Fast Algorithm for Vectorcardiogram and Interbeat Intervals Analysis: Application for Premature Ventricular Contractions Classification

Transcription

1 Fast Algorthm for Vectorcardogram and Interbeat Intervals Analyss: Applcaton for Premature Ventrcular Contractons Classfcaton Irena Jekova, Vessela Krasteva Centre of Bomedcal Engneerng Prof. Ivan Daskalov - Bulgaran Academy of Scences 15 Acad. G. Bonchev Str., 1113 Sofa, Bulgara E-mal:rena@clbme.bas.bg, vesska@clbme.bas.bg Correspondng author Receved: August 8, 25 Accepted: November 3, 25 Publshed: December 16, 25 Abstract: In ths study we nvestgated the adequacy of two non-orthogonal ECG leads from Holter recordngs to provde relable vectorcardogram (VCG) parameters. The VCG loop was constructed usng the QRS samples n a fxed-sze wndow around the fducal pont. We developed an algorthm for fast approxmaton of the VCG loop, estmaton of ts area and calculaton of relatve VCG characterstcs, whch are expected to be mnmally dependent on the patent ndvdualty and the ECG recordng condtons. Moreover, n order to obtan ndependent from the heart rate temporal QRS characterstcs, we ntroduced a parameter for estmaton of the dfferences of the nterbeat RR ntervals. The statstcal assessment of the proposed VCG and RR nterval parameters showed dstngushng dstrbutons for N and PVC beats. The relablty for PVC detecton of the extracted parameter set was estmated ndependently wth two classfcaton methods - a stepwse dscrmnant analyss and a decson-tree-lke classfcaton algorthm, usng the publcly avalable MIT-BIH arrhythma database. The accuracy acheved wth the stepwse dscrmnant analyss presented senstvty of 91% and specfcty of 95.6%, whle the decson-tree-lke technque assured senstvty of 93.3% and specfcty of 94.6%. We suggested possbltes for accuracy mprovement wth adequate electrodes placement of the Holter leads, supplementary analyss of the type of the predomnant beats n the reference VCG matrx and smaller step for VCG loop approxmaton. Keywords: Heartbeat Classfcaton, Holter ECG Leads, VCG loop, RR ntervals. Introducton The automatc analyss of the electrocardogram (ECG) has become a standard tool for fast dagnostcs of dfferent cardac dysfunctons by computer-based algorthms for detecton of dfferent heartbeat types. The occurrence of multple abnormal cardac contractons assocated wth exctaton from ectopc center n the ventrcles (known as premature ventrcular contractons (PVC) or ventrcular extrasystols) s consdered clncally mportant, snce t s a sgn for dsturbance n the depolarzaton process precedng n many cases the appearance of malgnant cardac arrhythma [15]. The problem for automatc detecton of PVCs s wdely dscussed n the lterature and usually the rules for automatc heartbeat classfcaton are based on the specfc features of the QRS complex, beng the most sgnfcant wave n the ECG. In comparson to the normally exted beats, the PVCs have wder QRS complex (>.12s) and bzarre waveforms due to the abnormal prolongaton of the conducton path through the ventrcles. The study of the nterbeat RR ntervals shows that the sngle PVC appears earler than the normal RR nterval, followed by a prolonged RR-nterval (usually complete compensatory pause, rarely ncomplete compensatory pause), or t may appear sandwched n between two normal beats (the so called nterpolated PVCs). 82

2 Automated heartbeat classfcaton was tradtonally performed usng the morphologcal descrptors of the QRS waveform [1, 2, 4, 5, 8, 1, 12], as well as the RR ntervals [2, 14]. In some works, these parameters were processed wth relatvely complcated methods as neural networks [4, 7, 14], lnear dscrmnants usng lkelhood functons [2], operaton on vectors n the multdmensonal space [5, 12], etc. Obvously, the performance of the morphologcal and the temporal QRS descrptors has been extensvely studed and hghly optmzed. In contrast, the spatal behavor of the cardac electrcal vector, represented by the vectorcardogram (VCG), has not been wdely nvestgated n the aspect of PVC detecton, except n the works of few authors [1,3,4,5,8]. It could be expected that the PVC vectorcardographc loops dffer n shape and orentaton from the normal QRS loops n the Frank orthogonal leads. Usng Holter recorders, however, the researchers face the lmtaton of ECG sgnal acquston from a small number of chest electrodes, whch are not suffcent for reconstructon of ndependent orthogonal VCG leads. Amng to overcome ths lmtaton, Cha et al. [3] used two ECG leads acqured from four chest electrodes placed over the heart n a cross orentaton, thus achevng sngle-plane VCG projecton, whch s close to the orthogonal one. The authors then appled a VCG-based trggerng algorthm for effectve rejecton of PVC beats. Another studes for PVC recognton [4, 5] nvestgated the performance of two VCG parameters (the magntude of the maxmal vector and ts angle) calculated for the non-orthogonal leads of the publcly avalable MIT-BIH arrhythma ECG database [11]. Although the PVC detecton accuracy wth patent-specfc local learnng set was promsng [4], the tranng wth a global learnng set reduced the accuracy [8]. A pont of nterest becomes the nvestgaton of Holter non-orthogonal VCG leads and extracton of VCG parameters, whch are ndependent from the shape and orentaton of the VCG-loop among the ndvduals. In ths study we nvestgated the relatons between the area spatal dsplacements of the VCG loops for N and PVC beats n the two non-orthogonal ECG leads from Holter recordngs avalable n the MIT-BIH database. We propose an algorthm for fast approxmaton of the VCG loop and calculaton of ts area. By estmaton of the common area wth a constructed reference VCG loop, we ntroduce relatve VCG characterstcs, whch are expected to be mnmally dependent on the patent ndvdualty and the ECG recordng condtons. Moreover, we nclude a relatve assessment of the temporal heartbeat characterstcs by calculaton the dfferences of the nterbeat RR ntervals. A dscrmnant analyss and a decson-tree-lke approach are appled for assessment of the PVC classfcaton ablty of the proposed VCG and RR nterval parameters. Materals and Method ECG Sgnals The study nvolved all 48 ECG recordngs from the MIT-BIH arrhythma database. The recordngs were dgtzed at 36 samples per second per channel wth 11-bt resoluton over a 1 mv range. Each recordng has a duraton of 3 mn and ncludes two leads 42 recordngs wth the modfed lmb lead II and V1, 3 recordngs wth the modfed II and V5 (1, 114, 123), 2 recordngs wth V2 and V5 (12, 14), 1 recordng wth the modfed lead II and V4 (124) [9]. The heartbeats were recognzed by the fducal ponts n the database and the orgnal database annotatons were taken nto account. Snce we focused only on the PVC classfcaton, we accepted two heartbeat groups Normal (N) group and PVC group. The N group s a knd of summary group, whch ncluded: () all beats, whch were annotated as Normals (approxmately 7% of all beats n the database); () some types of abnormal beats, whch are predomnant n the patent recordng (left and rght bundle branch blocks and paced 83

3 beats); () premature atral contractons. We excluded from the study all remanng heartbeat types, whch comprse about 2% of all beats n the database, ncludng: aberrantly conducted beats, nodal premature beats, nodal or atral premature beats, nodal escape beats, nodal ectopc beats, atral ectopc beats, fuson premature ventrcular contractons, ventrcular flutter waves, ventrcular escape beats, blocked atral premature beats, mssed beats and questonable beats. In addton, we avoded the processng of all low ampltude heartbeats (<15µV from peak-to-peak n one of the ECG leads), snce the low-ampltude sgnals corrupt the VCG loop to the pont that the analyss s compromsed. In future real-tme mplementaton, montorng of the ECG sgnal ampltude s expected. In case of low ampltude detecton for more than 1s n one of the ECG channels, an alarm for bad electrode contact has to be generated. Thus, the presence of Holter ECG data wth compromsed qualty s drected to the operator s attenton. On the other hand, the appearance of short duraton event of PVCs wth low ampltude projecton n one of the leads would not dsable the heartbeat analyss. Preprocessng The appled dgtal flterng prevents aganst power-lne nterference, tremor nose and baselne drft dstortons of the nput ECG sgnal that mpede the accurate measurement and classfcaton of the heartbeats. The mplemented preprocessng fltraton procedures have been accepted prevously n [4] and have the advantage for real-tme operaton. The followng procedures are realzed: a notch flter for elmnaton of the power-lne nterference, mplemented by movng averagng of samples n one perod of the nterference; a low-pass flter for suppresson of the tremor nose, realzed by movng averagng of samples n 3 ms tme-nterval, thus havng a frst zero at about 35 Hz; a hgh-pass recursve flter for drft suppresson [6] wth cut-off frequency of 2.2 Hz. Lead 1 Lead 2 Fducal Pont (V) Lead 2 (V).6 5 Lead 2 (V).4.2 Y max (V) (ms) zoom Y mn (ms) Lead 1 (V) X Lead 1 (V) mn X max (a) (b) (c) Fg.1. Illustraton of 2-lead ECG and the sngle-plane VCG loop approxmaton. (a) - Lead1 and Lead2: The extracted QRS segment of fxed sze around the fducal pont; (b) - VCG plane: Dvson n square regons wth a sde of 1µV. The VCG-loop of the selected QRS complex s drawn and hghlghted by the red square. (c) - Zoomed vew of the approxmated VCG loop: the approxmated area s drawn wth grey squares (correspond to the VCGMatrx TEST elements wth value 1). The orgnal VCG loop samples are marked wth ( ). 84

4 VCG analyss Extracton of the QRS complex The heartbeats were extracted n segments of 9 samples (25 ms) wth the fducal pont at the 3 th sample. Thus, the wndow of fxed-sze around the fducal pont (about 8 ms before and 16 ms after) guarantees the selecton of the complete QRS wave, even n the worst case of prolonged ventrcular extrasystole. An llustraton of the two ECG leads representng the extracted QRS segment and the poston of the fducal pont s shown n Fg. 1a. Calculaton of VCG Matrx of the tested QRS complex We ntroduce the VCG Matrx as an approxmaton of the VCG loop spatal poston, amng to facltate the assocated calculatons. For the sake of the approxmaton, the VCG plane, formed by Lead1 and Lead2 s dvded n square regons havng a sde of 1µV. Thus, a number of 1x1 squares fll the entre VCG plane, takng nto account the mnmal and the maxmal ECG sgnal ampltude of 5V and +5V, respectvely (Fg. 1b). All squares form the VCGMatrx TEST (sze 1x1) and they could take a value or 1, n dependence on the spatal poston of the VCG loop of the tested QRS complex. The VCGMatrx TEST elements are assgned as follows: Intalzaton all elements are set to ; Selecton of the VCGMatrx TEST elements, whch belong to the rectangle defned by [X mn, X max, Y mn, Y max ], where X = mn( Lead1); X = max( Lead1); Y = mn( Lead2); X = max( Lead2); mn max Each of the selected VCGMatrx TEST elements s tested whether t s nternal or t s external for the VCG loop. We apply a fast algorthm [13], whch calculates the number of crossngs of the ray between an nternal pont for the VCG loop and the element center (coordnates X c, Y c ), accordng to Condton (1). The odd number of crossngs ndcates an outsde element (value set to ), whle the even number ndcates an nsde element (value set to 1). An example of calculated VCGMatrx TEST elements s presented n Fg. 1c. mn max ( Lead2 and X c Y c and Y Lead1 j < Lead2 j c < Lead2 Lead1 Lead2 j ( Y ) or ( Lead2 c Lead2 j Y c and Y ) + Lead1 c < Lead2 ), (1) where, j are the consecutve ndexes of the VCG loop samples ( = 1,2,... N; j = N,1,2,... N 1). Calculaton of Reference VCG Matrx The reference VCG Matrx (VCGMatrx REF ) s calculated as a mean of the VCG Matrxes of the fve prevous QRS complexes, accordng to Equaton (2). The values of ts elements are rounded towards the nearest nteger and are equal ether to or to 1. Thus, the reference VCG Matrx has ones only n these elements, whch represent VCG loop spatal dstrbuton repeated n 3 or more QRS complexes from the 5 reference beats. The example of ECG sgnal wth N and PVC beats (see Fg. 2a) represents the calculated reference VCG Matrx nvolvng beats wth ndexes from T-5 to T-1 (see Fg. 2b). It s evdent that the black area (the ones n the reference VCG Matrx) concdes wth the elements, whch are nternal for at least three VCG loops. 85

5 T 1 5 VCGMatrx VCGMatrx REF = T = round( 5 ), (2) where T s the consecutve ndex of the tested QRS-complex n the fle. N PVC PVC N N * # Lead 1 Lead 2 Lead 2 (V) T-5 T-4 T-3 T-2 T-1 T (a) Tested Beat Beat: T-5 Lead 2 (V) Beat: T-4 Lead 2 (V) Beat: T-3 Lead 2 (V) Lead 1 (V) Lead 1 (V) Lead 1 (V) Beat: T-2 Lead 2 (V) Beat: T-1 Lead 2 (V) Reference Lead 1 (V) Lead 1 (V) Lead 1 (V) (b) Fg.2. Illustraton of the reference VCG Matrx calculaton (a) - Example of ECG sgnal wth N and PVC beats. (b) - The VCG loops and ther approxmated areas for the beats wth ndexes from T-5 to T-1. The last subplot llustrates the calculated reference VCG Matrx, whch summarzes the VCG loop spatal dstrbutons of all fve QRS complexes. 86

6 Calculaton of Common VCG Matrx We ntroduce the Common VCG Matrx (VCGMatrx COMMON ) for estmaton of the area of overlappng between the reference VCG Matrx and the VCG Matrx of the tested QRS complex. Snce the matrxes elements are bnary ( or 1), ther common parts could be easly calculated wth logcal AND, as wrtten n Equaton (3). VCGMatrx COMMON = VCGMatrx & VCGMatrx (3) REF TEST Fg. 3 s an llustraton of the Common VCG Matrx (grey squares) placed over the reference VCG Matrx and the VCG Matrx of the tested QRS complex (black squares + grey squares). It s well seen that n case of N beat, the reference VCG Matrx (Fg. 3a) and the tested VCG Matrx (Fg. 3b) overlap wth more than 8%. In case of PVC beat, the Common VCG Matrx flls less than 3% of both the reference VCG Matrx (Fg. 3c) and of the tested VCG Matrx (Fg. 3d). Lead 2 (V) Reference Lead 2 (V) Tested Beat (N *) Lead 1 (V) (a) (b) Lead 2 (V) Reference Lead 2 (V) Tested Beat (PVC #) Lead 1 (V) (c) Lead 1 (V) (d) Lead 1 (V) Fg.3. Illustraton of the common VCG Matrx calculaton. The Common VCG Matrx s shown wth grey squares, placed over the black squares of the reference VCG Matrx (a,c) and the VCG Matrx of the tested QRS complex (b,d). (a,b) The tested QRS complex s N beat (marked wth * n Fg.2a) (c,d) The tested QRS complex s PVC beat (marked wth # n Fg.2a) 87

7 Calculaton of VCG parameters We estmate the VCG areas, defned by the ntroduced above VCG matrxes as the sum of ther elements, presented n Equaton (4): X max Y max VCGArea = VCGMatrx( x, y), (4) x= X mn y= Y mn where: VCGMatrx = VCGMatrx, VCGMatrx, VCGMatrx ; TEST REF COMMON X = mn( Lead1); X = max( Lead1); Y = mn( Lead2); X = max( Lead2); mn max mn These VCG areas are greatly dependent on the ECG recordng condtons, whch for example nfluence the ECG sgnal ampltude. Thus, the values of the calculated areas may dffer even for one and the same patent, whch makes them ncomparable. In order to provde relatve VCG characterstcs, whch are expected to be mnmally dependent on the patent ndvdualty and the ECG recordng condtons, we ntroduce the relatve VCG parameters, named DA and DB, calculated accordng to Equatons (5) and (6): max VCGAreaTEST + VCGAreaREF 2VCGArea DA = VCGArea + VCGArea TEST REF COMMON (5) 1 VCGAreaCOMMON VCGAreaCOMMON DB = ( + ) (6) 2 VCGArea VCGArea TEST REF Interbeat RR ntervals The nterbeat RR ntervals are nvolved n the computaton of the parameter RRDff, whch represents the dfference between the duratons of the two RR ntervals, surroundng the tested heartbeat (wth ndex T). The normalzaton towards the mean value of the prevous fve consecutve RR ntervals s appled, n order to acheve a value ndependent from the heart rate. RRT RRT 1 RRDff = T.1 (%), (7) T 2 ( RR ) / 5 = T 7 where RR T = Fducal Pont T+1 - Fducal Pont T RR T-1 = Fducal Pont T -Fducal Pont T-1. Beat classfcaton Stepwse dscrmnant analyss Stepwse dscrmnant analyss was appled on the defned above parameters RRDff, DA and DB to dfferentate between N and PVC beats. Two lnear dscrmnant functons of the n- dmensonal vector x (n=3) were calculated F (Equaton 8) and F (Equaton 9). 88

8 n F ( x) = w x + a (8) =1 n F ( x) = w x + a (9) =1 Here w,w and a, a are the correspondng dscrmnant coeffcents and constants. F relates to the possblty the heartbeat descrbed by vector x to be N, whle F gves the possblty to be PVC. These two dscrmnant functons were computed for the tested heartbeat and t was labeled as correspondng to one of the two classes: N or PVC, dependng on the hgher value among F and F. Decson-tree-lke classfcaton algorthm We defned several condtons, whch are appled consecutvely for classfcaton of the tested heartbeat as N or PVC. The analyss s based on the defned parameters RRDff, DA and DB and ncludes the followng steps: Step 1: If (RRDff<-5%) the QRS s classfed as N; Step 2: If (DA<.5) AND (RRDff<2%) the QRS s classfed as N; Step 3: If (DA.5) AND (RRDff 2%) the QRS s classfed as PVC; Step 4: If (DB<.5) the QRS s classfed as PVC; Step 5: If (.65<DB<.85) AND (25%<RRDff<7%) the QRS s classfed as PVC; Step 6: If (DA>.7) the QRS s classfed as PVC; Step 7: If (DA>.3) AND (RRDff>8%) the QRS s classfed as PVC; Step 8: Else the QRS s classfed as N. The values of the thresholds for RRDff, DA and DB were chosen by descrptve statstcal analyss of the parameters dstrbutons, as shown below n secton Results, followed by teratve testng of the detecton accuracy and adjustng the threshold values. Results All procedures, ncludng the ECG sgnals preprocessng, the VCG analyss algorthm and the RR ntervals dfferences measurement, were mplemented n software utlty, usng the software package MATLAB 7.. The computed parameters RRDff, DA and DB for all beats n the MIT-BIH database were analysed wth STATISTICA 6. software. The relablty for PVC detecton of the parameters RRDff, DA and DB was estmated ndependently wth two classfcaton methods - the dscrmnant analyss and the decsontree-lke approach. The accuracy was evaluated wth the statstcal ndces Senstvty (Se) and Specfcty (Sp), calculated as follows: Correctly Detected PVC beats Se = (1) Total Number of PVC beats Correctly Detected N beats Sp = (11) Total Number of N beats 89

9 The results acheved wth the stepwse dscrmnant analyss, whch ncludes at each step the hghest ranked parameter, are presented n Table 1. Table 1. Stepwse dscrmnant analyss specfcty (Sp), senstvty(se) and dscrmnant functons F (N beats) and F (PVC beats) on each step. Step Parameters Sp(%) Se(%) F (for N beats) F (for PVC beats) 1 DA *DA *DA DA, RRDff *DA-.232*RRDff DA, RRDff, DB *DA+.98*RRDff +1225*DB *DA+.11*RRDff *DA+.213*RRDff+1 235*DB-632 (a) (b) (c) Fg.4. Categorzed dstrbutons of the parameters: (a) DA, (b) DB, (c) RRDff and (d) Scatterplot DA- RRDff, for the two heartbeat groups - N and PVC. The decson-tree-lke algorthm was desgned n accordance wth the results from the statstcal analyss of the parameters RRDff, DA and DB, whch were assessed for all beats n the database. The estmated statstcal dstrbutons for the two categores of N and PVC beats are presented n Fg. 4. They were used as a bass for settng the decson rules and the effcacy of each step s presented n Table 2. (d) 9

10 Table 2. The percent of correctly and erroneously classfed N and PVC beats estmated towards the total number of N and PVC beats at each step of the decson-tree-lke algorthm. Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8 True N 3.6% 87.6% % False N - -.4% 3.9%.9%.3%.9% - True PVC % 14.5% 7.3% 1.1%.8% - False PVC.1% 3% % The total accuracy acheved wth the decson-tree-lke classfcaton approach s shown n Table 3. The best results, whch were attaned at the second step of the dscrmnant analyss, are also added for comparson. Table 3. Specfcty (Sp) and senstvty(se) obtaned wth the dscrmnant analyss (second step) and the decson-tree-lke classfcaton method. Classfcaton Method Sp(%) Se(%) Dscrmnant analyss (DA, RRDff) Decson-tree-lke method Dscusson The study was focused on the wdely dscussed problem for automatc dscrmnaton between normal and premature ventrcular contractons. We nvestgated the projecton of the cardac electrcal vector on the sngle-plane formed by two Holter chest leads, amng to fnd out sgnfcant dfferences between the VCG loops of the N and the PVC beats n one patent. Snce the analyss of the spatal correlaton of two VCG loops s assocated wth tmeconsumng and relatvely complcated mathematcal procedures, we appled an approach for approxmaton of the VCG loops, thus reducng the complexty to operatons wth bnary matrxes. The sze of the ntroduced VCG matrx defnes the step of approxmaton. In our study, the operaton wth matrxes 1x1 s suggested to be a ratonal compromse between complexty and accuracy. Thus, we can apply a relatvely fast algorthm for searchng the VCG matrx elements, whch are nternal for the VCG loop, and then easly calculate the correspondng VCG loop area. Moreover, we proposed an approach for relatve assessment of the spatal dsplacement of the tested VCG loop area from the poston of a reference VCG loop area. The calculated VCG area parameters are mnmally dependent on the patent ndvdualty and the ECG recordng condtons and, therefore, ther statstcal comparson for all beats n the database becomes reasonable. We should menton that the number of QRS complexes, whch are nvolved n the estmaton of the reference VCG loop, depends on the avalable memory, speed and computaton resources when quas-real tme operaton s expected. In our study, a number of fve consecutve beats for the reference VCG matrx proved to be approprate. The proposed algorthm for PVC detecton, whch reles on fast VCG analyss, uses fxed-sze wndow of 25ms around the fducal pont for the VCG loop constructon. Thus, we guarantee that the selected ECG segment contans the hgh ampltude QRS complex samples, whch specfy the most sgnfcant part of the VCG loop area. The advantage of ths approach s that we avod the complcated procedures for fndng the onsets and the offsets of the ECG waves. 91

11 The VCG analyss tself does not contan nformaton about the temporal ECG characterstcs. In order to obtan ndependent from the heart rate temporal QRS characterstcs, we ntroduced a parameter for relatve assessment of the nterbeat RR ntervals dfferences. The smple calculaton of ths parameter corresponds to the concept for quas-real tme operaton of the algorthm. The statstcal assessment of the proposed VCG and RR nterval parameters showed dstngushng dstrbutons for N and PVC beats (see Fg. 4). As expected, the parameter DA (Fg. 4a), summarzng the areas of the reference VCG loop and the tested VCG loop, whch are outsde ther common area, has predomnantly low values for the N beats (<.3) and hgh values for the PVC beats (>.5), for more than 8% of the cases. The parameter DB (Fg. 4b), whch estmates the rato between the common area and the sze of both the reference VCG loop and the tested VCG loop, feature wth hgh values for the N beats (>.7) and relatvely low values for the PVC beats (<.7). Ths proved the assumpton that the N-beats beng the predomnant beats n the calculaton of the reference VCG loop matrx, would repeat the spatal VCG loop dstrbuton of a tested N-beat (see Fg.3 a,b) and just n opposte, the reference VCG loop would be hghly dstngushable from a casual PVC beat (see Fg.3 c,d). The more complcated cardac dysfunctons assocated wth frequent alteraton of N or PVC beats (more than 3 PVCs n a number of 5 consecutve beats), would lead to calculus of a reference VCG matrx wth predomnant PVC loops, that could mslead the values of DA and DB parameters typcal for N and PVC beats. We consder that a supplementary analyss of the type of the predomnant beats n the reference VCG matrx would mprove the accuracy by classfyng the tested beat as belongng or not to the type of the predomnant beats n the reference set. The analyss of the RR ntervals shows that the parameter RRDff (Fg. 4c) has well expressed peak between 2% and +2% for the N beats, whch appear normally n regular RR ntervals. In contrast, the PVC beats have wder RRDff dstrbuton, resultng from the varety of PVCs wth complete compensatory pause, non-complete compensatory pause or lack of compensatory pause. Moreover, we should take nto account that the RRDff parameter has smlar values for the ventrcular and the atral premature contractons, the last beng part from the N group. The dscrmnant analyss classfed the VCG descrptor DA as the top-ranked parameter n the studed parameter set (see Table 1). The use of RRDff n combnaton wth DA mproved the accuracy to 95.6% n recognton of N beats and 91% n recognton of PVC beats. The dscrmnant analyss was unable to provde better results when ncludng the second VCG parameter DB. The desgned decson-tree-lke classfcaton algorthm operates together wth DA, DB and RRDff parameters, and acheves more balanced results wth specfcty of 94.6% and senstvty of 93.3%. The results do not outmatch the reported accuracy of other methods for heartbeat classfcaton, but there are several factors, whch mght nfluence the performance of the analyzed parameters. For example, the Holter MIT-BIH database was collected wth dfferent combnatons of chest leads, whch obvously changed the projectons of the electrcal vector on the relevant VCG planes. Thus, the relaton between the spatal dsplacements of the VCG loops for N and PVC beats could vary - n some leads beng very enhanced, whle n others beng hardly vsble. Therefore, the use of Holter electrodes placement, whch ensures almost ndependent orthogonal VCG leads, s a potental approach for mprovement of the relablty of the studed VCG parameters. Another acton towards results mprovement s the use of smaller step for VCG loop approxmaton, whch however s assocated wth large VCG matrxes and more heavy computaton procedures. 92

12 Concluson We proposed an approach for PVC detecton from 2 non-orthogonal Holter leads, whch was based on a fast algorthm for VCG loop analyss and nterbeat RR ntervals dfferences calculaton. The performed statstcal assessment of the extracted parameter set proved the potental ablty of the VCG loop and RR ntervals analyses to provde a relable tool for PVC classfcaton n Holter ECG recordngs. We suggested possbltes for accuracy mprovement wth adequate electrodes placement of the Holter leads, supplementary analyss of the type of the predomnant beats n the reference VCG matrx and smaller step for VCG loop approxmaton. References 1. Bortolan G., I. Jekova, I. Chrstov (25). Comparson of four methods for premature ventrcular contractons and normal beats clusterng, Computers n Cardology, 32 (n press). 2. Chazal P., M. O Dwyer, R. B. Relly (24). Automatc classfcaton of heartbeats usng ECG morphology and heartbeat nterval features, IEEE Transacton on Bomedcal Engneerng, 51, Cha J. M., S. E. Fscher, S. A. Wcklne, C. H. Lorenz (2). Performance of QRS detecton for cardac magnetc resonance magng wth a novel vectorcardographc trggerng method, Journal of Magnetc Resonance Imagng, 12 (5), Chrstov I., G. Bortolan (24). Rankng of pattern recognton parameters for premature ventrcular contracton classfcaton by neural networks, Physologcal Measurement, 25, Chrstov I., I. Jekova, G. Bortolan (25). Premature ventrcular contracton classfcaton by the K th nearest neghbours rule, Physologcal Measurement, 26, Daskalov I. K., I. A. Dotsnsky, I. Chrstov (1998). Developments n ECG acquston, preprocessng, parameter measurement and recordng, IEEE Engneerng n Medcne & Bology, 17, Gómez-Herrero G., A. Gotchev, I. Chrstov, K. Egazaran (25). Heartbeat classfcaton usng ndependent component analyss and matchng pursuts, IEEE, Int. Conf. Acoustcs, Speech and Sgnal Processng, ICASSP, Phladelpha, USA, 4, Jekova I., G. Bortolan, I. Chrstov (24). Pattern recognton and optmal parameter selecton n premature ventrcular contracton classfcaton, Computers n Cardology, 31, Mark R., G. Moody (1988). MIT-BIH Arrhythma database drectory. Cambrdge: Massachusetts Insttute of Technology. 1. Mllet J., M. Perez, G. Joseph, A. Mochol, J. Chorro (1997). Prevous dentfcaton of QRS Onset and Offset s not essental for classfyng QRS complex n a sngle lead, Computers n Cardology, 24, MIT-BIH Arrhythma database Moreas J., M. O. Sexas, F. N. Vlan, E. V. Costa (22). A real tme QRS complex classfcaton method usng Mahalanobs dstance, Computers n Cardology, 29, O'Rourke J. Computatonal Geometry n C, Tspouras M.G., D.I. Fotads, D. Sders. (22). Arrhythma classfcaton usng the RR nterval duraton sgnal, Computers n Cardology, 29, Wagner S.G. (1994). Marrott s practcal electrocardography, 9-th ed., Wllams and Wlkns, Baltmore, Maryland. 93