Pulmonary vessels, MR, Technical aspects, Haemodynamics / Flow dynamics
J. Weir-McCall1, A. Kamalasanan1, D. Cassidy2, A. D. Struthers1, B. lipworth1, J. G. HOUSTON1; 1Dundee/UK, 2Scotland/UK
The compliance of the pulmonary artery is a key component in decoupling the right ventricle from the pulmonary bed,
allowing the right ventricle to work at maximum efficiency and protecting the microcirculation from large pressure gradients.[1–3] Indeed the stiffness of the pulmonary artery is a strong determinant of right ventricular function, and increased stiffness causes distal pulmonary arterial endothelial dysfunction and inflammation.[5,6] Increased stiffness is independently associated with reduced functional capacity, and higher mortality than the pulmonary artery pressures or pulmonary vascular resistance.[8–12] While stiffness is partially dependant on underlying distending pressures,[13,14] multiple studies have shown the intrinsic stiffness of arteries to be increased in pulmonary hypertension independently of these.[15–19] Stevens et al.
demonstrated in patients with pulmonary hypertension a curvilinear relationship between RV function and PA distensibility with extensive loss of pulmonary artery distensibility before a rapid decompensation of the right ventricle occurred. Pulmonary stiffness is increased early in pulmonary hypertension development,
and is increased even in those with isolated exercise induced pulmonary hypertension.[17,20–22] This combination of features suggest pulmonary arterial stiffness as a promising biomarker for detection of early disease and as a potential therapeutic target before end stage arterial remodelling occurs with dire consequences for the failing right ventricle.
The majority of measurements of arterial stiffness require knowledge of the arterial pressures to calculate the stiffness. In the arterial circulation this is not a significant issue as the brachial arterial pressures are readily obtained with a sphygmomanometer,
as mean BP and diastolic BP are relatively constant throughout the large arteries. However in the pulmonary arteries this provides a significant hurdle as an external measurement of the pressures is not readily available.
Pulse wave velocity (PWV) is an entirely non-invasive technique for measuring arterial stiffness that does not require knowledge of the arterial pressures. This can be measured using MRI,
and can be assessed using one of two methods: the transit time (TT) technique which measures the time it takes for the pulse wave to travel between two separate points along the vessel; and the flow-area (QA) technique which measures the change in cross sectional area and flow across the vessel at this point to derive the pulse wave velocity.
The results of these techniques have been shown to correlate well with one another in the pulmonary circulation,
and each individual technique has been shown to have good same day interscan reproducibility.[25,26] However the reproducibility of the two techniques has not been directly compared in the pulmonary circulation. In addition the effects of age or physiological flow states on PWV has not been elucidated.
The aim of this study is thus to assess and compare the reproducibility of the two MRI methods for measuring PWV in healthy volunteers,
and to assess the effects of age and exercise on these measures.