Assessment of the viscosity of the pulmonary artery wall

Abstract

Large artery wall viscosity reduces the efficiency of heart/vessel coupling. The aim of the present study was to assess pulmonary artery wall viscosity through comparison of the static (Dst) and dynamic distensibility (Ddyn) of the vessel wall.Right pulmonary artery pressure and diameter was measured in 13 patients and eight healthy volunteers. Ddyn was calculated as the relative change in end-diastolic diameter induced by the pressure pulse, and Dst as the relative change in mean diameter induced by the change in mean pressure during steady-state exercise.Dst The arterial wall responds to stress through both elastic and viscous behaviour, the former being related to the elastin and the latter to the collagen and smooth muscle content of the vessel wall [1,2]. During systole, the elastic behaviour of the arterial wall allows its diameter to increase proportionally to pressure; in the case of a purely elastic arterial wall, the total amount of energy stored would be released during diastole. In reality, however, part of the energy that corresponds to viscous deformation is dissipated within the arterial wall. Wall viscosity reduces the efficiency of heart/vessel coupling, and may place extra load on the ventricle [3].Wall viscosity is responsible for the difference that exists between the static (Dst) and dynamic distensibility (Ddyn) of a given vessel [1,2,4,5]. Under static conditions, the change in diameter in response to a given change in distending pressure is determined only by the elastic properties of the vessel wall. Under dynamic conditions, however, when changes in pressure are pulsatile and the time available for distension is limited, wall viscosity impedes distension, resulting in less change in diameter for the same change in pressure [6]. The greater the wall viscosity, the greater the difference between Dst and Ddyn. Dst has been found to be considerably higher than Ddyn in systemic arteries, the difference being accounted for by wall viscosity [1,2,4,5].Information on pulmonary artery wall viscosity is limited and controversial. In the perfused dog lungs, the slope of the static curve relating volume to pressure (i.e. lung compliance) was found to be 1.5 times steeper than that of the dynamic curve [7], indicating considerable vessel wall viscosity. In humans, no direct comparison of Dst and Ddyn of the pulmonary artery has been made, but the close similarity of pressure and diameter curves recorded from the pulmonary artery has been taken as a sign of low vessel wall viscosity [8]. The age dependence of pulmonary arterial wall distensibility is another controversial issue. Although in vitro studies have shown that the pulmonary arterial wall stiffens with age [9,10], in vivo studies have produced equivocal results; a negative correlation between pulmonary compliance and age was reported in one study [11] but no such relation was found in another [12].In view of the above uncertainties, the relationship between Dst and Ddyn of the right main pulmonary artery and its age dependence was ...

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