Cardiac spectral power reflects parasympathetic but not sympathetic nervous system activity in a clinical population

Journal of the

Autonomic Nervous System ELSEVIER

Journal of the Autonomic Nervous System 61 (1996) 201-203

Short communication

Cardiac spectral power reflects parasympathetic but not sympathetic nervous system activity in a clinical population Eric R. Muth a,*, Gary R. Morrow b, Wei Jiang

b, Robert M.

Stern a, Brent Dubeshter b

Department of Psychology, The Pennsylcania State Universio, University Park, PA 16802, USA b Behavioral Medicine Unit, Unit,ersity of Rochester School of Medicine and Dentisto', Cancer Center, Rochester, NY 14627, USA Received I I March 1996; revised 10 June 1996; accepted 12 June 1996

Abstract

The purpose of this short communication is to report our clinical findings regarding the use of the low frequency (LF, 0.02-0.15 Hz) and high frequency (HF, > 0.15 Hz) components of the spectral decomposition of heart-rate as indices of sympathetic (SNS) and parasympathetic nervous system (PNS) activity, respectively. Thirty-two females with histologically confirmed ovarian cancer, ranging in age from 46-72 years, participated in an autonomic assessment protocol consisting of a resting heart rate recording and several ANS function tests. The LF, HF and total power measures from the spectral decomposition were highly correlated with one another. In addition, the spectral components were most highly correlated with measures of PNS activity, i.e. standard deviation of heart rate at rest and the ratio of the six longest to the six shortest R - R intervals during deep breathing (E: I ratio). It is concluded, as other researchers have stated, that the use of the HF component of the HR spectrum as a measure of PNS activity is warranted, but caution must be used when interpreting the LF component. Keywords: Autonomic nervous system; Spectral analysis; Heart rate variability; Cardiovascular reflexes

The purpose of this short communication is to report our clinical findings regarding the use of the low frequency (LF) and high frequency (HF) components of the spectral decomposition of heart-rate (HR) as indices of sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) activity, respectively. Recent developments in cardiovascular behavioral medicine [8,14,19] have led to the use of the spectral decomposition of cardiac variability as an index of both PNS and SNS [5,21]. The HF component of the spectral decomposition of HR ( > 0.15 Hz) has been found to reflect the breath to breath variations in HR, known as respiratory sinus arrhythmia (RSA) and thus PNS activity [14]. Evidence is amassing to support RSA as a measure of PNS activity regardless of the particular analysis method used [9,10]. However, under certain conditions, it is questionable whether measures of RSA reflect PNS activity [7]. The LF component of the HR spectral decomposition (0.02-0.15

* Corresponding author. Hershey Medical Center, Gastroenterology Division, Room C5800, Box 850, Hershey, PA 17033-0850, USA. Tel.: + 1 717 5314504; fax: + 1 717 5316770; e-mail: emuth @med.hmc.psu.edu.

Hz) has been used as an index of SNS functioning. However, the relationship between the LF component and SNS activity is not clear and the LF component is thought to be a mixed ANS measure [17,18]. In fact, several reports directly question the validity of using the LF component as a measure of SNS activity [1,13]. Thirty-two female patients with histologically confirmed ovarian cancer were studied. Patients ranged in age from 4 6 - 7 2 years with a median age of 61. None had diagnosed cardiac, diabetic or alcoholic pathology that would compromise ANS activity. None smoked or drank coffee on the day of assessment. All signed approved informed consent documents. Patients participated in an ANS assessment after admission to the Clinical Research Center for chemotherapy treatment. Assessment took place at the patient's bedside, prior to the administration of any medical treatment. HR and respiration were recorded during the entire assessment and blood pressure was recorded before and after standing and before and during the cold pressor test. Patients lay supine, breathing at their normal rate, for one-half hour. Patients then moved from a supine to a standing position (orthostatic stress). After blood pressure measurements were taken, patients were asked to

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E.R. Muth et al. / Journal of the Autonomic Nervous System 61 (1996) 201-203

sit and perform a Valsalva maneuver by blowing into a mouthpiece connected to an aneroid manometer and holding a pressure of 40 mm Hg for 30 s. After a further 5 rain rest, the patient's dominant hand was immersed in 4°C ice water for one minute (cold pressor). Patients were then asked to resume the supine position, and after another 5 min rest, commenced deep breathing at 6 breaths per minute for 2 rain. Considerable evidence exists that HR and blood pressure changes in response to the above challenges are indices of ANS function. The assumption in all of these tests is that damage or impairment within a particular reflex reflects damage or change elsewhere in the ANS [4]. The ANS function test results and spectral decomposition of HR were scored as follows: E : I r a t i o ( E : I ) - originally described by Sundkist et al. [22], the ratio of the six longest R - R intervals to the six shortest R - R intervals measures changes in RSA during deep breathing at 6 breaths/rain over 2 rain. M a x i m u m - m i n i m u m values ( m a x - m i n ) - the difference between the fastest and slowest heart rates (HR) over a 2 min period while patients were breathing at 6 breaths/min [16]. 3 0 : 1 5 r a t i o normal subjects show tachycardia or shortening of the R - R interval when going from a supine to a standing position. The interval is minimal around the 15th beat after standing followed by relative bradycardia or lengthening of the R - R interval which is maximal around the 30th beat after standing. A ratio of less than 1.0 is usually considered abnormal [3,6]. V a l s a l v a r a t i o - first described by Levin [15], the most common scoring for the Valsalva maneuver is the Valsalva ratio. This is the ratio of the longest R - R interval after the Valsalva maneuver to the shortest R - R interval during the maneuver. C o l d p r e s s o r t e s t - as originally described by Hynes and Brown [12], the increase in blood pressure in response to immersing one hand in ice water for a period of one minute reflects SNS activity. P o s t u r a l e f f e c t s - an intact baro-reflex pathway in the brainstem and intact sympathetic splanchnic vasoconstrictor fibers are essential for the maintenance of blood pressure. Differences between supine and standing systolic blood pressure (SBP) and diastolic blood pressure (DBP) were used to assess SNS activity (supinestanding). In addition, the orthostatic response of HR to standing from a supine position was measured. S t a n d a r d deL, i a t i o n ( S D ) o f H R - this measure was the standard deviation of successive R - R intervals over a 5 rain period during the supine period. At rest in a supine position, SD of HR is primarily under PNS control [3]. S p e c t r a l d e c o m p o s i t i o n o f r e s t i n g H R - autonomic activity was assessed through Fourier mathematical analysis of millisecond variations in the heart's successive R - R intervals. Fourier analysis of R - R rate enables the separation of LF and HF frequencies [20]. Approximately 4.5 min periods were analyzed according to the methods of Akselrod et al. [2] yielding power estimates for the total power (TP) range of 0.02 to 0.50 Hz. LF was defined as the power falling

Table 1 Summary of spectral analysis results and clinical autonomicfunction tests Variable Mean Standard Normalrange/ deviation response a SD of heart rate

1.98 1.08 1.23 0.19 Max-rain heart rate 19.57 12.94 Valsalva ratio 1.42 0.53 30:15 ratio 1.01 0.07 Orthostatic stress-HR 9.27 13.67 Orthostatic stress-SBP - 0.13 12.77 Orthostatic stress-DBP 1.38 7.72 Cold pressor-SBP - 1.75 9.42 LF (low frequencypower) 793 946 HF (high frequency power) 237 349 TP (total power) 1425 1503 E:I ratio

0.6-2.7 1.12-1.43 varies 0.97-1.99 1.01-1.19 increases no change 7 mm/Hg increase 12-15 mm/Hg varies varies varies

Normal ranges are based on reports from Dunlap and Pfeifer [3], Gautschy et al. [6] and Thomson and Melmon [23].

a

between 0.02 and 0.15 Hz. HF was defined as the power falling between ___0.06 Hz of the respiratory frequency as determined through Fourier analysis of the respiratory signal [1 1]. The average responses, their standard deviations, and the normal ranges [3,6,23] for the ANS function tests and the spectral components from the decomposition of HR are found in Table 1. As can be seen, the SD of HR, E: I ratio, m a x - m i n HR, Valsalva, 30:15 ratio and response of HR to standing are all within normal limits. The average response of SBP to standing showed little change, and the range of - 0 . 1 3 _+ 12.77 indicates that the subjects showed a normal response as a decrease of more than 30 m m HG is considered abnormal. Likewise, a fall in DBP of greater than 10 m m HG upon standing is abnormal and thus, a range of 1.38 ___7.72 in our patients is considered normal. The cold pressor test yielded inconsistent results, probably because of the difficulty in performing the test consistently, i.e. patients had difficulty keeping their hand immersed for one minute and varied greatly in their tolerance of the procedure. Correlations were performed within the spectral components and between the spectral components and the ANS function test results (Table 2). The LF, HF and TP components of the spectral decomposition of HR were all highly, positively correlated. Standard deviation (SD) of HR was highly, positively correlated with both the LF and TP components and moderately, positively correlated with the HF component. The E : I ratio and max-rain of HR were moderately, positively correlated with the HF and TP components but not highly correlated with the LF. The response of HR to standing was also correlated with the LF, HF and TP components. However, this correlation was low and only marginally significant. The high, positive correlations between the LF, HF and TP components from the spectral decomposition suggests that all the spectral components are influenced by the same factor. The correlations between the ANS function tests

E.R. Muth et al. / Journal of the Autonomic Nervous System 61 (1996) 201-203

Table 2 Correlations of LF, HF and TP spectral components from the HR decomposition with the ANS function tests

LF HF TP SD of HR E:I Max-min HR Valsalva ratio 30:15 Cold pressor-SBP Orthostatic stress-SBP Orthostatic stress-DBP Orthostatic stress-HR

LF

HF

---0.83 0.33 0.24 0.09 -0.05 0.25 - 0.02 -0.19 0.35

0.57 --0.42 0.56 0.34 0.20 -0.36 0.22 - 0.09 -0.04 0.31

*** * NS NS NS NS NS NS *

TP ***

0.94 0.69 -0.84 0.50 0.34 0.12 -0.11 0.26 - 0.11 -0.17 0.32

** *** * NS * NS NS NS *

*** *** *** *** * NS NS NS NS NS *

NS - not statistically significant. * = P<0.1. ** = P < 0 . 0 5 . *** = P < O . O 1 . a n d t h e s p e c t r a l m e a s u r e s s e e m to i n d i c a t e t h a t t h e s p e c t r a l c o m p o n e n t s are m o s t h i g h l y r e l a t e d to P N S m e a s u r e s . S D o f H R , a l t h o u g h a m i x e d m e a s u r e , is p r i m a r i l y u n d e r P N S c o n t r o l at r e s t [3], t h e s t a t e o f o u r p a t i e n t s w h e n S D o f H R was

measured.

SD

of HR

consistently

accounts

for a

s i g n i f i c a n t a m o u n t o f v a r i a n c e in all o f t h e s p e c t r a l c o m p o nents. The only other measure

t h a t is h i g h l y c o r r e l a t e d

w i t h t h e s p e c t r a l c o m p o n e n t s is t h e E: I r a t i o w h i c h is a l s o a measure of RSA and therefore PNS activity. Max-min H R a n d r e s p o n s e o f H R to s t a n d i n g a r e a l s o P N S m e a s u r e s a n d are m o d e s t l y c o r r e l a t e d w i t h t h e s p e c t r a l c o m p o n e n t s , albeit, t h e y are l e a s t c o r r e l a t e d w i t h L F . F u r t h e r m o r e , t h e r e s p o n s e o f D B P to o r t h o s t a t i c s t r e s s , w h i c h r e f l e c t s S N S activity

is n o t s i g n i f i c a n t l y c o r r e l a t e d

with any

of the

s p e c t r a l c o m p o n e n t s . T h e s e c o r r e l a t i o n s l e n d s u p p o r t to t h e n o t i o n t h a t t h e s p e c t r a l c o m p o n e n t s o f H R a s s e s s e d in this study

reflect

searchers have

PNS

function

and

stated [1,9,13,18],

that,

as

previous

re-

caution must be used

w h e n a t t e m p t i n g to d e r i v e S N S m e a s u r e s f r o m t h e s p e c t r a l decomposition of HR.

Acknowledgements This work was supported by research grants NR9905 from the National Center for Nursing Research, RR00044 Clinical Research Center, DHHS,

and PBR43A

from the

American Cancer Society.

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