In vivo measurement of proximal pulmonary artery elastic modulus in the neonatal calf model of pulmonary hypertension: development and ex vivo validation

Abstract

Developing clinical work suggests that vascular stiffening plays a role in the progression of pulmonary hypertension (PH), while recent studies in animal models of hypoxic PH have found significant proximal vascular stiffening in the diseased population. Here, we develop and validate a minimally invasive, clinically realizable method to estimate the local elastic modulus of the proximal pulmonary arteries from pressure-diameter (PD) data. PD measurements were made in the main pulmonary arteries of 16 calves; lumen diameter was assessed using color M-mode tissue Doppler imaging ultrasound, while pressure was measured via catheter. Two methods corresponding to thin-walled pressure vessel theory ("thin") and Lame's equation for a thick-walled cylinder ("thick") were used to approximate the artery elastic modulus from PD measurements. The harvested arteries were tested independently to determine their "true" ex vivo elastic modulus and stiffness. Both approximations displayed excellent correlation with ex vivo elastic modulus of the calf main pulmonary artery (thin r(2) = 0.811; thick r(2) = 0.844; both P < 0.01). Bland-Altman analysis indicated that the thick-walled approximation has better overall agreement with ex vivo modulus. The approximations displayed quantitatively distinct regression slopes that were statistically different (P = 0.02). The elastic modulus of the main pulmonary artery can be reasonably estimated from combined color M-mode tissue Doppler imaging ultrasound and catheter pressure measurements in calves. Such measurements may be a valuable tool in the diagnosis and treatment of human PH.

Fig. 1.

Software interface for the processing of in vivo color M-mode tissue Doppler images of wall motion. The brightness-mode image appears as a triangular sector at the top of the image, M-mode data is at the center, while pressure (yellow) and ECG (blue) traces appear at the bottom. Also seen in the M-mode image are upper and lower main pulmonary artery wall definitions (red lines) and superimposed diameter (blue line).

Fig. 2.

Example cross-sectional image of hypertensive calf main pulmonary artery used for determination of diameter and wall thickness.

Fig. 3.

Representative measured pressure-diameter (PD) curves obtained from control (○) and hypertensive (+) calves.

Fig. 4.

Group comparisons of two elastic modulus approximations [thick-walled plane strain (E_TWPS), pressure-strain (E_PS)] and the ex vivo elastic modulus (E_ex). Values are means ± SD. *P < 0.005; #P < 0.001.

Fig. 5.

The two elastic modulus approximations [E_TWPS (A), E_PS (B)] regressed against E_ex of the calf MPA. Error bars refer to propagated measurement uncertainty. Open and shaded circles are indicative of control and hypertensive animals, respectively.

Fig. 6.

Bland-Altman agreement analysis between the elastic modulus approximations [E_TWPS (A), E_PS (B)] and the E_ex of the calf MPA. Each solid middle line is the bias, and the bounding dashed lines indicate the limits of agreement (± 1.96 SD). Open and shaded circles are indicative of control and hypertensive animals, respectively. Each plot has a single outlier, indicated by a dashed circle (see results for details).