(Chest. 2001;120:489-495.)
© 2001
American College of Chest Physicians
Spirometry Reference Values for American Indian Adults*
Results From the Strong Heart Study
M. Susan Marion, PhD;
Gary R. Leonardson, PhD;
Everett R. Rhoades, MD;
Thomas K. Welty, MD, MPH and
Paul L. Enright, MD
*
From the Aberdeen Area Tribal Chairmen’s Health Board (Drs. Marion, Leonardson, and Welty), Rapid City, SD; the College of Medicine, University of Oklahoma Health Sciences Center (Dr. Rhoades), Oklahoma City, OK; and the University of Arizona Health Sciences Center (Dr. Enright), Tucson, AZ. Supported by National Heart, Lung, and Blood Institute contracts U01-HL41642, HL41652, and HL41654.
Correspondence to: Paul Enright, MD, University of Arizona, HSC 2342, 1501 North Campbell Ave, Tucson, AZ 85724; e-mail: lungguy{at}aol.com
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Abstract
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Study objective: To derive spirometry normative values
from a large population of American Indian adults and compare them to
reference values for white adults.
Design: Pulmonary
function was assessed using spirometry in participants of the Strong
Heart Study, a multicenter, community-based, prospective study of
cardiovascular risk factors and disease in American Indians, utilizing
American Thoracic Society guidelines and a vigorous quality assurance
program.
Setting: Central Arizona, southwestern
Oklahoma, central South Dakota, and northeastern North Dakota.
Participants: Acceptable spirometry results were obtained
from 1,619 women and 1,005 men aged 45 to 74 years.
Results: Internal reference values and normal ranges for
FEV1, FVC, and the FEV1/FVC ratio were derived
from a healthy subgroup of 253 women and 190 men, identified by
excluding participants with factors associated with a lower
FEV1. Ten percent of the entire cohort (269 of 2,624
subjects) had airways obstruction, as defined by an
FEV1/FVC below the lower limit of the normal (LLN) using
the internal reference equations. After allowing for measurement
"noise," 31 participants were below the LLN using reference
equations for white adults from the large National Health and Nutrition
Examination Study (NHANES) III study but were normal using the internal
reference equations (1.3% false-positive), while 27 participants were
classified as normal using NHANES III equations but had airways
obstruction using the internal reference equations (1.2%
false-negative). Similarly low misclassification rates were seen for a
low FVC (prevalence, 17.6%).
Conclusion: For clinical
purposes, NHANES III spirometry reference equations for white adults
may be used when testing American Indian women and men aged 45 to 74
years.
Key Words: American Indians • normative values • reference equations • spirometry
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Introduction
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Spirometry
is probably the most important tool in screening for pulmonary disease
and is the most frequently performed pulmonary function
test. Reference equations used by most North American pulmonary
function laboratories to determine the normal range of spirometry
results for adults are derived from one of three major
studies,1
none of which included American Indians. New
spirometry reference equations for children and adults from the
National Health and Nutrition Examination Study (NHANES) III also did
not include American Indians.2
The Strong Heart Study (SHS), a multicenter, community-based,
prospective study of cardiovascular risk factors and disease in
American Indian adults aged 45 to 74 years,3
provided an
opportunity to derive spirometry normative values from a large
population of American Indian adults. Spirometry was included in phase
II of the SHS (from 1993 to 1995) to study pulmonary disease in
American Indians and because the FEV1 is an
excellent predictor of cardiovascular and pulmonary morbidity and
mortality.4
This analysis of spirometry results was
conducted in order to determine if the spirometry values from healthy
American Indian adults differ substantially from those of healthy white
adults in the United States.
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Materials and Methods
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Recruitment
Participants in the second examination (phase II) of the SHS,
July 1993 to November 1995, were originally recruited during the years
1989 to 1992 from 45- to 74-year-old enrolled resident members of the
following tribes: Pima/Maricopa/Tohono O’odham tribes of central
Arizona, seven tribes of southwestern Oklahoma (Apache, Caddo,
Comanche, Delaware, Fort Sill Apache, Kiowa, and Wichita), and the
Oglala and Cheyenne River Sioux of South Dakota and the Spirit Lake
tribe of North Dakota. Eligibility criteria and enrollment procedures
for the study have been previously published.5
Of those
living at the time of the second examination, 89% of the initial 4,549
participants returned. The research protocol was reviewed and approved
by the institutional review board for human studies of each clinical
center, and informed consent was obtained from all participants.
Interview and Clinical Examination
Spirometry and other measurements were scheduled throughout the
morning of the examination, which included seated BP, resting 12-lead
ECG,6
venipuncture for determination of blood
glucose7
and lipoprotein levels,8
and
physical examinations including impedance measurements, all performed
by a trained nurse. Anthropometric measurements included standing
height without shoes, weight, and hip and waist circumference. Standing
height was measured to the nearest centimeter (rounding up when 
0.5
cm) in a standardized fashion with the participant standing erect with
his or her back against the vertical wall-mounted stadiometer.
Participants were encouraged to stand as straight as possible with feet
flat on the floor. The chest was examined for evidence of surgery and
auscultated for rhonchi or rales. Pedal edema was recorded as absent,
mild, or marked. Trained interviewers obtained answers from each
participant to a subset of the standardized American Thoracic Society
(ATS) DLD-78 Respiratory Questionnaire9
and to a medical
history questionnaire.
Participant Preparation
The FVC maneuver was both explained and demonstrated.
Participants were seated during spirometry. Nose clips were used only
if leaks were observed during testing. At the end of every FVC
maneuver, acceptability and reproducibility checks were applied,
quality control messages were displayed, and the best three flow-volume
curves from the test session were displayed.10
The FVC
maneuver was repeated up to eight times or until at least three
acceptable maneuvers were obtained, with the highest and second highest
FEV1 measurements matching within 5% or 200 mL,
in accordance with 1994 ATS recommendations.11
Technicians were trained and certified prior to the start of the study.
Each week all test sessions were reviewed at the Pulmonary Function
Reading Center by a single quality control supervisor. Flow and volume
quality grades indicated the reliability of the
FEV1 and FVC, respectively. A test session with
three acceptable maneuvers, with the two largest
FEV1 measurements matching within 5% and 0.1 L,
received a flow quality grade of B. Test sessions with flow quality
grades of C (two acceptable maneuvers, but the two largest
FEV1 measurements did not match within 5%), D
(only one acceptable maneuver), or F (no acceptable maneuvers) were
mailed with comments to the technician. A monthly report summarized
technician performance over the previous 3 months.
Equipment Design and Calibration
A dry volume-sensing spirometer (Mijnhart) was selected
with Pulmotest software (S&M Instruments; Doylestown, PA), which
assisted the technician with quality control of maneuvers, calculated
the pulmonary function variables, suggested interpretations, printed
reports, and compressed graphics data for transmission and archival
storage.12
The performance of this spirometry system was
validated by third-party testing and found to meet ATS
standards.13
Leak checks and spirometer volume checks
using a 3-L syringe were repeated every morning of use on each of the
six spirometry systems.
Processing of Results
Time zero of each maneuver was determined using the
back-extrapolation technique. The FEV1, FVC, peak
flow, back extrapolated volume, and forced expiratory time were all
computed by standardized techniques, with resolutions of 10-mL volume,
10 mL/s flow, and 1-ms forced expiratory time. The peak expiratory flow
time was defined as the time in milliseconds from the back-extrapolated
time zero until the time of the peak flow. The results were corrected
to body temperature (body temperature and pressure, saturated
conditions) using the spirometer temperature sampled at the beginning
of each test session. Measurements from the three acceptable FVC
maneuvers with the highest sum of FVC plus FEV1
were stored by the spirometry system. The largest
FEV1 and the largest FVC from the three stored
acceptable FVC maneuvers were reported.
Statistical Analysis
Results from participants whose spirometry test sessions did not
meet the ATS standards for acceptability and reproducibility were
excluded from all analyses. In order to define a subsample of SHS
participants with "healthy" lungs, we performed a stepwise multiple
linear regression analysis, first entering standing height, age, and
gender, and then participant characteristics that might be predictors
of FEV1, based on a previous study.23
FEV1 was used as the dependent variable since it
is reduced as a result of both restrictive and obstructive pulmonary
disorders. Variables that were significant negative predictors of
FEV1 were used to exclude participants from the
healthy subset (Table 1 ). Many participants were excluded for cigarette smoking > 10
pack-years, asthma or wheezing, diabetes, or indexes of obesity.
Several variables in Table 1
are dependent on the answers to groups of
questions. Subjects excluded due to > 10 pack-years smoking were
either current or ex-smokers as determined by their responses to the
smoking questions in phase I and phase II. In subjects excluded under
the auscultation variable, rales or rhonchi were detected in any lobe
of the lungs. Subjects excluded under major ECG abnormality had ECG
findings that indicated left ventricular hypertrophy. Subjects excluded
under the cough variable usually coughed four to six times a day or
4 d/wk, when getting up, most days for at least 3 months, and
brought up phlegm.
Two additional factors that were very weak predictors of
FEV1, but were considered to be clinical
indicators of disease, were also used as exclusions. Subjects who could
not perform at least one acceptable spirometry maneuver (flow grade or
volume-quality grade D or F) were excluded from these healthy subgroups
since their results were considered to be inaccurate. About one sixth
of the entire cohort remained in the healthy group. All p values
< 0.05 were considered statistically significant. Analyses were
performed using statistical software (SPSS Base 7.5 for Windows; SPSS;
Chicago, IL).14
We estimated the misclassification rate that would have occurred if our
entire cohort had undergone a spirometry test in their doctor’s
office, but the spirometer used predicted values determined from white
adults participating in the NHANES III study. For this analysis, we
assumed that the internally derived reference equations (determined
from the SHS cohort) were correct (the "gold standard"). We
determined two types of spirometry abnormality rates, termed
airways obstruction, as defined by a
FEV1/FVC ratio below the lower limit of the
normal (LLN), and restriction, as defined by a FVC below the
LLN.
There is inherent "noise" when measuring the
FEV1 and FVC of about 100 mL, since the
within-test session FEV1 measurements match
within 5% or 200 mL, and the short-term within-subject repeatability
of FEV1 measurements is similar. We believe that
reported misclassification rates should not use "razor thin" cut
points, but should take this measurement error into consideration;
therefore, we did not consider a result misclassified if it was within
100 mL of the threshold.
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Results
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Demographics of the healthy group and the entire cohort are given
in Table 2
. Spirometry results were available for 3,253 of the 3,638 participants
(1,384 men and 2,254 women), but 571 subjects (18%) did not meet the
1994 ATS spirometry test session quality standards. Another 58 subjects
were removed due to inaccurately measured heights (as determined by
comparison to height measured during their third examination), leaving
a total of 2,624 participants for this analysis. The mean spirometry
values for the healthy group and for the entire cohort of 2,624
participants are given in Table 3
. Mean FEV1 and FVC values from the entire cohort
were about 10% lower than mean values from the healthy subgroup. The
age distribution of the women and men in the healthy subgroup, as shown
in Table 4
, demonstrates adequate numbers of participants in each of the age
groups for both men and women aged 45 to 75 years.
The spirometry reference equations from the healthy American Indian
women and men are given in Table 5
. Height, age, and gender were significant predictors of
FEV1, FVC, and FEV1/FVC.
The addition of second-order terms for age and height to the regression
model for healthy participants did not significantly improve the
strength of the prediction equations.
Estimated misclassification rates for airways obstruction and a low FVC
when using the NHANES III reference equations for white adults for
testing American Indian adults from the SHS cohort are shown in Table 6
.
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Table 6.. Comparison of Spirometry Abnormality Rates From the
Entire SHS Cohort (n = 2,640), Using the SHS Reference Equations From
Table 5
vs the NHANES III White Adult Reference Equations
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Discussion
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The largest previous spirometry study of American Indians was done
> 100 years ago, following the Civil War.15
More than
500 full-blooded Iroquois Indian men from a reservation near Buffalo,
NY were tested as part of an anthropometric study. The investigator
noted that the mean vital capacity (VC) of the Indian men was the same
as white soldiers (185 cubic inches). A 1978 study16
of
textile job applicants including 48 American Indian men and 19 women
suggested that their VC was slightly lower than the white subjects
tested. Crapo and coworkers17
tested 506 healthy,
never-smoking American Indian adults (151 men and 149 women), mostly
from the Navajo tribe. They found no statistically significant
differences between the spirometry values when comparing the American
Indian women with healthy white women tested previously. The
FEV1 values from the Navajo men were slightly
lower than from white men, but the differences were not clinically
significant. A population-based respiratory study18
of
Oklahoma residents in 1970 found a lower prevalence of airways
obstruction (FEV1/FVC < 70%) in American
Indians when compared to whites subjects (31% vs 48%, respectively).
In comparison with these previous studies, the SHS has the advantages
of a large population-based sample of the adults from several American
Indian communities in three states, using spirometry techniques that
exceed ATS recommendations.
In order for a set of reference equations to apply broadly to American
Indian patients, they should ideally be derived from healthy subjects
from several communities with climatic and socioeconomic diversity, and
should not be based on hospitalized patients or those seeking medical
care.19
We believe that the SHS cohort meets these
criteria, although many American Indian tribes were not represented.
A "healthy" group is difficult to define. Previous studies have
used different criteria. The ATS spirometry interpretation
workshop2
stated only that subjects should be "never
smokers, free of respiratory symptoms and disease." A
priori, we decided to exclude participants from the healthy group
who had characteristics that independently, significantly, and
negatively influenced pulmonary function (FEV1)
in the entire cohort. Since the mean FEV1 results
from the entire cohort were only 10% lower than those from the healthy
subgroup, small variations in the exclusion criteria would result in
differences in the predicted values of much less than 10%.
Diabetes mellitus is common in American Indians. Our analysis found
that diabetes was associated with a lower FEV1
(and a lower FVC, results not shown), so we excluded > 1,000 of the
study participants with diabetes from the healthy subgroup. Other
investigators20
have also reported this relationship in
other ethnic groups.
Some investigators21
have found that the addition of
nonlinear height terms to the regression equation (such as height
squared or height cubed) significantly improved their predictive power
of their equations, but we did not, despite the broad height range of
our healthy group. Most cross-sectional studies2
21
22
of
lung function show FVC and FEV1 linear age
regression coefficients of - 25 to - 28 mL/yr from the age 30 to 65
years. Our FEV1 age coefficients were in the same
range: - 26 and - 24 mL/yr for women and men, respectively. A small
degree of nonlinearity in the downward slope of lung function with age
(acceleration of decline) was noted in a previous longitudinal
study,22
but addition of a nonlinear age term did not
significantly improve the strength of our regression equations.
In Figures 1
2
3
, we visually compare the spirometry results from the healthy subset of
SHS participants with three previously published spirometry reference
studies: the study of Navajo adults by Crapo and
coworkers,17
the NHANES III study of Hankinson and
coworkers2
(using the equations for white subjects), and
the Cardiovascular Health Study23
of elderly white
subjects. The predicted values from our current study appear very
similar to those from the recent NHANES III and Cardiovascular Heart
Study, which used equipment and methods very similar to ours.
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Figure 1.. Predicted FVC values over the observed range of
heights for healthy women and men at age 58 years, comparing American
Indians from the SHS with NHANES III white adults,2
Navajo
Indians,17
and Cardiovascular Heart Study
participants.23
FVC is shown in liters and height in
centimeters.
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Figure 2.. Predicted FEV1 values (liters) vs age
(years) for healthy women (mean height, 159 cm) and men (mean height,
173 cm) in the SHS, compared with the healthy subjects from three other
studies. The age range of Cardiovascular Heart Study participants was
65 to 85 years.
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We estimated very low misclassification rates that would have resulted
if the SHS cohort had been tested in the clinical setting using the
NHANES III reference equations for white adults. Currently available
spirometers do not have separate reference equations for American
Indians, and the addition of the SHS reference equations in new
spirometers would add cost and perhaps complicate testing; therefore,
we believe that such an addition would not be worthwhile. We encourage
other investigators to determine if NHANES III spirometry reference
equations for white subjects can be used without a substantial risk of
misclassification when testing American Indian children and young
adults. In conclusion, we recommend that the NHANES III spirometry
reference equations be used when testing 45- to 74-year-old American
Indian adult patients in clinical settings.
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Acknowledgements
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The authors thank Nick Bazil for adding quality
assurance checks to the spirometry software, Pam Boyer-Pfersdorf for
training the pulmonary function technicians and monitoring their
quality, and all of the SHS pulmonary function technicians and nurses
for enthusiastically working with the study participants, including
Betty Jarvis, Martha Stoddart, Beverly Price, and Marcia O’Leary. We
also thank Millicent Higgins for actively promoting the use of
spirometry in cardiovascular epidemiology studies, the Indian Health
Service for the use of their clinics, and the following Indian
communities: Ak-Chin Papago/Pima, Apache, Caddo, Cheyenne River Sioux,
Comanche, Delaware, Spirit Lake, Fort Sill Apache, Gila River and Salt
River Pima/Maricopa, Kiowa, Oglala Sioux, and Wichita.
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Footnotes
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Abbreviations: ATS = American Thoracic Society;
LLN = lower limit of the normal; NHANES = National Health and
Nutrition Examination Study; SHS = Strong Heart Study; VC = vital
capacity
The opinions expressed in this article are those of the authors and do
not necessarily reflect those of the Indian Health Service.
Received for publication December 30, 1999.
Accepted for publication February 20, 2001.
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Abstract
Introduction
