From the E.Wolfson Medical Center, Holon (*), and the Pain Center of Sick Fund "Leumit", Ramat-Gan (**), Israel

Reprint requests: Dr. E.Dvorkin, P.O.Box 3167, Bat-Yam 59131, Israel

We had read with interest the communication of C. Thalhammer, H. Haller, F.C. Luft "Aurikulomedizin im angiometrischen Test" (1).

These authors measured the movement of the arterial wall of the radial artery by an A-mode ultrasonic device (NIUS 02, Omega, Bienne, Switzerland and Capital Medical Services, Paris, France). The device also incorporated a photoplethysmographic Finapres system that allowed blood pressure to be recorded from a finger ipsilateral to the radial artery examined. This high-resolution echo-tracking device is specially designed to measure noninvasively the vessel's morphology and mechanics (2). The vascular autonomic signal (VAS) was investigated using light applications on the skin of the arms, face, and forehead (59 experiments), and pain stimulus at the earlobe (4 experiments). In every case the authors did not register any changes in the elastic properties of the arterial wall and/or changes of the arterial lumen.

For many years we have investigated the hemodynamic aspects of the VAS phenomenon and the possibilities for objectivation of VAS. We are pleased with the interest of our colleagues in this very interesting phenomenon, and wish to make the following comments.

The vascular autonomic signal was established by Dr. Paul Nogier more than 30 years ago as a way to detect the movement of the pulse waves by palpation of radial artery smooth muscle tone variation.

Nogier had interpreted VAS as a measure of the autonomic response of an integrated biological system, such a body, to any incoming stimuli.

Today the existence of the VAS phenomenon had been proven. Many authors (Bricot, 1978, Navach, 1980, etc.) established the fact, that pulse waveform may demonstrate characteristic changes in consequence with Nogier's conception of VAS (3). Nevertheless the objective evaluation of VAS for practical needs is a very difficult problem.

In our opinion the main difficulties for objective evaluation of VAS are the following:

1. VAS interpretation is subjective and based on the individual physician's skill. Possible hemodynamic mechanism of palpating VAS are difficult to define in the metrological aspects of VAS objectivation.

2. Attempts at objectivation of VAS were often made using inappropriate technique and equipment.

Possible hemodynamic mechanism of palpating VAS

All objective investigations of the VAS phenomenon identified the subjective perceptions of the stronger and weaker beats when palpating radial artery pulse with objective changes in the diastolic part of the pulse waveform. We do not agree with Nogier's explanation of the VAS in the concept of a standing wave in an artery. Hypothesis of standing waves was proposed by Hamilton and Dow in 1939 as result of analysis of pulse waveform in aorta in experiments on dogs. Today researchers do not accept it at all as a normal state. It is documented that standing waves could occur only if there were a complete reflection of the wave, and if there were no interaction of the reflected waves from scattered arterial terminations. In human arterial trees, the wave reflection is too low and attenuation too high for resonance and standing waves to occur (4).

From our standpoint, the following interpretation of the VAS is more reliable. The subjective palpating perception depends on the pressure of the palpating finger, and the radial pulse may be found differently "large" in spite of unchanged intravasal pulse pressure.

Korner (5) examined the amplitude of the extravasal volume pulse in relation to a continually diminished coupling pressure of a photoelectric pulse detector. The author consideres that it is exactly the same course one gets by palpating the radial pulse with the finger. However the intravasal pressure amplitudes showed a different kind of course at the same time. These differences between extra- and intravasal pulsation usually can be seen in patients with undistributed circulation.

Having a vasoconstriction in the radial artery the course of the extravasal volume pulse-curve appears different and the sensation of the palpating finger, too. In this case the pulse is palpable evenly small. The differences can be explained by the change in the pressure-built elastic tension of the arterial wall and the stronger contraction of the vessel muscles in vasoconstriction. Thus, the diagnosis of vasoconstriction can be made by palpating the radial pulse.

On the other hand, smooth muscle relaxation may be solely due to changes in vessel geometry and/or dependent on changes in wall stiffness (6).

All publications of objective evaluation of VAS demonstrate that VAS is expressed by contour pulse changes in the diastolic part of the arterial pulse wave (3). Theoretically, those changes in pulse waveform are usually due to changes in vascular muscle tone influencing the compliance and on the velocity of reflected waves (7).

Could beat-to-beat changes in muscle tone be detected by echo-tracking and Finapres methods?

In clinical practice, the compliance cannot be measured directly. Its indirect determination requires the measurement of the magnitudes of arterial pressure and the fluctuations in volume. The result is thus affected by the specific errors incurred in both these measurements.

In recent years large artery function and structure could be assessment in clinical practice, by means of investigation of artery compliance and arterial wall thickness. For assessment of the radial artery compliance, the radial artery diameter and artery pressure are simultaneously acquired over the systolic-diastolic blood pressure range (beat-to beat finger measurement) and by an echo-tracking device, and compliance is expressed using Langewouters formula as the integral of the area under the compliance/ blood pressure curve normalized for pulse pressure.

Distensibility is obtained as compliance divided by vessel diameter. The method is operator-dependent. Intra-observer variabilies for elastic modulus are near 18% (8).

The compliance assessment requires accurate measurement of the diameter of the artery during all of the cardiac cycle, not only of the onset of the cardiac cycle, as done usually. The distention must be measured previously during all the cardiac cycle also, which is required of the local pulse pressure measurements instead of the finger pressure measurements (9).

In clinical practice physicians usually use distensibility as a ratio of the actual radius rise (dR) (as a measure of volume rise) to the accompanying rise in arterial pressure (dPA); dR/dPA is small and very precise methods are required for such measurements. However, the resolution of echo-tracking technique is relative poor for precise and detailed assessment of the changes of the pulse wave contour because the signal-to-noise ratio of the original signal is low. For precise measurements, the echo-tracking signals are sampled over 8 bits and averaged five times. Averaging of the beat-to-beat changes using in echo-tracking devices make them unable to detect changes over one cardiac beat.

In addition, cross-sectional dimension of the arterial lumen is also affected by spiral and longitudinal movements of the vessel wall, that could not be reflected by the echo-tracking methods correctly.

On the other hand, usage of the vascular unloading method as Finapres for beat-to-beat monitoring arterial pressure could not be recommended for precise assessment of beat-to-beat changes in vascular smooth muscle tone. Accuracy and precision can not be guaranteed by Finapres method even for trends in arterial pressure. Correlation between Finapres and direct intra-arterial pressures in artery is only (r = 0.82-0.78; r = 0.68-0.72, and r = 0.78-079 for systolic, diastolic and mean pressures respectively. Moreover, Finapres pressure may be unexplicably increased whilst invasive pressure was decreased (or vice versa) in 17-19% of measurements. The authors who investigated the accuracy of the Finapres concluded, that Finapres cannot be recommended for pressure monitoring in clinical practice or clinical research (10, 11).

Taking into account all those factors, the method used by Thalhammer et al is not adequate for detection of VAS (1).

Consequently, the conclusion of the authors is that they could not register VAS is not wonderful because the echo-tracking method cannot be accepted for VAS-registration. We agree, that for registration of the VAS the precise methods for beat-to-beat assessment in real time changes in the pulse waveform, are referable.

REFERENCES

1. Thalhammer C, Haller H, Luft FC. Aurikulomedizin im angiometrischen test. AKU, 1997, 25(1): 70

2. Girerd X, Chamiot-Clerc P, Copie X, et al. Effects of norepinephrine on the mechanical properties of the human radial artery in vitro. Am Heart J, 1998, 136: 624-31

3. Kenyon J. Electronic pulsography. In: Modern techniques of acupuncture. Vol 2, Thorsons Publishing Group, Wellingborough, UK, 1983, pp 82-93

4. O'Rourke MF. The arterial pulse in health and disease. Am Heart J, 1971, 82: 687-702

5. Korner M. Studies on the palpable intensity of the radial pulse.Anaesthesist, 1976, 25: 470-4

6. Bank AJ, Kaiser DR. Smooth muscle relaxation: effects on arterial compliance, distensibility, elastic modulus, and pulse wave velocity. Hypertension, 1998, 32: 356-9

7. Nikols WW, O'Rourke MF. McDonald's blood flow in arteries: theoretic, experimental and clinical principles. 3-rd Ed. Lea & Febiger, Philadelphia, 1990, p 432

8. Hansen F, Bergqvist D, Mangell P, Ryden A, Sonesson B, Lanne T. Non-invasive measurement of pulsatile vessel diameter change and elastic properties in human arteries: a methodological study. Clin Physiol, 1993, 13: 631-43

9. Hoeks AP, Brands PJ, Reneman RS. Technical aspects of compliance assessment. Arch Mal Coeur Vaiss, 1991, 84(3): 77-81

10. Stokes DN, Cluttion-Brock T, Patil C, et al. Comparison arterial pressure using the Finapres. Br J Anaesth, 1991, 67: 26-35