Chiropractic and HRV

Embed Size (px)

Citation preview

  • 7/27/2019 Chiropractic and HRV

    1/5

    161ournalof Manipulativeand PhysiologicalThernpeuticsVolume 22' Number 3' March/April 19990161-1754/99/$8.00 0 76/1/96841e 1999JMPT

    Rationale or Assessinghe Effects of ManipulativeTherapy on Autonomic Tone by Analysisof HeartRateVariabilityAle.t v!. Eingorn. Dca and George . Mtths. DCb

    ABSTRACT .Background:or more han 100years, hiro-practorshaveasserted hat overall healthcanbe mproved hrough he useof spinalmahip-ulative therapy.The autonomicnervoussys- (tern is known to control and regulate allinvoluntaryphysiologic activitiesby control-ling the activities of the internal organs,glands, and circulation. Recent studies docu-

    ment a potential relationship between he verte.bral subluxationcomplex,autonomic one, and car. '-diac function.Objective: his discussion eviews how it is possible o useheart ate variability analysis o calculatea quantitative ndex ofautonomic unction, which accurately eflects he sympatheticand parasympatheticoneand he sympathovagal alance.Discussion:he technique f heart ateanalysis nownasheartrate variability could be extremelyuseful n assessmentf treat-

    ment outcomes in clinical chiropractic practice.At present, heart rate variability is in wide-spread use in the fields of neurology, cardiol-ogy, psychology, psychophysiology, obstet-rics, anesthesiology, and psychiatry.Conclusion:Further studies in this area maylead to a better understanding of the effects ofspinal manipulation on (I) the general healthof an individual, (2) an individual's susceptibil-ity to lowered immunity and recuperative capaci-ty, and (3) conditions that lie outside the scope of---' musculoskeletalherapeuticsndaremore n line with

    classical chiropractic concepts. This can also contribute to a bet-ter-informed interprofessional cooperation between allopathicand chiropractic health care providers. (J Manipulative PhysiolTher 1999;22:161-5)Key ndexing erms:Autonomic Nervous System: Vagus Nerve;

    Chiropractic Manipulation

    Walko and Janouschek14showed thennographic coolingafter manipulative therapy of a patient with cervicothoracicpain. Grayson15discussed the case of a patient displaying apostmanipula~ve decrease in sympathetic output. Further-more, spinal joint dysfunction has been shown to affect car-diac function,9.1o and the somatovisceral autonomic reflexsystem.11.16-19atol9 has shown that noxious stimulation canresult in an increase n reflex somatocardiac sympathetic ac-tivity. Stimulation of vertebral joints produces a decrease nboth heart rate and blood pressure. SThe quality and directionof influence of SMT and ANS activity may depend on the spe-cific spinal level that is manipulated.1oOn the other hand, thebrain stem oscillators of the central autonomic network areintegral to the modulation of autonomic output!1 and Sato'swork indicates that the cardiac reflex response to somaticstimulation demonstrates a strong segmental organizationthat is suppressedby supraspinal structures}1.23Therefore themechanism by which manipulation may affect autonomicfunction must also consider possible suprasegmentaleffects.

    INTRODUCTIONFor more than 100 years, chiropractors have asserted hat

    overall health can be improved through the use of spinalmanipulative therapy (SMT).1-4 This concept is integral totraditional chiropractic philosophy and is continuouslyalluded to in numerous anecdotal accounts.l.4.SThe auto-nomic nervous system (ANS) is known to control and regu-late all involuntary physiologic activities by controlling theactivities of the internal organs, glands, and circulation.6-s

    Recent studies document a potential relationship betweenthe vertebral subluxation complex, autonomic tone, and car-diac function.S.9-11 n addition, in the osteopathic literaturemanipulation has been shown to have an effect on the ANS,specifically on lowering blood pressure n hypertensive indi-viduals and positively affecting the fibrinolytic system.12The same study showed that SMT produced an insignificanteffect in normotensive individuals. Another study iridicatedthat autonomic neurovascular changes seem to bccur withsuboccipital dermatomyotonic stimulation as measured withdigital strain gauge plethysmography. 13

    "Researchassociate,New York Chiropractic College, SenecaFalls, New York.bAssistant professorof clinical services,University of Bridge-port Collegeof Chiropractic,Bridgeport,Connecticut.Submit reprint requests o: Alex M. Eingom, DC, Better HealthChiropractic,PC, 825 SeventhAve, NewYork, NY 10019.

    PapersubmittedMay 21, 1998; n revised onn July 8, 1998.

    DISCUSSIONTraditionally, autonomic one hasbeenassessedpproxi-mately, on the basisof subjectivedata suchas pupil dilata-tion!4.2S istal skin in temperature!O eart ate, and sympa-thoadrenergicbiochemical criteria such as blood levels ofadrenaline, oradrenaline, nd corticosteroids.26-29The heart rhythm electrocardiographicR-to-R interval

    (RR) has ong beenconsidered o be ndicativeof cardiova-

  • 7/27/2019 Chiropractic and HRV

    2/5

    162 Journal of Manipulative and Physiological TherapeuticsVolume 22 .Number3. March/April 1999Manipulation and Autonomic Tone.Ein8o~

    ~Fig I. Methodofdetennining the RR nterval rhythmogram.(CourtesyofHeart Rhythm nstruments, Inc.

    analysis, and short-duration recordings of 5 minutes that usefrequency domain statistical analysis.Time domain analysis of HRV is based on calculation ofthe beat-to-beat interval measurements between adjacentQRS complexes, tenned the nonnal-to-nonnal (NN) intervalwith the usual electrocardiogram reference being theRR.36.42A statist ical variable that can be applied to timedomain HRV data is the standard deviation of the NN inter-vals. The significance of the result of the standard deviationof the NN interval analysis lies in that it is equal to the totalpower of the spectral analysis and is therefore an estimate ofthe overall HRV. Another time domain method of statisticalanalysis is therefore an estimate of the overall HRV. Anothertime domain method of statistical analysis is the standarddeviation of the average NN intervals. This measurementestimates the long-term or low-frequency (LF) variationswithin the heart rate data. Yet another time domain statisticalanalysis is the ror.mean square of the standard deviation ofsuccessive NN intervals. This measurement estimates theshort-term or high-frequency (HF) variations within theheart rate data. Geometric methods are also useful for analy-sis of time domain HRV data. The density distribution his-togram of NN intervals can be appraised with the HRV tri-angular index, which is also an estimate of the total power ofthe overall HRV.36.42

    Frequency domain analysis of the tachogram yields anestimate of the power spectral density, which reflects howpower, or variance, distributes as a function of frequency.Power spectral density can be calculated by either paramet-ric or nonparametric methods. The nonparametric methodsare simpler and faster, and usually use a fast Fourier trans-fonn. The fast Fourier transfonn analysis quantifies the pro-portion of one frequency relative to that of another frequen-cy within a signal.3DThe parametric methods yield smootherspectral components with easily identifiable central frequen-cies of the LF and HF components and provide an accurateestimation of power spectral density even on a small numberof samples. Short-duration frequency domain recordingsyield very low frequency, LF, and HF spectral components,whereas long-duration recordings yield an additional ultralow frequency component along with the verY low freQuen-~

    gal activity.3o Heart rate variability (HRV) is an analysistechnique that examines these beat-to-beat oscillations inheart rate. This analysis is targeted at what was formerlyconsidered electrocardiographic "noise..'31 Through the useof mathematical analysis, the oscillations can be broken intoa series of periodic components consisting of various fre-quency ranges.31-35 he earliest clinical use of HRV was inobstetrics as a marker for fetal distress.36At present, HRV isin widespread use n the fields of neurology, cardiology, psy-chology, psychophysiology, obstetrics, anesthesiology, andpsychiatry.36 Thus the qualitative analysis of the spectralcomponents inherent within the heart rate is an acceptedmeans of examining the functioning of the sympathetic andparasympathetic divisions of the ANS, which is known toreflect the health status of the individual.32-40

    HRV's usefulness in predicting risks and in early detec-tion of an abnormal physiologic state has been documentedin the literature.38.4o ikewise, the Framingham Heart Studyindicates that ambulatory monitoring of HRV in elderly sub-jects offers prognostic information beyond that of traditionalevaluation methods in relation to mortality risk.41 It shouldalso be noted that the normal heart dynamics, as reflected byHRV analysis, indicate the absence of a disease process andthus the presence of good health.32-35

    Integral to any HRV analysis is the collection of a heartrate rhythmogram by means of an electrocardiogram so thatthe inherent beat-to-beat oscillations, as measured by con-secutive RR intervals (Fig 1), can be readily examined. Foraccurate interpretation of this electrocardiogram tachogram,any ectopic beats or signal interference within the collectedcardiac rhythm data must be removed before performing !be.-various methods of statistical analysis on the data. //

    In 1996, Malik and the task force on HRV42 published athorough review of HRV in the ournal Circula!{on. The arti-cle described in detail the measurement methods and pro-posed standards for HRV analysis. The article also reviewsthe known physiologic interpretation of HRV data with aneye toward clinical application. Malik et al42 discussed theuse of two methods of HRV recording and analysis to stan-dardize physiologic and clinical studies: long-durationrecordings of 24 hours that use time domain statistical~

  • 7/27/2019 Chiropractic and HRV

    3/5

    Journal of Manipulative :md Physiological TherapeuticsVolume 22. Number 3. M:lrch/ApriI1999Manipulation and Autonomic Tone. Eingorn and /.fuJI.S

    163

    Rh ythm StrIp

    a)

    b)

    Fig 2. A, Representative R nterval HRV rhythmogram.B, Spectral unction of the HRV rhythmo-gram. (Courtesyof Heart Rate nstruments.nc.)

    spectrum up to the respiratory frequency of about 0.3 Hz,the HF spectral component is considered to be a combina-tion of both the vagal and sympathetic activity. In contrast,the LF spectral component with a peak of about 0.1 Hz isconsidered to be representative of bursts of sympatheticvasomotor activity.31.42 s noted earlier, the interpretation ofthe very low frequency and the ultra low frequency spectralcomponents of HRV is still unclear. Therefore analysis of

    ~he HF and LF spectral bands layered within the compoundHRV waveform provides an indication of changes in thesympathovagal balance.31.42.43t should also be noted that inaddition to frequency, the amplitude of the spectral functionshould also be considered when analyzing the sympathova-gal interaction.43It is therefore possible to use HRV analysis to calculate aquantitative index of autonomic function that accuratelyreflects the sympathetic and parasympathetic tone, as well asthe sympathovagal balance.31.44.4Sn conjunction with thisthe orthostatic test has been shown to be a reliable method ofANS provocation with predictable normal results.46.47Anorthostatic stress test is used to produce a vasodilatory stim-ulus. A simple slow upright tilt from a supine position to 80degrees or 90 degrees over a minimal period of 10 to 20 sec-onds can be used to elicit an orthostatic response.42.48nsome patients with adrenergic failure, a routine tilt test willnot elicit presyncope or an orthostatic drop response. Fouradditional methods of orthostatic challenge are sublingualtrinitroglycerin, postexercise tilt, lower body negative pres-sure, and a prolonged tilt.48 In addition, some patients willexhibit an orthostatic responseafter a meal or a warm bath.48

    cy, LF, and HF spectral bands. So far, only the LF and HFspectral components produce useful data.42Many of the time and frequency domain analysis methodsyield results that strongly correlate with each other.42This isof clinical significance because the frequency domain mea-sures are easier to perform and are more practical in a clini-cal setting because hey do not require the 24-hour recordingduration of the time domain methods.

    The different spectral components of HRV reflect the beat-to-beat modulation of various oscillators on the sinus node ofthe heart. The sympathetic and vagal efferent activity is aproduct of both the central nervous system oscillators of thevasomotor and the respiratory centers and the peripheral os-cillations of the arterial pressure and the respiratory motion.The heart rate and blood pressure waves that are coupled tothe respiratory frequency are termed Traube-Hering wavesand have a peak period of about 0.3 HZ.31ThI1 waves that arenonrespiratory frequency coupled waves are called Mayer-waves and consist of slower frequencies between O.O~~d0.15 Hz, with a peak period of about 0.1 HZ.30.31 hjlreforeHRV reflects the state and function of the central osFillators,peripheral responses o the central autonomic drive;6y meansof sympathetic and vagal efferent activity, humoral factors,and the state of the sinus node itself.3O.42

    The HF component of the HRV compound waveform (Fig2) can be abolished by a cholinergic blockade with atropine,leaving only the LF component with a peak of about 0.1 Hz.This remaining LF component can then be abolished with asympathetic blockade by propranolol.36 Although the vagalefferent activity is the major contributor to the entire HF

  • 7/27/2019 Chiropractic and HRV

    4/5

    164 Journal of Manipulative and Physiological TherapeuticsVolume 22. Number 3. March/April 1999Manipulation and Autonomic Tone. Eingom and Muhs

    ACKNOWLEDGMENTSWe would like to expressour thanks to Dr AlexanderRiftine of Heart Rhythm Instruments, Inc; Dr AnthonyRosner of the Foundation or Chiropractic Education andResearch; nd Mary Ellen Bowen and Krystyna Kossarskaof the University of BridgeportWahlstromLibrary.

    Perhaps two of the most elegant examples of autonomicassessment with HRV were the presentations of Lipsitz etal49and Pruvot et aI,50 n which the relationship between theparasympathetic and the sympathetic nervous systems innormal subjects and those subjects with vasovagal syncopewere studied by using the orthostatic test to provide provo-cation to the ANS. These studies revealed that patients withvasovagal syncope have an increase n parasympathetic toneon oI1hostatic provocation as reflected in an elevation of HFcomponents of the HRV analysis.

    Various general physiologic correlations have been notedin relation to HRV. Spectral analysis of HRV indicates aprevalent increase in LF during a 90-degree tilt, standing,mental stress, moderate exercise, moderate hypotension,physical activity, and occlusion of the coronary or commoncarotid arteries and is generally higher during the day. Anincrease in the HF component is seen as the result of con-trolled respiration, a cold stimulus to the face, rotationalstimuli, and is generally increased at night.42 Pathologicreduction in HRV has been noted in relation to myocardialinfarction, diabetic neuropathy, cardiac transplantation,myocardial dysfunction, tetraplegia, and in the identificationof individuals with the possibility of increased risk of sud-den cardiac death.42.51

    In the clinical setting HRV analysis must be used in con-junction with other clinical assessment modalities whendetermining the specific diagnosis of a patient's condition. Itis not a replacement for existing clinical evaluation methods.However, as noted earlier, ceI1ain rends in HRV fluctuationsare an accurate means for predicting and identifying thepresence of a serious disorder by a method independent ofthe patient's anecdotal report as to how he or she may bedoing. As such, it is a valuable and practical modality foraiding the clinical decision-making process.42CONCLUSION

    In summary,empirical and early quantitative evidencesuggestSMT may have an effect on a patient's autonomictone by modulationof the central control mechanisms ov-erning suchphysiologicprocesses.nasmuchasHRV analy-sis currently appears o be reliable and accuratemethod ofANS assessmentnd because o published esearchs in thecurrent literature that directly correlatesANS assessmentthrough HRV with either generalor specific SMT, we sug-gest hat further studies n this field may significantly~on-tribute o a betterunderstanding f the effectsof SMT n (I)the generalhealth of an individual, (2) an ndividu 's sus-ceptibility to lowered mmunity and recuperative pacity,and (3) conditions that lie outside the scope of muscu-loskeletal therapeuticsand are more n line with lassicalchiropractic concepts. It may also contribute to moreinformed interprofessional understanding and enhancedcooperation etweenboth allopathicand chiropractichealthcare providers. This includes the possibleeffectivenessofmanipulative ntervention n nonmusculoskeletal isorders,as reflected n the effectsof therapeuticmodulationof auto-nomic one.

    REFERENCES1. Palmer DD. Textbook of the science, art and philosophy of chi-

    ropractic. Portland (OR): Portland Printing House 1910. p.117,408.2. Palmer DD. The chiropractor. Los Angeles: Beacon LightPrinting Company; 1914.3. Leach RA. The chiropractic theories: a synopsis of scientificrese " . 2nd ed. Baltimore: William & Wilkins; 1986.

    4. Wardwe WI. Chiropractic: History and evolution of a newprofession St. Louis: Mosby; 1992.5. Gatterma MI. Foundations of chiropractic: subluxation. St.ouis: sby; 1995. p. 12, 150-68.6. Haratl Y, Machkhas H. Spinal cord and peripheral nervous sys-tem. In: Low PA, editor. Clinical autonomic disorders. 2nd ed.Philadelphia: Lippincott-Raven Publishers; 1997. p. 25-45.7. Guyton AC. Basic neuroscience: anatomy and physiology. 2nded. Philadelphia: WB Saunders: 1991. p. 273-82.8. Kandel ER, Schwartz JH, Jessell TM. Principles of neural sci-ence. 3rd ed. Norwalk (CT): Appleton and Lange; 1991. p.761-8.9. Jarmel MA. Possible role of spinal joint dysfunction on thegenesis of sudden cardiac death. J Manipulative Physiol Ther1989;12:469- 77.10. Jarmel M, Zatkin J. Improvement of cardiac autonomic regula-tion following spinal manipulative therapy. Chiropractic Foun-dation Conference Proceedings; 1995 JuI6-8; Washington (DC).11. Slosberg M. Effects of altered afferent articular input on sensa-tion, proprioception, muscle tone and sympathetic reflexresponses. J Manipulative PhysiolTher 1988;11:181-9.

    12. Celander E, Koenig AJ, Celander DR. Effect of osteopathicmanipulative therapy on autonomic tone as evidenced by bloodpressure changes and activity of the fibrinolytic system. J AmOsteopath Assoc 1968;67:1037-8.13. Purdy WR, Frank JJ, Oliver B. Suboccipital dermatomyotomicstimulation and digital blood flow. J Am Osteopath Assoc1996;96:285-9.14. lIalko EJ, Janouschek C. Effects of osteopathic manipulativetreatment in patients with cervicothoracic pain: pilot studyusing thermography. J Am Osteopath Assoc 1994;94:135-41.15. Grayson MF. Horner's syndrome after manipulation of neck.BMJ 1987;295:1381-2.16. Korr 1M. The neurobiologic mechanisms in manipulative ther-apy. New York: Plenum Press; 1977.17. Giles LGF. Paraspinal autonomic ganglion distortion due to ver-tebral body osteophytosis: a cause of vertebrogenic autonomicsyndromes? J Manipulative Physiol Ther 1992;15:551-5.18. Sato A. The reflex effects of spinal somatic nerve stimulation onvisceral function. J Manipulative Physiol Ther 1992;15:57-61.19. Sato A. Somatovisceral reflexes. J Manipulative Physiol Ther1995;18:597-602.20. Harris W, Wagnon RI. The effects of chiropractic adjustmentson distal skin temperature. J Manipulative Physiol Ther1987;10:57-60.21. Benarroch EE. The central autonomic network. In: Low PA,editor. Clinical autonomic disorders. 2nd ed. Philadelphia:Lippincott-Raven Publishers; 1997. p. 17-23.22. Atsuko H, Hideo 0, Akio S, et al. Somatocardiovascular re-flexes in anesthetized rats with the central nervous system intactor acutely spinalized at the cervical level. Neurosci Res 1995;???q7-~n~

  • 7/27/2019 Chiropractic and HRV

    5/5

    Journal of Manipulative and Physiological TherapeuticsVolume 22 . Number3. March/April 1999Manipulation and Autonomic Tone. Eingorri and Wuhs

    165

    23. SatoA. Neural mechanisms f autonomic esponses licited bysomatic sensorystimulation. Neurosci Behav Physiol 1997;27(5):610-21.24. Briggs L, Boone WR. Effects of chiropractic adjustmentonchangesn pupillary diameter:a model or evaluatingsomato-visceral esponse. ManipulativePhysiolTher 1988;11: 81-9.25. VandertopWP, de Vries WB, NotermansNC, Tulleken CAF,Gispen WHo Beneficial effect of an ACTH4-9 analogueonexperimentally nduceddiabetic autonomicneuropathy n theeye of the rate under generalanaesthesia. Auton Nerv Syst1995;5149-57.26. Woo MA, Stevenson WG, Moser DK, Middlekrauf HR.Complexheart ate variability andserumnorepinephrineevelsin patients with advancedheart failure. J Am CoIl Cardiol1994;23:565-9.27. aka H, Mochio S, SatoK, et al. Spectralanalyses f R-R nter-val and systolic blood pressuren diabeticautonomicneuropa-thy. JAuton Nerv Syst 1995;52:203-11. -~28. Ikuta Y, Simoda 0, Kano T. Quantitative assessment f theautonomicnervoussystemactivities during atropine nducedbradycardiaby heart ate spectralanalysis.J Auton Nerv Syst1995;52:71-6.29. WarnerMR. Time-courseand requencydependence f sym-pathetic stimulation evoked nhibition of vagal effects at thesinusnode.J Auton Nerv Syst 1995;52:23-33.30. NovakV, Novak P,Low PA.Time frequencyanalysisof cardio-vascular unction and ts clinical applications. n: Low PA, edi-tor. Clinical autonomic disorders. 2nd ed. Philadelphia:Lippincott-RavenPublishers;1997.p. 323-48.3 . Karemaker M. Analysis of blood pressure nd heart ate vari-ability: Theoreticalconsiderations.n: Low PA, editor.Clinicalautonomicdisorders.2nd ed. Philadelphia:Lippincott-RavenPublishers;1997.p. 309-22.32. GoldbergerAL. Fractals and he heart. Konink linjke Neder-landsevanWetenschappen990;93:409-18.33. GoldbergerAL, Rigney DR. Suddendeath is not chaos. n:Krasner S, editor. The ubiquity of chaos.Vol. 3. Washington:AAAS Press;1990.p. 23-34.34. GoldbergerAL. Fractalelectrodynamics f the heartbeat. nnAcad Sci 1990;591:402-9.35. GoldbergerAL, Rignery DR, West BJ. Chaos and fractals nhumanphysiology.Sci Am 1990;262(2):43-9.36. FreemanR. Noninvasiveevaluationof heart rate variability:the time domain. n: Low PA, editor.Clinical autonomicdisor-ders.2nd ed. Philadelphia:Lippincott-RavenPublishers;1997.p. 297, 298, 307.37. MolgraadH, HenllansenK, Bjerregaard.Spectralcomponentsof short-tenll R-R interval variability in healthy subjectsandeffectsof risk factors.Eur HeartJ 1994;15:1174-83.38. Itoh H, TakedaK, TanakaM, et al. Youngborderlinehyperten-sives are hyperreactive o mental arithmetic stress: spectralanalyses f R-R intervals.J Auton Nerv Syst 1995;~4: 55-62.

    39. lnohue K, Ogata H, Hayano J, Miyake S, Kamada T, Kuno M,et al. Assessment of autonomic function in traumatic quadri-plegic and paraplegic patients by spectral analysis of heart ratevariability. J Auton Nerv Syst 1995:54:225-34.40. Furlan R, Piazza S, Belivacqua M, Turiel M, Norbiato G,Lombardi F, et al. Pure autonomic failure: complex abnormali-ties in the neural mechanisms regulating the cardiovascularsystem. J Auton Nerv Syst 1995;51 223-35.41. Tsuji H, et al. Reduced heart rate variability and mortality in anelderly cohort: the Framingham Heart Study. Circulation1994;90:878-83.42. Malik M, et al. Heart rate variability: standards of measure-ment, physiological interpretation, and clinical use. Circulation1996;93:1043-65.43. Montano N, et al. Power spectrum analysis of heart rate vari-ability to assess he changes in sympathovagal balance during1ddedorthostatic ilt. Circulation 1994;90:1826-31.44. K in EM, Riftine AD, et al. Application of automated ystemsfor ssessinghe functional states f the humanorganism,Part1: nalysisof indicesof the functional states f personsn var-. us professions (white-and blue-collar workers). HumanPhysiology.Acad SciencesUSSR 1991;17(1):126-31.45. Kazin EM, Riftine AD, et al. Automatedsystems or the com-plex evalliation of the health and adaptive reservesof thehuman body. Human Physiology. Acad Sciences USSR1990;16(3):94-9.46. Sneddon F, CounihanAJ, Bashiry, HaywoodGA, Ward DE,CammAJ. Assessment f autonomic unction n patientswithneurally mediated yncope: ugmented ardiopulmonary aro-receptor responses o graded orthostatic stress.J Am CoilCardioI1993;21:1193-8.47. Shakespeare F, Katritsis D, Crowther A, Cooper C, ColtariJD, Webb-PeploeMM. Differences n autonomicnerve unc-tion in patients with silent and symptomatic myocardialischemia.Br HeartJ 1994;7122-9.48. Low PA. Laboratoryevaluation f autonomic unction. n: LowPA, editor. Clinical autonomicdisorders. nd ed. Philadelphia:Lippincott-RavenPublishers;1997.p. 179-208.49. Lipsitz LA, Mietus J, Moody GB, GoldbergerAL. Spectralcharacteristics f heart rate variability before and during pos-tural tilt: relations to aging and risk of syncope.Circulation1990;81:1803-10.50. Pruvot E, Vibe K, Vesin JM, SchlapferJ, FromerM, Kappen-bergerJ. Autonomic mbalanceassessedy heart ate variabil-ity analysis n vasovagal yncope.PACE PacingClin Electro-physioI1994;17:2201-6.51. Jarmel ME. Identifying and treating apparentlyhealthy ndi-viduals at increased isk for sudden ardiacdeath.ChiropractFamPract 1992;17(3):1-9.