Questions from the Field II – Pressure-Volume Catheters & Equipment
In part one of this recurring series, I reviewed some common questions I have encountered during lab visits over the past 11 years. In this installment, I will be reviewing a sample of general questions related to our pressure-volume catheters and ADV500 Pressure-Volume System:
Why is pressure measurement alone not enough?
This is one of the most common questions that we get from pressure users. Pressure is a fundamental parameter that can tell us a lot about general physiology and, when placed into a ventricle, it can report basic contractile information like dP/dt. However, dP/dt, like many contractile indices, is load-dependent and therefore will vary with the loading conditions (pre-load and after-load) on the heart. As a result, though a common index to report in literature, it is not telling the whole story.
By adding a volume measurement to the data set, we can derive orders of magnitude more information, such as stroke volume, ejection fraction and cardiac output, while also collecting pressure data at the same time. It allows for a more complex understanding of cardiovascular function, but again, is still not the entire picture. Pressure and volume being measured from a pump (in this case the heart) in a closed system still is affected by loading conditions on the heart. Fortunately, PV loop catheters, while allowing for the measurement of important load-dependent parameters, also afford you the option to do transient occlusions – most commonly to the IVC – which changes the loading conditions on the heart. These changes, colloquially called “occlusions”, allow the heart to unload and because we are measuring data from within, we can understand fundamental properties about the heart.
For anyone interested in learning more about the benefits of measuring volume in addition to pressure, I always direct them either to our “Why Study PV Loops” technical note, or even better, our “PV Loop Workbook” that answers these questions and many more.
What surgical approaches are commonly used and what is best for you?
With the understanding that PV catheters are deployed in the ventricle, there are a few general surgical approaches that are commonly considered – a peripheral approach through jugular (RV), carotid or femoral (for LV); or a direct approach through the ventricle apex (RV or LV). So, which is best for you? This is partly a scientific and/or protocol limitation-based question – meaning that some protocols demand a certain surgical approach to be viable. However, if we use the LV as an example, many rodent applications can equally use the carotid or the apex as both are available, surgically. In choosing one or the other, in this example, we are effectively choosing to open the chest or not, and this brings up a host of physiological debates that would be better served in a separate blog. For the purposes of this discussion, I would like to focus on the practical aspects of this decision, namely:
- Which approach is “easier” surgically?
Depending on the surgical expertise in the lab, many would find the carotid isolation, catheter insertion and catheter positioning (more on this below) more challenging than an open chest approach. Open chest protocols demand certain infrastructure though such as ventilation.
- Do either offer benefits in terms of catheter lifespan?
Catheter lifespan will be covered more in part III of this blog, but in general an apical insertion will yield a longer lifespan in terms of number of insertions. This is because the catheter does not need to navigate through vessels or valves, instead just the tip of the catheter is used each time.
- What approach is best for you?
As mentioned above, this is a complicated question. In most settings, a closed chest will be preferable to maintain physiological conditions. If you have questions as to which approach is best for your application, please be in touch and we can advise you further.
How to select an appropriate catheter for your model?
When choosing a catheter, you need to consider the following variables:
- Catheter French size
Pressure-volume catheters come in various diameters, or “French” sizes where one French is 1/3mm in diameter. Our catheters come in 1.2F, 1.9F, 3.5F, 5F and 7F models – each best suited for different applications. Most commonly – 99% of the time – the animal model alone will dictate the French size needed. For example, mouse applications are always 1.2F, rat applications 1.9F and so on – however a neonatal rat application might also see a 1.2F catheter used. True also is that large animal models can typically accept either a 5F or a 7F catheter. Therefore, the starting place is always to understand what animal model you are using and then from there, consider the additional variables below.
- Tip preference
Pressure-volume catheters typically have a straight distal tip, allowing for smooth passage through vessels and valves – this is true for all our 1.2F-3.5F catheters. In larger animal applications, catheters with either a straight or “pig-tail” tipped distal end can be utilized. Generally speaking, a pig-tailed catheter is helpful for stabilizing the catheter in the apex of the heart, keeping the catheter in the mid-line of the heart and also protecting against tip contact with the apex itself. Importantly, pig-tail catheters are not suitable for apical insertions and in these cases a straight tipped catheter would always be suggested. Therefore, tip preferences become important in larger models and when introducing the catheter retrograde through a valve.
- Electrode spacing
Pressure-volume catheters generate their volume signal using electrodes placed along the catheter’s length where there are two emitting and two receiving electrical information. For proper data collection, it is imperative to have these electrodes within the entire ventricle, especially the 2 receiving electrodes. For this reason, in addition to the catheter French size, PV catheters are also specified by their “electrode spacing.” The importance of electrode spacing is another complicated topic deserving its own discussion, but the best process for understanding your needs would be to measure the apex to valve length. We would take this information and suggest the best spacing for your needs.
What equipment is needed to collect these types of data?
Once a proper catheter is selected, it needs to be plugged into an amplifier for signal processing and output. Our ADV500 PV system accomplishes this task by collecting both the solid-state pressure data and electrical information from the receiving electrodes. From here, it processes and amplifies the signals prior to output in units of voltage. Although there are dozens of data acquisition providers that can physically collect and display PV data, there are only a few that have imbedded analysis programs. Therefore, although a person could write their own analysis software algorithms, we always suggest going with a provider with innate functionality. Examples include ADInstruments’s LabChart 7 and 8, iWorx’s LabScribe 1-4, DSI’s PONEMAH, Biopac’s AcqKnowledge, emka TECHNOLOGIES iOX and NOTOCORD.
Can I collect data from both ventricles at the same time?
Though researchers typically utilize one side of the heart or the other – infarct research focusing on the left and pulmonary on the right, for example – this is changing. We now have researchers understanding the importance of both and with this change in approach, comes innovation in technology. We offer equipment that can be used in both ventricles simultaneously, a topic discussed in a recent webinar.
Pressure-volume loops are an expansive topic and as such, we will be revising it again in the near future. Part III will delve deeper into the data that comes from this equipment asking questions such as – “how can I optimize catheter position”, “what is ‘phase’”, “what ‘phase’ number is best for my animal model”, “how to optimize catheter lifespan” and more. All the best until then, Cheers!