Abstract
As a result of the improved medical treatment of infectious diseases, these formerly leading causes of mortality in the United States have been supplanted in rank by chronic events as the major causes of death. The major causes include coronary artery disease, respiratory disease, and cancer. Static and dynamic pulmonary functions in the apparently healthy wheelchair user popula tion were measured and then modelled using stepwise regression. One hundred and nine wheelchair users (97 males, 12 females) with paraplegia (n = 77) or quadriplegia (n = 32) gave informed consent and participated in this study. Subjects ranged from being Olympic caliber wheelchair marathon racers to those who live sedentary life styles. Subjects performed three slow vital capacity (SVC) tests, three forced vital capacity (FVC) tests, and three maximal voluntary ventilation (MVV) tests while seated in their standard wheelchair. The order of the pulmonary function tests was randomized. Subjects also completed two functional residual capacity (FRC) measurements. Analysis of variance revealed significant differences in several pulmonary functions based on gender (FEVC, p = 0.0001, FEV1, P = 0.0001, FEVC 25-75%, p = 0.005, PEF, P = 0.002, FIVC, P = 0. 002, RV, P = 0.0001, MVV, P = 0.0001, SVC, P = 0. 001). The women's unforced prediction equations using age, height, weight did yield some significant correlations with predictions based upon ambulatory subjects. The men's unforced adapted prediction equations did reveal significant correla tions with the validation group for FEVC (r = 0. 66, p = 0.007), FEV1 (r = 0. 62, p = 0. 015), PIF (r = 0. 95, P = 0.015), MVV (r = 0. 57, P = 0.067), SVC (r = 0. 69, p = 0.019), and RV (r = 0. 67, p = 0.009). Pulmonary function in male wheelchair users should be predicted using equations which incorporate years with disability and level of impairment. Additional study is required to make any recommendations regarding women wheelchair users. However, study of pulmonary function in women should be made a priority. Pulmonary function is affected by the extent of physical impairment, and tends to decline as years with disability increase.
Introduction
Recent estimates show the prevalence of spinal cord injury (SCI) in the United States to be from 250,000 to 350,000 people, with annual incidents of new injuries at approx imately 10,000.1-4 As a result of the im proved medical treatment of infectious diseases, these formerly leading causes of mortality have been supplanted in rank by chronic events as the leading causes of death in the United States, including coronary artery disease, respiratory disease, and cancer. It had been reported that 21 % of the mortality rate among the SCI population is related to respiratory incidents which is not far behind cardiovascular incidents (25%), the leading cause of mortality. 1 Improve ments in healthcare and continued research on pulmonary function will provide infor mation for physicians, therapists and most importantly for individuals with mobility impairments which could increase the life expectancy of such individuals.
Introduction
The increased numbers of wheelchair users, who are experiencing prolonged life present an important public health issue both in terms of resource allocation for long term management and of considerations for rehabilitation programs aimed at increasing functional aerobic work capacity. 5.6 Be cause of their reduced levels of maximal aerobic work capacity resulting from loss of muscle function with concomitant physical deconditioning due to inactivity, the wheel chair user population appears to be espe cially disadvantaged for long term survival without a program of aerobic fitness training and testing.
Introduction
It is well known that the level of physical impairment (spinal cord injury) affects the functions of the various aspects of the respiratory apparatus such as the intercostal muscles and diaphragm. Lower pulmonary functions have been documented in wheel chair users by several researchers.
l.2.7�12
More specifically, individuals with higher levels of impairment (quadriplegia) have poorer pulmonary functions than people with paraplegia.
l.2.7�12
Often the pulmonary function values for the wheelchair user are compared with normative tables obtained from the able bodied population 13 which are based on gender, age and simple anthropometry. These tables are useful for initial evalu ations of an individual's pulmonary func tions. However, evaluation of pulmonary function among wheelchair users should be based on data collected from a wheelchair using population.
1,14
The purpose of this project was to de velop pulmonary function prediction equa tions based upon measured performances of apparently healthy wheelchair users. A secondary objective was to initiate the development of wheelchair user pulmonary function normative values including: forced expired vital capacity (FEVC) , forced ex pired volume after one second (FEV1), forced expired vital capacity from 25 to 75% (FEVC 25-75%), peak expiratory flow (PEF) , forced inspired vital capacity (FIVC), peak inspiratory flow (PIF), residPulmonary function in wheelchair users 561 ual volume (RV), maximal voluntary ventilation (MVV) , slow vital capacity (SVC), tidal volume (TV), and the ratio of FEV1jFEV.
Methods

Subjects
One hundred and nine wheelchair users (97 males, 12 females) with paraplegia (n = 77) or quadriplegia (n = 32) gave informed consent and participated in this study. The subjects' levels of chronic physical activity ranged from that of an Olympic caliber wheelchair marathon racer to that of a person leading a completely sedentary life style. Our subjects represent a balanced cross-section of chronic physical activity. Subjects were culturally and ethnically di verse and ranged in age from 17 to 69 years. Forty-nine percent of the men reported to be non smokers, whereas 75% of the women reported to be non smokers. Fifty seven percent of the men who identified themselves as long term smokers reported that they had quit smoking. All subjects were free from illness at the time of parti cipation in the study. This research was approved by the CSUS Committee for the Protection of Human Subjects.
Experimental protocol
Subjects performed three slow vital capacity (SVC) tests, three forced vital capacity tests (FVC) , and three maximal voluntary vent ilation (MVV) tests while seated in their standard wheelchair. The order of the pul monary function tests was randomized. Spirometry was performed using either a dry rolling seal or a water seal spirometer (Vacumetrics, Ventura, CA or W.E. Col lins, Braintree, MA). Spirometers were calibrated with a 3 I syringe before testing. Other selected pulmonary functions were calculated from these tests. All tests were performed according to the recommenda tions for measurement of respiratory func tion of the American Thoracic Society.
Statistical analysis
Standard descriptive statIstIcs were calcu lated for the pulmonary function data.
Statistical analysis
These data were also expressed as percen tiles for the men. There were insufficient numbers of women to present meaningful percentile values. A 2 x 3 ANOVA (p < 0.05) was performed to determine the existence of any significant differences in pulmonary functions between men and women across disability groupings (upper level injury, C5-T3, middle level injury, T4-T1O, lower level injury, Tll and lower). Sheffe's post hoc test was performed to determine within group differences with a significance level of p < 0.05. Forward step wise regression (forced and unforced) was used to develop models for predicting pul monary function from age, height, weight, injury level, and years of injury. Due to .the low number of female subjects all of their data were included for development of the female measurement models. Correlation analyses were performed for both males and females using prediction equations from Crapo et al. [15] [16] [17] Data from randomly selected male subjects whose pulmonary test results were not used to develop the male measurement models were acquired for the validation procedures. One way ANOV A (p < 0.05) was performed for each of the predicteds for the prediction and validation groups. When validating the models it was decided, a priori, that a type II error may be more serious than a type I error. Therefore a significance level of p < 0.10 was chosen for regression analysis in an attempt to minimize the probability of committing a type II error, ie not detecting potentially significant correlations.
Results
Descriptive statistics for pulmonary function data for both men and women appear in Tables I, II women. Because of these differences separ ate models were developed for men and women.
Results
Between levels of injury there were sig nificant differences in some pulmonary functions. Maximal voluntary ventilation measurements were significantly different across men in the upper and lower level injury (p < 0.05). Slow vital capacity data were different across men in the upper and middle groups (p < 0.05) as well as across men in the upper and lower groups (p < 0.05).
Pulmonary function percentiles
Percentile values for select pulmonary functions for men by impairment level are presented in Table IV . Percentile values were not presented for women as there were insufficient data to create meaningful cells. Table IV shows a gradual increase in pul monary function as the level of impairment decreases. The range of values across the percentiles is indicative of the broad spec trum of people tested.
Pulmonary function percentiles
Regression models for predicting pulmonary function Forward stepwise regression was used to determine prediction models for pulmonary functions of men and women wheelchair users based upon age, height, weight, injury level, and years of injury. Typically pulmon ary function prediction equations that have been developed for ambulatory individuals are based upon age, height and weight. Therefore these measurements were also examined as possible independent variables for predicting pulmonary function of men and women wheelchair users (Table V) . Level of impairment was not used in this analysis. In this analysis age had the highest and most frequent partial correlation coef ficient for men, whereas height appeared most often for women. Age had a negative partial correlation coefficient in nearly all equations in which it appeared for men and women. For men the equation for RV had the highest correlation (r = 0.42), whereas for the women the FEVC equation had the highest correlation (r = 0.85), (Table V) . Significant partial correlations were found Forced stepwise regression equations for men and women were also developed based upon age (years), height (centimeters) and weight (kilograms), independent of level of impairment (Table VI) . Again, for men the equation for residual volume (RV) had the highest correlation ( r = 0.46), whereas for women the FIVe equation had the highest correlation ( r = 0.87) ( Table VI) . The equations for women all had higher correla tions than the equations for men. However, there were substantially fewer women than men. Age had a negative partial correlation with pulmonary volumes (FEVe, FEVl, sve, MVV) and flows (PEF, PIF) for men. For women, age had negative partial cor relations with pulmonary volumes (FEVe, FEVl, FVe 25-75%, FIVe, RV, MVV, SVC) and flow (PIF).
Pulmonary function percentiles
The equations developed to predict pul monary functions of wheelchair users based upon age, height and weight did not yield substantial correlations. In addition, analy sis of variance indicated that for some variables there is some dependence upon injury (impairment) level. Because age is an important factor in predicting the pulmon ary function of ambulatory individuals, it can be argued that, if a person acquires a disability sometime during their lifetime, the years since injury could contribute to predicting their pulmonary function. Unforced stepwise regression analysis using age in years, height in centimeters, weight in kilograms, years injured in years, and injury level were used as independent variables to generate the equations for Table VII. Level of injury was numerically scaled by assigning a number based upon anatomical location of lesion, ie each verte bra was assigned a progressively higher number starting with the first cervical ver tebra. For men the years injured (YI) and injury level (1L) had the strongest partial correlation coefficients in nearly all of the equations. Years injured was most often negatively correlated with pulmonary func tion, whereas injury level was most often positively correlated. Residual volume (RV) was best predicted with height and weight. The results from Table V and Table  VII differ primarily for two reasons: (a) the independent variables were different which altered the partial correlations, and (b) the subject pool used to develop the equations was different because complete data were not available for all subjects. The stepwise regression analysis of male subject data revealed significant prediction equations for FEVC, FEVl, PEF, FIVC, PIF, RV, (Table VII) . All of the independent vari ables appeared in at least one equation. RV and MVV were inversely related to years injured, whereas FEVe was affected by injury level. (Table VIII) . Equations were not developed for women as there were insufficient data. The equation for FIVe accounts for the greatest variation in the data. Again the partial correlation for years injured remains most negatively correlated with nearly every pulmonary function variable. Age is posi tively correlated with some variables and negatively correlated with others.
Pulmonary function percentiles
Forced stepwise regression equations for men were also developed based upon age (years), height (centimeters), weight (kilo grams), years injured, and injury level
Model validations
Models were validated by using data from randomly selected subjects, whose pulmon ary function test data were not used to construct the models. One way ANOV A on each of the predicteds for prediction and validation groups showed the data were independent. In the validation group the models incorporating years of injury and level of impairment accounted for the greatest proportion of the total variance. The equations developed for women wheel chair users were not validated with the data collected. The men's unforced generic (without years of injury and injury level) prediction equations did result in significant correla tions with the validation group for RV (r = 0.78, P = 0.005). The men's forced The men's adapted prediction equations were able to account for a significant por tion of the variance for more pulmonary function variables in the validation group than the men's generic equations.
Model validations
Comparison with prediction equations from ambulatory subject data Pulmonary function values for women pre dicted by the equations developed by Crapo 11 The results of the male subjects who participated in this research support these previous findings. Therefore predic tion equations developed for ambulatory people may not be optimal for evaluating the pulmonary function of wheelchair users. The findings of this study indicate that for male wheelchair users time since injury and level of injury are more strongly related to pulmonary function than are age, height and weight, the typically utilized prediction vari ables. Furthermore, the results indicate that there are gender differences in pulmonary functions among wheelchairs users. The magnitude of this difference may or may not be the same as exists in the general popula tion. Among male and female subjects pulmonary function was somewhat depend ent upon level of impairment. In other words there was an inverse relationship between level of impairment and pulmonary function. This was likely related to viability of motor and sensory innervation of the primary muscles of ventilation and to better postural control.
Model validations
The equations developed using the adapted independent variables accounted for a greater portion of the variance in the male validation group than did the generic independent variables. This may be because the adapted independent variables account more for the effect of physical impairment, and that the effects of physical impairment on the pulmonary system may be greater than those accounted for by simple anthro pometry. The effects of aging are to some extent accounted for by years with dis ability. Hence, for wheelchair users years with physical impairment may contain more information related to pulmonary function than does age alone. Pulmonary function tends to decrease during natural aging. This holds true for wheelchair users as well. However, it appears to be dominated by years with physical impairment. The un forced models developed for men using the adapted prediction variables accounted for a significant portion of the variance in the measurements of the validation group for key pulmonary function parameters (FEVC, MVV, RV and SVC).
Model validations
Having a mobility impairment tends to reduce pulmonary function when compared to the ambulatory population norm, and pulmonary function declines with each year since the impairment began. The original decline may be due to the loss of function associated with the physical impairment. The dependence of further decline based upon years of injury, rather than just age, suggests that other mechanisms may be in effect. One of these mechanism may be the decreased level of physical activity often observed in the wheelchair user population. Greater chronic physical activity may pro mote greater pulmonary function among wheelchairs users, or alter the rate of loss of pulmonary function.18-22 A close examina tion of Table IV indicates that some men are capable of exceeding the ambulatory popu lation norms. These tended to be those who were more active.
Conclusion
Pulmonary functions of male wheelchair users should be predicted using equations which incorporate years with disability and level of impairment. Additional study is required to make any recommendations regarding women wheelchair users. How ever, study of pulmonary function of women should be made a priority. Pulmonary func tion of wheelchair users needs further investigation. With larger sample sizes the effects of smoking status could be investig ated and included in the development of prediction equations.

{'paper_id': '19162425', 'title': 'Prediction of pulmonary function in wheelchair users', 's2_url': 'https://api.semanticscholar.org/CorpusID:19162425', 'source': 'S2ORC'}