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Synergies in the Lab – Transit-Time Flow Measurement

By Cole McLarty | 08 Jul 2021

In my last blog, I touched on Pressure-Volume loop data collection when used in conjunction with other volume-based modalities such as echocardiography, MRI, and our vascular flow probes. It is not uncommon for technologies to disagree with one another, for various reasons. However, though these differences can be explained technically, they do not change the fact that they confound scientific conclusions. This was specifically true with PV loop data which would classically deviate from other technologies when it came to the calibration of volume. Through technological advancement, Transonic Scisense developed a PV technology that delivers volume calculations that better track with other modalities and in so doing, allows for greater confidence in your data.

Today I would like to touch on another technology that works synergistically with other systems, but from a different viewpoint.

 

Transit-Time Flow Probes

Our Transit-Time flow probes have been the gold-standard for vascular and tubing-based flow measurement for over 35 years. On the vascular side, Flowprobes in a variety of configurations are suitable to be placed around vessels 0.3mm and larger in diameter which allows researchers to measure anything from a mouse’s carotid to a cow’s aorta. These probes are designed to be implanted in a perivascular fashion, calculating flow without having to disturb blood flow or directly interact with the blood. This allows for the fluid dynamics to be maintained, as well as the patency of the vessel. The technology has been described in previous blogs and webinars; the resulting data is highly resolved and accurate. It is common for this technology to be utilized as a standalone measurement, for example in carotid thrombosis where flow measurement is used to quantify thrombosis progression by looking at changes in blood flow. However, this technology can also be meaningful in various settings where it can work cohesively to deliver new parameters and optimize others. Let’s take a look:

 

Blood Flow and Pressure

Pressure and flow are two fundamental characteristics in a closed system that includes a pump, such as our cardiovascular system. By conducting vascular flow measurements with our probes and blood pressure with one of our Scisense pressure catheters, researchers can better understand the properties of this system.

For scientists interested in cardiovascular function, a pressure catheter can be placed in the left ventricle of the heart – either via retrograde insertion or through the apex. From pressure data, you can calculate important indices such as dP/dt and maximum/minimum pressure gradients. When partnered with an aortic flow probe, you can also ascertain measurements of cardiac output and stroke volume. When used together, you get a comprehensive picture of load-dependent contractile performance, see the figure on the left below.

Flowprobe and Pressure Catheter on Heart

Taking a similar approach with our flow probe placed on the aorta, a pressure catheter can be placed in both the arterial and venous branches of the circulatory system. This approach allows you to calculate systemic vascular resistance (SVR), also known as total peripheral resistance, see the figure on the right above. SVR is the calculation of cardiac output (CO), central venous pressure (CVP), and mean arterial pressure (MAP). Understanding this relationship allows you to better understand the global properties of the cardiovascular system as well as the loading conditions on the heart.

The probes are designed to minimize any disturbance of the blood flow under study for the most accurate measurements possible. As they are implanted in a perivascular fashion and don’t directly interact with the blood, the patency of the vessel is maintained and the fluid dynamics are in as natural of a state as possible.

Blood Flow and PV Loops

Biventricular PV Catheters in HeartBlood flow can also be used to compliment pressure-volume loop data collection. As we learned last time, with our ADV500 PV system and catheters, a researcher can input the live stroke volume calculations from the flow probe into the PV system’s hardware, allowing for a tighter data collection process. In addition to this benefit, a researcher can also utilize these two technologies for longitudinal cardiovascular assessment. To accomplish this, a flow probe is placed on the ascending aorta for chronic measurements of cardiac output and stroke volume. This data can be used to track load-dependent changes in conscious animals over a period of weeks or months. PV loops can then be introduced as a data endpoint where the flow probe can be used to optimize the PV data and the PV system can be used to track load-independent data parameters.

Tubing and Pressure

Tubing Flow and fluid filled pressureThus far we have focused on vascular applications, however, our transit-time technology is also suitable for various preparations where fluid flow measurement through tubing is required, such as isolated heart and organ preservation studies, pulsatile pumps, valve leakage determination, and many more. In these applications, a flow probe is used either within (in-line) or clamped around (clamp-on) the tubing, and accurate measurements of volumetric flow can be obtained with our 400-series TS10 tubing flow meters. Just as flow and pressure are two sides of the same coin in the cardiovascular system, the same holds true for tubing applications – for these scenarios, a pressure sensor can be used to better understand pressure gradients in the line. When pressure measurement is needed, our SP430 pressure amplifier is partnered with our TS410 flow module to allow for an accurate measure of pressure from anywhere in the line. Using either our solid-state high-fidelity Scisense pressure catheters, or an inexpensive fluid-filled catheter, a researcher can look at specific locations or general tubing pressures, respectively. Figure above shows one of our clamp-on tubing flow probes and a fluid-filled pressure sensor in action.

 

Transonic understands what it takes to collect meaningful data in your lab and strives to present opportunities for synergistic data collection. If we can help answer any questions about this topic or any other, please be sure to check out our webinar series that covers best practices related to data collection with our technologies as well as interesting new surgical approaches and models. We can also be reached here. Cheers!

 

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