Sleep 7.3 High vs. Low Frequency Coupling in Cardiopulmonary Physiology

This talk explains respiratory sinus arrhythmia, the vagus nerve’s role, and the balance between sympathetic and parasympathetic systems

Video Breakdown:

00:00 High Frequency Coupling Explained

02:01 Autonomic Nervous System: Sympathetic vs Parasympathetic

03:00 Low Frequency Coupling & HRV

04:52 CPC Mechanisms

Now, what is the physiology behind that? What is the physiological basis between high frequency coupling, and low frequency coupling? So let's first focus on high frequency coupling.
Now the answer to this is relatively simple because it has to do clearly with respiratory sinus arrhythmia. This is the changes in the heart rate that happen with respiration. So as you inhale, your heart rate increases, as you exhale, your heart rate decreases. This is a cycle that happens, every 3~7 seconds, fitting into the high frequency range that are used in the coupling and, heart rate variability measures. So the question is what's causing this rise in heart rate with inhalation? And that simply has to do with the fact that it is a withdrawal of the vagus nerve, and this is an actual recording of the vagus nerve in a live human individual.
They actually put a needle into the neck, into the vagus nerve, around the carotid artery in the jugular vein, so it's very, dangerous. But, you could see that during inhalation, you could see the vagus nerve shut down. And so what's happening is as you inhale, your vagus nerve, the vagal break is removed, and so your heart rate goes up and as you exhale, your vagus nerve comes back in.
Respiratory sinus arrhythmia is all mediated through the vagus or the parasympathetic nervous system. And the reason the parasympathetic nervous system is involved and not the sympathetic nervous system is involved, it has to do with the dynamics that happens at the SA node.
So this is a study that was done in dogs, I believe. They stimulated the vagus nerve here with electrical stimulation. And you can see that the heart rate immediately decreases, then it immediately recovers after you stop the electrical stimulation. The sympathetic nervous system on the other hand, so if you activate the sympathetic nerve to this SA node, you can see that this electrical stimulation does cause an rise in the heart rate, but there is a lag here, sometimes five seconds or so, and then the rise in heart rate doesn't happen immediately, it sort of has a gradual change. And even after you stop the electrical stimulation, the heart rate doesn't decrease right away, there's a lag, and then the decrease is reduced. So again, the sympathetic nervous activity has a delayed onset, slow execution, and long recovery time. And so this is why the respiratory sinus arrhythmia that you see, which falls into the high frequency range is all purely parasympathetic, and the sympathetic is too slow to fit into this high frequency range.
So when we're talking about this high frequency coupling, this indicates the parasympathetic nervous activity and it doesn't reflect sympathetic nervous activity. So what about the low frequency coupling? So we know for low frequency coupling in terms of heart rate variability, not cardiopulmonary coupling. In terms of heart rate variability, it has to do with a barrel reflex mechanism. And it's just simply the fact that, the barrel reflex is a, a mechanism to control blood pressure, so if there's a rise in blood pressure, you have signals in the glossopharyngeal and the vagus nerve going to the Nucleus Tractus Solitarius, that then sends signal to the vagus nerve, which then says, alright, let's reduce the heart rate when there's a high blood pressure. Because the vagus nerve is myelinated, this happens pretty quickly, almost in a second. The sympathetic nervous system sends signals to both the SA node to increase the heart rate or to decrease the heart rate. In this case, if there's an increase in blood pressure, it reduces the heart rate.
And it also causes the heart to relax so that the inotropic, the strength of the contraction is reduced and it also sends a signal to the vessels to relax so that the blood pressure, is reduced. But there's a time lag for this, for a number of reasons, which I won't go into detail here.
But the time lag is about seven to 20 seconds and this corresponds to this low frequency range of about 0.04 to 0.15 hertz. So low frequency, has been thought traditionally to incorporate a lot of the sympathetic. There is some parasympathetic as well, but a lot of it being sympathetically mediated.
So a note here though, when we're talking about heart rate variability, we understand this. But with cardiopulmonary coupling, there's no respiratory component involved in this. It's not like your heart or your respiratory rate changes because your blood pressure increases or decreased, like a person with hypertension doesn't necessarily have a faster respiratory rate.
So what's happening here? The things that are happening during cardiopulmonary coupling is that during the CAP, remember the cyclic alternative pattern, which indicates unstable sleep, you're seeing these phase A activations, phase A, phase B, deactivations, and you can see here that you have respiratory changes that occurs during the activation or arousal periods, and then you also see the changes in heart rate too.
As a result, the cardiopulmonary coupling that is happening at the low frequency range has to do with the on and off periods that are occurring with these activations or arousals. And so, again, it has to do with this repeated cycle of sympathetic nervous system activity during Phase A, followed by parasympathetic recovery. This cyclical time to frequency calculation suggests,
so the period of CAP cycles from one Phase A to the next is about 20 to 40 seconds. And so 40s cycle corresponds to a frequency about 0.025 hertz, 20 seconds to 0.05 hertz. And this falls into the low frequency band. So the slow rhythm of arousals and recovery, during these CAP couples to this low frequency domain.
But it's important to note for cardiopulmonary coupling, the band that is used here in 0.01 to 0.1 Hertz is not the same as the heart rate variability, low frequency band, which is 0.04 to 0.15 hertz. And the reason this is broader and at lower frequencies is probably because this is slower, like the lag between these two things are slower.
So that is an explanation a little bit of what is the low frequency cardiopulmonary coupling indicate physiologically.

Course episodes

Sleep 7.1 Understanding Cardiopulmonary Coupling in Sleep Studies

Sleep 7.2 Cardiopulmonary Coupling: Sleep Staging Classifications

Sleep 7.3 High vs. Low Frequency Coupling in Cardiopulmonary Physiology

Sleep 7.4 Cardiopulmonary Coupling and Sleep Apnea Detection

Sleep 7.5 Traditional vs. Modern CPC Methodology

Sleep 7.6 Cardiopulmonary Coupling in Sleep: Strengths, Limitations & Future