Incidence and Hemodynamic Characteristics of Near-Fainting in Healthy 6- to 16-Year Old Subjects

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1 JACC VoL 25, No SYNCOPE Incdence and Hemodynamc Characterstcs of Near-Fantng n Healthy 6- to 16-Year Old Subjects CATHERINE C. E. DE JONG-DE VOS VAN STEENWIJK, MD,*t WOUTER WIELING, MD, PI4D,t JUDITH M. JOHANNES, KAREL H. WESSELING, PHD~ Utrecht and Amsterdam, The Netherlands MD,t MARK P. HARMS, MD,t WIETSE KUIS, MD, PHD,* Objectves. We studed the ncdence and hemodynamc characterstcs of near-fantng under orthostatc stress n healthy "chldren and teenagers. Background. Orthostatc stress testng s ncreasngly used to dentfy young subjects wth unexplaned syncope. However, the assocated ncdence of syncope and hemodynamc responses n normal young subjects are not well known. Methods. Eghty-four healthy subjects 6 to 16 years old performed forced breathng, stand-up and 7 tlt-up tests. An ntravenous lne to sample blood for bochemcal assessment of sympathetc functon was ntroduced between the stand-up and tlt-up tests. Fnger arteral pressure was measured contnuously. Left ventrcular stroke volume was computed from the pressure pulsatons. Results. Sxteen of the 84 subjects were excluded because of techncal problems. The ncdence of a near-fantng response n the remanng 68 subjects was 1% (7 of 68) for the stand-up test and 4% (29 of 68) for the tlt-up test. Baselne parasympathetc and sympathetc actvty of nonfantng and near-fantng subjects was not dfferent. Near-fantng was characterzed by attenuated systemc vasoconstrcton and exaggerated tachycarda that occurred as early as 1 mn after return to the uprght poston. On tlt-up, plasma adrenalne levels ncreased by a factor of 2, wth slghtly hgher ncrements n the near-fantng subjects. Conclusons. Inadequate vasoconstrcton s the common underlyng mechansm of near-fantng n young subjects. The remarkably hgh ncdence of near-fantng durng the tlt-up test after ntravaseular nstrumentaton rases serous doubts about the utlty of ths procedure n evaluatng syncope of unknown orgn n young subjects. (J Am Col Cardol 1995;25: ) Fantng orgnates from a precptous declne n blood pressure wth an nadequate flow of blood to the bran and s assocated wth reflex systemc arterolar vasodlaton and cardac vagal slowng (1-4). In chldren and teenagers fantng s a relatvely common but poorly documented phenomenon. We nvestgated healthy young subjects ntended to serve as a control group for studes dealng wth true autonomc and cardovascular dysfuncton n chldren and teenagers. The present study reports the hgh ncdence and hemodynamc characterstcs of near-fantng n these healthy subjects durng tlt-up testng wth ntravascular nstrumentaton for blood samplng. These data are needed because of the ncreased use of tlt-table testng n young subjects wth syncope of unknown orgn (5,6). From the *Unversty Hosptal for Chldren and Youth, "Het Wlhelmna Knderzekenhus," Utrecht; and?department of Internal Medcne and TNO BoMedcal Instrumentaton, Academc Medcal Centre, Amsterdam, The Netherlands. Ths study was supported n part by grants from the Foundaton for Chldren wth a Chronc Dsease n Relaton to Exercse, Utrecht and the Rutnga-van Sweten Foundaton, Amsterdam, The Netherlands. Manuscrpt receved June 13, 1994; revsed manuscrpt receved January 12, 1995, accepted January 26, Address for corresnondence: Dr. Catherne C. E. de Jong-de Vos van Steenwjk, Unversty Hosptal for Chldren and Youth, "Het Wlhelmna Knderzekenhus" Neuwe Gracht t37, 3512 LK Utrecht, The Netherlands. Methods Subjects. Eghty-four young subjects (48 grls, 36 boys; 6 to 16 years old) were studed at the Unversty Hosptal for Chldren and Youth, "Het Wlhelmna Knderzekenhus." They were recruted from prmary and hgh schools n the cty and regon of Utrecht, The Netherlands. M1 subjects were n good health, dd not smoke, ate a normal det wthout salt restrcton, used no medcaton and dd not vst a hosptal clnc regularly. They had no hstory of postural complants or frequent fantng. Sxteen grls and nne boys had physcal sgns of puberty accordng to the crtera of the Tanner score system. Subjects and parents gave wrtten nformed consent. The study protocol was approved by the Ethcs Commttee of the hosptal. When t appeared that the ncdence of near-fantng was unexpectedly hgh, we communcated our fndngs to the Ethcs Commttee, after consultaton t was decded to complete the study. Measurements. Blood pressure was measured contnuously by nonnvasve recordng of fnger arteral pressure wth an Ohmeda 23 Fnapres montor (7). Ths nstrument measures blood pressure changes durng orthostatc stress testng as well as durng hypotensve perods wth suffcent accuracy (8,9). A black Ohmeda cuff was wrapped around the rght mddle fnger. Cuff sze was chosen accordng to the manufac by the Amercan College of Cardology /95/$ (95)56-A

2 1616 D~ JONG-DE VOS VAN STEENWIJK ET AL. JACC Vol. 25, No. 7 NEAR-FAINTING IN HEALTHY YOUNG SUBJECTS turer's recommendatons. The cuffed fnger was held at heart level to avod hydrostatc pressure errors. An electrocardogram was obtaned (Hewlett-Packard model 7895A) and connected to a cardotachometer to obtan nstantaneous heart rate responses. Blood pressure, heart rate and an event marker were dsplayed on a Nhon Kohden WS 681G thermal array recorder (Tokyo, Japan) to montor cardovascular changes durng the experments. The sgnals were dgtzed on-lne and stored n a personal computer for off-lne analyss. Free catecholamne levels were determned radoenzymatcally (1). The nterassay coeffcents of varaton for noradrenalne and adrenalne levels determned prevously (1) were 1.1% and 1.3%, respectvely. The ntraassay coeffcents of varaton for noradrenalne and adrenalne levels were 4.1% and 5.4%, respectvely. Protocol. At least 1 h before the start of the study, the subjects consumed a lght breakfast wthout coffee or tea. The nvestgaton began at 9 AM n a room wth a constant ambent temperature of 22 C. The subject's age, gender, heght and weght were recorded. A general medcal nvestgaton was performed next, and the results were normal n all subjects. The subjects then had a 3-mn perod to become famlar wth the equpment and the maneuvers. Maneuvers. The experment began at about 1 AM and was always performed n the same order. Forced breathng and stand-up tests were performed frst as an ntal evaluaton of neurocardovascular functon (11). Ths part of the protocol was performed wthout the venous catheter because ntravascular nstrumentaton may tself nfluence the responses studed (1). Next, a venous catheter was nserted, and the tlt-up test was performed. Tlt-up testng s preferred to stand-up testng because the expermenter has more control and allows rapd return to the supne poston n case of mpendng syncope n the uprght poston. Forced breathng. After 5 mn of supne rest, the subjects performed sx maxmal nspratons and sx expratons at a rate of sx breaths a mnute to evaluate cardac vagal control (11). Stand-up testng. After another 5 mn of supne rest, subjects were gven a verbal command to stand up. The subjects stood up n 2 to 3 s and remaned standng for 5 mn. If premontory symptoms or sgns of near-fantng occurred (see later), or f an abnormally large decrease n blood pressure was observed, the subjects were nstructed to le down mmedately. Venpuncture. A 2-gauge polyethylene catheter was placed n the antecubtal ven of the left arm. Local anesthesa was not used. The subjects then watched a calmng nature move n the supne poston. After 3 mn, a 1-ml venous blood sample was collected for bochemcal assessment of sympathetc functon. After blood samplng, another 5-mn rest perod was nsttuted to obtan baselne values of blood pressure and heart rate. Tlt-up testng. A 7 tlt-up test was then performed for 3 s on a tlt table wth foot support. After 5 mn n the head-up poston, a second venous sample was taken. Because of the tme needed for blood samplng, subjects remaned n the uprght poston for 6 to 7 mn. However, subjects who experenced near-fantng were tlted back mmedately, and a second blood sample was obtaned mmedately after tlt-back. Premontory symptoms and sgns of near-fantng ncluded nausea, abdomnal dscomfort, lght-headedness, weakness, blurred vson, pallor, yawnng, sghng and sweatng. A persstent decrease n blood pressure on orthostatc stress >2 mm Hg for systolc blood pressure or >5 mm Hg for dastolc blood pressure was consdered abnormal (11). Postural tachycarda was defned as an ncrease n heart rate above ts age-adjusted 95% confdence nterval of >35 beats/mn after 2 ran of standng (11). Analyss of data. Heart rate varaton durng forced breathng was expressed as the nspraton-expraton dfference n beats/mn and used to assess efferent vagal cardac control (11). We evaluated the recordngs contnuously durng a 3-s supne control perod before and durng the frst 3 s mmedately after the start of the two orthostatc stress tests. Durng the 5 mn n uprght posture, we also evaluated 1-s averages of varables at the end of each mnute. Durng the tlt-up test we addtonally analyzed the last mnutes before termnaton of the test. The arteral pressure sgnal was analyzed by software developed by the physology department of the Academc Medcal Centre (12). The fnger arteral pulsewave was analyzed by a pulse contour method that computes changes n left ventrcular stroke volume from the pulsatle systolc area. We used the mproved pulse contour method of Wesselng et al. (13) n whch pulsatle systolc area s consdered proportonal to stroke volume. The proportonalty constant s the aortc characterstc mpedance, and ts value s taken as the group mean statstc only, not for each ndvdual. The method therefore lacks absolute ndvdual calbraton, but changes from baselne set at 1% can be determned wth precson (12-16). Cardac output was computed as the product of stroke volume and heart rate. Systemc vascular resstance was computed as mean blood pressure dvded by cardac output. A 5-s movng average was used n the computaton of systemc vascular resstance to account for the delays between changes n nstantaneous cardac output at the aortc root and ther transformaton to changes n tssue perfuson flow due to the Wndkessel bufferng effect of the arteral system (15). Statstcs. Results are expressed as dfferences from baselne (supne) measurements for heart rate n beats/ran; for systolc and dastolc blood pressure n mm Hg; for stroke volume, cardac output and total systemc vascular resstance as percent change. Pared and unpared Student t tests were performed when approprate. As a correlaton measure, we used the Pearson product moment correlaton (r). Results are presented as mean value _ SD; p <.5 was consdered sgnfcant. Results In 16 of 84 subjects fnger arteral pressure measurement on stand-up or tlt-up testng was of poor qualty as a result of

3 JACC Vol. 25, No. 7 DE JONG-DE VOS VAN STEENWIJK ET AL NEAR-FAINTING IN HEALTHY YOUNG SUBJECTS Table 1. Supne and Hemodynamc Value Changes After 1 mn of Stand-Up and 1 mn of Tlt-Up Testng Stand-Up Test Tlt-Up Test Near-Fantng Near-Fantng Nonfantng Group Group Nonfantng Group Group (n - 61) (n = 7) (n = 39) (n 29) Gender (M/F) 33/28 3/4 24/15 12/17 Supne SBP (mm Hg) 12 (13) 99 (12) 112 (3) 19 (11) Supne DBP (ram Hg) 51 (9) 49 (5) 59 (9) 57 (6) Supne HR (beats/mn) 75 (9) 78 (6) 74 (11) 72 (9) ASBP 1 (11) 3 (12) 8 (12) 2 (1)* zxdbp 15 (7) 1 (7)t 17 (8) 11 (9)* 2~HR 19 (1) 26 (8)1" 15 (1) 25 (11)* SV (% change) -35 (9) -36 (7) -38 (1) -37 (7) CO (% change) -19 (9) -13 (1) -25 (1) -16 (11):1: SVR (% change) 46 (18) 27 (15) 6 (26) 34 (2) *p <.5,?p <.5, :~p <.1, p <.1, nonfantng versus near-fantng subjects. Data presented are mean value (SD) or number of patents. CO = cardac output; DBP = dastolc blood pressure; F = female; HR = heart rate; M - male; SBP = systolc blood pressure; SV = stroke volume; SVR = systemc vascular resstance; A absolute dfference from supne level; % change = percent change to supne level. artfacts caused by movements of the fnger or hand. These subjects were excluded from analyss. In 4 of the remanng 68 subjects nserton of the ntravenous lne was not successful for techncal reasons. However, these 4 subjects remaned n the study group. Of the 16 subjects excluded from the study, 1 developed a near-fantng response durng stand-up testng and 2 durng tlt-up testng. Stand-up testng. After 3 to 5 mn of stand-up testng, near-fantng was observed n 7 of the 68 subjects (3 grls, 4 boys), of whom 6 had premontory symptoms or sgns and a large decrease n blood pressure. Only one subject had a large decrease n blood pressure wthout sgns or symptoms. None of the subjects actually fanted. Age, gender and supne control values for blood pressure and heart rate dd not dffer between the near-fantng and nonfantng groups (Table 1). Heght (146 _+ 9 cm n the near-fantng group, 153 _+ 15 cm n the nonfantng group) dd not dffer, but body weght was sgnfcantly lower n the near-fantng group (35 _+ 5 vs. 44 _+ 14 kg, p <.5). In general, the ntal crculatory patterns were smlar n the near-fantng and nonfantng groups (Fg. 1). However, dfferences n some hemodynamc responses to the orthostatc stress of actve stand-up testng could be observed wthn 2 s after the onset of the maneuver. After 1 mn of standng, dastolc blood pressure n the near-fantng group was sgnfcantly lower and heart rate hgher (Table 1). Systemc vascular resstance ncreased n both groups n response to stand-up testng. In the non-fantng group the average ncrease was 46% after 1 ran of standng, whereas n the near-fantng group the ncrease was only 27% (p <.1). No dfferences n cardac output were observed. Durng contnued standng, the dfferences n arteral blood pressure and systemc vascular resstance contnued to ncrease slowly. The dfference n heart rate remaned unchanged durng prolonged standng. In 2 of 7 subjects n the near-fantng group and n 5 of 61 n the nonfantng group, postural tachycarda was observed. Venpuneture. None of the subjects developed a nearfantng response durng the nserton of the ntravenous lne or durng the supne samplng of blood. Tlt-up testng. After 1 to 7 ran of tltng, 29 (12 grls, 17 boys) of the 68 subjects developed a near-fantng response. In the seven subjects wth near-fantng durng stand-up testng, near-fantng also occurred durng tlt-up testng. In the four subjects that dd not receve an ntravenous lne, near-fantng was not observed. In 26 of the 29 near-fantng subjects, a large decrease n blood pressure was accompaned by premontory symptoms or sgns. In three subjects a large decrease n blood pressure wthout premontory symptoms or sgns was observed. None of the subjects actually fanted. As n the stand-up test, supne control values, gender, age and heght dd not dffer. In contrast to the stand-up test, there was also no dfference n body weght. The nspraton-expraton dfferences durng forced breathng were almost dentcal at 28 _+ 6 beats/mn n the near-fantng group and at beats/mn n the nonfantng group. The characterstc blood pressure and systemc vascular resstance decrease observed n the frst 2 s of actve standng (Fg. 1) were attenuated or not present on tlt-up testng (Fg. 2). These dfferences n the ntal crculatory adjustments between actve standng and passve tltng are well known (15,17) and are not dscussed further. The hemodynamc devatons from control n the near-fantng group after 1 mn n the tlt-up poston were smlar to those durng stand-up testng; blood pressure ncreased less and heart rate ncreased more than n the nonfantng group (Table 1). The decrease n cardac output n the near-fantng group after 1 ran of tlt-up testng was sgnfcantly less than that n the nonfantng group (p <.1). Systemc perpheral resstance ncreased n both

4 1618 D~ JONG-DE VOS VAN STEENWIJK ET AL. JACC Vol. 25, No. 7 NEAR-FAINTING IN HEALTHY YOUNG SUBJECTS =: 2 E ~s 1 I E Q,. (n 2 1 o 5 Q- ta 2 (1. 2 Q. 1 5 E Q,. 1 5O I I 2O t,o 1 5 ::b t~ 1 5 2O 2O I I 1 t,.) 5 2O 1 5 I I,,, Tme (see) Fgure 1. Characterstc hemodynamc responses to stand-up testng are plotted as group averages for 61 nonfantng subjects (sold lne) and 7 near-fantng subjects (dashed lne). Systolc blood pressure (SBP), dastolc blood pressure (DBP) and heart rate (HR) are expressed n absolute values, whereas stroke volume (SV), cardac output (CO) and systemc vascular resstance (SVR) are expressed n relatve changes to supne control values. The maneuver started at s. None of the subjects had premontory sgns or symptoms of nearfantng wthn the frst 3 ran after the onset of stand-up testng. bpm = beats/mn. groups but only 34% n the near-fantng group compared wth 6% n the nonfantng group (p <.1). Eght of the 29 subjects n the near-fantng group developed postural tachycarda compared wth 4 of the 39 subjects n the nonfantng group. In the last mnute before tlt-back (Fg. 2) n the t~ to) 1 5 2O IO 5O, II,, Tme {see) Fgure 2. Crculatory responses to tlt-up testng n 39 nonfantng subjects (sold lne) and 29 near-fantng subjects (dashed lne). The maneuver started at s. Near-fantng started after 1 ran of tltng. Vertcal dashed lne = last 6 s before tlt-back. Abbrevatons as n Fgure 1. near-fantng group, blood pressure levels rapdly decreased, heart rate no longer responded wth an ncrease but decreased to a level below that n the nonfantng group. The combnaton of a constant stroke volume and a decrease n heart rate resulted n a decrease n cardac output. At the moment of tlt-back, systolc blood pressure n the near-fantng group had decreased to 71 _+ 12 mm Hg and dastolc blood pressure to mm Hg, whereas n the nonfantng group systolc blood pressure had ncreased to 118 _+ 18 mm Hg and dastolc blood

5 JACC Vol. 25, No. 7 DE JONG-DE VOS VAN STEENWIJK ET AL NEAR-FAINTING IN HEALTHY YOUNG SUBJECTS Table 2. Plasma Noradrenalne and Adrenalne Levels n Nonfantng and Near-Fantng Subjects (mean -+ SD) Noradrenalne (nmol/lter) Adrenalne (nmol/lter) Near- Near- Nonfantng Fantng Nonfantng Fantng Group Group Group Group (n : 36) (n : 28) (n : 36) (n : 28) Supne 1.7 (.62) 1.52 (.59).16 (.11).14 (.11) Standng 2.62 (.75) 2.3 (.7).34 (.23).49 (.48) Increase.93 (.71).78 (.67).17 (.19).35 (.43)* *p <.5, near-fantng versus nonfantng subjects. pressure to 73 +_ 12 mm Hg (all p <.5). At the moment of tlt-back, systemc vascular resstance had decreased to the supne control level n the near-fantng group but remaned 31% above control levels n the nonfantng group (p <.1). Bochemcal assessment of sympathetc functon. Supne plasma noradrenalne levels n nonfantng and near-fantng subjects dd not dffer, and an dentcal ncrease after 5 ran of tlt-up testng was observed (Table 2). Supne plasma adrenalne levels were nearly dentcal, but n the near-fantng group the ncrement from supne to uprght was sgnfcantly hgher than that n the nonfantng group, wth very pronounced ncreases n two near-fantng subjects (uprght levels 1.5 and 2.3 nmol/lter, respectvely). The correlaton between changes n plasma adrenalne levels and n systemc vascular resstance at the moment of tlt-back was negatve and sgnfcant (r = -.28, p <.5) but poor (r 2 =.8 [.e., 8% of the varance explaned]). Dscusson Incdence and predctve ndexes of near.fantng. Fantng s a common phenomenon n young subjects but s rare n adulthood (1,2,18). It has been estmated that as many as 15% of young subjects wll have a fantng experence before reachng adulthood (19). There s no explanaton for the hgh ncdence of fantng n chldren. In two recent studes (6,17) dealng wth the ncdence of fantng under orthostatc stress n 42 healthy chldren and teenagers, a 17% near-fantng response was observed. When the shorter duraton of our test (5 vs. 1 to 2 mn) s consdered, the ncdence of near-fantng responses of 1% n the stand-up test n our study s n accordance wth prevous studes. The absence of any ncdence of near-fantng n two other recent studes (2,21) can probably be explaned by the small number of young subjects nvolved. In young patents wth a suspected condton called neurocardogenc syncope (22), the ncdence of near-fantng durng orthostatc stress ranged from 43% to 57% (6,2,21). A slghtly hgher ncdence was calculated n a recent comprehensve data revew (23). The hgh ncdence (4%) of near-fantng n our study durng a tlt-up test of only 5 mn n duraton can be attrbuted to the ntravascular nstrumentaton and blood samplng ac- companyng the tlt-up testng (emotonal fant response) (1,2,24). Prevous nvestgators (1) have also demonstrated that nvasve methods predspose subjects to fantng durng orthostass. Stevens (25) found a near-fantng response n 41 (5%) of 82 healthy men (23 to 43 years old) durng a 2-mn tlt-up test wth nstrumentaton compared wth 15 (1%) of 152 subjects wthout nstrumentaton (25). We cannot exclude the possblty that the passve change n posture contrbuted to the hgh ncdence of near-fantng. However, ths factor s not expected to be of great consequence because the statc ncrease n skeletal muscle tone durng passve 7 tltng and quet standng s lkely to be smlar. Moreover, orthostatc responses on stand-up and 7 tlt-up testng are reported to be comparable n young adults (1). A separate study wll be needed to determne whether actve or passve changes n posture wthout nstrumentaton evoke dfferent orthostatc responses n chldren and teenagers. The present fndngs mply that the use of tlt-up testng n combnaton wth ntravascular nstrumentaton as a dagnostc procedure n young patents wth unexplaned syncope wll result n many false postve test results. Thus, a postve tlt-table test result cannot be nterpreted as sold proof of an nnocent cardonhbtory fantng response as the underlyng cause of the syncope. In agreement wth prevous studes n healthy young subjects (17), no sgnfcant dfferences n ncdence of nearfantng responses were observed between genders. Physcal heght, n contrast to a prevous report (1), was not a predctve ndex for orthostatc tolerance n our study. Body weght correlated wth near-fantng only n the actve stand-up test before venpuncture. Thus, gender and body buld appear to have at most only a small nfluence on orthostatc tolerance n normal young subjects. Heart rate varatons durng forced breathng dd not dffer between near-fantng and nonfantng subjects and were normal for age (11,26). Thus, we could not confrm wth ths test a recent observaton (27) that fant-prone young subjects have a hgh vagal actvty. Control values for blood pressure, heart rate and plasma catecholamne levels were comparable n the near-fantng and nonfantng groups, agan suggestng smlar average basal sympathetc outflow before the orthostatc stress test. Hemodynamc characterstcs of near-fantng. The key factor responsble for the gradual decrease n arteral pressure before an mpendng fant was early progressve decrease n systemc vascular resstance (Fg. 1 and 2, Table 1). However, ths phenomenon s not easly explaned because there s strong crcumstantal evdence that sympathetc outflow before the actual fant s hgh (28-31). It has been suggested that a reflex trggered by ventrcular receptors stmulated by the combnaton of a low ventrcular volume and a hgh notropc state s nvolved n vasovagal fantng (2,22). Such a reflex mechansm could theoretcally be responsble for the progressve decrease n systemc vascular resstance. However, we dd not fnd a pronounced decrease n stroke volume n the early phase of near-fantng. Moreover, the lnk between ventrcular receptor

6 162 D~ JONG-DE VOS VAN STEEN!VIJK ET AL. JACC Vol. 25, No. 7 NEAR-FAINTING IN HEALTHY YOUNG SUBJECTS June ~1995: actvaton and neurally medated syncope has not been frmly establshed (2,4,24,32). Adrenalne has been proposed as a humoral vasodlatng agent n the vasovagal response (3,33) and has been reported to be elevated durng fantng nduced by tlt-up testng n adults (3,3,33,34). In the present study the ncrease n venous plasma adrenalne levels after 5 ran n the uprght poston was marked (Table 2). However, pronounced ncreases n adrenalne levels were also found n the nonfantng group, and the correlaton between adrenalne level and systemc vascular resstance responses n both groups was poor. On the whole, the data suggest that the elevated uprght adrenalne level reflects arousal (35) but does not act as an mportant medator of fantng. The forearm plasma noradrenalne samples taken from nonfantng and near-fantng subjects at the moment of nearfantng dd not dffer sgnfcanty. Ths seems nconsstent wth the fndng that n muscle sympathetc nerve recordngs n adults, sympathetc slence has been observed at the moment of a near-fantng, ndcatng complete cessaton of sympathetc drve to skeletal muscle vessels (2,28-3). A strkng reducton n noradrenalne spllover to plasma s also noted n other major vascular areas, such as the mesenterc, renal and cardac beds (35,36). Ths nconsstency was noted earler (33,34) and has been attrbuted to two factors: 1) Changes n plasma levels of noradrenaln are too sluggsh to document sudden changes n sympathetc nerve frng rates (4,34,35); and 2) not only adrenalne (Table 2), but also noradrenalne s released from the adrenal medulla durng hypotensve stress (34,37). Conclusons. The mechansm leadng to near-fantng n healthy young chldren and teenagers s complex and uncertan and needs to be consdered n any nvestgaton of the role of the autonomc nervous system n pathologc condtons. 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