Normal Prediction Equations of Spirometric Parameters in 799 Healthy Iranian Children and Adolescents

Mohammad Golshan MD*, Mehdi Nemat-Bakhsh MD**

*Department of Pulmonary Medicine, Isfahan University of Medical Sciences, **Department of Physiology, Isfahan University of Medical Sciences, Isfahan, Iran

Keywords • Adolescent • children • FEV • reference values • vital capacity • spirometry standards

Introduction

Pulmonary function tests (PFTs) are valuable tools increasingly used to provide objective measures of lung function for various purposes.^1-2 They include quantifying respiratory impairment, monitoring the evolution of lung disease and monitoring pulmonary injuries induced by drugs, occupational or environmental exposure.^3,4 Improved methods have also led to extension of the field to pediatric practice.^5-16

PFT results are reference values obtained from quantified studies of normal values in healthy non-smokers.^16-18 Due to differences among the populations and technical and procedural issues, equations used to predict normal lung function are diverse.^2,16 Thus it is important to improvise appropriate prediction equations as reference for a given population.¹⁶ Many published reference values are available and the American Thoracic Society (ATS) has published guidelines for making such choices.^1-16 However, the inappropriate use of control data obtained from non-native populations regarding variations in physical fitness and genetic factors, is a major cause of misinterpreting PFT results. Besides, many of the universally used standardized prediction equations, probably due to relatively small population samples offer less reliability on the extremes of the status and age spectrums. These shortcomings can be overcome by defining normal values for different populations.

The purpose of the present study was to extract and calculate prediction standards based on the measurements of ventilatory function in 799 healthy never-smoker children and adolescents living in Isfahan, central Iran.

Patients and Methods

During a 3-year period (March 1995 to March 1998), all the staff of a bank were referred for check-up. All employees and also every 20th patient of a pulmonary clinic in Isfahan, Iran were asked to refer their family members including parents, spouse, and children for medical evaluation including obtaining meticulous medical history, physical examination and spirometry. If the subject would refuse to refer his family, the next patient (21st) was offered so. In the patients group, those who were expected to have pulmonary problems were not included in the series. The check-up cases and family members of the patients were meticulously interviewed and physically examined by an experienced pulmonologist. Any of the subjects who had respiratory complaints or history of ever smoking, serious pulmonary or cardiac disease, respiratory symptoms or physical findings related to cardiopulmonary diseases, or evident chest deformity were excluded from the study. Height was measured by standing against a wall with socks only. All spirometeries were performed in the sitting position with a nose clip. Forced vital capacity maneuver was performed in a standard fashion using 2 similar electronic spirometers (Cybermedic, Lousiville Co. 80027 updated with new software ver.3.8D which were calibrated daily). All spirometeries were performed by two experienced technicians well aware of the implicit sources of failure in this procedure and capable of instructing subjects to make proper efforts. Barometric pressures, measured daily, showed a range of 631 to 635 mm Hg, and room temperature was between 21 to 25 degrees centigrade. BTPS corrections were automatically made by the spirometer’s software.

All of the spirometric parameters were derived from a single best test. Three percent (3%) of the test reports of these youths did not meet AST criteria for standardization of PFTs and were discarded. The remaining 799 reports were saved in a FoxPro database for analysis. These data were later analyzed with SPSS software and a prediction

equation was derived for each of the measured parameters by a multiple regression analysis.

Results

Of the 1,019 young subjects initially invited for interview, 61 (6%) refused or failed to attend for medical interview. At the medical interview 10 boys (1%) who were smokers were excluded. The remaining 948 (92%) cases reported no respiratory problems (including chronic cough, dyspnea, wheezing, asthma, shortness of breath, chronic and/or recurrent respiratory illness) and physical examination revealed no abnormality. Forty-eight subjects (5.5%), mostly young children, could not cooperate or refused to provide pulmonary function tests, and 25 (3%) of the tests did not meet the AST criteria and were discarded. The remaining 799 tests were kept for analysis.

Three children under four years of age (40-46 months) successfully completed acceptable tests, and were included in the series.

Demographic characteristics in relation to measured spirometric parameters are shown in table 1. Measured FVC and FEV₁ of the subjects in relation with age are shown in figure 1. All the parameters have been collected each from a single subject. The extracted prediction equations are demonstrated in table 2.

Discussion

Prediction equations derived from large populations are important both for daily clinical spirometry needs and screening or epidemiologic research.¹⁶ This survey of a large population sample with a wide age range provides suitable setting for developing such equations for the Iranian population. Since the physique of the people in the Middle East region seems to be similar, the equations may be applicable to other neighboring countries as well.

While most of the PFT studies performed in non-European populations have shown greatly decreased lung volumes compared to Europeans, our results show that the lung volumes and flow rates in Iranian children are nearly similar to or only slightly lower than most of the international scales.

A major problem conflicting prediction equations is the fact that mathematical equations derived from regression analysis of the measurements are less accurate for the measurements performed in cases with independent variables close to lower or higher limits of the regression line.² The presence of three children younger than 4 years of age, expands the availability of valid predictions for children beyond all of the previously reported ranges. The pulmonary function tests in the mentioned three children fulfilled the AST criteria and the results, though not perfect, seemed reasonable. This may help to obtain better estimates for older children by keeping them away from the lower edge of the regression line.

Another concern is that the complex process of growth affects the relationship between indices of body size and spirometric measurements in children and adolescents. In fact, in children and adolescents the relationship between ventilatory function and height or age is not linear.² This fact makes it an obligation for the prediction models to use either divided smaller age groups or develop non-linear complex equations.² We have chosen the first option and have developed two sets of prediction equations for children under and above the age of 12. As in children there is a close relationship between age and height. The regression equations using only one of these independent variables suffices, though the equations containing both height and weight render more promising results. Moreover, most of the published models have used a constant in equations but in our series using a constant, results in serious decline in the correlation coefficient (r²), and we decided not to use a constant for most of the equations.

In conclusion, we have presented for the first time, a set of prediction equations of ventilatory function for healthy children and adolescents of native population that can be used as reference values in Iran and neighboring countries.

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