Anesth Analg 2000;91:398-402
© 2000 International Anesthesia Research Society
REGIONAL ANESTHESIA AND PAIN MEDICINE
The Current Perception Thresholds Vary Between Horizontal and 70° Tilt-Up Positions
Osamu Shimoda, MD, PhD*, and
Yoshihiro Ikuta, MD, PhD
*Department of Anesthesiology, Kumamoto University School of Medicine; and
Surgical Center, Kumamoto University Hospital, Kumamoto, Japan
Address correspondence and reprint requests to Osamu Shimoda, MD, Department of Anesthesiology, Kumamoto University School of Medicine, 1-1-1 Honjo, Kumamoto 860-8556, Japan. Address e-mail requests to shimoda{at}kaiju.medic.kumamoto-u.ac.jp
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Abstract
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We investigated the influence of posture on current perception threshold (CPT). The subjects consisted of 20 healthy male volunteers (23–31 yr old). At both the horizontal and the 70° tilt-up position (TUP), the CPTs (5, 250, and 2000 Hz) of the middle finger were determined by using the Neurometer CPT/C (Neuropteran, Baltimore, MD). Autonomic nervous activities were evaluated by heart rate variability (HRV) analysis and spontaneous baroreflex sensitivity analysis at the two postures previously mentioned. The three CPTs at the 70° TUP were significantly lower than those at the horizontal posture (5 Hz, P < 0.05; 250 Hz, P < 0.001; 2000 Hz, P < 0.05). The changes in HRV and spontaneous baroreflex sensitivity at the 70° TUP indicated decreasing parasympathetic tone. The CPTs of 5 and 250 Hz were significantly correlated with mean systolic blood pressure at the 70° TUP. The CPT of 2000 Hz was significantly correlated with the 0.15–0.4 Hz component in HRV at both postures. The regression analysis of the difference of 5 Hz CPT with that of the mean systolic blood pressure showed a significant correlation (P < 0.001). To evaluate the clinical course of peripheral nerve disorders, the comparison of CPTs measured during the same posture is important. This suggests that CPTs must be measured at the horizontal posture.
Implications: Current perception thresholds at the 70° tilt-up posture were significantly lower than those at the horizontal posture. When the compensatory mechanism for preserving blood pressure was emphasized, the current perception thresholds would have a relational connection to mean systolic blood pressure, similar to the concept of hypertension-induced hypoalgesia.
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Introduction
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The current perception threshold (CPT) device has been commercially available and widely used to clinically evaluate peripheral nerve function. However, CPTs vary with every peripheral nerve (e.g., trigeminal, median, and tibial nerve) and have a large intersubject variance even in normal subjects (NeuvalTM Database II [Neurotron, Baltimore, MD] normative data). The factors influencing CPT variance other than age and sex in normal subjects have not been explored (1,2).
Decreased pain sensitivity has been observed in hypertensive men (3,4), in borderline hypertensive men (5), and in normotensive men with a parental history of hypertension (6). The relationship between increased blood pressure and decreased pain perception was found even in normotensive subjects (7). The nature of this interaction is unclear, yet several studies have suggested that the baroreflex control of cardiovascular regulation influences antinociception in humans and animals (4,8). Furthermore, hypertension-induced hypoalgesia is normalized by treatment with enalapril (9), whereas diuretics or ß-blockers did not alter the pain sensitivity in hypertensive patients (10). These studies suggest that the renin-angiotensin-aldosterone system may also have some effect on the regulation of pain perception.
We recently found that the CPT values measured every day from the same peripheral nerve in healthy subjects was not always fixed. Considering the concept of hypertension-induced hypoalgesia previously mentioned (5–10), we hypothesized that blood pressure, autonomic nervous activity, and the renin-angiotensin-aldosterone system may also influence CPT, because the three frequencies of the sine wave current are assumed to activate the characteristic subpopulations of peripheral nerve fibers, C fibers by 5 Hz, A-
fibers by 250 Hz, and A-ß fibers by 2000 Hz (11,12). We investigated the changes in CPT with postural change intending to activate the compensatory mechanisms, i.e., the autonomic and the renin-angiotensin-aldosterone system for preservation of blood pressure.
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Methods
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The study was approved by the investigative review board of our hospital. The subjects consisted of 20 healthy male volunteers ranging in age from 23 to 31 yr (mean, 27.4; SD, 3.3 yr). The subjects were randomly selected from hospital staff and medical students. No subject had any sign of autonomic dysfunction or cardiovascular disease detected by screening medical examination results or previous history. Preexamination blood pressure was in the normotensive range (<140/90 mm Hg) and there was no parental history of hypertension. All subjects refrained from caffeine and nicotine for at least 2 h preceding their arrival at the laboratory. The study was explained in detail and informed consent was obtained from all subjects.
The present experiment was conducted in a quiet laboratory room where the temperature was maintained at 25–26°C. First, the subject sat on a comfortable chair. Two plate surface electrodes of the Neurometer CPT/CTM (Neurotron) were placed on both lateral sides of the right middle finger. The lead II electrocardiogram (ECG) and beat-to-beat blood pressure were monitored by using a Nippon Colin BP-508 (Komaki, Japan). A tonometer sensor was attached to the left wrist over the radial artery, and calibrated at 5-min intervals by an oscillometric cuff attached to the left upper arm. ECG and beat-to-beat blood pressure waves were simultaneously input into a personal computer (PC9801 NS; NEC, Tokyo, Japan) via a wave form regulator (Nihon Kohden, Tokyo, Japan). The digital R-R interval data with a time resolution of 0.5 ms and the blood pressure wave with a pressure resolution of 0.125 mm Hg were then stored on a floppy disk for later analysis. The details of both measurements of R-R interval and blood pressure wave were reported previously (13). To accustom the subject to the CPT testing, CPTs of 5, 250, and 2000 Hz were determined by using the forced-choice CPT testing mode. The details of the Neurometer CPT/CTM and the testing mode have been reported elsewhere (1,2) (Neurometer CPT/CTM operating manual). The subjects were then, randomly assigned to two groups of 10 subjects each by coin toss.
The subjects of one group (n = 10) were placed in the horizontal position on a comfortable bed. The subjects practiced breathing synchronously with a 15-cycle/min metronome signal and were allowed at least 20 min to adjust to the environment. After confirming that the heart rate and blood pressure were stable, the ECG and beat-to-beat blood pressure wave were simultaneously recorded for 3 min. Then, CPTs of 5, 250, and 2000 Hz in a randomized order were determined by using the forced-choice CPT testing mode. Each frequency of the CPT test was performed after at least a 3-min interval. Blood pressure monitoring and cuff calibrations were interrupted during the determination of the CPT. Subsequently, the posture slowly changed to the 70° tilt-up posture (TUP). The forearms were placed in armrests adjusted to the level of the subject’s xiphoid process. The body balance of the subject at the 60° to 70° TUP resembled the upright posture. The compensatory mechanisms for preservation of blood pressure are similar between the two postures (14,15). Because the body fluctuation is less at the 60° to 70° TUP than during the upright posture, the former is appropriate to measure physiological variables (14,15). ECG and beat-to-beat blood pressure recording and determination of the CPTs of 5, 250, and 2000 Hz were again performed in a similar fashion. Subjects of another group (n = 10) were similarly examined except for reversal of the posture order; i.e., first, at 70° TUP; second, at the horizontal posture. We excluded from further analysis one subject who showed hypotension >20 mm Hg at 70° TUP compared with that at the supine posture.
The R-R interval and blood pressure were analyzed by using a personal computer. The mean systolic blood pressure (SBP) was calculated using every SBP. Spontaneous baroreflex sensitivity was calculated by using both the R-R interval and beat-to-beat systolic pressure measurements recorded simultaneously. The algorithm of spontaneous baroreflex sensitivity has been reported elsewhere (16). R-R intervals were paired with the SBP value of the preceding beat. All sequences of three or more pairs, which showed simultaneous increases or decreases in both the R-R interval and systolic pressure, were selected. Each sequence was analyzed by linear regression. The slopes of all regression lines were averaged. The averaged slope was considered the spontaneous baroreflex sensitivity and expressed in ms/mm Hg. Heart rate variability during each 3-min period was analyzed by fast Fourier transmission. The details of heart rate variability analysis have been reported previously (17). We considered the 0.05–0.15 Hz components to be the low frequency component and the 0.15–0.4 Hz components to be the high frequency component. We calculated the respective band area values as the power and expressed the values in ms (2). Then, the low/high ratio was calculated.
The measured values were presented as mean (SD). Postural change in the mean SBP was assessed by using Student’s paired t-test. Postural changes in CPTs and autonomic data were assessed by using the Wilcoxon’s signed rank test. Linear regression analyses were performed on each CPT value versus the mean SBP and autonomic data at the two postures. According to the differences in each CPT between the two postures, the subjects were classified to the supine >70° TUP group, the supine = 70° TUP group, or the supine <70° TUP group. Fisher’s exact test was used to clarify the relationship between the postural difference of the CPT value and that of the mean SBP and autonomic data. If a significant relationship was found, linear regression analysis was performed. Finally, after classification to the previously mentioned groups at every three frequencies of CPT, the mean SBP of the supine >70° TUP group and the supine <70° TUP group was compared by using Student’s paired t-test (intragroup) and unpaired t-test (between the groups). In addition, the autonomic data of the two groups were compared by using Wilcoxon’s signed rank test (intragroup) and Mann-Whitney’s U-test (between the groups). A probability value <0.05 was considered significant.
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Results
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Figure 1 shows the individual changes in 5 Hz CPT (left panel), 250 Hz CPT (middle panel), and 2000 Hz CPT (right panel) at the two postures. The three CPTs at the 70° TUP were significantly lower compared with those at the horizontal posture. The mean SBP did not change between the two postures. The spontaneous baroreflex sensitivity and the high frequency component in the heart rate variability at the 70° TUP were significantly smaller than those at the horizontal posture. The low/high ratio in heart rate variability at the 70° TUP was significantly larger than those at the horizontal posture (Table 1).
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Figure 1. The individual changes in 5 Hz CPT (left panel), 250 Hz CPT (middle panel) and 2000 Hz CPT (right panel) at the two postures. The ordinate units are the CPT values One CPT value = 10 micro-A, CPT = current perception threshold.
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Table 1. Current Perception Threshold (CPT), Mean Systolic Blood Pressure (SBP) and Autonomic Indices in the Two Postures
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A significant correlation, calculating with linear regression analyses of the three CPTs versus the mean SBP and autonomic data at the two postures, was observed in 1) 5 Hz CPT (x axis) versus the mean SBP (y axis) at the 70° TUP (y = 96.4 + 0.56x, r = 0.66, P = 0.002), 2) 250 Hz CPT (x axis) versus the mean SBP (y axis) at the 70° TUP (y = 94.9 + 0.38x, r = 0.57, P = 0.011), 3) 2000 Hz CPT (x axis) versus the high frequency component in heart rate variability at the horizontal posture (y = -9.55 + 0.09x, r = 0.54, P = 0.018) and at the 70° TUP (y = -3.28 + 0.03x, r = 0.54, P = 0.016).
Table 2 shows the incidence of the differences of the CPT between the two postures. There was a significant relationship only between the postural difference of 5 Hz CPT and that of the mean SBP (P = 0.006). The regression analysis of the difference of 5 Hz CPT (x axis) with that of the mean SBP (y axis) showed a significant correlation (y = 1.56 + 0.64x, r = 0.60, P < 0.001) (Fig. 2). In the 5 Hz CPT, the supine >70° TUP group (n = 14) showed a significant difference in mean SBP (119.9 mm Hg, 7.4 SD at the supine and 110.6 mm Hg, 9.3 SD at the 70° TUP, P = 0.0013), whereas the supine <70° TUP group (n = 5) did not show a significant difference in the mean SBP (119.2 mm Hg, 7.8 SD at the supine and 124.6 mm Hg, 15.4 SD at the 70° TUP). There was a significant difference in the mean SBP at the 70° TUP between the groups classified by the postural difference of 5 Hz CPT (P = 0.023). In the 250 Hz CPT and 2000 Hz CPT, there was no significant relationship between the differences in CPT and that of either the mean SBP or the autonomic data.
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Figure 2. The regression analysis of the difference between the two postures at 5 Hz CPT (abscissa) with that of mean SBP (ordinate). The abscissa units are the CPT values. One CPT value = 10 micro-A, CPT = current perception threshold, SBP = systolic blood pressure.
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Discussion
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In normotensive volunteers, CPTs of the three frequencies in the 70° TUP were significantly lower than that in the horizontal posture. The CPTs of 5 and 250 Hz were significantly correlated with the mean SBP at the 70° TUP. The CPT of 2000 Hz showed a significantly inverse correlation with the high frequency component in heart rate variability at both postures. There was a significant relationship between the postural difference of 5 Hz CPT and that of the mean SBP.
At the upright posture, approximately 500–700 mL of blood is pooled in the lower limbs and in the splanchnic and pulmonary circulation (18). In response to these reductions of venous blood return, the cardiopulmonary, aortic, and carotid baroreceptor are stimulated, leading to increased sympathetic outflow and decreased parasympathetic activity (14,15,19,20). The high frequency component and the low/high ratio in the heart rate variability are commonly used as indices of vagal activity and sympathovagal interaction, respectively (21). The spontaneous baroreflex sensitivity test permits measurement of baroreflex sensitivity under natural conditions compared with vasoactive drug-induced analysis (15,22). In the current study, the changes in heart rate variability and spontaneous baroreflex sensitivity at the 70° TUP indicated decreasing parasympathetic tonus.
Prolonged upright posture activates the renin-angiotensin-aldosterone system and arginine vasopressin to maintain blood pressure (23). The hypertension-induced hypoalgesia is normalized by treatment with enalapril (9), whereas diuretics or ß-blockers did not show altered pain sensitivity in hypertensive patients (10). These findings suggest that the renin-angiotensin-aldosterone system has some effect on pain perception regulation, and its mechanism is not only through normalization of hypertension. We did not measure the serum concentration of the vasopressive peptides. However, the compensatory mechanism for preservation of blood pressure, including autonomic and hormonal reaction, should be activated during the 70° TUP testing.
The three frequencies of the sine wave current have been assumed, in general, to activate the characteristic subpopulations of peripheral nerve fibers: C fibers by 5 Hz; A- fibers by 250 Hz; and A-ß fibers by 2000 Hz (11,12). Somatic pain sensation is mainly conducted to the spinal cord via C fibers and A- fibers. An inverse correlation between SBP and pain rating has been reported even in normotensive subjects (5,7,24). Therefore, our perspective before the study was that sympathetic and autonomic activation at the 70° TUP might increase the CPTs; however, our findings were paradoxical.
The mean SBP and the CPTs of 5 and 250 Hz showed a liner relationship at the 70° TUP. Furthermore, 14 subjects showed reduction of 5 Hz CPT concurrent with decreased mean SBP at the 70° TUP, whereas the remaining five subjects who showed increased 5 Hz CPT at the 70° TUP, showed unchanged or slightly increasing mean SBP. These findings resembled the relationship between blood pressure and pain perception (5,7,24). However, these relationships were not observed in the horizontal posture. When the compensatory mechanism for preservation of blood pressure was emphasized at the 70° TUP, the 5 and 250 Hz CPTs would have a relational connection to mean SBP in a similar manner with the concept of hypertension-induced hypoalgesia. At the 70° TUP, the autonomic variable did not differ between the supine >70° TUP group and the supine <70° TUP group in the 5 Hz CPT despite the significant difference in mean SBP. This finding suggests that the relationships between blood pressure and the renin-angiotensin-aldosterone system markedly influence the 5 Hz CPT rather than those between blood pressure and autonomic balance in the control system of the heart rate.
The present findings do not clarify all aspects of postural effect on the CPTs, because the present preliminary study did not intend to explore these mechanisms. The analysis of the postural effect on the CPTs needs to be examined by measuring the serum concentration of the vasopressive peptides and direct autonomic neural discharges. Because the mechanism of hypertension-induced hypoalgesia has not been established, further analysis is required.
The CPT device has frequently been used as an index of functional restoration and disorder of peripheral nerves. Therefore, if the CPTs were assessed incorrectly, a clinician might misinterpret the efficacy of the treatment. To evaluate the clinical course of peripheral nerve disorders, our findings indicate that the comparison of the CPTs measured during the same posture is important. Because the CPTs of 5 and 250 Hz showed no significant correlation with the mean SBP or autonomic indices at the horizontal posture, the CPT must be measured at the horizontal posture. To measure the CPT of 2000 Hz, simultaneous measurement of autonomic activity is also necessary, because the CPT of 2000 Hz showed a significantly inverse correlation with the high frequency component in heart rate variability, even in the horizontal posture.
In conclusion, the current study confirmed that posture influences CPTs. The comparison of CPTs measured during the same posture is important. This suggests recommending that CPTs must be measured in the horizontal posture.
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Acknowledgments
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Supported, in part, by Grant 10671423 for Scientific Research from the Ministry of Education, Science and Culture, Japan.
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Accepted for publication April 28, 2000.
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Abstract
Introduction
