THE APPLICATION OF COMPOUND EMERGENCY INDICATOR AS A SOFTWARE ALTERNATIVE FOR THE PATIENT BUTTON IN LONG-TERM CARDIAC MONITORS. Augustyniak P.

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1 THE APPLICATION OF COMPOUND EMERGENCY INDICATOR AS A SOFTWARE ALTERNATIVE FOR THE PATIENT BUTTON IN LONG-TERM CARDIAC MONITORS Augustyniak P. AGH Univrsity of Scinc and Tchnology, 30, Mickiwicz Av Krakow Poland, mail: august@agh.du.pl Sarching for th conomy and opportunitis for applications of th human xprinc yildd a dvlopmnt of a rqust-drivn ECG intrprtation mthod. It assumd that th procssing of th acquird ECG is triggrd by th xpiry of data validity priod or by th mrgncy dtctor continuously suprvising th basic paramtrs of th signal. This papr focuss on th corrct choic of th mrgncy dtctor procdur covring a wid rang of disass but not rquiring much computational powr in th patint-sid warabl systm. Aftr th invstigation of th numrical complxity for initial stags of ECG procssing and of th rprsntation of most common disass in basic cardiac paramtrs, w propos th us of a compound rhythm- and contour-basd indx as th mrgncy indicator. Th computation is simpl nough to b prformd in a limitd-rsourcs nvironmnt (.g. cll phon) without considrably influncing its autonomy tim. INTRODUCTION Th dvlopmnt of a thortical background and th xprimntal dtrmination of validity tim valus for particular diagnostic paramtrs of th ECG ld to th implmntation of th on-dmand intrprtation of rcords. This mod shows its particular advantags in cardiac monitoring ntworks. Th avoidanc of unncssary data procssing and transmission lads to a considrabl rduction of powr consumption and to th xtnsion of autonomic oprating tim. Th additional bnfit coms from th managmnt of th datastram transmittd to th suprvising cntr through th wirlss digital link [1]. In th on-dmand procssing schm, th signal intrprtation in th rmot rcordr is initiatd by th xpiry of prvious data validity or by th alrt from th mrgncy dtctor. This ida mimics th natural rlation btwn th patint and his doctor. It assums th schduling of xaminations in th mdically justifid tim intrvals (priodic xaminations) or as a rsult of dtrioration of th patint's status subjctivly prcivd as pain or far. Th validity tim is dtrmind for particular ECG diagnostic paramtrs on th basis of currnt valus bing componnts of th mdical dscription of th patint. In this contxt, with rgard to th diagnostic rlvanc of paramtrs, th survillanc systm automatically builds a hirarchical list of valus rquird to b calculatd with thir tmporal attributs dtrmining nxt tim points of th rquird updat. In cas th mdical dscription of th patint is corrct and his or hr ral status gts wors suddnly, rlativly long validity tim spcifid for particular diagnostic paramtrs caus th loss of monitoring continuity. This scnario, vry probabl in th cardiac monitoring, rquirs th prompt updat of th patint dscription in th systm vn if th validity intrvals - basd on th prvious valus of diagnostic paramtrs - ar not yt xpiring. This functionality is supportd by th patint's (vnt) button in traditional long-trm (Holtr) rcordrs and in looping vnt rcordrs it is additionally usd for triggring th acquisition and intrprtation of th signal strip [2]. Th us of th patint button dpnds on subjctiv flings (pain or far), oftn having th origin indpndnt of th rcordd pathology. On th othr hand, som lif-critical vnts may happn without clar symptoms and thus will b missd in th vnt st. Although th implmntation of th patint button and its functionality is rquird by th intrnational standards [3], in author's opinion it should not b lft as th uniqu option for initiating of th data acquisition and intrprtation. Thrfor, w postulat to mbd th mrgncy dtctor in th rcordr softwar. Its rol is to dtct th patint's status This work was supportd by th AGH Univrsity of Scinc and Tchnology in th yars undr th grant no

2 dtrioration and triggring of th acquisition and intrprtation of a signal strip. Stting th long validity tim for diagnostic paramtrs would no longr prvnt th immdiat adaptation of th monitoring systm to th suddn changs of th patint's status. Th proposd solution consists in a continuous limitd-rang signal intrprtation, as ncssary for th dtction of th mrgncy. Th functionality of th dtctor mimics th patint's flings of pain and is orintd to spot th abnormality. Unlik th pain, th automatic dtction and rcording of vnts is th objctiv and rproducibl procss, and thrfor is mor rliabl than th patint button. Additionally, th dtctor dos not rquir any intraction from th monitord prson. Th mrgncy dtctor consists of th slctd subst of th ECG intrprtation subroutins. Th slction is basd on th compromis btwn two contradictory critria: - support of a wid rang of pathologis causing th alrt, - low computational complxity, th us of only th initial stps of ECG intrprtation procss hlps avoiding th rror propagation and improvs th dtction rliability. For initial stags of th on-dmand ECG intrprtation tsting, th mrgncy dtctor was basd on th absolut valu of succssiv RR intrval diffrncs [4]. It was proposd as a rsult of studis on th mdical standards and consultd with th cardiologists. In cours of furthr rsarch it was ncssary to optimiz th ratio of th dtction quality to th computational complxity which is th main subjct of this papr. a) b) Fig.1. Dtction of th mrgncy; (a) diagnostic paramtrs validity tim ar long basd on th prvious st of diagnostic rsults; th mrgncy dtctor capturs th chang of patint's status and initiats th intrprtation of currnt signal; (b) th valus of diagnostic paramtrs rprsnt th pathology and shortn th corrsponding validity tim what rsults in a mor frqunt intrprtation of th signal. METHODS A. Estimation of th complxity for a diagnostic procdur Common complxity stimators may b usd for th assssmnt of computing powr ncssary to prform th dtction of mrgncy [5]. In ral applications, th ECG intrprtation uss various algorithms and oftn particular procssing stags ar difficult to b distinguishd. Th invstigation of algorithms is additionally burdn by th protction of manufacturr's intllctual proprty. Th procdurs ar usually not availabl in th sourc cod (.g. C/C++, Dlphi tc.) that could b asily analyzd, instad th usr gts targt platform-orintd prcompild libraris. Although th majority of manufacturrs wr not intrstd in disclosing of thir cod for th scintific purpos, gnral conclusions drawn from crtain availabl procdurs ar intrsting from th viwpoint of th softwar dsign. Th fficincy of diagnostic procdurs highly dpnds on th us of rsourcs dtrmind individually for ach targt platform and on th input signal and th probability of disass occurrnc. For this rason, altrnativly to th us of thortical complxity stimators calculatd from th cod analysis, w dsignd a prototyp dtctor application. In ordr to limit th rang of considrd mobil platforms (cll phons, SmartPhon, PDA computrs, tc.), w slctd a wll documntd 2

3 dvlopmnt kit of th PXA-270 procssor [6] (fig. 2), alrady applid in a wid rang of PDA computrs of various brands (Asus P-565, Sharp Zaurus, Motorola A780, Acr n50, Compaq ipaq 3900). Microprocssor PXA-27X with updatd XScal cor is th Marvll's implmntation of fifth gnration ARM architctur (codnamd Bulvrd). It supports on of th slctabl clock frquncy: 312, 416, 520 i 624MHz. It is an intgratd Systm-on-Chip with no intrnal mmory dsignd to mobil applications of high dynamics and low powr rquirmnts, thanks to a vry high ratio of th computing powr to th nrgy consumption raching MIPs/mW. Fig. 2. Th dvlopmnt kit of th PXA-270 microprocssor (ral siz: 67.6 x 36.7 x 5.2 mm) B. Invstigations of paramtrs variability in prsnc of disass Studis of rcommndations and guidlins for ECG signal intrprtation issud by th profssional organizations of cardiologists (.g. Amrican Hart Association [7]) ld to th conclusions on dpndnc of th variability of diagnostic paramtr valus and rprsntd pathologis. Taking into account th varity of standards in us, w dcidd to xprimntally invstigat th archiv ECG rcords and th variability of calculatd paramtrs. Th xprimnt was xpctd to yild th assssmnt of th rlativ diffrnc of th paramtrs and thir rspctiv physiological valus obsrvd in prsnc of particular most common pathologis. Considring additionally th probability of occurrnc for ths pathologis, w can optimiz th choic of diagnostic paramtrs bing a basis of th mrgncy dtctor, for th achivmnt of maximum rliability. In th st of total N archiv rcords capturd in proportion to th natural occurrnc of rprsntd disass, th avrag valu of absolut diffrnc btwn paramtr valus from normal n and pathologic a rcord (or two parts of th sam rcord) from th particular patint was takn as th first approach to th Variability Indx VI 1 N n a VI = = i i 1 i (1) N W aimd at finding th paramtr 1 attributd with th maximum valu of VI and othr paramtrs n rprsntativ to th pathology, but statistically indpndnt from th prviously slctd. During th slction th valus of variability indx wr wightd by th complmnt of absolut cross corrlation valu in ordr to favorit th mutually indpndnt paramtrs: n : n {, 1 (1 c 1 n, n 1 n 1 = max( VI } n = ) (1 c n, n 2 ) max : k {, k ( VI } 1 n 1 ) ) L (1 c n,1 ) (2) 3

4 C. Compound mrgncy indx Th dscription of th mrgncy dtctor so far was simplifid by th assumption of th statistical indpndncy of particular diagnostic paramtrs. In a ral ECG intrprtation algorithm this assumption is not fulfilld. Th rcordd signal is th sourc of all diagnostic paramtrs and th procdurs at th initial stags of procssing influnc svral diagnostic paramtrs. On th othr hand, th mrgncy dtctor basd on a singl diagnostic paramtr was snsitiv only in a limitd rang of pathologis, thrfor w propos th Emrgncy Indx EI as a compound paramtr, calculatd out of svral slctd mta-paramtrs 1... n availabl in th ECG intrprtation procss, dspit of thir mutual corrlation.. EI = + + K + (3) n Th appropriat choic of th componnt paramtrs is important in th aspct of uniform snsitivity of th dtction in a wid rang of cardiac pathologis with rgard to thir acutnss and minimal rquirmnts for th computation powr. First of ths rquisits raiss th qustion of variability of ECG diagnostic paramtrs causd by th pathologis in contxt of thir possibl consquncs and occurrnc probability. Th xpctd snsitivity in prsnc of th pathologis of various origin, suggsts th slction of mta-paramtrs basd on thir minimum cross corrlation. Th dscription of computation powr rquirmnts in portabl systms applying advancd powr managmnt solutions, is much mor practical with th us of dirct powr units (microwatts) than with th gnral polynomial-basd stimat of th complxity. This dscription is also usful in th assssmnt of th contribution from th continuously running mrgncy dtctor algorithm to th total powr consumption of th portabl ECG rcordr. RESULTS A. Estimation of th computation complxity For all procdurs of which w hav availabl th sourc cod, th computational complxity was assssd thortically and xprssd as th most probabl count of machin cycls prformd in th cours of xcution. This valu was nxt multiplid by th nominal powr rquirmnt for a singl machin cycl spcifid in th microprocssor application not what yilds a first approach of th total powr rquird for th xcution of ach ECG intrprtation procdur. In cass w hav availabl svral implmntation variants basd on diffrnt algorithms, th avrag valu of xpctd powr consumption was calculatd (tab. 1). Tab.1. Computation complxity of ECG intrprtation procdurs (singl-lad ECG signal of 10s lngth, sampling frquncy 500Hz, avrag HR=78 bpm) procdur purpos (in th xcution ordr) implmntation variants thortical complxity xpctd powr consumption [µw] dtction of hart bats izolin lvl dtction calculation of th hart rat classification of hart bats morphology dtction of wav bordrs dtction of wav axs dtrmination of th dominant rhythm arrhythmia dtction SUM B. Paramtrs variability in prsnc of disass This rsarch was aimd at indicating th diagnostic paramtrs showing th highst variability in prsnc of chang of th patint's status in a wid rang of pathologis. In rsult of th invstigations, 4

5 prsntd in dtails in [8], twlv most frqunt pathologis (normal sinus rhythm, sinus tachycardia, sinus bradycardia, possibl AV block, vntricular scap bats, atrial fibrillation, AV conduction dfct, infarct of any localization, atrial nlargmnt, vntricular hyprtrophy, lft and right bundl branch blocks) wr found in 1730 studid rcords. Ths rcords wr now invstigatd again in ordr to find th most "univrsal" diagnostic paramtr 1 and furthr indpndnt diagnostic paramtrs 2... n of valus varying in prsnc of pathologis in thir possibl widst rang. Th normalizd rsults of paramtrs variation for slctd 12 pathologis ar displayd in tabl 2. C. Optimal rprsntation of th mrgncy Th rsults from tabl 2 wr usd to dtrmining of rangs for th pathologis rprsntd by a risn variation of particular diagnostic paramtrs. As it may b noticd from tabl 2, th variation of th hart rat is particularly high within th group of rhythm-rlatd disass, whras th variation of th QRS contour is mor important within th group of contour-rlatd pathologis. Consquntly, dsigning th mrgncy dtctor basd on th diagnostic paramtrs spcifically varying within such disjoint groups of pathologis, w xpct to mad it snsitiv nough for all ths pathologis. For th purpos of th rsarch, w assum that th positiv mrgncy dtction rquirs th valu EI to b qual or gratr than EI n + std(ei n ), whr EI is th valu of mrgncy indx (qn. 3) for th pathology, EI n is th valu of mrgncy indx for th physiological signal, and std() stands for th standard dviation. Th probability of missing th mrgncy vnt in cas of th us of simpl mrgncy indicator and two variants of compound mrgncy indicators is displayd in tabl 3. Tab. 2. Corrlation of variation of ECG diagnostic paramtrs in prsnc of most probabl pathologis. pathology typ 1 - variation of th hart rat 2 - variation of th QRS contour 3 - variation of th wavs lngth normal sinus rhythm sinus tachycardia sinus bradycardia possibl AV block vntricular scap bats atrial fibrillation AV conduction dfct infarct (any localization) atrial nlargmnt vntricular hyprtrophy lft bundl branch block right bundl branch block Tab. 3. Rsults of invstigations for th optimal compound mrgncy indicator (singl-lad ECG signal of 10s lngth, sampling frquncy 500Hz, avrag HR=78 bpm) paramtrs usd 1 1 & 2 1 & 2 & 3 tru positiv tru ngativ fals positiv fals ngativ snsitivity spcificity positiv prdictiv valu thortical computational complxity powr rquird [µw]

6 DISCUSSION Intrprtation of th rsults displayd in tab. 3. lads to th conclusion that th componnts of th optimal compound indicator ar: - hart rat variations ( 1 ) and - QRS contour variations ( 2 ). Th additional considring of wavlngth variations ( 3 ) is not rcommndd, bcaus this dos not significantly incras th dtctor rliability and rquirs much of th additional computation powr, rising its total consumption to 524µW. Significant fficacy of th mrgncy dtctor composd of th hart rat variations and QRS contour variations is a rsult of a vry low corrlation btwn ths paramtrs. Thanks to th mutual indpndnc, ths paramtrs covr almost disjoint rangs of pathologis (s tab. 2). This tabl also hlps in xplaining a low incras of th dtctor fficacy in rsult of additional us of wavlngth variations ( 3 ). This paramtr, rquiring considrabl computational powr, dos not significantly corrlat with any of th 12 most frqunt pathologis (maximum valu of corrlation is 0,37 for th myocardial infarction). On of th most surprising rsult of th xprimnt carrid out with th prototyp rcordr is th low lctrical powr rquird for th intrprtation of a singl lad 10s ECG rcord, hardly xcding 0.5 mw. This is possibl thanks to th application of a powrful microprocssor working at a low clock frquncy. Unfortunatly in a ral rcording systm th rquird powr is significantly highr du to th ncssity of support of th oprating systm as wll as powring th priphrals: mmory, communication modul and low-signal analogu circuitry. Tsts of th mrgncy dtctor softwar supporting th patint button, prformd with us of artificial long trm signals drivd from th CSE databas showd th snsitivity of and th spcificity of whil th powr consumption raisd du to th signal intrprtation by only 0.24 mw. Ths rsults wr confirmd in furthr tsts using custom 24-hours annotatd Holtr rcordings. Th prformanc of th rcording or monitoring systm no longr rlis on subjctiv prcption of pain or somthing uncommon, for th patint's button th rportd snsitivity is of th ordr of 70% and th spcificity hardly xcsss 55%. In Holtr systms, th rliability of vnt marks accompanying th rcord may b significantly improvd thanks to th softwar support of patint button without limiting th rcordr autonomy tim. In th looping vnt rcordrs, th mrgncy alrt may rmotly chang currnt mdical procdur or triggr a rscu action. Similar tchniqu is also usful in auto-adaptiv ECG intrprtation or distant on-dmand rcording for changs of th softwar architctur and functionality. BIBLIOGRAPHY 1. Augustyniak P. "Th Us of Slctd Diagnostic Paramtrs as a Fdback Modifying th ECG Intrprtation" Proc. Computrs in Cardiology (IEEE-EMB), Vol 33, 2006, pp Zarba W., Maison-Blanch P., Locati E.H. "Noninvasiv Elctrocardiology in Clinical Practic" Futura Publishing Co. Nw York, IEC "Mdical lctrical quipmnt: Particular rquirmnts for th safty, including ssntial prformanc, of ambulatory lctrocardiographic systms." Augustyniak P. "Rqust-Drivn ECG Intrprtation Basd on Individual Data Validity Priods". Procdings of th 29th IEEE EMBS Intrnational Confrnc, 2007, pp Papadimitriou C.H. "Computational Complxity". Addison-Wsly, p (visitd on March 31, 2008) 7. AHA ECG databas. Availabl from Emrgncy Car Rsarch Institut, 5200 Butlr Pik, Plymouth Mting, PA Augustyniak P. "Pursuit for th Knowldg of a Cardiology Exprt a Hiddn Poll Mthodology" Procdings of th 30th IEEE EMBS Intrnational Confrnc, 2008, pp J.L. Willms "Common standards for quantitativ lctrocardiography: 10th CSE progrss rport". Luvn, Blgium: ACCO,