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Engineering the Dynamics of Pulsating Blood Flow

Fri, 04/13/2012 - 6:12am
Satya Karri, Ph.D.

With cardiovascular diseases as the leading cause of death worldwide, device manufacturers are under immense pressure to produce products to address these health concerns. However, prior to launching such devices, the technologies need to be properly tested to ensure success. This article looks at setting up a system to simulate cardiac flow for testing these medical devices.

According to the World Health Association and American Heart Association, cardiovascular diseases (CVD) are the leading causes of death in the world. CVDs include coronary artery disease, cerebrovascular disease, peripheral arterial disease, rheumatic heart disease, congenital heart disease, deep vein thrombosis, and pulmonary embolisms. A variety of medical devices exist for the treatment of these clinical scenarios, including heart valves, stents, atrial septal occuluders, coils for treating aneurysms, and balloons for deploying stents and treating angioplasty. The in-vitro testing (or R&D) of these devices requires simulating physiologically relevant conditions (i.e., correct systolic over diastolic pressures at appropriate cardiac outputs).

Figure1
Figure 1: A simplified circuitry for simulating cardiac flows

A simplified circuitry for simulating cardiac flow consists of a pulsatile pump, a vascular or cardiac model, a compliance chamber, and a resistive valve as shown in Figure 1. The entire network can be thought of and modeled as an electrical network—also termed a windkesel model—with the compliance chamber serving to modulate the pressures and the resistive valve serving to control the overall mean pressures. The workhorse of the entire system is the pulsatile pump that needs to be well engineered to simulate various physiological conditions, including control of the stroke volume, beat rate, and control of the physiological waveform.

The pump utilized for driving the flow needs to be a positive displacement pump that can accurately follow a given position waveform. Cardiovascular flows, although pulsatile (consisting of a primary beat rate), when decomposed into its Fourier components, is comprised of multiple frequencies that create the complex physiological waveform. The pump should, therefore, be capable of generating and following arbitrary waveforms to simulate physiological conditions.

Pneumatic systems create an inherent lag between the driving waveform and the output waveform or can create a dampening of the system. A flow controlled system, as that obtained with a positive displacement pump, will be more reliable and repeatable for performing in-vitro bench top testing. In comparison, pressure controlled systems will have inherent issues pertaining to repeatability of the system as the performance of the drive mechanism will be response-time dependent. However, in cases where robust control of flow is not required, a pressure controlled system is acceptable (e.g., accelerated durability testing of heart valves and occluders). In both systems, repeatability will be a primary concern as in-vitro testing equipment should be able to make objective comparisons between various test articles.

Other criteria that need to be taken into account include the measurement equipment used, such as pressure transducers and flowmeters. These should be chosen such that the accuracy and the frequency response are sufficient to capture the inherent intricacies of the flow and pressure dynamics. Data collection and algorithms used for analysis should also be thoroughly validated in order to create a robust system for evaluating the performance of these cardiovascular devices.

Satya Karri, Ph.D. serves as the director of laboratory services at ViVitro Labs Inc. (www.vivitrolabs.com), a world leader in cardiovascular device testing equipment and services. Dr. Karri is responsible for investigating and evaluating the performance and durability of cardiovascular devices in order to meet regulatory requirements.

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