In this video Dr. Singer discusses the role of cardiovagal function as a key component of autonomic function testing.
Alright, on to cardiovagal function. We are interested in cardiovagal function for a number of reasons. We know cardiovagal function is impaired early in the autonomic neuropathies, including diabetic and amyloid neuropathies, so it’s very sensitive and the reason for that is rather simple. Just like a length-dependent neuropathy starts in nerves that are longest, so nerves that supply the foot, so the vagal nerve is the longest cranial nerve and so it’s early affected in nerve disease as well, and it’s very sensitive and contains a lot of parasympathetic fibers that we’re interested in studying. It is also convenient because there’s nothing easier than assessing cardiovagal function.
You really, all you need is a 3-lead EKG and some form of standardized stimulus and you can derive cardiovagal function. There are a number of ways to do that. The ones that are highlighted here in red are the ones that we do as part of the autonomic reflex screen. Why? Again, it goes back to sensitive, specific, reproducible, and all those other attributes, and so those are the ones I will be focusing on here.
When one looks at the underlying physiology it actually gets quite complicated. You can write a book about possible and real actual components and constituents to the response to deep breathing. It goes back to reflexes such as the Hering-Breuer reflex, the Bainbridge reflex may be involved. There is some component of the Baroreflex that has a contribution. There may also be some central neural coupling and even cardiac stretch reflexes may play a role. While interesting, for practical reasons all you need to know, that taking deep breath in and out results in modulation of the cardiovagal outflow from the nucleus ambiguous, and that results in respiratory sinus arrhythmia. Increase in heart rate when you take a deep breath in and decrease in heart rate when you breathe out. And that’s what we quantify here. We do think that the lung stretch reflex is actually the major component but really, it doesn’t matter, you assess cardiovagal function either way.
And, this is how we do it. On the top panel you see beat-to-beat heart rate, in the bottom panel respiration, and we look at the best 5 consecutive responses to 5 breaths. So, we do a total of 8-10 breaths and then choose the best 5 consecutive heart rate responses and quantify those. We basically look at the difference between the maximum and minimum heart rate, do that 5 times and take the average. Why are we doing the best consecutive 5? Well, back then, that was thought to be the best way of doing it, the most unbiased way of doing it, and all our normative data are derived based on that and so we are stuck with that now. Maybe taking the highest 5 would have been just as good but this is what we have normative values on and so that’s what we are using.
And again, there are a number of factors other than disease that can affect heart rate responses to deep breathing. Age clearly important variable. Posture is important. Your heart rate response is acutely diminished when you obtain in the standing position, so have your subjects in a standardized position, and do we all know if the data that are derived is supine, so we all have to be lying supine when we do the test. The rate of breathing plays a role. We have compromised on 6 breaths, 5 seconds in, 5 seconds out, that seems to give the maximum heart rate response for the majority of people. The depth of breath is very, very important. If patients do not take a maximum breath in and out, you have spuriously low responses. It’s very important. There are good studies that support that, even 80% is not good enough; 100% effort. And again medications.
Again, there are normative values…if you can see the age effect here…gender does not really seem to play a significant role in heart rate responses to deep breathing. And the other values are listed here, and they are published. Here is an example of a normal heart rate response, heart rate in red on top and a blunted virtually absent heart rate response to deep breathing on the bottom in the patient with diabetic autonomic neuropathy.