these variables, including height may be affected by other factors including osteoporosis and this will confound the interpretation of comparisons made to a predicted normal value for lung capacity that has been determined based upon a patients’ height.

In terms of height Sorkin et al. [5] found that in healthy Caucasians a longitudinal decline in height, with aging, becomes apparent from the age of 40 years. An initial rate of decline of 1cm per year accelerates after the age of 60 years with women losing more height than men. However, Capderou et al., (2011) [6] in a large single centre study comparing historic height with current measured height concluded that significant height loss becomes important after the age of 70. In patients with COPD height loss may occur at an earlier age due to the high prevalence of osteoporosis in smokers [7, 8]. Osteoporosis is also a recognised complication of regular treatment with oral corticosteroids which are often used to treat exacerbations in COPD [9, 10]. Although there is little systemic absorption of inhaled corticosteroids some concern exists that they might increase the risk of osteoporosis [11, 12]. Hence height may not be the most accurate measure to use, to determine predicted ‘normal’ lung volume in patients suffering from COPD. Using current height to determine the predicted values of FEV₁ may mean that the degree of impairment of FEV₁ is subject to confounding particularly in an elderly population.

This paper has therefore considered the use of armspan as an alternative measure for the estimation of height and in the prediction of ‘normal’ lung volume. Armspan is quick and easy to measure, is low cost and a useful alternative when standing height is difficult to measure such as in patients with disabilities who cannot stand, bedridden patients and those with bony deformities [7-13].

The research was carried out using a cohort of COPD patients recruited in a primary care setting in the North East of England. Members of the cohort were first examined in 1999-2002. The second examination occurred in 2007-2008.

2. METHODS

In the study reported here the cohort was first evaluated between September 1999 and December 2002, when patients with suspected COPD, attending primary care asthma clinics, were identified by performing spirometry according to the British Thoracic Society guidelines (BTS) [14].

Patients were classified as having COPD utilising the absolute post-bronchodilator FEV₁/FVC ratio ≤ 0.7. Exclusion criteria were: FEV₁/FVC >0.7; age <40 years and a clear diagnosis of asthma based on BTS criteria of an increment over baseline FEV₁ of more than 15% and more than 200mls with the bronchodilator and patient with any obvious bony deformity that may alter armspan or height measurements.

Patients in the non COPD control group were classified as those with FEV₁% predicted values of >80% and with no symptomatic evidence of obstructive disease such as cough and shortness of breath.

Height was recorded using a single stadiometer without shoes and arm span was measured. Armspan is defined as the maximum distance from the tip of the middle finger of one hand to the tip of the same finger of the other hand with outstretched arms and the subject standing erect. Spirometry (Vitalograph 2120); before and after bronchodilatation was also undertaken.

FEV₁% predicted was determined for measured height and for estimated height based upon armspan (armspan/1.03 and armspan/1.01 in males and females respectively) [15].

In 2007-2008 survivors from this cohort were asked to attend for review.

Sunderland Local Research Ethics Committee approved this study.

3. RESULTS

From the survivors of the original cohort 104 patients with COPD and 42 without COPD attended for review in 2007-2008, Table 1 presents their demographic characteristics.

All members of the COPD cohort showed a significant loss in measured height, mean 2 cm from 1999-2002 to 2007-2008. This can also be seen in Fig. (1) however this difference in height is not shown when height is determined using armspan.

Fig. (2) shows that for the cohort, loss of height, increased with age. This loss of height with age is also reflected in the change in armspan to height ratio (Fig. 3) although the results here are more variable.

FEV₁% predicted in 2007/8 was significantly (p<0.001) lower when using height determined from armspan – 53.3±17.2% against 52.1 ±16.8% in males and 57.5±16.1 vs 55.7±17.7 in females within the COPD group. However, this difference was only found with the female members of the control cohort.

Table 2 shows the distribution of patients classified according to GOLD criteria in both 19992-2002 and 2007-2008. There was no significant movement between stages as a result of recalculating FEV₁% predicted from height estimated from armspan. Whilst this would suggest that there is no need to change the management plan and/or prescriptions for the group overall there will be a significant impact upon those individuals whose classification does change.

DISCUSSION

The accurate classification of COPD is important in that strategies need to be developed for the management of these patients. Therefore if impairment of FEV₁ is to be used to classify the stage of disease progression there is a need for accurate predictions to be made for normal lung volumes. These are currently calculated using a regression equation in which current height is one of the variables. However, current height can be confounded by a number of other factors including osteoporosis.

In this study we have shown that patients both those with confirmed COPD and the controls exhibit a significant reduction in mean height of 2cm over a median period of observation of about 8 years. This variation of height in turn influences the predicted values of lung function, calculated to diagnose and classify the stages of COPD, thus underestimating the degree of severity in these patients. The reduction of standing height was more significant in female subjects suggestive of postmenopausal degenerative bony changes in women [16]. This is important as more females are now reporting with COPD.

It is an established problem that standing height loss occurs in elderly people and this is supported by our data (Fig. 2) which shows the mean height loss in different age groups over time. The underlying causes that are responsible for height reduction may include narrowing of vertebral space, age related vertebral fractures mainly due to osteoporosis, regular use of corticosteroid, female sex and lower body weight [19,20].

Armspan has traditionally been used to estimate predicted lung volumes in pulmonary function laboratories especially when the subjects have a spinal deformity or are unable to stand. Armspan is correlated with height and the normal phenomenon is for armspan to slightly exceed height. A number of studies have examined this relationship [13,17,18] and to estimate height from armspan measurements we chose ratios of 1.03 and 1.01 in men and women respectively as appropriate for a Caucasian population [13].

The information presented here shows that the calculated height measured by armspan is similar when estimated over time. However, the standing heights, measured using a stadiometer, are different at baseline and follow up (Fig. 1) indicating a possible under- estimation of normal lung volume if measured height is used to calculated FEV₁. If this is the case the severity of COPD progression would be underestimated.

The conclusion that can be reached from these results is that the use of current height will mean that COPD progression is underestimated if measured height is used. The use of armspan to determine height and hence normal lung volume will give a more accurate measure of the impairment of lung function and hence COPD progression.

There are some limitations to the study. The number of subjects both in the COPD and control groups is low however it was possible to demonstrate the impact of height loss across the cohort. Furthermore, historical heights were measured by the clinical staff in their local clinical settings in contrast to the work of Kjensli and colleagues [21]. Kjensli and colleagues reviewed whether height reduction influences interpretation of lung function in COPD patients. However they used the highest remembered height and compared this with current height. They showed that the spirometric values were influenced by height losses of greater than 5.9 cm. This was especially true for those with a low FEV₁.

Future longitudinal studies are needed in order to validate our finding with a larger number of subjects.

REFERENCES