Fig. 6. The figure depicts changes in respiratory parameters during the infusion of alfentanil. A  illustrates that the reduction in respiratory rate was not related to the plasma concentration within the range of studied plasma concentrations (r²< 0.01). The inset  graph demonstrates that within twin-pair differences in the reduction of respiratory rate were not related to within-pair differences in plasma concentrations (r²< 0.01). B  illustrates that the increase in carbon dioxide was not related to the plasma concentration within the range of studied plasma concentrations (r²< 0.01). The inset  graph demonstrates that within twin-pair differences in the increase of carbon dioxide was not related to within-pair differences in plasma concentrations (r²< 0.01). These findings indicate that plasma concentrations were not a relevant factor affecting estimates of heritability and familial aggregation.