Interpretation of Spirometry: Selection of Predicted Values and Defining Abnormality

Abstract

Spirometry is the most frequently performed investigation to evaluate pulmonary function. It provides clinically useful information on the mechanical properties of the lung and the thoracic cage and aids in taking management-related decisions in a wide spectrum of diseases and disorders. Few measurements in medicine are so dependent on factors related to equipment, operator and the patient. Good spirometry requires quality assured measurements and a systematic approach to interpretation. Standard guidelines on the technical aspects of equipment and their calibration as well as the test procedure have been developed and revised from time-to-time. Strict compliance with standardisation guidelines ensures quality control. Interpretation of spirometry data is based only on two basic measurements -the forced vital capacity (FVC) and the forced expiratory volume in 1 second (FEV 1 ) and their ratio, FEV 1 /FVC. A meaningful and clinically useful interpretation of the measured data requires a systematic approach and consideration of several important issues. Central to interpretation is the understanding of the development and application of prediction equations. Selection of prediction equations that are appropriate for the ethnic origin of the patient is vital to avoid erroneous interpretation. Defining abnormal values is a debatable but critical aspect of spirometry. A statistically valid definition of the lower limits of normal has been advocated as the better method over the more commonly used approach of defining abnormality as a fixed percentage of the predicted value. Spirometry rarely provides a specific diagnosis. Examination of the flow-volume curve and the measured data provides information to define patterns of ventilatory impairment. Spirometry must be interpreted in conjunction with clinical information including results of other investigations.

Relationship between Body Surface Area and Pulmonary Functions in Patients of Silicosis

“…Previous studies have implicated that lung function may theoretically be affected by several directly measured or derived anthropometric variables including standing height, trunk-leg ratio, weight, body surface area, and body mass index. 9 Das and Jahan found that an increase in BSA is associated with an increase in lung volumes and PEFR both in males and females. 10 However, the effect of BSA on the pulmonary functions in patients of silicosis is an arena hardly explored.…”

Espirometría en pacientes infectados con virus de la inmunodeficiencia humana

“…Por otro lado, pacientes que acudieron al área de banco de sangre y/o consejería del Hospital Víctor Lazarte Echegaray de la red asistencial Es Salud -La Libertad durante el periodo de recopilación de datos fueron seleccionados como controles, debiendo cumplir con los siguientes criterios de inclusión: Personas de ambos sexos sin patologías pulmonares o respiratorias agudas o crónicas, ni enfermedad pulmonar activa. Para ambos grupos se excluyó a pacientes con diagnóstico previo de Enfermedad Pulmonar Obstructiva Crónica, pacientes Oncológicos, cardiópatas, gestantes, con patologías pulmonares agudas (no más de 2 semanas), con Insuficiencia cardiaca congestiva: NYHA I, II, III y IV, fumadores con un índice de tabaco >10 (26,27) , usuarios de Oxigenoterapia, TBC actual, en UCI, asmáticos o con vasculitis de algún tipo, pacientes con trauma torácico o antecedente similar, con defectos de caja torácica, o patologías diafragmáticas, tiroideas o pleurales. (28,29) A todos ellos, una vez seleccionados, se les solicitó previo consentimiento informado, su participación en el estudio.…”

A Case for Using Indian Prediction Equations for Pulmonary Function Tests in Indians

“…These find application in confirmation of diagnosis, evaluation of functional impairment, assessment of therapeutic response, monitoring for occupational and environmental lung damage, pre-operative risk stratification and for following the course of disease. 1 PFTs also find application in studies on public health as well as in settling legal claims for lung injuries. Being non-invasive, relatively less expensive and almost entirely without any hazards to the patient, these tests are also carried out periodically to aid in management decisions.…”

“…Thus, every person will have a different "normal" value and that is not constant but ever-changing with time. 1 The normal value is calculated using "prediction" equations that take into account the known and the unknown predictors or determinants of the parameter of interest. These equations are developed by studying lung function of a large sample of carefully selected and well-defined "normal" subjects.…”

“…It has been strongly recommended that due to the well-known differences in lung functions between subjects of different ethnic origins, equations developed in the population with the same ethnicity as the subject being tested must be selected. 1,2 The equations also need to be gender-specific. That means using Indian equations for Indian subjects.…”