Spirometry and chest wall dimensions in achondroplasia

Standard values for pulmonary function in short-limbed dwarfism are not available. Therefore, chest diameters and expiratory spirograms were measured in 58 female and 44 male subjects between 7 and 60 years of age with achondroplasia, the most common form of dwarfism. Standing height in adults was 49.6±3.2 (SD) inches with a sitting/standing height ratio of 0.66 (normal 0.52-0.53). Despite extremely short stature, only AP chest diameters in males were smaller than control subjects of similar age. The following equations were derived for forced vital capacity (FVC): males (under 25 years), FVC(L)= -3.56 + 0.162 X sitting height (in) +0.067 X age (yrs); males (over 25 years), FVC(L)= -0.73 + 0.162 X sitting height (in) -0.047 X age (yrs); females (under 20 years), FVC(L)= -3.56 + 0.150 X sitting height (in) + 0.067 X age (yrs); females (over 20 years), FVC(L)= -1.92 + 0.150 X sitting height (in) -0.016 X age (years). Similar prediction equations were derived for FEV₁ and FEF25-75%: FEV₁/FVC % was 84.2 (±6.5) for females and 88.0 (±6.5) for males. We also compared the observed FVC measurements to values calculated using standing heights derived from the subject's sitting height, assuming a normal body proportion. The observed vital capacity in achondroplasia was only 67.6 (± 19.2) percent of that predicted for normally proportioned females and 72.4 (± 13.6) percent for males, suggesting reduced vital capacity in achondroplasia, due to reduced chest wall compliance or abnormal lung growth.