Vascular autonomic signal (VAS): 40 years after
E. Frinerman, MD, PhD
(from the 14th MSAIMA conference)     

Short history

From ancient times the pulse has been recognized as the most fundamental sign of live. The modern outstanding scientist and clinician wrote «There is in clinical medicine no physical sign more basic or important than the arterial pulse». For many centuries in both, the oriental and western medicine, the arterial pulse has been a well-known sensitive and reliable indicator of health and deviation from the health standard. It can give us important diagnostic information about the cause of this deviation.  Nevertheless, in all these modalities the diagnosis was grounded on the most stable pulse properties. Subtle beat-to-beat changes in the pulse properties were ignored.
 
Forty years ago, Dr. Paul Nogie was the first reported the diagnostic value of the subtle variations of the of the arterial pulse properties. That phenomenon was established by the palpation of the radial pulse as a response to local auricular pressure. Dr. Nogie named this phenomenon «reflex auriculo-cardiale» (RAC). Additional investigations established that this phenomenon is a broad cutaneos-vascular reflex that involves autonomic nervous system. Therefore, Dr. Nogie changed the name of this phenomenon to Vascular Autonomic Signal (VAS). Because the biological role of the cardiovascular system includes informative and regulatory functions, the VAS is a more broad and significant than just a cutaneos-vascular reflex. The VAS seems to offer a real window to the most significant regulatory systems of the body. 

The diagnostic parameters of the phenomenon known today as VAS, or Nogie reflex, differ from both the diagnostic parameters used in academic and oriental medicine arterial pulse. The VAS is useful to predict a therapeutic reactivity of a patient to different substances, and thus, helps to decide what medication to use in a variety of clinical situations. Nevertheless, the academic medicine has not appreciated VAS. This is because the vascular autonomic signal is a subjective tool and its validity has not been accurately assessed in accordance with the Western medicine scientific protocol. Some serious attempts were made to resolve this problem using the novel technical achievements. Unfortunately, the problem is not resolved until now.
 
In my opinion, there are two main reasons for this. Firstly, the objective pulse properties that form the subjective sensation named VAS are not known.

The first Nogie hypothesis was that is the pulse wave amplitude changes. Nevertheless, while the VAS was first recorded by Bricot in 1976 the pulse wave amplitude changed only in 20% of the persons in whom the pulse palpation data were collected.

Secondly, Nogie described VAS as a standing wave. The standing wave theory, proposed in the 50th of the previous century has not been confirmed yet.  Thus, the theoretical grounding of the VAS is not based.

The next series of the scientific studies of the VAS phenomenon was performed by Dr. Joe Navach during 1977-1985. Dr. Navach performed a series of animal experiments, which proved that the VAS phenomenon is indeed a valid physiological function. In clinical setting, Dr. Navach established that every patient had a characteristic vascular autonomic signal at rest. The patients demonstrated specific clinical characteristics that were extremely consistent with the classification of the VAS variations with a statistical accuracy of 97%. Dr. Navach established that substances such as adrenalin or acetylcholine can produce a response in the pulse at a certain distance from the external auricule both in animal subjects and in human subjects. According to the research of Dr. Navach, VAS represents a systemic response of living organisms to changes in the environment. This response is presumably mediated by the limbic system, and intended to function beyond conscious control. Dr. Navach believed that analysis of the VAS can be used as an indirect measure of the response of neurohormone to changes in both the internal and external environment of a vertebrate subject. Furthermore, the VAS characteristics can be examined and subtle changes in neurohormone can be predicted by the VAS response to auricular acupuncture.

In 1980 Dr. Navach developed a computer analysis program for isolating VAS. The main idea of this analysis is that VAS is expressed as a change in the area size of the diastolic part of the arterial pulse wave.  

What is new?

General conception

Today, VAS is classified as an adaptive neurovegetative reaction that originates in the central nervous system and transmitted to the arterial system. Changes in the palpated radial artery pulse properties allow for assessing the body reaction caused by any external stimuli.

By definition of Dr. Raphael Nogier, VAS corresponds to a set of individual variations in the style or type of arterial pulsation itself. Despite the uncertainty of this definition, it really seems impossible to describe the palpable beat-to-beat changes in radial artery pulse in a more definite way.

New theory

Recent achievements in the vascular physiology may give a new vision of the VAS phenomenon. From the perspective of the biological oscillation theory, the author offers a new hypothesis of the role of the cardiovascular system (CVS) as an informative and regulatory autonomic system.

Biological rhythms are ubiquitous and have been demonstrated at any level of organization in living matter. A myriad of biological oscillators sited in peripheral organs are organized and synchronized by special structures, i.e. biological clocks, mostly located in central nervous system. The cardiovascular system plays a harmonizing role in parallel with the biological oscillators, thus, forming a single dominant frequency. The CVS is a very sensitive oscillator that continually strives for optimal flow wave patterns required by humans. It fine-tunes all the parts of the system in the adaptation of the living organism. The CVS is self-organizing entity that acts as a very precise analog system. The anatomical organ of this system is the vascular endothelium. Endothelial cells behave as hemodynamic sensors that transform mechanical information from the blood into biochemical signals, thus, providing adherence and integrity of the CVS. The vascular system is autoregulated by vascular wall shear stress / smooth muscle tone interactions, predominantly through nitric oxide (NO) mechanisms. NO is the main "hormone" of vascular regulation, and its release is modulated by wall shear stress, acting directly on vascular tone, regulating blood flow distribution and influencing both central and autonomic nervous regulation.

The VAS is a measurable manifestation of this integration analog system and is expressed by changes caused primarily by the backward waves in the pulse. It seems to be not causally dependent on cardiac output and is primarily related to the peripheral vascular tone changes. The VAS is a true technique for assessing the functional state of the bio-oscillator system. It summarizes information concerning oscillating systems and can identify system variables and parameters, as well as the interactions among the variables.

The variability in cardiovascular signals reflects the hemodynamic interplay between disturbances to cardiovascular function and the dynamic response of the cardiovascular regulatory systems. Interactions among the respiratory, cardiac and metabolic rhythms have a functional, hierarchical structure, and cardiovascular phase relationships may play a harmonizing role.

In my opinion, the problem of VAS is in its complexity. Interplay of different variable oscillatory patterns, which act simultaneously, cause short-term changes in the radial artery pulse.
 
In cardiovascular physiology there are well known oscillations in vascular tone or vascular diameter that occur both in vivo and in vitro and are not a consequence of the heart beat, respiration or neuronal input. All the oscillations act synchronously and expressed by different parameters. These oscillations are named vasomotion. The main known vasomotion oscillatory patterns are the following:
1.  High-frequency (- 0.3 Hz. or 0.27 per beat) components,
2.  low- frequency (0.11 Hz, or 0.1 per beat), and
3.  Ultra-low frequency (< 0.02 Hz) component.

The first frequencies are a marker of vagal activity in the heart. The second, are a marker of sympathetic activity in the heart and vessels. Thus, both components are markers of ANS outflow acting predominantly at the heart and secondary at the arteries.

The ULF components relate exclusively to vascular system, which means, to intrinsic properties of the arteries to spontaneous diameter oscillations due to vascular tone changes. As opposed to the first two, the ULF components have much higher amplitude and seem to be little influenced by VAS-induced factors.

All the oscillations act simultaneously and are superimposed modulating pulse wave form in the radial artery. The individual mechanical properties of the arterial wall also influence the shape of the pulse.
      
It seems interesting to compare between the arterial vasomotion (AVM) and VAS and to investigate the role of the AVM in the VAS-phenomenon.

AVM seems to be an intrinsic property of the vascular system. Metabolic oscillations (in glycolysis) could be responsible for vasomotion

VAS occurs in each artery of the body
(The same is AVM)

VAS is expressed by changes in vascular smooth muscle (VSM) tone
(The same is AVM)

VAS is expressed by variations in blood flow 
(The same is AVM)

In principle the VAS changes in the tone are non-linear and highly irregular
(The same is AVM) 

Methodological aspects

Recently, a new approach to assessment of hemodynamics has been developed. Instead of standing wave theory of the VAS it seems rationally to use a modern theory named wave-intensity analysis. This method seems be able to resolve the problem of understanding VAS.

Wave-intensity analysis: a new approach to assessment hemodynamics

Fig.1 Pressure, flow velocity, and wave intensities in the radial artery from a single volunteer. Top: pressure is top waveform and flow velocity is bottom waveform. Bottom: forward-traveling compression wave (S), backward-traveling compression wave (R), and two forward traveling expansion waves (X, D). Such distinct waves (S, R, X, and D) were identifiable at each site in every great conduit artery
The S wave is generated by myocardial contraction, resulting in the opening of the aortic valve, increased arterial pressure, and acceleration of arterial flow
The R wave is a backward-traveling reflection of the S wave.
The X wave is a forward-raveling expansion wave
The D wave is an expansion wave generated at the end of systole by a decrease in myocardial shortening rate

It's seems rational to compare the palpation method with the WIA data. What a properties changes in the pulse wave «educated thumbs» are palpated?
 
Many clinicians who used the VAS in their practice believe that educated thumbs are more sensitive than any device for pulse investigation. The VAS phenomenon is usually felt by the practitioner as a qualitative variation of perception of the pulse. It starts after 1 to 3 cycles of application of a stimulus. This signal occurs without any alteration of heart rate and can last for up to 8 to 15 cardiac cycles. 

Thumb perception; principal anatomy-physiological data

The cutaneous system receives sensory inputs from mechanoreceptors that respond to mechanical stimulation (force). 

Cutaneous sense – receptors in skin

• Touch – skin
• Ouer layer – epidermis, inner layer – dermis
• Corpuscular endings – 4 types, touch and kinesthesis / proprioception
• Free nerve endings – pain and temp

Corpuscular endings

• Merkel receptor (SA1) – disk shaped, close to surface, slow adapting, small receptive field, responds best to pressure
• Meissner corpuscle (RA1) – stack of flattened cells w / nerve fiber weaving thru, close to surface, rapid adapting, small r.f., responds best to tapping or fluttering
• Ruffini cylinder (SA2) – many branched fiber in cigar shaped capsule, deep in dermis, slow adapting, large r.f., responds best to stretching
• Pacinian corpuscle (RA2) – layered, onion-like, deep in dermis, fast adapting, large r.f., responds best to vibration

The spacial capacity is 1.4 mm. to minimum 1 mm. Delay in processing information is 0.4-120 m/sec

The discriminative properties of tactile sensation are mediated by a class of fast-conducting myelinated peripheral nerve fibers--A-beta fibers--whereas the rewarding, emotional properties of touch are hypothesized to be mediated by a class of unmyelinated peripheral nerve fibers--CT afferents (C tactile)--that have biophysical, electrophysiological, neurobiological, and anatomical properties that drive the temporally delayed emotional somatic system.
Mechanoreceptors are responded to general energy principle

Pulse qualities defined by palpation VAS:
Superficial palpation   Deep palpation
Volume changes          Intensity changes
Amplitude                      Strength
Force            

Such amplitude changes are only in 20% of VAS palpating, Force and Strength   must be the main factors forming VAS sensation. In such conditions wave intensity analysis may be the most adequate method for VAS demonstration.

Mechanoreceptors respond to general energy principle. It seems possible that «educated thumb» could distinct energy changes in the R wave that is a backward-traveling reflection of the S wave. The D wave that is an expansion wave generated at the end of systole also may be detected.

This hypothesis partly is confirmed by both by the authors who defined VAS as changes in wave velocity in the early systole and the authors who described VAS as changes in late diastole.