Questions from the Field – Pressure Catheters and Equipment
As a Sales Representative and Product Manager for the last 11 years, I have run into a wide range of questions from our customers in the lab. As part of an on-going series, I will review some of these questions with the hopes that they are helpful to you, in your lab. For the first blog, I will be covering some general questions related to pressure catheters and their amplifiers:
What is a “solid-state” catheter?
The measurement of pressure under physiological conditions (in-vivo, ex-vivo or in-situ) is a common requirement for various studies. To acquire these types of data, it is common to deploy a catheter that has a distally mounted window allowing the catheter access to the fluid pressure. The manner in which the catheters sense and transmit these pressure changes differs, with two common catheter types being used – “fluid-filled” and “solid-state.”
In general terms, “fluid-filled” catheters are just that, tubing filled with a fluid or gel material that can have a force applied at one end, then this force is transmitted through the fluid medium to the proximal side of the tubing where the data is amplified for output. “Solid-state” catheters, on the other hand, operate differently; using Figure 1 below you can immediately see the catheter has a purposeful design. Of its several interesting features, the most relevant to this discussion is the sensing surface (Fig 1. A) where blood exerts a force that is measured by a piezoresistive membrane (not shown). As the membrane bends, resistors flex and send a signal up the catheter’s internal wiring to a paired amplifier that processes the signal for output. There are several benefits of this technology, but the most important one is the extremely high damping coefficient and response rates which make measurements like dP/dt more accurate, especially in rodents.
Why do I need to “hydrate” my catheter before use? What is being hydrated?
All “solid-state” catheters need to be hydrated before use by submerging the catheter tip into body-temperature fluid (usually saline) for at least 30-minutes prior to each use. As you can see in Figure 1. C, a flexible Silastic membrane separates the sensing surface from fluid and therefore must be largely impermeable to fluid. In reality, Silastic materials have some degree of permeability, meaning that the flexible membrane absorbs some amount of fluid and swells when exposed to water and other fluids (blood). The swelling will change the dimensions of the flexible membrane and exert force on the sensing surface. The changing output from the sensor during this event is called “hydration drift”. Hydration drift will continue until the Silastic material has absorbed as much water as it can - typically half an hour in a saline solution. By doing this process on the bench-top, any drift due to hydratation can be mitigated and removed before experimental measurements start.
What other equipment do I need if I want to measure pressure using one of your pressure catheters?
Catheters are workhorses; however, they do not operate by themselves. As alluded to above, the signal that comes from the catheter sensor is small and needs to be amplified and conditioned before display on a computer. Scisense catheters are either plugged into the SP200 or SP430 and from there, these amplifiers output analog data (+/-5V) for display on most commercially available data acquisition systems.
My catheter will not respond to pressure changes and/or cannot be balanced to “0mmHg.”
If a catheter cannot be balanced to 0mmHg before use, or if the catheter is not responding to pressure changes, this can indicate an issue. The easiest place to start is to confirm that all connections are made firmly and that there are no fluids or residue in any of these connections. Also, confirm that the catheter’s tip does not have any residual biological tissue on it damping or offsetting the signal (refer to our cleaning guides). It is always best practice to inspect your catheter daily to ensure that no tissue is building up over time. If all these steps have been taken and the issue hasn’t been resolved, contact a member of our team and we can perform additional assistance.
Is pressure measurement alone adequate for interrogating ventricular contractility?
Typically, researchers know exactly what they need in terms of data types and it is our responsibility to provide the correct equipment and catheter sizes to meet their needs. For vascular applications, this process is straight forward. For ventricular applications, there are some considerations that are not always evident to customers at the outset, namely the load dependency of classic contractile measurements like dP/dt. For applications where contractility is the focus, we suggest using a pressure-volume catheter that can provide both ventricular pressure – using the same solid-state technology – and a volume signal. From these measures, you can derive cardiac output, calculate ejection fraction and dP/dt, in addition to load-independent measures such as ESPVR and EDPVR. To learn more about the possibilities of PV studies, review our “Why study PV loops” technical note.
The above 5 questions are not exhaustive, so if you find that you have additional questions for us related to your application, please be in touch. Also, check back for the next installment in March when I will review common questions related to pressure-volume data collection. All the best until then, Cheers!