Comparison between muscle sympathetic nerve activity and calf vascular resistance with head-up tilting in humans

Journal of the Autonomic Nereous System, 33 (1991) 277-282

277

1991 Elsevier Science Publishers B.V. 0165-1838/91/$03.50 JANS 01152

Comparison between muscle sympathetic nerve activity and calf vascular resistance with head-up tilting in humans Shin-ichi Tsunoda 1 Kazumasa Shindo ~, Zenji Shiozawa 1 and Tadaaki Mano 2 I Department o/Medicine, Neuroloyd,, Yamanashi Medical UnieersiO~and 2 The Research Institute of Ent,ironmental Medicine, Nage4va Uni~,ersiO,, Japan

(Received 24 April 1990) (Revision received 26 December 1990) (Accepted 28 December 1990)

K e y words: M u s c l e s y m p a t h e t i c n e r v e a c t i v i t y : C a l f v a s c u l a r r e s i s t a n c e : M i c r o n e u r o g r a p h y ;

I m p e d a n c e p l e t h y s m o g r a p h y ; H e a d - u p tilting

Abstract The purpose of this study was to determine the relationship between muscle sympathetic nerve activity (MSA) to calf muscle and calf vascular resistance with head-up tilting in humans. In nine healthy volunteers, we performed simultaneous measurements of MSA using microneurography and calf vascular resistance using electrical impedance plethysmography in the same leg. The volunteers were laid on a tilt table and the examinations with head-up tilting (0 o, 30 o, 60 o) were performed. Corresponding to the increase in angles of head-up tilting, MSA and calf vascular resistance significantly increased. A statistically significant positive linear correlation was demonstrated between MSA and calf vascular resistance. These data suggest that impedance plethysmographic measurement of calf vascular resistance can be reliably used to assess the status of MSA with head-up tilting.

Introduction M i c r o n e u r o g r a p h i c r e c o r d i n g of m u s c l e s y m p a t h e t i c n e r v e a c t i v i t y ( M S A ) is useful for e v a l u a t ing the p e r i p h e r a l s y m p a t h e t i c n e r v e f u n c t i o n s in h u m a n s [2,5,9,12,13]. A l t h o u g h it is g e n e r a l l y a c c e p t e d that M S A i n c r e a s e s v a s c u l a r r e s i s t a n c e f o l l o w i n g b a r o r e f l e x m o d u l a t i o n [11], the a c t u a l relationship between MSA and vascular resistance is d i f f i c u l t to assess. T o clarify this r e l a t i o n s h i p w i t h h e a d - u p tilting, we p e r f o r m e d e x a m i n a t i o n s

of m i c r o n e u r o g r a p h y (MSA) and impedance p l e t h y s m o g r a p h y (calf b l o o d f l o w a n d v a s c u l a r r e s i s t a n c e ) s i m u l t a n e o u s l y in the s a m e leg.

Materials and Methods Subjects

T h e i n v e s t i g a t i o n was p e r f o r m e d , w i t h w r i t t e n consent, on nine healthy male volunteers, aged 1 9 - 5 8 y e a r s old. Measurements

Correspondence: S. Tsunoda, Department of Medicine, Neu-

rology. Yamanashi Medical University, Tamaho-cho. Yamanashi-ken, 409-38, Japan.

M i c r o n e u r o g r a p h i c r e c o r d i n g of c a l f M S A a n d i m p e d a n c e p l e t h y s m o g r a p h i c m e a s u r e m e n t of c a l f b l o o d flow w e r e p e r f o r m e d s i m u l t a n e o u s l y in the

27S

right leg of the subjects. Electrocardiogram (ECG) was recorded by surface electrodes on the chest wall and blood pressure was measured from the middle finger of the right upper extremity using Finapres (a non-invasive continuous blood pressure measurement system, Ohmeda, U.S.A.). The middle finger was always kept at the right atrial level to avoid fluctuations of blood pressure with head-up tilting.

Impedance plethysmograph.v Measurements were made using an impedance p l e t h y s m o g r a p h (AI-601G, N i h o n K o h d e n , Japan). Two pairs of self adhesive aluminium tape electrodes were attached around the right calf circumferentially. A constant sinusoidal current of 350 /*A at 50 kHz was introduced between the outer electrodes. The total basal impedance (Z0) and the change of segmental impedance (zlZ) were recorded at the inner electrodes. The waveform of d Z / d t was obtained using a differentiator (ED. 601G, Nihon Kohden, Japan). To avoid the influences of venous occlusive plethysmography on MSA, calf blood flow was measured using a non-venous occlusive method (pulsatile blood volume measurement) [3], the one similar to the method for measuring cardiac output [6]. An example of an experimental recording is shown in Fig. 1. Calf blood flow was calculated using the formula shown below. To avoid the blood resistivity constant and the distance of sensing electrodes, this formula was modified as described by Mohapatra et al. [7].

M icroneurography Signal of MSA to calf muscle was recorded by a tungsten microelectrode, percutaneously inserted into peroneal nerve fascicles at the right knee. Nerve signals were amplified with a preamplifier, then fed through a bandpass filter (500 2000 Hz) and an R-C integrator with a time constant of 0.1 s to obtain mean voltage neurograms. MSA was monitored with an oscilloscope and a loudspeaker throughout all experiments. Concerning the procedure for identifying MSA and the recording system of nerve signal, we have previously described this in detail [10]. MSA was expressed as the burst rate (bursts/rain) and the total MSA (calculated by bursts/min × mean burst amplitude/min). The total MSA at the head-up tilting 0 ° was given the value of 1.0 unit and all other total MSA were expressed by relation to this value.

ECG

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where Z0=total b a s a l i m p e d a n c e (~2); ( d Z / d t ) m i n = m i n i m u m v a l u e o f dZ/dt(g2/s) f r o m t h e b a s e l i n e ; ET= v e n t r i c u l a r e j e c t i o n t i m e o b t a i n e d f r o m d Z / d t w a v e f o r m (s); RR = R R int e r v a l o f E C G (s).

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Fig. 2. Burst rate of MSA (muscle sympathetic nerve activity), total MSA (bursts/minXmean burst amplitude/min), calf vascular resistance, calf blood flow, heart rate and mean blood pressure with head-up tilting. Corresponding to the increase in tilting angle, MSA (burst rate and total MSA) and calf vascular resistance increased and these values were correlated to the sine function of tilting angle.

Fig. 3. Comparison between MSA (muscle sympathetic nerve activity) and calf vascular resistance. Top figure shows relationship between the burst rate of MSA and calf vascular resistance. Lower figure shows relationship between the total MSA and calf vascular resistance. Both the burst rate of MSA and the total MSA were significantly correlated to calf vascular resistance with head-up tilting.

Calf vascular resistance was obtained by dividing the mean blood pressure (diastolic pressure + 1/3 pulse pressure) by calf blood flow and was expressed in units.

Protocol T h e s u b j e c t s w e r e l a i d o n a tilt t a b l e a n d t h e experiments with head-up tilting were performed.

280 The angles of head-up tilting were 0 °, 30 °, 60 ° and the order of head-up tilting was randomized. Measurements of MSA, ECG, blood pressure and impedance of lower extremity were made at the steady state in each tilted position.

Analysis The experimental data were recorded on an ink-jet recorder with a paper speed of 25 m m / s . The records were divided into 1-min periods and all experimental values were obtained using a digitizer. Statistical analysis was performed using the mean values of the last l-rain period at the steady state in each position (0 o, 30 o 60 ° ). Results were expressed as mean + SEM and statistical analysis was performed by using Student's t-test. Values of P < 0.05 were considered to be significant.

Results M S A (the burst rate of MSA and the total MSA) and calf vascular resistance significantly increased with increase in head-up tilting and these values were statistically correlated with the sine of tilting angles (Fig. 2). N o t only the burst rate of MSA but also the total MSA showed a signifcant positive linear correlation with calf vascular resistance with head-up tilting (Fig. 3). Corresponding to the increase in tilting angles, calf blood flow decreased and heart rate and mean blood pressure slightly increased. There was no evidence that the signals of M S A were influenced by the impedance plethysmographic procedures. Also, the dZ/dt waveform was not influenced by respiratory m o v e m e n t of the chest wall.

Discussion Although it is generally accepted that M S A is related to the vasoconstriction of skeletal muscles [11], there are few reports analysing the relationship between M S A and vascular resistance [1,8]. In this study, by analysing calf M S A and its vasoconstrictive effect, as defined by calf vascular resistance on the homolateral leg with head-up

tilting, we demonstrated that vascular resistance increases in a parallel fashion with MSA. It was clearly shown that not only the burst rate of M S A but also the total M S A indicate the vasocorlstrictive effect of MSA. Moreover, it was clear that the burst rate of MSA, the total M S A and the calf vascular resistance were simultaneously correlated to the sine function of tilting angles. Iwase et al. [4] demonstrated that the burst rate of M S A increased linearly and was correlated to the sine of tilting angle. They explained that the sine of tilting angle is related to the b o d y axis c o m p o n e n t of gravity. Furthermore, our findings suggest that MSA causes peripheral vasoconstriction against the b o d y axis c o m p o n e n t of gravity, thus contributing to the systemic blood pressure homeostasis. In the present study, we used non-venous occlusive impedance plethysmography, a beat-to-beat pulsatile blood volume measurement, because this method is relatively simple for measuring continuous calf blood flow and vascular resistance together with microneurographic recording of MSA in the same leg. It was clear that impedance plethysmographic measurement of calf vascular resistance can be reliably used to assess the status of calf M S A with head-up tilting in normal subjects. Also, clinically, simultaneous recordings of MSA and calf vascular resistance may provide new information concerning M S A and its vasoconstrictive effects.

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