Mohammad W. Mohiuddin

The arterial system pressure–volume loop

Although the ventricular P-V loop has become a popular tool to characterize aspects of the performance of the heart, an arterial system P-V loop has not yet been described. In principle, the volume stored in the arterial system (V) could be calculated by integrating the difference between inflow and outflow. In practice, however, flow out of the innumerable arterioles cannot be measured directly. To overcome this obstacle, it has been shown that outflow can be approximated by input pressure divided by total peripheral resistance. Recently, the classical Windkessel model was generalized with the concept of apparent arterial compliance (C(app)), the transfer function relating pressure and volume expressed in the frequency domain. The arterial system P-V loop serves as a time-domain representation of C(app). This simple technique provides the first known characterization of an arterial system P-V loop.

Increasing pulse wave velocity in a realistic cardiovascular model does not increase pulse pressure with age

The mechanism of the well-documented increase in aortic pulse pressure (PP) with age is disputed. Investigators assuming a classical windkessel model believe that increases in PP arise from decreases in total arterial compliance (C(tot)) and increases in total peripheral resistance (R(tot)) with age. Investigators assuming a more sophisticated pulse transmission model believe PP rises because increases in pulse wave velocity (c(ph)) make the reflected pressure wave arrive earlier, augmenting systolic pressure. It has recently been shown, however, that increases in c(ph) do not have a commensurate effect on the timing of the reflected wave. We therefore used a validated, large-scale, human arterial system model that includes realistic pulse wave transmission to determine whether increases in c(ph) cause increased PP with age. First, we made the realistic arterial system model age dependent by altering cardiac output (CO), R(tot), C(tot), and c(ph) to mimic the reported changes in these parameters from age 30 to 70. Then, c(ph) was theoretically maintained constant, while C(tot), R(tot), and CO were altered. The predicted increase in PP with age was similar to the observed increase in PP. In a complementary approach, C(tot), R(tot), and CO were theoretically maintained constant, and c(ph) was increased. The predicted increase in PP was negligible. We found that increases in c(ph) have a limited effect on the timing of the reflected wave but cause the system to degenerate into a windkessel. Changes in PP can therefore be attributed to a decrease in C(tot).

Two-day control of pulmonary blood flow with an adjustable systemic-pulmonary artery shunt.

Our laboratory has developed an adjustable systemic-pulmonary artery shunt (AS) to provide improved control of pulmonary blood flow (PBF) after neonatal palliation of hypoplastic left heart syndrome (HLHS). Six piglets of 6–10 kg underwent left thoracotomy and placement of a 3.5 mm polytetrafluoroethylene (PTFE) shunt from the left subclavian artery to the left pulmonary artery (LPA). The LPA was ligated at its origin. An AS was placed on the PTFE graft after both anastomoses had been performed. A flow probe was placed on the LPA distal to the shunt insertion. The AS was adjusted every 2 hours (0.1 mm increments over 18 minutes) from fully open to an estimated 60% flow reduction throughout the 44-hour test period, similar to delayed sternal closure (DSC). At any shunt setting, standard deviations of normalized blood flow in each piglet were ranged from 1.34% to 8.05% indicating consistent and stable relationship between shunt setting and flow over the DSC. These data lend strong evidence that the device would perform successfully in a human infant during the DSC. Clinical trials are necessary to determine whether mechanical control of PBF results in improved clinical outcomes.