Head orientation should be considered in ultrasound studies on carotid artery distensibility

Introduction: During ultrasound distensibility assessment of the carotid artery, the patient's head is usually rotated sideward and slightly upward to optimize visibility of the carotid segment. Head rotation may affect vessel length and thus the longitudinal strain of the arterial segment. Because the longitudinal and circumferential mechanical behaviour of an artery are intrinsically related, head rotation may influence circumferential mechanics and thereby measured distensibility. Methods: In 12 apparently healthy volunteers (age 22 +/- 3 years, mean +/- SD, 6 men/6 women), we investigated whether head rotation led to a change in absolute and relative distension of the common carotid artery (CCA) by performing ultrasound examinations with the head in two orientations. Additionally, CCA length was measured in both orientations with MRI to assess whether indeed a change in length occurred because of head rotation. Rotation-induced longitudinal strain was calculated from these lengths. Results: We found a significant decrease of 0.054 mm (6.8%, P = 0.001) and 0.007 (5.6%, P = 0.019) in absolute and relative distension with head rotation, respectively. MRI measurements showed a significant rotation-induced longitudinal strain of 1.7 +/- 2.3% (P = 0.032). Conclusion: We conclude that consistent head rotation during a CCA ultrasound assessment causes a significant and clinically relevant bias in carotid artery distension measurements. The impact of unstandardized use of head rotation in studies with carotid distensibility as an outcome measure can therefore not be neglected; thus, standardization is highly recommendable.