Genetic and environmental influence on inspiratory resistive load detection

To differentiate genetic factors from environmental forces in determining threshold for resistance load detection (RLD), 62 healthy adolescent twins (mean age = 16 years) and 74 healthy adult twins (mean age = 34 years) were studied by the standard psychophysical technique. The zygosity was determined by blood groups, finger prints, and physical appearances. Mean values for age, height, weight, pulmonary functions (FVC, FEV1, FEV1/FVC, Raw, and FRC) were not different between monozygotic (MZ) and dizygotic (DZ) pairs either in adolescent or adult twins. Threshold for RLD was analyzed in terms of added resistance divided by airway resistance plus apparatus resistance (Weber fraction) and mouth pressure (P) at the threshold. Both Weber fraction and P were equivalent between MZ and DZ either in adolescent or adult twins. In adolescence, within-pair variance for P was significantly greater in DZ than in MZ, but within-pair variances for Weber fraction were similar between MZ and DZ, the former indicating that threshold for RLD is predominantly influenced by genetic factors. In adults, within-pair variances for Weber fraction and P were not different between MZ and DZ, indicating that environmental factors predominate over genetic factors. When adolescent and adult twins were lumped together, mean values for Weber fraction and P were not different between male and female. Relationship between P and age was significant both in male and female. P (male) = 0.281 + 0.013 X age (years) +/- SD 0.36 cm H2O, and P (female) = 0.235 + 0.013 X age (years) +/- 0.48 cm H2O. These results indicate that the threshold for RLD is influenced predominantly by genetic factors in adolescence, but environmental factors predominate in the adult. Although sex difference is not clear, P at the threshold increases with age in both sexes.