Feasibility and agreement in inspiratory capacity measured by spirometry and body plethysmography

Inspiratory capacity (IC) is the maximum volume of air (measured in liters) that can be taken in a full inspirations and without hesitation from a point at the end of a normal expiration or respiratory tidal volume. Is quite sensitive for quantification of improvement after therapeutic interventions, predicts dynamic hyperinflation and exercise limitations. It can be measured by spirometry (the vital capacity maneuver or VC), or by body pletismography (BP). The aim of this study was to determine the feasibility and agreement in IC measured by spirometry or BP in a large sample of normal subjects and patients with different ventilatory patterns. Methods and measurements: Healthy subjects, of 18 years of age and older, non-smokers and non-history of respiratory disease. A second sample of patients referred by their physicians for a routine pulmonary function testing. They were stable and had no symptoms of acute respiratory infection in the previous two months. Results: A total of 58 healthy subjects and 163 were recruited for the study. Four subjects were not able to complete acceptable maneuvers. The final sample was 56 normal subjects (55.4% males) and 163 patients, 64% males. Mean age for normal subjects was 34while for patients was 56.8 years. In both groups, a high correlation (r >0.90) was found comparing spirometry and plethysmography measurements. The agreement analysis showed an CCI = 0.90, although agreement limits were wide (600 mL). Discusion: Despite the high correlation and agreement between two methods for measuring inspiratory capacity, they can have and significant variability within and between methods.