Table 1.
Baseline characteristics of study subjects.
Fig 1.
Inner jugular vein diameters measured ultrasonographically in monozygotic (a) and dizygotic (b) A and B twins, in the sitting and in the supine body position, without and with a controlled 60 mmHg Valsalva. Antero-posterior and lateral diameters were averaged for the left and right sides for each person. Note very close mean values for two members of the twin pairs in both groups. Distensibilities computed for monozygotic (c) and dizygotic (d) A and B twins. Distensibilities for the left and right inner jugular veins measured between 0–60 mmHg Valsalva were averaged for each person.
Fig 2.
Intertwin correlation of average diameter, compliance and distensibility for monozygotic and ditzygotic twins in the sitting and in the supine positions without and with a 60 mmHg controlled Valsalva attempt.
At right the significance levels of the correlation coefficients are shown (valid for monozygotes, n = 40, but almost identical for dizygotes, n = 42). Note higher correlation values for monozygotic twins (hinting for genetic factors). Note limited correlation of values sitting (measured at lower initial pressure).
Table 2.
Age and sex adjusted parameter estimates for additive hereditary (A), common environment (C) and unique environmental influences (E) of mean internal jugular vein diameter by structural equation modeling.
Table 3.
Age and sex adjusted parameter estimates for additive hereditary (A), common environment (C) and unique environmental influences (E) of compliance, distensibility and full distension range (values averaged for both sides) by structural equation modeling.