Correlation of Pulse Wave Transit Time with Pulmonary Artery Pressure in a Porcine Model of Pulmonary Hypertension

Abstract

For the non-invasive assessment of pulmonary artery pressure (PAP), surrogates like pulse wave transit time (PWTT) have been proposed. The aim of this study was to invasively validate for which kind of PAP (systolic, mean, or diastolic) PWTT is the best surrogate parameter. To assess both PWTT and PAP in six healthy pigs, two pulmonary artery Mikro-Tip™ catheters were inserted into the pulmonary vasculature at a fixed distance: one in the pulmonary artery trunk, and a second one in a distal segment of the pulmonary artery. PAP was raised using the thromboxane A2 analogue U46619 (TXA) and by hypoxic vasoconstriction. There was a negative linear correlation between PWTT and systolic PAP (r = 0.742), mean PAP (r = 0.712) and diastolic PAP (r = 0.609) under TXA. During hypoxic vasoconstriction, the correlation coefficients for systolic, mean, and diastolic PAP were consistently higher than for TXA-induced pulmonary hypertension (r = 0.809, 0.778 and 0.734, respectively). Estimation of sPAP, mPAP, and dPAP using PWTT is feasible, nevertheless slightly better correlation coefficients were detected for sPAP compared to dPAP. In this study we establish the physiological basis for future methods to obtain PAP by non-invasively measured PWTT.

Keywords: pulmonary artery pressure (PAP); pulmonary hypertension (PH); pulse arrival time (PAT); pulse wave transit time (PWTT); pulse wave velocity (PWV).

Figure 1

PAT is the sum of PEP (pre-ejection period—details see below) and PWTT. The pulse arrival time (PAT) is defined as the time interval between the R-spike of the ECG (green) and the pulse arrival at the distal pulmonary artery catheter (red). In this example the timespan between the R-spike of the ECG and the pulse arrival in the proximal pulmonary artery (blue) is equivalent to the PEP, as the catheter is placed right behind the pulmonary valve. The pulse propagation from the proximal pulmonary artery (blue) to the distal pulmonary artery (red) is called pulse wave transit time (PWTT). This PWTT, as well as the PAT, can be calculated without knowledge of the exact localization of the respective pressure probes. When the exact distance between the proximal and the distal catheter position ($$\overline{\mathrm{PD}}$$) is known, pulse wave velocity (PWV) can be calculated as: PWV = $$\overline{\mathrm{PD}}$$/PWTT [10].

Figure 2

X-ray of the thorax to assess correct positions of catheters. A conventional pulmonary artery (Swan-Ganz) catheter was placed in the pulmonary artery for gold standard measurement of pulmonary artery pressure (circle). The sensors of two Millar Mikro-Tip catheters are located in a proximal (filled triangles) and a distal branch of the (open triangles) pulmonary artery (, X-ray taken with the C-Arm Ziehm Vision, Ziehm Imaging, Nuremberg, Germany).

Figure 3

The pulse wave transit time between the proximal and distal pulmonary artery. The proximal pressure curve of the pulmonary artery is shown in blue, the distal one in red. Solid lines represent the pulmonary artery pressure in mmHg. Local pulse arrival was determined by intersection of the tangent of the local maximum derivative (dotted lines in blue and red) and the previous dPAP. Pulse wave transit time (PWTT) was calculated as the time difference between selected pulse arrival from the proximal and distal pressure curve (black dotted lines).

Figure 4

Inverse linear correlation between sPAP, mPAP, dPAP and PWTT during thromboxane A2-induced PH in six pigs. Average correlation coefficient was calculated via Fisher’s-Z transformation (sPAP r = 0.742 ± 0.104, mPAP r = 0.712 ± 0.078 and dPAP r = 0.609 ± 0.179). Linear regression was performed by least square method.

Figure 5

Inverse linear correlation between sPAP, mPAP, dPAP and PWTT during hypoxic pulmonary vasoconstriction in five pigs. Average correlation coefficient was calculated via Fisher’s-Z transformation (sPAP r = 0.809 ± 0.081 *, mPAP r = 0.778 ± 0.082 and dPAP r = 0.734 ± 0.090, *: Significant difference between sPAP vs. dPAP, p ≤ 0.05, rmANOVA). Linear regression was performed by least square method.

Figure 6

Hyperbolic correlation between sPAP, mPAP, dPAP and PWV during thromboxane A2-induced PH ((A), n = 5, catheter distance unavailable in one animal) and hypoxic pulmonary vasoconstriction ((B), n = 5). PWV was calculated as pressure sensor distance divided by PWTT given in m/s. For each animal an individual colour was used.