A Comparison of Hydrostatic Weighing and Air Displacement Plethysmography in Adults With Spinal Cord Injury

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ORIGINAL ARTICLE

A Comparison of Hydrostatic Weighing and Air Displacement Plethysmography in Adults With Spinal Cord Injury Jody L. Clasey, PhD, David R. Gater Jr, MD, PhD ABSTRACT. Clasey JL, Gater DR Jr. A comparison of hydrostatic weighing and air displacement plethysmography in adults with spinal cord injury. Arch Phys Med Rehabil 2005; 86:2106-13. Objectives: To compare (1) total body volume (Vb) and density (Db) measurements obtained by hydrostatic weighing (HW) and air displacement plethysmography (ADP) in adults with spinal cord injury (SCI); (2) measured and predicted thoracic gas volume (VTG); and (3) differences in percentage of fat measurements using ADP-obtained Db and HW-obtained Db measures that were interchanged in a 4-compartment body composition model (4-comp %fat). Design: Twenty adults with SCI underwent ADP and VTG, and HW testing. In a subgroup (n⫽13) of subjects, 4-comp %fat procedures were computed. Setting: Research laboratories in a university setting. Participants: Twenty adults with SCI below the T3 vertebrae and motor complete paraplegia. Interventions: Not applicable. Main Outcome Measures: Statistical analyses, including determination of group mean differences, shared variance, total error, and 95% confidence intervals. Results: The 2 methods yielded small yet significantly different Vb and Db. The groups’ mean VTG did not differ significantly, but the large relative differences indicated an unacceptable amount of individual error. When the 4-comp %fat measurements were compared, there was a trend toward significant differences (P⫽.08). Conclusions: ADP is a valid alternative method of determining the Vb and Db in adults with SCI; however, the predicted VTG should be used with caution. Key Words: Body composition; Densitometry; Disabled persons; Plethysmography; Rehabilitation. © 2005 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

From the Department of Kinesiology and Health Promotion (Clasey), Department of Physical Medicine and Rehabilitation (Clasey), and the Spinal Cord and Brain Injury Research Center (Clasey), University of Kentucky, Lexington, KY; and VA Ann Arbor Healthcare System (Gater), Department of Physical Medicine and Rehabilitation (Gater), University of Michigan, and University of Michigan Model Spinal Cord Injury Care System (Gater), Ann Arbor, MI. Preliminary results presented to the American College of Sports Medicine, June 2000, Indianapolis, IN, and the 6th International Symposium In Vivo Body Composition Studies, October 2002, Rome, Italy. Supported in part by the National Center for Research Resources (grant no. NCRR NIH M01 RR02602), the University of Kentucky Body Composition Core Laboratory, the National Institute on Drug Abuse (grant no. K12 DA14040-03), the Veterans Affairs Rehabilitation Research and Development (grant no. B2247V), and National Institutes of Health (grant no. K23 RR16182-01). No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s)or upon any organization with which the author(s) is/are associated. Reprint requests to Jody L. Clasey, PhD, Dept of Kinesiology and Health Promotion, 216 Seaton Ctr, University of Kentucky, Lexington, KY 40506-0219, e-mail: [email protected]. 0003-9993/05/8611-9928$30.00/0 doi:10.1016/j.apmr.2005.06.013

Arch Phys Med Rehabil Vol 86, November 2005

REPORTED 400,000 PERSONS with spinal cord injury A (SCI) live in the United States, and approximately 11,000 new cases of SCI are reported each year. SCI may cause many 1

different adverse physiologic alterations, including blunted cardiovascular, ventilatory, thermoregulatory, metabolic, and hormonal responses at rest and during and after stimulatory testing procedures.2 In addition, significant changes in body composition often occur after paralysis caused by SCI. Bone mineral density, geometric, and structural properties reportedly decrease within 2 years after injury, particularly in body regions below the level of injury.3,4 Total body water is reportedly substantially lower in adults with SCI than in able-bodied persons5 and measurements of fat-free mass (FFM) as determined by whole-body potassium (K40) counting are also reduced after SCI.6 The primary constituents of the FFM (specifically the water, mineral, and protein) proportions and densities may be adversely altered, perhaps substantially, depending on the level and completeness of injury, and the time since injury. Such alterations may contribute to the reported decrease in basal or resting metabolic rate.7-9 The decrease in caloric expenditure at rest and during physical activity may contribute to an increase in total and regional fat mass. Several rehabilitative strategies have been used in adults with SCI (pharmacologic therapies, function stimulation, passive weight-bearing sessions, and exercise training intervention) to improve patients’ physiologic status and thus their quantity and quality of life after paralysis.10 Outcome measures of body composition alone may provide valuable information or may be used in combination with a wide variety of functional and metabolic measures to assess treatment effectiveness in persons with SCI. If baseline and intervention-induced changes in body composition of people with SCI are to be accurately assessed, multicompartment body composition models must be used because significant changes in the proportions and densities of the primary constituents (water, mineral, protein) of the FFM are likely to occur because of paralysis.11,12 A prerequisite of most multicompartment body composition models is a measurement of total body density (Db), which is used in the equation for computation of the fat mass and the FFM. Hydrostatic weighing (HW; sometimes called underwater weighing) has long been considered the criterion standard for determining the measurement of total body volume (Vb) that is used to determine Db. Although HW has been successfully used to determine Vb in a variety of populations, using this method for certain groups of people is difficult and at times impossible. In particular, those persons who have a fear of water and those who suffer from burns or injury that prevent their submersion in water have been excluded from studies that require HW. Several investigators have attempted to overcome many of the limitations of HW by developing alternative methods of determining Vb (air displacement, helium dilution, acoustic plethysmography), but the results have not been satisfactorily valid or reliable.13,14 Recently, a new air-displacement plethysmography (ADP) method of determining Vb was developed. Although few studies have been completed to date to determine the validity and reliability of this new method, preliminary

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BODY COMPOSITION IN ADULTS WITH SCI, Clasey Table 1: Physical Characteristics and Injury-Related Information of the Adults With SCI (Nⴝ20) Subject

Sex

Age (y)

Level of Injury

Time Since Injury (y)

Weight (kg)

Height (cm)

BMI (kg/m2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Mean ⫾ SD

M M M F M F M M F M M F M M M F M M F M

32.8 27.1 27.4 19.7 35.3 53.5 31.6 21.8 34.1 18.5 45.7 29.9 41.9 44.0 45.9 33.1 42.0 38.1 56.4 43.6 36.1⫾10.5

T3 T4 T4 T4 T4 T5 T5 T6 T7 T8 T10 T10 T10 T11 T11 T11 T12 L2 L2 L1

1.3 2.2 1.5 3.7 16.0 34.0 16.2 3.0 1.4 0.8 1.8 14.0 25.2 9.0 18.0 4.5 14.0 2.0 17.0 18.6 10.2⫾9.5

91.5 71.1 86.1 50.4 82.6 60.5 62.8 83.3 52.2 77.4 110.1 51.5 75.0 79.5 59.0 51.9 128.4 121.1 53.4 71.1 76.0⫾23.0

182.5 178.7 199.7 167.4 176.3 151.7 173.3 171.5 158.0 182.2 178.5 169.5 169.0 173.2 178.0 163.5 181.8 182.9 160.6 178.7 173.3⫾10.7

27.5 22.3 21.6 18 26.6 26.3 20.9 28.3 20.9 23.3 34.6 17.9 26.3 26.5 18.6 19.4 38.8 36.2 20.7 27.5 24.8⫾6.0

Abbreviations: BMI, body mass index; F, female; M, male; SD, standard deviation.

results in several populations (children, young and older adults, obese persons, athletic persons) differ, often showing small but systematic bias, somewhat dependent on the population tested.15-24 We are unaware of any study to date comparing traditional Vb and Db measurements obtained by HW and ADP in adults with SCI. Although HW is possible for persons with SCI, ADP may be a valid and reliable method of determining Vb and Db in this population and requires less effort on the part of the subject and the investigator. The primary purpose of this study was to compare measurements of Vb and Db, obtained by HW and ADP in adults with SCI. Because the ADP method requires a measured or predicted thoracic gas volume (VTG) for subsequent computation of Vb, and the measured VTG can be problematic in adults with higher level SCI, the measured and predicted thoracic gas volumes were also compared. In addition, in a subgroup of the SCI population examined, we investigated the difference in total body percentage of fat (%fat) obtained by using the 2 Db measures in a 4-compartment body composition model (4comp %fat). METHODS Participants Twenty adults with SCI (14 men, 6 women; age range, 18.5–56.4y) participated in this study. Subjects were recruited from the SCI and medicine clinics of the University of Kentucky, the Cardinal Hill Rehabilitation Hospital, and the Lexington Veterans Affairs Medical Center for participation in a variety of ongoing clinical collaborative research efforts of the University of Kentucky Body Composition Core Laboratory, the Spinal Cord Injury Exercise Laboratory, and the General Clinical Research Center (Lexington, KY). The subjects’ physical characteristics and other relevant demographic information are presented in table 1. Before participation, all subjects provided a detailed medical history, underwent a physical examination, and had their level of injury and American Spinal Injury Association (ASIA) impairment classification deter-

mined by a single board-certified SCI medicine physician (DRG). Inclusion criteria included SCI below the T3 level at least 6 months previously and motor complete (ASIA grade A or B) paraplegia. Subjects with Ashworth Scale spasticity score greater than 2 or with pressure ulcers greater than grade II were excluded from the study.25 Each subject provided written informed consent in accordance with the guidelines of the University of Kentucky Institutional Review Board for the Protection of Human Subjects (Medical) and the Veterans Administration Human Subjects Committee. Testing Procedures Body weight and recumbent length. Body weight was determined with the subjects seated in a modified chair suspended from a 225-kg load cell force transducera connected in series with a digital display INFCS Infinity C strain gauge meterb and an OmniScribe chart recorderc calibrated to the nearest .045kg before each testing session. During measurements of body weight and Vb, subjects wore a standard Lycra racing swimming suit.d Recumbent length was determined with a flexible steel tape measure while the subjects were lying supine on a firm padded table. Every effort was made to ensure full extension of contracted limbs, and the measurement (to the nearest 0.1cm) was made from the most distal aspect of the head to the heel of the most extended limb. ADP determination of Vb and Db. All Vb and Db measurements were obtained during a single testing session. The ADP total body volume (ADPV) and density (ADPDb) measures were acquired using an ADP system.e The ADP measurements were determined before the HW measurements of body volume and density to comply with the manufacturer’s recommendation that the subjects should be dry during the procedure. Before testing began, the ADP system was calibrated using previously described methods26 with a cylinder of a known volume (49.975L). During the testing procedure, the subjects wore standardized swimming suits provided by Jantzen Inc and a swim cap provided by the manufacturer of the ADP system. Subjects who normally wore an external or indwelling urethral Arch Phys Med Rehabil Vol 86, November 2005

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BODY COMPOSITION IN ADULTS WITH SCI, Clasey

catheter were asked to remove them before measurements and to wear a condom during the remainder of the testing; none of the current subjects used suprapubic catheters or urostomies for bladder management. After body weight and height measurements had been obtained, the subjects were assisted in transferring into the ADP chamber, were properly positioned in an upright seated position, and were given instructions for the testing procedure. Body weight was determined by the load cell technique previously described and was manually entered into the ADP system software. Manual input of body weight was required rather than by using the electronic scale connected in series to the ADP system because the scale was inaccessible to the subjects. The subjects were instructed to sit quietly in the sealed ADP chamber for 50 seconds while the ADPV was determined. The between-day variability in the ADP measurements for this group of subjects could not be determined because the subjects were only available for 1 ADP testing session. In addition the between-trial ADPV reproducibility could not be determined because, once stored in the ADP software, individual trials cannot be retrieved. Thus, we relied on the manufacturer’s error tolerance range (⬍0.3% and no greater than 150mL to be acceptable) for the within-trial error determination. After the ADPV measurement had been completed, VTG measurement (used to correct ADPV for VTG) was performed. For this measurement, the subjects were instructed to wear a nose clip and place a breathing tube (connected to the internal wall of the chamber) into their mouths, to provide 2 or 3 tidal volume breath cycles, and then to perform 3 quick, gentle “huffs” by contracting and relaxing the abdominal and intercostal muscles. The ADP design and operational procedures have been described elsewhere.27 Several trials were typically required to obtain ADP merit measurements of less than 1.0 to ensure accurate determination of VTG. Several additional adults with SCI were excluded from this study because the nature and level of their SCI prevented them from being able to perform the VTG determination procedure successfully. HW determination of Vb and Db determination. We used a previously described HW method28 to provide a measure of Vb (corrected for residual lung volume and 0.1L assumed for gastrointestinal gas volume) and thus Db. Briefly, the subjects’ underwater weight was determined using the chair, load cell force transducer, and recording system. The water temperature was maintained between 36° and 38°C to prevent hypothermia or autonomic dysreflexia. As a safety precaution, an investigator entered the underwater weighing tank with the subject to assist when necessary. Each subject wore a weighted vest to reduce buoyancy and to provide stability in the chair. The tare weight, which was determined with the assisting investigator in the tank, accounted for the weighing apparatus and the weighted vest. Underwater weight was measured at residual lung volume for 3 to 8 trials. Because the HW was very labor intensive and fatiguing for many of the subjects, we chose to terminate this portion of the testing when 3 consistent (⬍.05kg between trials) measurements were obtained. We used the average of the 2 underwater weight measurements most closely in agreement to compute HWV and HWDb. Residual lung volume was measured with a nitralyzerf immediately before the HW outside the underwater weighing tank with subjects in a grossly similar seated position as achieved during HW using an oxygen dilution technique.29 The between-day variability in the HW measurements for this group of subjects could not be determined because the subjects were only available for 1 testing session. However, the between-trial variance presented as a percentage mean difference between the 2 trials was less than 1%, using the absolute value of the trial differences. Arch Phys Med Rehabil Vol 86, November 2005

A subgroup of 13 subjects included in this investigation underwent additional testing to allow computation of %fat using a 4-comp %fat technique proposed by Heymsfield et al.30 Because substantial changes in the proportions and densities of the constituents of the FFM are likely to occur following SCI, this method of determining %fat has been proposed to provide a more accurate determination than more traditional methods that rely on assumptions of consistency in FFM constituents.11,12 This model uses the measurements of Db, total body water (TBW; in liters) and total body bone mass (TBM; in kilograms), and body weight to calculate 4-comp %fat. We used the Db measures from HW and ADP to determine 4-comp %fat using this body composition model so that the magnitude of the difference in %fat measurements could be evaluated. The subgroup (n⫽13) of subjects was admitted to the University of Kentucky General Clinical Research Center on the evening before the TBW measurement procedure. After an overnight fast, TBW was measured in the morning by a previously described deuterium oxide (D2O) dilution technique31 modified for use with plasma samples. After a baseline blood sample was taken, a precisely determined amount (measured to the nearest .0001g; ⬇30g per subject), measured by a calibrated electronic scale,g of the isotope was administered orally (between 8:00 and 8:30 AM) and 3 hours later a second blood sample was collected. Concentrations of D2O in the prepared plasma water samples, using a previously described incubation methodology,28 were determined by voltage readings (.0001mV) obtained 90 seconds after sample injection into a single-beam infrared spectrometer with a 4-mm fixed filter with a temperature-controlled flow-through sample cell.h This cell was connected in series to a True-rms 87/E III Multimeter with .0001mV digital display.i Individual regression equations converting voltage readings to D2O concentration (in ppm) were derived by using 6 prepared standard dilutions (164.7, 346.3, 479.7, 697.5, 1078.0, 1226.0ppm) for each subject. Baseline and equilibrated prepared plasma water samples were analyzed in duplicate, with an error tolerance of .0004mV within a given sample. When the error tolerance was determined in liters of water, the percent mean difference between the sample replicates, using the absolute value of the sample replicate difference, was .99⫾.26L. Corrections were made for nonaqueous hydrogen exchange (5%), and the density of water at body temperature was assumed to be .99371kg/L for the TBW computation.30 Dual-energy x-ray absorptiometry. TBM (in kilograms) estimates were required to compute percentage fat using the multicompartment body composition model. The TBM was estimated from bone mineral ash measured by total body dualenergy x-ray absorptiometryj (DXA) scans. Bone mineral ash was multiplied by 1.279 to estimate TBM (the sum of osseous mineral and cell mineral). In addition, total body bone ash was multiplied by .4180 to compute total mineral volume.30 The subjects were instructed to remove all objects such as jewelry and eyeglasses and wore only a standard hospital gown during the scan procedure. In accordance with the DXA manufacturer’s recommendation, the scan mode selected for each subject was based on body depth (sagittal diameter) at the thickest portion of the body. A single trained investigator (JLC) certified by the state of Kentucky and the International Society of Clinical Densitometry (ISCD) analyzed all DXA scans. Of note, nearly all the subjects included in this investigation had spine stabilization rods due to their injuries. Because of the location, varying dimensions, and composition of these rods, elimination of or correction for the influence of these rods on TBM was not possible. In accordance with the recommenda-

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BODY COMPOSITION IN ADULTS WITH SCI, Clasey Table 2: HWv and HWDb Versus ADPv and ADPDb of the Adults With SCI (Nⴝ20)

HWVv (L) ADPv (L) HWDb (g/mL) ADPDb (g/mL)

73.73⫾5.20 74.28⫾5.22* 1.0329⫾0.0045 1.0244⫾0.0043*

R2

TE

.99*

.79

.77*

.0126

*P⬍.01 vs HW.

tion of the ISCD, precision for the total body bone mineral content measurements was accomplished by performing total body scans twice for an independent sample of 27 able-bodied subjects. The resulting precision of the 2 scan measurements was .0219kg when reported as standard deviation (SD) (in kilograms), .0061 when reported as coefficient of variation (CV), and .61% when reported as percentage of CV (%CV). Statistical Analyses The mean difference between the ADPV and HWV, and ADPDb and HWDb measurements of the 20 subjects were calculated with paired t tests.k In addition, shared variance, determination of total error (TE), and the Bland and Altman32 plotting procedure were used to examine the strength of the relationship between the 2 methods of determining Vb and Db. We used similar statistical analyses to determine the strength of the relation between measured and predicted VTG measurements required by the ADP method, and to determine the effect of using ADPDb or HWDb to compute %fat by the 4-comp %fat. The ␣ level for statistical significance was P less than .05. RESULTS Comparison of Vb and Db Measurements The relation between the HWV and ADPV measurements, and between the HWDb and ADPDb measurements are presented in table 2. Significant shared variances (P⬍.01) were found between the Vb (r2⫽.99) and the Db (r2⫽.77) measurements. Group mean comparisons showed that HWV measurements were significantly lower than ADPV measurements (mean difference ⫾ standard error [SE], ⫺.55⫾.13; P⬍.01) and HWDb measurements were significantly greater than ADPDb measurements (mean difference ⫾ SE, .0085⫾.0022; P⬍.01). For 16 of the 20 subjects tested, HWV was lower than ADPV, and HWDb was greater than ADPDb. The TEs around the line of identity for the volume and density measures were .79L and .0126g/mL, respectively. The HWV minus ADPV relative difference, expressed as a percentage of the mean difference for each subject, ranged from ⫺2.17% to 1.95% (mean ⫾ SE, ⫺.78%⫾.21%). The HWDb minus the ADPDb relative difference, expressed as a percentage of the mean difference for each subject, ranged from ⫺1.87% to 2.18% (mean ⫾ SE, .82%⫾.21%). Figure 1 shows the Bland-Altman plot for the HWV versus the ADPV the measurements. The mean difference for volume measurements was ⫺.55L and the 95% confidence intervals (CIs; ⫾2 SDs) ranged from ⫺1.70 to .60L. Figure 2 shows the Bland-Altman plot for the HWDb versus the ADPDb measurements. The mean difference for density measurements was .0085g/mL and the 95% CIs (⫾2 SDs) ranged from ⫺.0109 to .0279g/mL. With the exception of 1 subject, all volume and density differences fell within the 95% CIs.

Discrepancy Between the Total Body Volume Measures (HW – AP; L)

Mean ⫾ SE

2.25 1.50 0.75 0.00 -0.75 -1.50 -2.25 -3.00 20.00

40.00

60.00

80.00

100.00

120.00

140.00

Mean Total Body Volume Measures (HW + AP / 2; L)

Fig 1. Bland-Altman plot for Vb determined by ADP versus Vb determined by HW in adults with SCI (Nⴝ20). Solid line represents mean volume measurement differences (HW ⴚ ADP); dashed lines represent ⴞ2 SDs of the mean measurement differences.

Comparison of Measured and Predicted VTG Because the predicted VTG equation used in the ADP was developed using able-bodied persons and may be inappropriate for application in adults with SCI, the measured VTG and predicted VTG were compared. Pilot testing in our laboratory had previously shown the inability of subjects with SCI above T3 to perform the VTG measures because of intercostal and abdominal muscle paralysis. A weak, yet significant (P⬍.01) shared variance (r2⫽.37) was found between the measured VTG and predicted VTG and a group mean comparison found no significant differences (mean difference ⫾ SE, .17⫾.15L; P⫽.27) between the measured VTG (mean ⫾ SE, 3.95⫾.19L) and the predicted VTG (mean ⫾ SE, 3.78⫾.12L). The TE for the measured and predicted VTG was .68L. The measured VTG minus the predicted VTG relative difference, expressed as a percentage of the mean difference for each subject, ranged from ⫺38.6% to 33.1% (mean ⫾ SE, 3.1%⫾3.9%). Figure 3 shows the Bland-Altman plot for the measured and predicted

0.0500 Discrepancy Between the Total Body Density Measures (HW – AP; g/mL)

Measures

3.00

0.0375 0.0250 0.0125 0.0000 -0.0125 -0.0250 -0.0375 -0.0500 0.9800

1.0000

1.0200

1.0400

1.0600

1.0800

Mean Total Body Density Measures (HW + AP / 2; g/mL)

Fig 2. Bland-Altman plot for Db determined by ADP versus Db determined by HW in adults with SCI (Nⴝ20). Solid line represents mean density measurement differences (HW ⴚ ADP); dashed lines represent ⴞ2 SDs of the mean measurement differences.

Arch Phys Med Rehabil Vol 86, November 2005

BODY COMPOSITION IN ADULTS WITH SCI, Clasey

3.00 2.00 1.00 0.00 -1.00 -2.00 -3.00 2.00

2.50

3.00

3.50

4.00

4.50

5.00

5.50

6.00

Mean Thoracic Gas Volume (Measured VTG + Predicted VTG / 2; L)

Fig 3. Bland-Altman plot for predicted VTG versus measured VTG in adults with SCI (Nⴝ20). Solid line represents mean VTG (measured VTG ⴚ predicted VTG); dashed lines represent ⴞ2 SDs of the mean measurement differences.

VTG. The 95% CIs (⫾2 SDs) for the measured and predicted VTG ranged from ⫺1.18 to 1.52L. With the exception of 1 subject, all VTG differences fell within the 95% CIs. Comparison of 4-Comp %Fat Measurements Using HWDb or ADPDb The relation between the 4-comp %fat measures using Db measurements obtained by HW (4-comp %fat HW) and ADP (4-comp %fat ADP) was examined in a subgroup (n⫽13) of the 20 subjects included in this study. There was a significant shared variance (r2⫽.93, P⬍.01) between the 4-comp %fat HW and the 4-comp %fat ADP measurements. Group mean comparisons showed a trend toward a significant mean difference (mean difference ⫾ SE, ⫺1.4⫾0.7; P⫽.08) between the HW 4-comp %fat measurements (mean ⫾ SE, 27.7⫾2.8) and the ADP 4-comp %fat measurements (mean ⫾ SE, 29.1⫾2.9). The TE around the line of identity for the 4-comp %fat measurements was 2.9%. Figure 4 shows the Bland-Altman plots for the HW 4-comp %fat versus the ADP 4-comp %fat measurements. The mean difference between the 4-comp %fat measurements was ⫺1.2%, and the 95% CIs (⫾2 SDs) ranged from ⫺6.6% to 3.8%. With the exception of 1 subject, all 4-comp %fat differences fell within the 95% CIs. DISCUSSION Until recently, the only practical method of obtaining Db measurements for use in body composition models was to perform an HW technique. Using standardized validated procedures, combined with appropriate residual lung volume measures, this method provides an accurate measure of Vb and thus Db. Although HW has been successfully used to determine Vb in a variety of populations, this method is difficult and at times impossible to use in certain people. The HW method can be used for adults with SCI, but the procedures are often labor intensive for both the subject and the investigators. The HW technique imposes additional risks of aspiration and autonomic dysreflexia to the SCI population, and may also require significant modifications to the HW system to allow for stable transfers and measurements during testing. The recent development of ADP offers an alternative method of obtaining Db measurements. This method eliminates many of the difficulties encountered during testing of persons with SCI or other physArch Phys Med Rehabil Vol 86, November 2005

ical disabilities. Only a limited number of studies have been performed to validate the ADP system, and the results of these studies often conflict and may depend on the population examined. Comparison of the results of this study with others using the ADP and comparing results with results from HW is complicated by the fact that the adults with SCI had great variance in physical characteristics and injury-related information. Table 1 shows the diversity in factors including age, weight, height, body mass index, and perhaps most important in this SCI population, the level and time since injury. Despite the diversity of this SCI population, the percentage mean differences for the subjects’ Vb and Db measurements varied very little. Specifically, the greatest individual difference in Vb measurements was ⫺2.17% and 2.18% for Db. The significant group mean measurement differences further show that ADPVb measurements were higher and ADPDb measurements were lower than HW measurements in a majority (80%) of the adults with SCI. These findings are similar to those of Wagner et al,24 who used similar techniques and criteria to determine the validity of ADP in assessing the body composition of a group of young black men. In that study, ADPDb measurements were only slightly lower than HWDb measurements, but the difference was statistically significant, and the Db was underestimated in 73% of the 30 subjects tested. In contrast, Collins et al16 found that ADPDb was significantly greater than HWDb in a group of collegiate football players, and Lockner et al21 found that ADPDb was significantly greater than HWDb in a group of children aged 10 to 18 years, with the largest overprediction for the smallest children. In addition, Nunez et al23 found strong correlations and no significant group mean differences between ADPDb and HWDb measurements in a group of adults and children; however, ADPDb was underestimated in the younger children. These findings were similar to those of another study17 that found no significant mean differences or bias in ADP and HW in children, but the 95% confidence limits were rather large. Fields et al19 found that, in a group comprised solely of women, the ADP and HW %fat measurements did not significantly differ. Biaggi et al15 reported no significant differences in the mean %fat measurements obtained by ADP and those obtained by HW in a group of adult men and women. However, when

Discrepancy Between the 4-Compartment Model %Fat Measures (4-Comp %Fat HW - 4-Comp %Fat AP)

Discrepancy Between the Thoracic Gas Volume Measures (measured VTG – predicted VTG; L)

2110

12.0

8.0

4.0

0.0

-4.0

-8.0

-12.0 0.0

10.0

20.0

30.0

40.0

50.0

Mean 4-Compartment Model %Fat (4-Comp %Fat HW + 4-Comp %Fat AP / 2)

Fig 4. Bland-Altman plot for 4-comp %fat using Db measures from ADP versus 4-comp %fat using Db measures from HW (nⴝ13). Solid line represents mean %fat measurement differences (4-comp %fat HW ⴚ 4-comp %fat ADP); dashed lines represent ⴞ2 SDs of the mean measurement differences.

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BODY COMPOSITION IN ADULTS WITH SCI, Clasey

compared with HW, ADP underestimated %fat measurements in men and overestimated women. Similar group mean and gender differences were reported by Levenhagen et al20 who found no group mean differences in ADP and HW %fat measures, but found that ADP %fat measurements were significantly less in men and significantly greater in women than %fat by HW. The diversity among the findings, relative to the magnitude and direction (lower or higher) of the difference between HWDb and ADPDb measurements, further shows that results and conclusions from the comparisons between the 2 methods may be population specific. A recent review by Fields et al33 reports the impact of several population-specific factors, including age, size, and sex, that may influence the results of the comparison between ADP and HW. The present study is unique in that it demonstrates and addresses the magnitude of the difference, and the relatively large bias in the number of subjects (16/20) with a common directional difference between ADP and HW measurements, in a group of adults with SCI. The ADP measurements require a measured VTG, which can be impossible for some adults with SCI to complete because the level and the completeness of the injury result in compromised use of assistive respiratory and abdominal muscles necessary for this measurement, particularly for persons with injuries above T6. Alternatively, a predicted VTG provided by the manufacturer can be used to determine ADPV and has been used in numerous studies when measured VTG could not be successfully completed, or to reduce the time spent testing. In a study specifically designed to determine the affect of using the predicted VTG, McCrory et al34 compared measured and predicted VTG in a group of able-bodied adults. Despite finding no significant differences between the measured and predicted VTG , the authors suggested that measured VTG remain a part of standard experimental and clinical practice and that the results of their study may not necessary apply to other groups. In the present study, the group mean measured and predicted VTG did not differ significantly. However, the rather low (albeit significant) shared variance (r2⫽.37) and the large range of relative mean differences between measured and predicted VTG lead us to concur with the recommendations of McCrory34 and we suggest that the predicted VTG should be used with caution. The need for caution is further shown when the TE of VTG measures (.68L) is compared with the TE for the Vb measures (.79L). Persons with impaired intercostal and abdominal mus-

cles due to paralysis would not only have difficulty performing the VTG, but would actually have greater true VTG than predicted because of restrictive lung disease and reduced diaphragmatic excursion, leading to spurious reductions in Db and overestimates of percentage body fat. Further population-specific refinement of the predicted VTG equation would be necessary to ensure accuracy of its future use. To date, only 2 studies have taken the novel approach of substituting HWDb for ADPDb in a multicompartment model to examine the resulting %fat measurements. Millard-Stafford et al22 performed such an analysis with young, healthy, ablebodied subjects, in whom the proportions and densities of the primary constitutes FFM are likely to vary less from the theoretical standard reference man than are those of adults with SCI. That study found that ADPDb was greater than HWDb, and when ADPDb was substituted for HWDb in a 4-comp %fat model, the %fat measurement was significantly lower. Nevertheless, the authors concluded that the small bias in the resulting 4-comp %fat measurements (SD of the difference, 2.3%) showed that the substituting of ADPDb for HWDb in a multicompartment model is an acceptable practice. In a group of children, Fields and Goran18 found no significant difference in mean fat mass determinations obtained using a 4-compartment model when ADPDb and HWDb were interchanged. The authors also reported that HW, DXA, and total body water techniques demonstrated significant bias in fat mass measurements, whereas ADP did not. This finding offers further support for the use of ADP to measure body composition in children. In a subgroup of subjects (n⫽13), our findings show that there was significant shared variance (r2⫽.93, P⬍.01) between the 4-comp %fat HW and the 4-comp %fat ADP measurements when ADPDb was substituted for HWDb in a multicompartment body composition model. Although the 4-comp %fat group mean measurements did not differ significantly, a trend toward significance (P⫽.08) was observed and further analysis suggests that the reduction in the number of subjects (from 20 to 13 subjects) may have influenced the significance of these findings. In addition, the decrease in the shared variance between HWDb and ADPDb (r2⫽.77) compared with HWV – ADPV (r2⫽.99), despite the use of an identical body weight to calculate Db, suggests that the changes to the water and mineral fractions of the FFM (due in part to the SCI and subsequent paralysis) may have had a significant impact on the comparison

Table 3: Physical Characteristics, Injury-Related Information, and 4-comp %fat HW Characteristics of the Adults With SCI (nⴝ13)

Subject

Sex

Age (y)

1 2 3 4 5 6 7 8 9 10 11 12 13 Mean ⫾ SE

M M M F M M M M M M M F F

27.1 27.4 35.3 53.5 31.6 21.8 18.5 45.7 41.9 44.0 45.9 33.1 56.4 37.1⫾3.3

Level of Injury

T4 T4 T4 T5 T5 T6 T8 T10 T10 T11 T11 T11 L2

Time Since Injury (y)

Weight (kg)

2.2 1.5 16.0 34.0 16.2 3.0 0.8 1.8 25.2 9.0 18.0 4.5 17.0 11.5⫾2.9

71.1 86.1 82.6 60.5 62.8 83.3 77.4 110.1 75.0 79.5 59.0 51.9 53.4 73.3⫾4.4

Height (cm)

4-Compartment Model %Fat* (%)

Aqueous Fraction of the FFM* (%)

Mineral Fraction of the FFM* (%)

178.7 199.7 176.3 151.7 173.3 171.5 182.2 178.5 169.0 173.2 178.0 163.5 160.6 173.6⫾3.2

29.2 19.8 29.4 42.0 9.9 25.3 9.0 34.9 29.8 31.2 26.0 35.7 38.2 27.7⫾2.8

71.6 78.2 70.2 71.4 72.6 72.0 81.1 78.0 75.2 69.9 70.0 84.0 71.5 74.3⫾1.3

6.7 7.6 5.7 6.2 6.0 6.6 5.6 6.3 7.2 5.9 6.2 6.8 7.6 6.5⫾0.2

*Calculated using body density measurements by HW.

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BODY COMPOSITION IN ADULTS WITH SCI, Clasey

of 4-comp %fat measurements. The group means for the water fraction (mean ⫾ SE, 74.3%⫾1.3%) and the mineral fraction (mean ⫾ SE, 6.5%⫾0.2%) of the FFM are shown in table 3 and deviate little from the water fraction (73.8%) and the mineral fraction (6.8%) of the FFM proposed in the theoretical standard reference body. However, the large individual variability of these FFM fractions, combined with the small differences in Db by ADP and HW, showed that the 4-comp %fat HW minus 4-comp ADP relative difference, expressed as a percentage of the mean difference for each subject, ranged from ⫺24.8% to 15.4% (mean ⫾ SE, ⫺5.4%⫾2.7%). We do not conclude from this additional analysis that ADPDb should not be considered for use in multicompartment body composition analyses in adults with SCI; however, using ADPDb interchangeably with HWDb in a single study should be avoided. CONCLUSIONS Unfortunately, there have been no other reported studies comparing ADP with HW in adults with SCI and other disabilities. The reduction in time, effort, and risk provided by the ADP measurement make it an attractive alternative measurement for HW, and the findings of this study support the use of ADP as a valid method for Vb and Db in adults with SCI below T3. We further conclude that the use of predicted VTG should be avoided if possible until further SCI population specific refinement of this equation can be accomplished. In addition, these findings suggest that ADP is a valid alternative method of determining the Db for use in multicompartment body composition models to calculate percentage fat in future studies designed to examine baseline measurements and treatment effects in adults with SCI. Finally, we suggest that the ADP technology should be refined to include a wheelchair-accommodating scale connected in series with the system, and include a method or device that would accurately assess the thoracic lung volume of persons who are physically unable to perform the current procedure. Such refinement will undoubtedly increase the usefulness of ADP as a body composition assessment tool in adults with SCI and other disabilities. Acknowledgments: We thank Jantzen Inc (Portland, OR) for generously donating the swimming suits used during body composition assessment. We also thank Linda Rice, Tammy Ellis, Phoebe Brown, and Susan Berryman of the University of Kentucky General Clinical Research Center for expert clinical care. We thank Flo Witte for editorial assistance, and Craig Winsor, Adrienne Janowiak, and Carol Beth Mize for their technical support. References 1. DeVivo MJ. Epidemiology of traumatic spinal cord injury. In: Kirshblum SK, Campagnolo DL, DeLisa JA, editors. Spinal cord medicine. Philadelphia: Lippincott Williams & Wilkins; 2002. p 69-81. 2. Bauman WA, Spungen AM, Adkins RH, Kemp BJ. Metabolic and endocrine changes in persons aging with spinal cord injury. Assist Technol 1999;11:88-96. 3. de Bruin ED, Dietz V, Dambacher MA, Stussi E. Longitudinal changes in bone in men with spinal cord injury. Clin Rehabil 2000;14:145-52. 4. Clasey JL, Janowiak AL, Gater DR. Relationship between regional bone density measurements and the time since injury in adults with spinal cord injuries. Arch Phys Med Rehabil 2004;85: 59-64. 5. Cardsus D, McTaggart WG. Total body water and its distribution in men with spinal cord injury. Arch Phys Med Rehabil 1984;65: 509-12. Arch Phys Med Rehabil Vol 86, November 2005

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25. Ashworth B. Preliminary trial of carisoprodol in multiple sclerosis. Practitioner 1964;192:540-2. 26. McCrory MA, Gomez TD, Bernauer EM, Mole PA. Evaluation of a new air displacement plethysmograph for measuring human body composition. Med Sci Sports Exerc 1995;27:1686-91. 27. Dempster P, Aitkins S. A new air displacement method for the determination of human body composition. Med Sci Sports Exerc 1995;27:1692-7. 28. Katch F, Michael ED, Horvath SM. Estimation of body volume by underwater weighing: description of a simple method. J Appl Physiol 1967;23:811-3. 29. Wilmore JH. A simplified method for the determination of residual lung volumes. J Appl Physiol 1969;27:96-100. 30. Heymsfield SB, Lichtman S, Baumgartner RN, et al. Body composition of humans: comparison of two improved fourcompartment models that differ in expense, technical complexity, and radiation exposure. Am J Clin Nutr 1990;52:52-8. 31. Modlesky CM, Cureton KJ, Lewis RD, Prior BM, Sloniger MA, Rowe DA. Density of the fat-free mass and estimates of body composition in male weight trainers. J Appl Physiol 1996;80: 2085-96. 32. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 1:307-10.

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33. Fields DA, Goran MI, McCrory MA. Body-composition assessment via air-displacement plethysmography in adults and children: a review. Am J Clin Nutr 2002;75:453-67. 34. McCrory MA, Mole PA, Gomez TD, Dewey KG, Bernauer EM. Body composition by air-displacement plethysmography by using predicted and measured thoracic gas volumes. J Appl Physiol 1998;84:1475-9. Suppliers Interface Inc, 7401 E Butherus St, Scottsdale, AZ 85260. Newport Electronics Inc, 2229 S Yale St, Santa Ana, CA 92704. Houston Instruments, 8500 Cameron Rd, Austin, TX 78753. Jantzen Inc, PO Box 3001, Portland, OR 97208. BOD POD Body Composition System, software version 1.80; Life Measurement Instruments, 1850 Bates Ave, Concord, CA 94520. f. Model 505 Nitralyzer; Med Science, St. Louis, MO 63132. g. Scientech SA 120; Scientech, Inc, 5649 Arapahoe Ave, Boulder, CO 80303. h. Miran 1FF; Invensys Process Systems, 33 Commercial St, Foxboro, MA 02035. i. Fluke Corp, 6920 Seaway Blvd, Everett, WA 98203. j. Lunar DPX-IQ; software version 4.3; GE Medical, 726 Heartland Trl, Madison, WI 53717. k. StatView; SAS Institute Inc, 100 SAS Campus Dr, Cary, NC 27513. a. b. c. d. e.

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