Heart 1997;77:417-422

Signal averaged P wave compared with standardelectrocardiography or echocardiography forprediction of atrial fibrillation after coronarybypass grafting

P J Stafford, S Kolvekar, J Cooper, J Fothergill, F Schlindwein, D P deBono, T J Spyt,C J Garratt

AbstractObjective-To define the clinical value ofthe signal averaged P wave (SAPW) andto compare it with the standard electro-cardiogram (ECG), echocardiogram, andclinical assessment for the prediction ofatrial fibrillation after coronary bypassgrafting (CABG).Design-Prospective validation cohortstudy.Setting-Regional cardiothoracic centre.Patients-201 unselected patients under-going first elective CABG were recruitedover six months. Patients requiring con-comitant valve surgery were excluded.Main outcome measures-Age, sex, car-diothoracic ratio, and cardioactive drugswere noted. P wave specific SAPWrecordings, ECG, and M mode echocar-diograms from which left atrial diameterwas measured were performed within 24hours of surgery. Filtered P wave dura-tion (SAPWD), spatial velocity, andenergy were calculated from the SAPW.From the ECG, lead II P wave duration, Pterminal force in lead Vl, total P waveduration, and isoelectric interval weremeasured. Patients had Holter monitor-ing for 48 hours postoperatively and dailyECGs until discharge.Results-Two patients died (1%) and 10were unsuitable for analysis (5%). Of theremaining 189, 51 (27%) had atrial fibril-lation (AF) lasting > 1 hour at a mean of 2(0.5 to 7) days after CABG. Of the vari-ables examined, only SAPWD (AF group148 (SD 12), v 142 (14) ms, P = 0-008) andmale sex (AF group 96%, v 78%, P < 0.01)were significantly different. A prospec-tively defined SAPWD of > 141 ms pre-dicted atrial fibrillation with positive andnegative predictive accuracies of 34% and83%. Logistic regression analysis identi-fied both male sex and SAPWD as signifi-cant independent predictors of post-operative atrial fibrillation.Conclusions-Signal averaged P waveduration was a better predictor of atrialfibrillation after coronary bypass graftingthan standard electrocardiographic orechocardiographic criteria. The predic-tive value of this test is such that it islikely to be useful in the design ofprospective trials of prophylactic antiar-rhythmic treatment but is of limited use

using current techniques in the clinicalmanagement of individual patients.

(Heart 1997;77:417-422)

Keywords: coronary bypass grafting; atrial fibrillation;signal averaged electrocardiography

Although atrial fibrillation remains the mostcommon postoperative arrhythmia that occursafter coronary artery bypass grafting (CABG),definite clinical predictors have not been iden-tified. 1-3 The incidence of atrial fibrillationafter CABG varies between 10% and 40%.While this arrhythmia is rarely fatal, it cancause significant discomfort from palpitations,dyspnoea, and occasionally chest pain, it isassociated with a thromboembolic risk, and itdelays hospital discharge.

Evaluation of the characteristics of the sur-face P wave from the standard electrocardio-gram has been found by Buxton andJosephson to be moderately predictive of post-operative atrial fibrillation.5 Recently twogroups have investigated signal averaged elec-trocardiography of the surface P wave as a pre-dictor of atrial fibrillation after CABG.67 Inone of these investigations patients with valvesurgery were included in the patient groupstudied,6 and in the other only a small numberof patients were examined.7 The predictivevalue of the signal averaged P wave was mod-erate in both of these studies and it has beensuggested that analysis of the standard electro-cardiogram may be as good as analysis of themore technically difficult and time consumingsignal averaged electrocardiogram performedin these investigations.8The aim of the prospective investigation

described here was to compare directly thepredictive value of standard electrocardiogra-phy, left atrial dimension derived from Mmode echocardiography, and P wave charac-teristics after P wave specific signal averagingfor the development of atrial fibrillation afterCABG in a large group of patients undergoingfirst elective CABG.

MethodsSTUDY DESIGNTwo hundred and one patients were recruitedover a six month period from those attendingour institution for first elective isolated

Glenfield Hospital,Leicester, UnitedKingdom: Departmentof CardiologyP J StaffordJ CooperD P deBonoC J GarrattDepartment ofCardiothoracicSurgeryS KolvekarT J SpytUniversity ofLeicester, Leicester,United Kingdom:Department ofEngineeringJ FothergillF SchlindweinCorrespondence to:Dr P J Stafford, Departmentof Cardiology, ClinicalSciences Wing, GlenfieldHospital, Groby Road,Leicester, LeicestershireLE3 9QP, United Kingdom.Accepted for publication28 January 1997

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CABG. Patient recruitment was limited by theavailability of Holter monitoring recorders. Noother selection criteria were applied. Patientswith minor degrees of valve disease that didnot require operative intervention were notexcluded, nor were those with previousarrhythmia. All patients gave informed con-sent to the study, which was approved by thehospital ethics committee.

In the 24 hours before CABG, patientsunderwent standard 12-lead electrocardiogra-phy, echocardiography including colourflowexamination, and P wave specific signal aver-aging. Demographic variables, the presence orabsence of clinical signs of cardiac failure, thecardiothoracic ratio from the erect preopera-tive PA chest x ray, preoperative cardiac med-ication, previous myocardial infarction, andprevious cardiac arrhythmia were alsorecorded. Postoperatively patients were moni-tored continuously by both telemetry andHolter monitoring for the first 48 hours aftersurgery. Thereafter they had daily electrocar-diograms, with additional studies if symptomswere reported. Sustained atrial fibrillation wasdefined as atrial fibrillation that lasted at leastone hour on Holter monitoring, or which wasdocumented on two standard electrocardio-grams taken one hour apart. At discharge thepatient's cardiac rhythm was noted and theirdrugs recorded. Special attention was paid to/3 blocker withdrawal during the perioperativeperiod.4

STANDARD ELECTROCARDIOGRAPHYStandard electrocardiograms were performedusing a Hewlett Packard Pagewriter machinewith high pass and notch filters switched off.Analysis was performed on magnified leads I,II, III, and Vi as described by Buxton andJosephson5 and Morris et al.9 Lead II P waveduration was defined as the time from the ear-liest onset of P wave activity in lead II to thelast P wave activity in this lead. Total P waveduration was the time from the earliest onsetof P wave activity in any of leads I, II, or III tothe last P wave activity in any of these leads.The isoelectric interval was the differencebetween lead II P wave duration and total Pwave duration (fig 1). Lead Vi was used tomeasure the P terminal force in lead Vl(PTFVl), defined as the duration of the terminal(negative) part of the P wave in lead VI in sec-onds multiplied by its depth in millimetres(assuming normal calibration) (fig 2). If theterminal P wave was positive the duration andamplitude of the portion of the P wave afterthe notch usually seen towards the centre ofthe waveform was measured. All measure-ments were performed by two independentobservers. Interobserver variability was 10 ms(7-8%) for lead II duration, 9 ms (6-6%) fortotal P wave duration, 5 ms (33%) for isoelec-tric interval, and 0 005 (33%) for PTFV1.

ECHOCARDIOGRAPHYAll patients underwent standard M mode,cross sectional, and colourflow echocardiogra-phy (HP Sonos 1500 or Sonos 2000).Diastolic left atrial diameter was measured

A = Lead 11 durationB = Total P wave durationIsoelectric interval (IEI) = B - A

B

Figure 1 Measurement ofP wave duration, total P waveduration, and isoelectric interval. Magnified standardleads I, II, and III are shown. P wave duration (A) ismeasuredfrom standard lead II. Total P wave duration(B) is the interval between the earliest atrial activationseen in either leads I, II, or III to the latest deflection inany of these leads. The isoelectric interval is the differencebetween total P wave duration and P wave durationmeasuredfrom lead II (that is, B-A). After Buxton et al.5

from the M mode echocardiogram at the levelof the aortic root using on screen callipers.

SIGNAL AVERAGED ELECTROCARDIOGRAPHYOur signal averaging and P wave analysismethodology has been described previously.'0Subjects were recorded supine and relaxed in

PTFV1 = a(mm) x b(s)

Figure 2 Determination of the P terminalforce (PTF,).This measure of left atrial abnormality is defined as themaximum amplitude of the terminal negative portion of theP wave in lead VI in mm multiplied by its duration inseconds. In cases where the terminal portion of the P waveis not negative it is defined as extendingfrom the regionwhere the P wave is notched to its end and the PTFV, willhave a positive sign.

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Prediction of atrialfibrillation after coronary bypass grafting

Table 1 Clinical and demographic variables

AF (n (%/)) * SR (n (%)0) P

Age 62-8 (7 8) 60-6 (7 0) NSMale sex 49 (96) 108 (78) < 0-01Diagnosis 1: AVD 1: AVD NS

1:MVD 3:CCF NSAdmission drugs

j3blocker 38 (75) 98 (71) NSCalcium antagonist 33 (65) 91 (66) NSAmiodarone 1 (2) 4 (3) NSDigoxin 0 (0) 4 (3) NS

Previous arrhythmiaPAF 3 (6) 5 (4) NSPalpitations 10 (20) 27 (20) NSVF 0 (0) 2 (1) NS

Previous MI 32 (63) 72 (52) NSHeart failureJVP 1 (2) 7 (5) NSCrackles 1 (2) 4 (3) NSOedema 3 (6) 19 (14) NS

Discharge drugs/3blocker 7 (13) 25 (18) NSCalcium antagonist 5 (10) 16 (12) NSAmiodarone 5 (10) 1 (1) NSDigoxin 30 (59) 11 (8) < 0-001

3blocker withdrawal 32 (63) 78 (57) NS

*All values are n (%) except for age which is presented as mean (SD) in years. MVD, minormitral valve disease; AVD, minor aortic valve disease; CCF, congestive cardiac failure; PAF,previous paroxysmal atrial fibrillation; VF, ventricular fibrillation; MI, remote myocardialinfarction; JVP, raised jugular venous pressure.

a quiet room. A modified orthogonal lead sys-tem was used. Between 100 and 200 beatswere averaged to give a final filtered noise levelof < 0-2 juV and an estimated low pass cutofffrequency of at least 70 Hz. Analogue signalswere amplified between 4000 and 10 000times, and bandpass filtered between 1 and300 Hz. A fourth, trigger signal was derivedfrom one of the orthogonal leads and used toalign P waves during signal averaging. The lat-ter signal was bandpass filtered between 20and 50 Hz. Analogue data were then digitisedat 1 kHz with 12 bit resolution.

Voltage threshold triggering using the Rwave of the signal selected for the trigger chan-nel was used to identify each electrocardio-graphic cycle. However, P waves were thenaligned by template matching to an evenlyspaced 15 point P wave derived template (thatis, true P wave triggered averaging). An algo-rithm that automatically determined the mostfrequently occurring P wave morphology foreach subject was used to select the averagingtemplate. P waves with morphologies thatfailed to match this template accurately wererejected from the signal average. This tech-nique averaged the most common P wave

Table 2 Preoperative investigation results

AF SR P

Chest x ray CTR 0 49 (0 05) 0 49 (0 05) NSStandard ECG

Duration 128 (24) 127 (79) NSPTFv. -0-02 (0 03) -0-01 (0-03) NSTotal duration 139 (29) 136 (26) NSIEI 13-5 (17-11) 16-2 (18-5) NS

Left atrial diameter 3-6 (0 4) 3-6 (0 4) NSSignal averaged ECG

Noise 0-19 (0-08) 0-17 (0-08) NSDuration 148 (12) 142 (14) 0-008Mean SV 3-29 (0 86) 3-26 (0-89) NSPeak SV 12-35 (4-1) 12-46 (4-4) NSRatio peak/mean SV 3-74 (0 73) 3-78 (0 69) NSP20 34-1 (16-1) 34-4 (17-2) NSP30 19-8 (10-3) 20-9 (10-3) NSP40 8-56 (3-91) 9-76 (4-61) NSP60 2-80 (1-47) 2-72 (1-28) NSP80 1-20 (0 80) 1 17 (0 60) NS

Values are mean (SD). CTR, cardiothoracic ratio; SV, spatial velocity, P20, P30, etc, energy infrequency bands from 20, 30, etc Hz to 150 Hz after spectral analysis; PTFV,, P terminal force inlead VI after Morris et a19; IEI, isoelectric interval after Buxton et al.i

morphology for a particular individual to aclose tolerance, ensuring high fidelity of theresultant averaged waveform.

After averaging, P wave signals were highpass filtered at 40 Hz using a 30 term finiteimpulse response filter and a vector magnitudeplot was constructed. P wave limits were setautomatically by an algorithm that identifiedthe start of the P wave as the point at whichthe vector magnitude rose to more than threestandard deviations above its baseline valueand the P wave endpoint as the point at whichthe vector magnitude fell to within three stan-dard deviations of the baseline value of theminimum PR segment magnitude. These limitswere used to determine the P wave duration.Spatial velocity-the rate of change of the Pwave voltage with respect to time-was calcu-lated by digital differentiation between thelimits defined above and mean, peak, and theratio of peak to mean spatial velocity weremeasured. Spectral analysis comprised Fouriertransformation of the entire unwindowed Pwave after filtering at a high pass of 15 Hz.10From the resultant power density spectrum,the total energy contained in frequency bandsfrom 20, 30, 40, 60, and 80 to 150 Hz was cal-culated.

Using the above signal averaging system wehave found P wave duration to be a repro-ducible measurement" (coefficient of repro-ducibility 15 ms (11%)), although spatialvelocity and P wave energy were less repro-ducible (for example, mean spatial velocity1 44 mV/s (31%), P20 10-7 MV2/s (24.6%))

STATISTICAL ANALYSISCategorical variables are presented as n (%).Continuous data are presented as mean (SD).Univariate comparisons between patientsdeveloping atrial fibrillation and those who didnot were made by the x2 test for categoric dataand the unpaired t test for continuous data.Multivariate analysis was by logistic regres-sion.

ResultsPATIENT CHARACTERISTICSOf the 201 patients studied two (1%) died in theearly postoperative period and 10 (5%) wereexcluded from analysis because of power-fre-quency contamination of their signal averaged Pwave recordings leading to filtered noise levels ofmore than 0 3 MV. Of the remaining 189 patients51 (27%) developed sustained atrial fibrillationat two (0 5 to 7) days after CABG. The age ofthe patients developing atrial fibrillation was62-8 (1 1) years and of those remaining in sinusrhythm 60-6 (0 6) years (P = NS). There weresignificantly more males in the atrial fibrillationgroup than in the group who remained in sinusrhythm (49 (96%) v 108 (78%), P < 0-01). Noother significant differences in clinical variables,prevalence of preoperative arrhythmia, admis-sion drugs, discharge drugs, or /3 blocker with-drawal were noted apart from the expectedexcess of digoxin use at discharge in patientswho had developed atrial fibrillation (30 (59%) v1 1 (8%), P < 0 001) (table 1).

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of 48%, positive predictive accuracy of 34%,n in A tn roClirti7ro -atr1rafr17 nf RAO/^ (4fic 4

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P= 0.008 E4NS X Discussion

0_00NS _35E Our findings in a large homogeneous group of1 patients undergoing first elective CABG sug-1 gest that of the multiple preoperative clinical,

o t 1111 1 1 _ 3 X electrocardiographic, and echocardiographicX variables that we studied the only predictors of

atrial fibrillation after CABG were filtered P0 AECG ECG 2.5 wave duration, measured from the signal aver-

duration total duration diameter aged electrocardiogram, and patient sex. Afteradjustment for the difference in sex between

e 3 Difference in signal averaged P wave duration, entw o dIwave duration measuredfrom the standard patients who developed atrial fibrillation after)cardiogram, and left atrial diameter in patients CABG and those who did not by logisticping atrialfibrillation and those remaining in sinus regression analysis, P wave duration remained an after coronary bypass surgery. Mean values are significant independent predictor. However,ited. Only signal averaged P wave duration wascantly different between the groups. although signal averaged electrocardiography

was a better predictor of postoperative atrial- fibrillation than standard electrocardiography

or echocardiography in these patients, the pre-dictive ability of this technique was not high.

i.8- The value of the technique for routine preop-erative clinical use in individuals is therefore

.6 - likely to be limited. No differences in measures,------of P wave energy or spatial velocity were noted

>.4 _ / in this group of patients, in contrast to the dif-- ' ..... .- SAECG duration ferences that we and others have reported

ECG duration between patients with paroxysmal atrial fibril-1.2 ---------LA diameter lation and controls without arrhythmia.'213This may be related to the poor reproducibility

0 of these measures.'"0 0.2 0.4 0.6 0.8 1 Two previous groups have reported their

1-Specificity experience with the use of preoperative signal

e 4 Receiver operator curves for signal averaged P averaged electrocardiography to predict atrialduration, total P wave duration from the standard fibrillation after CABG.67 Steinberg and col-)cardiogram, and left atrial diameterfor prediction of leagues analysed preoperative signal averagedfibrillation after coronary bypass grafting. Sensitivity P wave recordings in 130 patients, 21 (16%)tted against 1- specificity for a range of cutoff valuesch variable. Deviation towards the top left of the of whom underwent valve surgery rather thanby the curve representing signal averaged P wave isolated CABG.6 In their series, signal aver-ion implies increased discriminant ability over the aged P wave duration was the only preopera-variables presented. tive predictor of postoperative atrial

fibrillation, with a sensitivity of 77%, speci-ficity of 55%, and positive and negative pre-

'PERATIVE ELECTROCARDIOGRAPHY AND dictive accuracies of 37% and 87%. Despite)CARDIOGRAPHY their inclusion of patients undergoing valvesignificant differences were observed surgery in this series (which is likely toeen patients developing atrial fibrillation improve the apparent predictive ability of thethose remaining in sinus rhythm in any vari- signal averaged P wave for atrial fibrillation),derived from the standard electrocardio- these values are similar to those reported in

l, or in left atrial diameter derived from the our current analysis. Additionally, the cutoffiode echocardiogram. Filtered signal aver- value for P wave duration employed byP wave duration was significantly longer in Steinberg was similar, at 140 ms, to thatnts who developed postoperative atrial fib- reported in our current study, despite the useion (148 (12) ms v 142 (14) ms, P = of a different P wave averaging system, sug-8) (table 2) (fig 3). gesting that P wave duration may be similar)gistic regression analysis was performed on between these two systems which both usedficant univariate predictors of postoperative non-recursive filtering of the signal averaged Pfibrillation. We have previously defined a wave before duration was measured. Klein et

ed signal averaged P wave duration of more al also found that P wave duration was predic-141 ms as predictive of paroxysmal atrial tive of atrial fibrillation after CABG.7 These

lation.'2 Both male sex (odds ratio 2-4 (1 1 investigators excluded patients who under-5-1), P = 0 02) and the prospectively went valve surgery, but analysed only 54 sub-led signal averaged P wave duration cutoff jects. In these patients, P wave durationLs ratio 1.5 (1-0 to 2 0), P = 0 04) were predicted postoperative atrial fibrillation withtified as significant independent predictors a sensitivity of 69%, specificity of 79%, posi-)stoperative arrhythmia. A P wave duration tive predictive accuracy of 65%, and negativeore than 141 ms predicted atrial fibrillation predictive accuracy of 82%. These values, in aCABG with a sensitivity of 73%, specificity small number of subjects, are considerably

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Prediction of atrialfibrillation after coronary bypass grafting

better than those reported by ourselves andSteinberg, but may reflect the small number ofpatients studied.

Recently Frost et al have reported a compar-

ison of P wave duration from the standardelectrocardiogram and from the signal aver-

aged electrocardiogram using a commercialsystem.'3 In contrast to our investigation andthose of both Steinberg et al and Klein et althese investigators found that P wave durationmeasured from the standard electrocardio-gram was slightly, but significantly, greater inpatients who developed postoperative atrialfibrillation, whereas signal averaged P wave

durations did not differ. These workers havepreviously published an analysis of the repro-

ducibility of their signal averaging techniquecompared to analysis of the standard electro-cardiogram, finding that the signal averagingtechnique they use (Aerotel HIPEC 200 sys-

tem) was no better than analysis of the stan-dard electrocardiogram. 14 The apparentdisparity between our results, together withthose of Steinberg et al and Klein et al, andthose of Frost's group is unlikely to be relatedto differences in the patients studied, since weand Klein et al both studied a very similarpatient group to Frost's. It seems more likely,therefore, that these differences are due to theuse of different signal averaging methodology.The results of our study and those of

Steinberg et al suggest predictive values for thepreoperative signal averaged P wave for post-operative atrial fibrillation similar to those pre-viously reported by Buxton et al for analysis ofthe surface P wave from the standard electro-cardiogram.5 These investigators performed an

analysis of P wave duration measured in stan-dard leads I, II, and III. Total P wave dura-tion, defined as the time from the earliest atrialactivation to the last P wave signal in any ofthese three leads, was moderately predictive ofpostoperative atrial arrhythmia, as was the iso-electric interval (total P wave duration minuslead II P wave duration). A combination ofprolonged total P wave duration and pro-

longed isoelectric interval predicted atrial fib-rillation with a sensitivity of 66%, specificity of70%, positive predictive accuracy of 48%, andnegative predictive accuracy of 83%. The sim-ilarity of these results, using the standard elec-trocardiogram, to those achieved by signalaveraged electrocardiography has led to thesuggestion that the more complex and costlysignal averaged electrocardiogram is no betterfor prediction of postoperative atrial arrhyth-mia than the simpler and cheaper standardtest.8 However, Buxton's series was analysedapproximately 13 years before those reportingthe results of signal averaged electrocardiogra-phy, and contained a greater proportion ofpatients who developed atrial flutter thaneither our sample or that of Steinberg. Directcomparisons of the two techniques from thesedata alone are therefore likely to be of limitedvalue.

Both Steinberg et al and Klein et al mea-

sured P wave duration in lead II of the stan-dard electrocardiogram, finding it to beinferior to signal averaged P wave duration for

the prediction of atrial fibrillation after CABG.However, this result can easily be inferredfrom Buxton's original data if only lead IIduration is measured. Frost measured total Pwave duration, but not the isoelectric interval,finding it to be a better univariate predictor ofpostoperative atrial fibrillation than the signalaveraged P wave duration because of anapparent correlation with patient age. Thus Pwave duration from the standard electrocar-diogram was not an independent predictor ofpostoperative atrial fibrillation when a multi-variate model including patient age wasemployed. In our present investigation wereport a comparison of variables derived fromthe signal averaged P wave with those derivedaccording to the methods described byBuxton, thereby affording a direct comparisonin the same patient group of the relative clinicalvalue of the two techniques. Our results showthat the signal averaged P wave is a better pre-dictor of postoperative atrial fibrillation thananalysis of the surface P wave from the stan-dard electrocardiogram. Moreover, our dataalso suggest that signal averaged P wave dura-tion is independently predictive of atrial fibril-lation after CABG, although its predictivepower remains only modest.

It is perhaps surprising that preoperativesignal averaged electrocardiography has anypredictive ability for postoperative atrial fibril-lation, since very few of the patients in ourseries had a history of past atrial arrhythmia.Previous studies have examined the signalaveraged P wave in patients with documentedatrial fibrillation, finding the P wave to belonger and to contain greater energy than incontrols.'5-17 Increased P wave duration inpatients awaiting CABG therefore seems todetect a subclinical propensity to atrial fibrilla-tion that becomes apparent after additionaltrigger factors associated with the operation.As such the preoperative signal averaged Pwave may identify a subgroup of patients whocould be targeted in controlled trials of pro-phylactic antiarrhythmic treatments. Furtherstudy of the postoperative signal averaged Pwave may be rewarding.

CONCLUSIONSProspective analysis of standard electrocardio-graphy, left atrial diameter from M modeechocardiography, and the signal averaged Pwave in patients about to undergo elective firstCABG showed that signal averaged P waveduration was the best predictor of postopera-tive atrial fibrillation. The diagnostic accuracyof this test might be sufficient to identify agroup of patients at increased risk of postoper-ative atrial fibrillation, but using current tech-niques it is of limited clinical value inindividual patients.

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2 Leitch JW, Thompson D, Baird DK, Harris PJ. The impor-tance of age as a predictor of atrial fibrillation and flutterafter coronary bypass grafting. Jf Thorac Cardiovasc Surg1990;100:338-42.

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3 Yousif H, Davies G, Oakley CM. Perioperative supraven-tricular arrhythmias in coronary bypass surgery. IntCardiol 1990;26:313-8.

4 Frost L, Molgaard H, Christiansen E, Hjortholm K,Paulsen P, Thompson P. Atrial fibrillation and flutterafter coronary artery bypass surgery: epidemiology, riskfactors and preventive trials. Int Cardiol 1992;36:253-61.

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7 Klein M, Evans SJ, Blumberg S, Cataldo L, BodeheimerMM. Use of P-wave-triggered, P-wave-signal-averagedelectrocardiogram to predict atrial fibrillation after coro-nary bypass surgery. Am HeartJY 1995;129:895-901.

8 Seifert M, Josephson ME. P-wave signal averaging. Hightech or an expensive alternative to the standard ECG?Circulation 1993;88:2980-2.

9 Morris IJ, Estes EH, Whalen RE, Thompson HK,McIntosh HD. P-wave analysis in valvular heart disease.Circulation 1964;29:242-52.

10 Stafford PJ, Denbigh PN, Vincent R. Frequency analysis ofthe P wave: comparative techniques. PACE 1995;18:

261-70.11 Stafford PJ, Cooper J, Fothergill J, Schlindwein F, deBono

DP, Garratt CJ. Reproducibility of the signal averaged Pwave. Heart 1997;77:412-16.

12 Stafford PJ, Robinson D, Vincent R. Optimal analysis ofthe signal averaged P wave in patients with paroxysmalatrial fibrillation. Br Heart 1995;74:413-8.

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14 Christiansen EH, Frost L, Pilegaard H, Toftegaard-NielsenT, Pedersen AK. Within- and between-patient variationof the signal averaged P wave in coronary artery disease.PACE 1996;19:72-81.

15 Yamada T, Fukunami M, Ohmori M, Kumagai K, SakaiA, Kondoh N, et al. Characteristics of frequency contentof atrial signal-averaged electrocardiograms during sinusrhythm in patients with paroxysmal atrial fibrillation.Am Coll Cardiol 1992;19:559-63.

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