Using the ViVitro Pulse Duplicator for Full Heart Testing

This FAQ will provide configurations to the Model Left Heart flow loop to enable right heart testing and connect two Pulse Duplicators to allow full heart testing.

The ViVitro Pulse Duplicator is composed of the clear acrylic Model Left Heart, SuperPump pulsatile pump, Flow Measuring System, and ViViTest data acquisition system. This flexible piece of testing equipment simulates physiological pulsatile flow in the heart using customizable waveform control, peripheral resistance, and compliance while allowing clear device visibility as well as pressure and flow measurement and user-friendly data collection.

Initially the system was designed to mimic conditions of the left side of a heart with the intent of testing prosthetic heart valves. However, since its inception, the ViVitro Pulse Duplicator has also been customized to perform hydrodynamic testing on a variety of novel cardiovascular devices. One common custom use of the Pulse Duplicator system is for Full Heart applications.

This FAQ will first cover Right Heart adjustments and implications followed by additional Full Heart adjustments and implications.

Right Heart:

Six areas to consider when configuring the standard ViVitro Labs Pulse Duplicator for Right Heart testing:

• Peripheral Resistance: The pressures in the Right Heart are significantly lower than those in the Left Heart. As shown in Figure 1, in order to reach the lower pressures of right heart conditions, it may be necessary to replace the Peripheral Resistance Controller from the flow loop with a simple pinch valve.

Figure 1. Standard Peripheral Resistance Controller (left) Pinch valve for resistance control of right heart conditions (right).

• Heat Exchanger: Similar to the Peripheral Resistance, the heat exchanger may require a substitution to obtain the necessary right heart pressures. Shown in Figure 2, ViVitro Labs offers a low pressure drop heat exchanger which, while being less efficient, allows the Pulse Duplicator to achieve right heart conditions

Figure 2. Standard heat exchanger (A) with higher efficiency and pressure drop. Low pressure drop heat exchanger (B) with lower efficiency and pressure drop.

• Compliance: The system compliance will affect the shape of the pressure waveform, including the spread of systolic and diastolic pressures. As with the left heart, for some conditions it may be necessary to adjust compliance volumes to reach the desired pressure waveforms. See Figure 3 and the Pulse Duplicator User Manual for more details on system compliance.

Figure 3. Compliance options for pressure waveform control. The aortic root (taller) and Windkessel (shorter) compliance tanks along with other options (not shown) may be necessary.

• Static Head: The static pressure created by fluid in the atrium will have a large effect on the systemic pressures in the system if right heart conditions are being targeted. It may be necessary to control this height to obtain consistent results. As shown in Figure 4, a line in the atrium chamber may be useful. For hypotensive conditions this fluid should be dropped as low as possible.

Figure 4. Controlling fluid level in the atrium chamber to control static pressure.

• Right Ventricle Outflow Tract (RVOT): Since the glass aorta for the standard ViVitro Labs Pulse Duplicator may not simulate the desired anatomy of the RVOT, the Aortic Conduit Mount (ACM) may be an effective alternative. Essentially, the ACM can be used to mount custom ventricle outflow tracts desired to emulate test site physiology. See Figure 5 below.

Figure 5. Model silicone aortic root mounted in the ACM accessory for the ViVitro Labs Pulse Duplicator.

• ViViTest Software: Currently, ViViTest software (for use with the Pulse Duplicator) utilizes specific terminology for Left Heart applications, mainly in reference to valve sites (aortic and mitral). This software is still effective for Right Heart testing, as all the pressure and flow measurement locations and subsequent calculations are still relevant. Just keep in mind that references to the aortic and mitral valves apply to the pulmonic and tricuspid valves, respectively. For a screenshot of the ViViTest user interface, see Figure 6 below.

Figure 6. ViViTest software user interface.

Full Heart:

Given that one Pulse Duplicator simulates the conditions in half the heart, two Pulse Duplicators can be connected to simulate a full heart. This experimental set-up is shown in Figure 7, below. The following points are things to consider when connecting two ViVitro Labs Pulse Duplicators for full heart testing:

Figure 7: Two connected Pulse Duplicators to test full heart conditions. Note extra tubing connections were for Ventricular Assist Device (VAD) testing and are not required.

• Tubing: To connect the flow path for one continual circuit, the tubing needs to be re-routed. You will likely need longer segments of ½” ID tubing to do this. Essentially, the aortic outflow of each Pulse Duplicator needs to feed into the atrium chamber of the other Pulse Duplicator. See Figure 8 below for a schematic of the full heart flow loop.

Figure 8. Full heart tubing connections (NOTE: This view is from the back face of the Pulse Duplicator, i.e. the side that connects to the SuperPump pulsatile pump).

• Synchronizing SuperPumps: You may want the two SuperPump pulsatile pumps driving the same synchronized waveform. To do this, designate one pump to be the master and one to be the slave. From output channel 0 of the master pump’s IO module, connect a BNC cable. Split this BNC cable with a T connector and connect the two free ends to the two “Waveform In” channels on the back of the two SuperPump controllers (see Figure 9 for schematic). Ensure the controller for the slave pump is set for external waveform control. This will allow both pumps to drive with the same waveform.

Figure 9. Schematic for using a master IO module to drive a synchronized waveform in two SuperPump pulsatile pumps.

• Synchronizing Electromagnetic Flow Meters: If you are using two electromagnetic flow probes (for example Carolina Medical) in the full heart system, they can interfere with each other. If you notice that one or both flow tracings are showing excessive amounts of cyclic noise when no flow is present, they are likely picking up the drive frequency of the other flow meter. This can be alleviated by synchronizing the flow meters. The Carolina Medical flow meter has a synchronize cable which plugs into the ‘SYNCH’ port on the back of the flow meter (see Figure 10). This will ensure that the drive frequencies of both meters are synchronized and will not interfere with each other.

Figure 10. SYNCH port on the back of a Carolina Medical flow meter.

• Air Compliance in the VIA: One or both Pulse Duplicators may have a VIA attached between the SuperPump pulsatile pump and Model Left Heart (see Figure 4). The VIA will increase ventricle compliance, reduce the rate of increase of ventricle pressure during ventricle contraction, and reduce oscillations in the ventricular pressure. However having air compliance in the VIA causes a disconnection between the pump stroke volume and the actual forward volume.

Assuming a valve has zero leakage: without air in the VIA, a stroke volume of 75mL per stroke will result in a forward volume of 75mL; with air in the VIA, a stroke volume of 75mL per stroke will result in a forward volume less than 75mL. So if either or both Pulse Duplicators in the full heart have air compliance in the VIA, their forward volumes will differ slightly and may result in a backlog of fluid in the atrium of the Pulse Duplicator with the lower forward volume. If this is the case, you may want to install a bypass tube between the two atrium chambers to allow the respective fluid levels to stabilize.

• Temperature: With two Pulse Duplicators connected, there is at least twice the test fluid to heat. It may be necessary to set the heat bath (or other heat source) higher than usual to ensure that the desired temperature is maintained throughout the entire system.

Whatever your application, the ViVitro Pulse Duplicator is a flexible and customizable piece of testing equipment. Contact us today to discuss how we can address your unique cardiovascular device testing needs.