If you are reading this blog, then you most likely know that Transonic is the flow measurement company. But the quantified measurements we provide are far more complex, intricate, and valuable than that single sentence would indicate. At times, we receive inquiries about our measurements—how they apply and what they reveal about a patient’s physiology, with CABG patency and its related metrics being most frequently questioned. To that end, we offer the information below, a Deep Dive into the world of CABG measurements by Transonic.

Simply, a graft is patent when it provides sufficient blood flow to the heart muscle. Normally, the coronary arteries handle this job; however, if any of those arteries become blocked or constricted (occluded), a CABG graft can surgically bridge the occlusion to supplement blood flow. Then, to oversimplify, Transonic’s tech precisely quantifies that new flow, helping to assure patency, and thereby also helping to detect nonpatency, if any exists. However, data is only as clear and useful as the lens through which it is viewed. In order to understand the metrics through which we interpret and report data, we must first understand basic CABG- and cardio-physiology.

When the heart’s muscle tissue contracts (systole), the coronary blood vessels surrounding and innervating that muscle are compressed by the pressure. So unlike the arteries in the rest of the body, the coronary arteries—and any CABG grafts across them—have highest flow rates when the heart relaxes (diastole). Consequently, changes in the diastolic flow profile (waveform) can indicate nonpatency.

Remember that fluids, like blood, are incompressible. Blood vessels, however, are slightly elastic, like a stiff balloon. So, when blood flows into a section of a vessel (such as a CABG graft) the volume that also flows out must be the sum of what flowed in, minus what is retained by the expansion of the vessel. (Side note: this principle is also important for proper probe positioning. As a graft expands, it accepts a greater amount of blood than is passing through, but exactly how much more blood depends on where along the length of the graft we’re measuring. Think about blowing up a balloon. It expands more in the middle than it does at the ends. That is why Transonic recommends always placing the probe one centimeter from the distal anastomosis, so the measurements remain consistent.) Furthermore, the elasticity of the heart’s vessels isn’t completely uniform. Veins are more elastic than arteries, which means when the heart contracts, clamping down on both the coronary arteries and veins, the arteries, being less elastic, receive less blood than the veins evacuate. When the heart relaxes, the vessels refill. Therefor, counterintuitively, the overall blood volume of the system drops during systole.

Next on our cardiac physiology exposé (because it is directly connected to flow volume) is vessel resistance. Vessels aren’t perfectly smooth on the inside, and these irregularities exert drag on the fluid passing through the vessel. To understand how this affects cardiac blood flow, let’s consider a box that is open on both ends to let fluid pass through. The box is three inches long on each side. This gives the box a volume of 27 cubic inches, and a surface area of 36 square inches (1.33 to 1 ratio). So when a fluid flows through it, one cubic inch of volume has to drag against 1.33 square inches of surface. Now consider a smaller box, two inches on a side. It has a volume of 8 cubic inches, and a total surface area of 16 square inches (2 to 1 drag ratio). Lastly consider a cube that is only one inch on a side. It has a volume of 1 cubic inch, but a surface area of 4 square inches (4 to 1 drag ratio). As you can see, the smaller the vessel, the greater the drag—i.e. the greater its resistance to blood flow.

Competitive flow must also be considered. Often, the artery that needs to be bypassed is not fully occluded. So when the new graft is complete, the partially-open native vessel continues to allow some blood flow, which robs the new graft of its full carrying capacity. Consequently, it is always prudent to block off the native vessel with finger pressure or an instrument (redirecting all the blood through the new graft) before checking flow. Furthermore, Bernoulli’s principle posits that as the speed of a fluid in a vessel increases, the pressure it exerts on the walls of the vessel decreases. So when a vessel is partially blocked, the pressure drops as the blood quickens past the occlusion.

This discussion of partial blockage raises another important clinical distinction. At rest, the heart only uses a small percentage of its potential maximum blood flow, which it controls by dilating or contracting the coronary vessels. Since the heart has a tremendous capacity for work, and thereby great capability for vessel dilation, the blood flows within those vessels can proceed unimpeded (at rest) when up to 68% of the vessel is blocked. In other words, flow isn’t the best determiner at a lower percentage of blockage.

The purpose of flow measurement is to take the conceptual explanations above and convert them to useful numbers upon which a surgeon can base sound clinical judgments. If any metric exceeds a particular threshold, it is recommended that the surgeon review the graft for potential revisions. Below, each metric is listed with a brief description of its utility.

Mean Flow (Qmean): This is the most straightforward metric. A blockage in a graft increases resistance, which reduces mean flow. So if mean flow is lower than expected, it may indicate a blockage.

Pulsatility Index (PI): This metric simply divides median flow by mean flow. The resulting number tends to increase with occlusion, however there are several factors that can effect this trend, so Transonic never recommends sole reliance on PI.

Diastolic Filling Percentage (DF%) and Diastolic/Systolic ratio (D/S-ratio): As stated above, occlusion can reduce diastolic dominance in a flow waveform, so both DF% and D/S ratio both quantify this problem. However, because diastolic dominance changes depending on where the graft is located and where the probe is placed on the graft, the threshold for a potential problem varies considerably.

Diastolic Resistance Index (DRI): This metric is arguably the most useful. Like DF% and D/S ratio, DRI informs the surgeon about changes in diastolic dominance that may indicate a graft blockage, but DRI is calculated from averages of flow rates rather than volume, so it is less susceptible to erroneous readings. As resistance to flow through the graft increases, DRI trends upward alongside it, making this metric more intuitive than others.

We hope this in-depth analysis proves helpful to you, whether you are a current customer, or considering becoming one. Regardless, thank you for reading,

             Transonic Systems, Inc

                                 The Measure of Better Results