To examine the relative importance of genetics and environments in determiing chemical and mechanical control of breathing, an age-, body size-, and sex ratio-matched study was conducted in 62 twins (mean age=16.4 years, 20 pairs of monozygotes (MZ) and 11 pairs of dizygotes (DZ) for ventilatory responses to hypoxia (A) and hypercapnia (S) along with thresholds for perception of added inspiratory resistance (0.38-5.11cmH
2O/1/sec measured at 0.51/sec 7ow rate). Both A and S were determined by a dual control system developed to automatically attain normocapnic hypoxia and normoxic hypercapnia.
The variable A was assessed by the hyperbolic equation:
V
E=V
O+A/(P
eAO2-32)
Where, V
E: minute ventilation in 1/min, V
O: asymptote for ventilation obtained by extrapolation, P
eAO2: endtidal P
O2 in mmHg and 32: P
eAO2 at which the slope of the V
E-P
eAO2 curve approaches an infinite.
The variable S was assessed by the equation:
V
E=S(P
eACO2-B)
Where, P
eACO2: endtidal P
CO2 in mmHg and B: extrapolated intercept on the abscissa (P
eACO2).
A and S were standardized by body surface area (BSA). The thresholds for inspiratory resistance was assessed by resistance value (ΔR) detected on 50% occasions and the mouth pressure (P) in which the subjects could detect the load. ΔR was analysed as Weber fraction (ΔR/Ro: Ro=airway resistance plus apparatus resistance). Since ΔR/Ro and P were distributed in lognormal fashion, In(ΔR/Ro) and lnP were used for the analysis. To examine the similarity between pairs, intrapair variance ratio (F
2=M
WDZ/M
WMZ) was used.
A/BSA and S/BSA averaged 97.5±64.2 (mean±SD) 1·mmHg/min/m
2 and 0.65±0.19 1/min/mmHg/m
2, respectively in MZ. They averaged 79.5±53.6 1·mmHg/min/m
2, respectively in DZ. There were no significant differences in these variables between MZ and DZ. In (ΔR/Ro) and lnP averaged -1.001±0.738 and -0.993±0.660, respectively in MZ. They averaged -0.826±0.705 and -1.005±0.736, respectively in DZ. There were no significant differences in these variables between MZ and DZ.
In contrast, intrapair variance for A/BSA and S/BSA were significantly smaller in MZ than in DZ (p<0.005 for A/BSA and S/BSA). Intrapair variance for lnP was significantly smaller in MZ than in DZ (0.01<p<0.025). There were no significant relationships between A/BSA, S/BSA, in (Δ[/Ro) and lnP.
These results indicate that 1) genetic influence on chemosensitivity to hypoxia and hypercapnia and mechanosensitivity to added inspiratory resistance is more evident than environmental influence in young healthy twins, and 2) perception of added inspiratory resistance is not directly related to chemical control of breathing.
