Transmural strain distribution in the blood vessel wall

Abstract

The transmural distributions of stress and strain at the in vivo state have important implications for the physiology and pathology of the vessel wall. The uniform transmural strain hypothesis was proposed by Takamyzawa and Hayashi (Takamizawa K and Hayashi K. J Biomech 20: 7–17, 1987; Biorheology 25: 555–565, 1988) as describing the state of arteries in vivo. From this hypothesis, they derived the residual stress and strain at the no-load condition and the opening angle at the zero-stress state. However, the experimental evidence cited by Takamyzawa and Hayashi ( J Biomech 20: 7–17, 1987; and Biorheology 25: 555–565, 1988) to support this hypothesis was limited to arteries whose opening angles (θ) are <180°. It is well known, however, that θ > 180° do exist in the cardiovascular system. Our hypothesis is that the transmural strain distribution cannot be uniform when θ is >180°. We present both theoretical and experimental evidence for this hypothesis. Theoretically, we show that the circumferential stretch ratio cannot physically be uniform across the vessel wall when θ exceeds 180° and the deviation from uniformity will increase with an increase in θ beyond 180°. Experimentally, we present data on the transmural strain distribution in segments of the porcine aorta and coronary arterial tree. Our data validate the theoretical prediction that the outer strain will exceed the inner strain when θ > 180°. This is the converse of the gradient observed when the residual strain is not taken into account. Although the strain distribution may not be uniform when θ exceeds 180°, the uniformity of stress distribution is still possible because of the composite nature of the blood vessel wall, i.e., the intima-medial layer is stiffer than the adventitial layer. Hence, the larger strain at the adventitia can result in a smaller stress because the adventitia is softer at physiological loading.