Studies have shown that slow and deep breathing (SDB) increases baroreceptor sensitivity (BRS), but the mechanisms are not clear. Is it due to the increased blood pressure oscillation during SDB? or due to increased parasympathetic tone? etc...?
Most of the evidence for baroreceptor augmentation during slow breathing is based on 'spontaneous' methods (alpha index in particular) calculated at the respiratory frequency. The problem with this approach is that respiratory sinus arrhythmia does not reflect baroreceptor function (See papers by Lipman, JA Taylor, CO Tan). All the evidence to date also fail to account for the fact that baroreflex sensitivity is frequency dependent. Therefore, the apparent rise in barorceptror sensitivity during slow breathing may simply reflect the frequency dependent nature of the barorelfex rather than any 'augmentation' due to slow-breathing per se. This is complex subject. I suggest you read our papers (Tzeng et al., 2009 JAP, Horsman et al., 2015 JAP).
I think that the basic mechanism is the answer chemoreceptor of a chemical with an increased paCO2 (ie. Loop gain)
The same mechanism that occurs in OSA
I think before asking the 'how' question, we need to first ask the question of whether the phenomenon actually exists - in this case, the proposition that 'slow breathing increases baroreflex sensitivity'. If you look at the literature carefully you will (should) see that the evidence is built up on the basis of one type of baroreflex measurement that is highly suspect. Therefore, the more imporant question is to first establish that the 'thing' that you're interested actually occurs. In the absence of a clear 'yes' to that question, the 'why' question is pointless.
In my opinion the question should concern baroreceptor stimulation rather than baroreceptor sensitivity, Baroreceptors are stimulated by increases of blood pressure which as a response induces a reduction of heart rate via vagal stimulaion and reduction of peripheral resistance via inhibition of the sympathetic discharge. In slow breathing there is an increase of blood pressure oscillation as Ali Gholamrezaei suggests. Increase in pressure oscillations means an increase of the ascending phase of the pressure itself brought about by an increase of venous return during a deeper inspiration. The increase of the ascending pressure phase is then responsible for a stronger stimulation of baroreceptor. However we must remember that in the atria the Paintal receptors are located, These receptors are stimulated by atrial distension (filling) which increases in inspiration and is proportional to the deepness of the inspiration itself. The reflex response is an increase in heart rate. At this point a question arises: how it is possible that reflexes that give opposit effects can reveal an increase in baroreceptor stimulation? The fact is that the distension of the right atrium occurs before that of the increase in blood pressure, so that baroreceptor stimulation is delayed with respect to atrial distension. Due to this delay, it cannot be excluded that the barorecetor induced deacrease in heart rate is underlined by the previous increase due to right atria distension.
I think that the basic mechanism is to be sought in changing the system of chemical control breathing.The control system baroreceptor is dependent on the chemical respiratory. Changes in respiratory rate result in a hypersensitivity of the control system ventilatory
A ventilatoty disturbance (Loop gain) ,change in expiratory minute ventilation, Δ VE disturbance, produces a change in alveolar PCO2 (Δ PaCO2). The amount change in alveolar PaCO2,depends on the properties of the plant sensivity. After a circulation delay, this ΔPaCO2 changes the PCO2 at the chemoreceptos. This change in chemoreceptors PCO2 produces a change in ventilation based on the sensitivity of the controller that acts to correts the initial changes.
Loop gain characterizes the sensitivity of the feedback system controlling ventilation and is defined as the size of a corrective ventilator response.
References:
Dempsey JA et al (2010) Physiol. Rev. 90,47-112;
Chowdhuri S et al (1985) J Appl Physiol ,109,1378-1383)
Here the results of our study on the effect of using inspiratory threshold load during slow deep breathing on cardiovascular parameters and baroreflex sensitivity. Increasing blood pressure fluctuation along the respiratory cycle does not further increase baroreflex sensitivity at 0.1 Hz. However, having a wider range of changes in blood pressure may better train the baroreflex if practiced over time, needs to be further investigated: https://doi.org/10.1111/psyp.13447
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