Action potential wavelength restitution predicts alternans and arrhythmia in murine Scn5a+/− hearts

Mice which are haploinsufficient in the Scn5a+/− gene have reduced cardiac sodium channel (Nav1.5) density and are used to model the Brugada syndrome. Conduction velocity restitution showed lower initial values and earlier points of failure during incremental pacing in the murine Scn5a+/− right ventricle (RV) epicardium particularly when treated with flecainide. The broadness of the conduction velocity restitution function was a poor indicator of arrhythmia or alternans. Conduction velocity alternans occurred abruptly and was more marked in the flecainide-treated Scn5a+/− RV epicardium. Introduction of wavelength restitution yielded functions that converged to a common instability condition in contrast to action potential duration (APD) or conduction velocity restitution. This occurred at significantly lower heart rates in Scn5a+/− RV epicardium following flecainide or quinidine challenge, corresponding to a smaller total wavelength (basic cycle distance) resulting from a reduction in conduction velocity. Wavelength restitution was superior at predicting alternans than either APD or conduction velocity restitution. Abstract Reductions in cardiac action potential wavelength, and the consequent wavebreak, have been implicated in arrhythmogenesis. Tachyarrhythmias are more common in the Brugada syndrome, particularly following pharmacological challenge, previously modelled using Scn5a+/− murine hearts. Propagation latencies and action potential durations (APDs) from monophasic action potential recordings were used to assess wavelength changes with heart rate in Langendorff-perfused wild-type (WT) and Scn5a+/− hearts. Recordings were obtained from right (RV) and left (LV) ventricular, epicardial and endocardial surfaces during incremental pacing, before and following flecainide or quinidine challenge. Conduction velocities (θ′), action potential wavelengths (λ′= APD ×θ′), and their corresponding alternans depended non-linearly upon diastolic interval (DI). Maximum θ′ was lower in Scn5a+/− RV epicardium than endocardium. Flecainide further reduced θ′, accentuating this RV conduction block. Quinidine reduced maximum θ′ in WT and caused earlier conduction failure in the RV of both Scn5a+/− and WT. Use of recovery wavelengths (λ′0= DI ×θ′) rather than DI, provided novel λ restitution plots of λ′ against λ′0, which sum to a basic cycle distance permitting feedback analysis. λ′ restitution gradient better correlated with alternans magnitude than either APD or θ restitution gradient. The large differences in θ′ and APD restitution contrasted with minor differences in maximum λ′ between epi- and endocardia of untreated hearts, and quinidine-treated WT hearts. Strikingly, all regions and conditions converged to a common instability point, implying a conserved relationship. Flecainide or quinidine decreased the pacing rates at which this occurred, through reducing basic cycle distance, in the Scn5a+/− RV epicardium, directly predictive of its arrhythmic phenotype. Ventricular arrhythmia, particularly ventricular fibrillation (VF), represents a major cause of death worldwide (Chugh et al. 2008). Risk of ventricular arrhythmia may be inherited, as in the Brugada syndrome (BrS). Previous work suggested that the mechanism of VF initiation involves breakdown of the propagating action potential (AP) wavefront into multiple wavelets, which cause scroll wave formation (Davidenko et al. 1995; Zaitsev et al. 2000). This leads, in a positive feedback manner, to further wavebreaks, the wavelets of which follow the chaotic and meandering conduction pathways that characterise VF. Wavebreak is proposed to result from heterogeneities existing within the cardiac tissue (Weiss et al. 2005). These may either be anatomical such as an ischaemic area or vascular structure, or they may be functional such as an increased dispersion of refractoriness resulting in pockets of inactivatable tissue. The travelling wave has an associated excitation wavelength (λ) given by the product of its action potential duration (APD) and conduction velocity (θ). The longer the λ, the less likely that areas of depolarisation and repolarisation will meet upon encountering tissue heterogeneity (Fig. 1C). There is normally a sufficient safety factor for the wave to completely pass over the heterogeneity, but decreases in λ below the extents of the heterogeneities increase the probability of wavebreak (Fig. 1D). Rapid heart rates decrease both APD and θ, as characterised by their respective restitution functions. Summary of wavelength calculations from primary electrophysiological data and their relationship to the concept of wavebreak re-entry A, typical monophasic action potential obtained from the Scn5a+/− RV epicardium upon which is superimposed indications of BCL, APD90, latency and DI of both the current (nth) and its preceding ((n–1)th) action potential. These form the basis for computations of the wavelength parameters representing λ′ and λ′0 (B). These sum together to give the which is by the pacing rate and conduction velocity of the tissue. wavelength encountering a heterogeneity which may cause a conduction block. This may be to wavelength alternans to formation or or anatomical to or the wavelength over the heterogeneity, is by the of the propagating to be the excitation wave to wavelength encountering the this the has the heterogeneity before the excitation has through the block. a propagating wave is which may in the a APD APD is against diastolic interval has previously been for arrhythmia This an feedback APD alternans from a in the pacing the function gradient between and is to be a major mechanism for an arrhythmic APD in areas of cardiac tissue of area has a APD and the a result in an APD gradient that area of with an as the at which alternans et al. within the the which the meet the when the area is and their less longer distance pathways the when the area has and of et al. This the to the the of a et al. et al. the cardiac tissue at a ventricular the of wavebreak and to VF. APD restitution has been in at predicting et al. a relationship et al. recordings obtained during the incremental pacing at and heart rates in the WT RV epicardium WT RV flecainide-treated WT RV epicardium Scn5a+/− RV epicardium and flecainide-treated Scn5a+/− RV epicardium The flecainide-treated Scn5a+/− RV epicardium longer latencies at both pacing rates and alternans at the channel Scn5a+/− have been used to model in which in of Scn5a+/− of the at heart rates and more VF following flecainide at heart rates et al. have in the right et al. Scn5a+/− further changes with in common with the condition et al. have an increased to APD alternans et al. in et al. This may their reduced channel et al. to in θ, APD and These will result in to the following alternans. of directly APD alternans and its relationship to the restitution function gradient in the Scn5a+/− that both APD alternans and restitution were in the right ventricular (RV) epicardium following flecainide challenge, APD restitution the relationship between the was and rather than with an This from the relationship in an of the magnitude of that the APD restitution is et al. than feedback such as et al. and θ may be alternans. of θ using a restitution function parameters with of velocity and and to a feedback relationship to using Scn5a+/− have cardiac conduction et al. The the haploinsufficient Scn5a+/− to a in of λ, for feedback to ventricular arrhythmogenesis. The θ restitution and alternans that arrhythmic into a novel λ restitution as in the The of this over to the of λ with rate et al. is that feedback and instability in the form of alternans. were by the of a for of cardiac were in and the Mice were in at to and to and Mice were by were and to the or were WT were of which were were and were Scn5a+/− were of which was were and were The have previously been in et al. and WT and Scn5a+/− et al. were used to electrophysiological the changes associated with in the et al. obtained following were and with and sodium with to a of at a rate of monophasic action were by a upon the left (LV) and right ventricular epicardial with the heart to a recordings were using an by of were for and or RV was at the of by recordings were to and at for using were at using a the ventricular to a using using pacing was used to recordings with and to et al. The incremental pacing was at a basic cycle for by a of This was showed either into or arrhythmogenesis. the was for the of and to be at to following in arrhythmia to and et al. of by of and Scn5a+/− showed VF and Flecainide or quinidine were to give of and as used in work et al. et al. et al. and with for before or of were their The was as the at the The λ′0 at the of between the magnitude of alternans and the restitution the predicting to alternans. was obtained by the of and the resulting function by The is the at the electrophysiological data from the were normally and were as and the restitution data from to the functions were normally data were as and in the form for differences between used which yielded values of of and were in of the data the WT and Scn5a+/− hearts, in the and of flecainide or at or epicardial or endocardial These using are as their and was as is the of the and is the of between regions and were as they Scn5a+/− and WT hearts. within given was to endocardial and epicardial recordings in in the to the for endocardial Recordings were in at from the were to either flecainide or and were used in and data the of the RV or epicardial or endocardial pharmacological further the obtained at for given with and flecainide or quinidine between WT and Scn5a+/− at for pharmacological they the of flecainide against quinidine at for were using an to values have been for values are in the with the are in data for as as and latency values are in data for and are in data for the RV are in the are in the A, of the θ conduction velocity λ wavelength restitution and wavelength alternans BCL, basic cycle the at which the at which the at which of the function restitution and alternans which is to diastolic interval gradient of the APD restitution is the diastolic interval conduction velocity of DI between repolarisation and magnitude of λ′ function θ conduction velocity restitution and λ wavelength maximum maximum gradient of the λ restitution maximum gradient of the λ alternans against λ′0 of of the function restitution and of of the function restitution and of the of the θ conduction velocity λ wavelength restitution and wavelength alternans initial used for θ, conduction θ′, latency which is to conduction velocity distance maximum conduction velocity by restitution λ, wavelength, distance wavelength, distance the λ′0 at which the λ restitution is at the of wavelength at the the of wavelength at which the wavelength is of maximum wavelength by restitution λ′0 corresponding to gradient in the λ restitution the wavelength at the between restitution and alternans of the θ conduction velocity λ wavelength restitution and wavelength alternans of The basic cycle distance the product of the and λ is the wavelength given by the product of θ and is the wavelength given by the product of θ and which the product of θ and is in form to and of restitution functions with a instability at gradient. were in RV and and of WT and Scn5a+/− before and following of either flecainide or typical recordings from WT and Scn5a+/− and RV before and following flecainide at and heart rates were increased at the pacing rate and following of flecainide. the of pharmacological latency alternans at the of the pacing such marked at longer and with in flecainide-treated Scn5a+/− RV epicardium. occurred in quinidine-treated WT RV endocardium. Conduction velocities were from latencies at the preceding pharmacological latencies were in Scn5a+/− RV epicardium than in either the Scn5a+/− RV or WT RV epicardium or and This a conduction within the the right in flecainide or quinidine conduction velocity in both as from their channel Flecainide increased the latencies in Scn5a+/− RV epicardium to This in an in the Scn5a+/− RV Quinidine increased conduction in all WT particularly in the RV to Quinidine increased Scn5a+/− RV epicardium conduction latencies to typical restitution plots of θ′ against DI through all the regions and Previous et al. as either or this to or arrhythmic Conduction velocity restitution of θ′ against DI obtained from typical WT and Scn5a+/− in the RV epicardium and RV before and following of flecainide or quinidine The data the of in the the and of such between Flecainide and quinidine both reduced conduction but this was marked in the flecainide-treated Scn5a+/− RV epicardium. in function were characterised by a was in the WT than in the corresponding epicardium was in the WT by either flecainide or quinidine to and to the of was in the WT epicardium than the corresponding Scn5a+/− These are with the RV arrhythmic in the Scn5a+/− et al. the of function broadness for arrhythmic θ′ restitution functions provided a maximum of θ′ at the heart rates was in WT RV epicardium and contrast was lower in Scn5a+/− RV epicardium than the corresponding Flecainide further reduced in the Scn5a+/− RV epicardium to Quinidine reduced in the WT RV and WT epicardium to and from to Quinidine in a between the WT RV epicardium and the of quinidine was smaller in the Scn5a+/− RV epicardium than the corresponding WT These the RV conduction in the Scn5a+/− and its by flecainide at heart rates such as during or the but the in the Scn5a+/− which may the dispersion in The DI at which conduction velocity to at heart rates was increased by flecainide in Scn5a+/− RV epicardium and corresponding to and from to Quinidine smaller in in Scn5a+/− RV epicardium and WT RV to and from to Flecainide conduction failure in the Scn5a+/− RV at pacing Quinidine in conduction failure in the RV of both Scn5a+/− and WT hearts. contrast with of of APD alternans with DI, θ′ alternans occurred abruptly before This and of alternans to θ′ restitution of θ′ alternans were at pacing rates (Fig. corresponding to murine heart was in magnitude of θ′ which was to in all at rates of corresponding to heart flecainide increased the magnitude of θ′ alternans in the Scn5a+/− RV epicardium to This occurred before the of refractoriness in this and plots the magnitude of in the RV at and heart rates the the the and the the alternans occurred at heart rates with all to and differences between the magnitude of θ′ alternans is increased by rate in θ′ alternans was in the Scn5a+/− RV epicardium following flecainide as by the These that θ′ alternans was marked in the flecainide-treated Scn5a+/− RV epicardium, which is the previously implicated in the initiation of arrhythmia et al. their in the the contrast between the of the θ′ alternans function with the θ′ restitution a relationship between The a of θ and APD parameters to give wavelength, which has previously been as a major for θ and APD were to give λ′ and this is against DI (Fig. λ′ decreased non-linearly with DI, as previously et al. in common with θ′ restitution further was by their and of λ′ against DI obtained from typical WT and Scn5a+/− in the RV epicardium and RV before and following of flecainide or quinidine The data the of in the λ′ was against a λ′0, permitting a feedback in which instability at a gradient of typical plots from WT and Scn5a+/− epicardium and before and following with flecainide and The resulting together with smaller to or λ′ restitution λ′ against λ′0 obtained from typical WT and Scn5a+/− in the RV epicardium and RV before and following of flecainide or quinidine The data the of in the together particularly at λ′0 values than the plots in either or or previously APD restitution at values of were differences in the of between WT or RV or or epicardial or endocardial before or following of either flecainide or This a a common or wavelength corresponding to the of positive feedback to The at which the common of occurred, showed predictive differences in arrhythmic Flecainide increased in Scn5a+/− RV epicardium to (Fig. Quinidine increased in both WT RV and the Scn5a+/− RV epicardium to and from to using λ′ restitution A, plots of λ′ restitution against the in the Scn5a+/− RV epicardium and Scn5a+/− RV epicardium treated with the corresponding to the was increased by flecainide in the Scn5a+/− RV epicardium This was to changes in the in and plots at and pacing in the the the and the between pacing, and between The the λ restitution at given pacing rate is given by the total wavelength or basic cycle distance This may a common instability in but a heart rate of (Fig. in untreated Scn5a+/− RV epicardium and were smaller than in the corresponding WT and Quinidine decreased in the WT RV to an heart rate of (Fig. in untreated Scn5a+/− RV epicardium was smaller than in WT RV epicardium There was an in the Scn5a+/− RV Flecainide reduced the in the Scn5a+/− RV epicardium to Quinidine marked in the WT. the in the quinidine-treated WT RV was smaller than in the corresponding Scn5a+/− the at the showed differences between or pharmacological a at a of Maximum values of excitation wavelength at heart showed differences in the untreated WT and untreated WT RV the untreated differences were in the RV was in the WT than in the corresponding Scn5a+/− Quinidine decreased in the WT RV to and WT to differences were in flecainide-treated untreated was in the than the epicardium. This is to conduction from the epicardium, but in heterogeneity existing the Flecainide and quinidine both this by reducing endocardial pacing the maximum of the λ′ restitution a of a of There were differences in the of λ′0 at which λ′ was This was in the flecainide-treated WT RV to the corresponding epicardium the at the was significantly in both or values arrhythmic and conditions with rate of λ restitution of the magnitude of λ′ alternans against λ′0 (Fig. were with the λ restitution with in the λ restitution were differences between the values of λ′0 that to a gradient of between were differences in maximum gradient differences in the at which the occurred These differences in the Scn5a+/− RV epicardium following of either flecainide to or quinidine to in with the λ restitution changes in was in the Scn5a+/− epicardium as to the corresponding The of λ alternans λ plots of the magnitude of λ′ against λ′0 in WT and Scn5a+/− RV epicardium and RV before and following of either flecainide or quinidine the of as all together with common The values of λ′0 at which the λ′ alternans plots and λ restitution plots to of propagating was in the of flecainide in the WT RV than in the corresponding epicardium was decreased by quinidine in the WT epicardium to This differences in the of caused by alternans following of channel the Scn5a+/− RV epicardium values of before and following of this at a significantly to with an earlier of conduction resulting from λ alternans. functions that the magnitude of alternans non-linearly increased with restitution gradient. that when over a between restitution of to the λ′ restitution relationship showed a and converged the of APD restitution significantly the magnitude of alternans and from that a function which θ is superior to APD λ′ restitution a alternans than was and was to a of chaotic feedback This or more to θ and APD which may the alternans at of the between the APD restitution gradient and the magnitude of APD alternans from et al. and between the gradient and the magnitude of This that between a gradient of and for either function the λ′ restitution a better with and a smaller from the the APD alternans from and a from the a relationship. The Brugada syndrome is an condition associated with in the the channel This the and its is associated with a to ventricular particularly and VF is characterised by a a right as as and wave in the right et al. and associated with are but may be to the through flecainide et al. contrast to have suggested that electrophysiological of arrhythmia using pacing a et al. et al. an of the the following quinidine may an et al. better of the in and The of haploinsufficient murine to this Scn5a+/− a reduction in RV et al. and This a arrhythmic particularly following flecainide et al. et al. Summary of the concept of wavelength restitution by the murine Scn5a+/− following flecainide A, of the in which form the basis of the arrhythmic in a channel reduction in the of the consequent in untreated WT untreated Scn5a+/− and flecainide-treated Scn5a+/− The Scn5a+/− RV epicardium longer latencies and when to WT. Flecainide latency but These differences depolarisation and repolarisation that are rate and by conduction velocity and APD restitution conduction in untreated Scn5a+/− to WT. This is by flecainide. data from et al. that untreated Scn5a+/− and WT have flecainide this in Scn5a+/− RV epicardium. both depolarisation and repolarisation to the λ restitution These may differences in maximum but to a common of instability the pacing rate to this is significantly lower in the Scn5a+/− RV epicardium when treated with flecainide. the of the λ′ restitution to the magnitude of λ′ alternans through a which over heart rates than previously for APD restitution. This in excitation wavelengths that may give to heterogeneities or of re-entry an existing heterogeneity, and consequent arrhythmia in the form of Previous the of APD and alternans with heart more with pacing in the of flecainide in the Scn5a+/− RV epicardium at lower rates than in or in the WT et al. of arrhythmic that untreated Scn5a+/− but WT showed of at pacing with Scn5a+/− showed of VF than WT at heart Scn5a+/− hearts, quinidine decreased the of at rates and increased the of VF at quinidine was in the WT at all heart with the at APD restitution from Scn5a+/− RV epicardium showed increased (Fig. This was by increased of APD alternans. the relationship between APD alternans and restitution through pacing rates was and the of alternans et al. This the of restitution in the of arrhythmic in et al. et al. that between APD restitution and alternans may that θ The the of heart rates θ conditions to in the a more wavelength restitution of the of wavebreak in in conduction latencies θ were characterised at and (Fig. of the broadness of the θ restitution function et al. poor of the arrhythmia and alternans. the maximum θ′ and the DI at which conduction correlated with arrhythmic the Scn5a+/− RV epicardium both before and following flecainide (Fig. θ′ alternans was less than previously for APD an at pacing rates Flecainide increased θ′ alternans in Scn5a+/− RV epicardium, in with the changes in maximum θ′ and conduction APD and θ′ were to a reduction of excitation wavelength with DI, to in et al. of DI, in a feedback against a distance, of using restitution that both upon and are This was by a further of the DI into a wavelength, given by the product of θ′ and DI, a novel λ restitution function (Fig. heart epicardial and maximum values of λ within the untreated hearts. Quinidine decreased the WT endocardial maximum λ and the Scn5a+/− with its in WT but in Scn5a+/− et al. The novel λ restitution APD that with heart positive feedback to failure of wave and feedback at a λ restitution gradient of Strikingly, this at and total in both all and before or following of either flecainide or This a common for the of The pacing rates at which this was were in untreated they were decreased by flecainide and quinidine in the Scn5a+/− RV epicardium, and by quinidine in the WT RV endocardium. a the at given pacing rate was reduced both by the Scn5a+/− and by the further of flecainide in the RV epicardium (Fig. was reduced by quinidine in the WT RV endocardium. is given by the product of the and θ′, are to in θ as was heart rate be an factor in The corresponding λ alternans showed common wavelengths of or this occurred at pacing rates in the Scn5a+/− RV epicardium following of either flecainide or are in with the λ restitution the of heart rates that were the gradient of the λ restitution function with the magnitude of its alternans more than APD restitution (Fig. for changes in conduction velocity by using λ restitution is to the basis of and is a superior of arrhythmia (Fig. instability points λ restitution the relationship between excitation and This the of either between channel and changes et al. or which channel through in and resulting in a product of APD and was less between the λ restitution function and λ restitution provided a of the alternans and the maximum gradient of the function was to in contrast to the obtained by APD restitution. This the of for changes in θ but the of a may the magnitude of alternans. such may be at or increased as this is to et al. et al. and et al. The were obtained in murine et al. permitting of and their to pharmacological et al. common with hearts, their ventricular excitation of a by (Nav1.5) and et al. which is by and and have more resulting in the function of the and the in repolarisation is by a current et al. et al. et al. and et al. et al. et al. 2008). and conduction et al. et al. et al. with the of in the tissue the common from et al. the for major conduction with changes in heart which result in in the θ restitution et al. These differences with that excitation and recovery wavelengths permitting wavebreak values may between and hearts. The were in Langendorff-perfused which et al. et al. for the of cardiac that be recordings a of latency and repolarisation with et al. that recovery with recordings in an et al. recordings or but values were in the current analysis. were a that the of the in with the the of the of with the of the distance and of the recordings cardiac have been used in and et al. et al. et al. The the of pharmacological in current or for The major and of flecainide is a of the but this current is in murine et al. the of the resulting in reduced such as flecainide the from to and or et al. in of flecainide all Quinidine with but major in et al. in to and The of are by the of conduction velocity to reducing This the basic cycle in the Scn5a+/− RV epicardium, which is to to its particularly reduced channel density et al. Flecainide less the to a of particularly in the quinidine in the particularly the RV endocardium. This is to changes in endocardial wavelength at lower pacing rates caused by channel in with in conduction velocity. The Scn5a+/− is from this to the of et al. The have for arrhythmia rate and of cardiac The that of flecainide increased in ischaemic heart the of reducing to for rate in arrhythmia et al. The that which θ be in or be to when are to in total wavelength This be by either of or the of which the at to reducing are a common cause of resulting from conditions such as Brugada syndrome or conditions such as ischaemic heart have a predictive and and electrophysiological using pacing to arrhythmia be used in but have been from the Brugada syndrome to of predictive of arrhythmic parameters such as APD restitution have These from of the of which repolarisation and depolarisation The current novel the of alternans and The resulting wavelength restitution and alternans were in Brugada syndrome and be used as in or their is likely to be to all arrhythmic be used in both of and more than current and the that for conduction velocity of the channel be in safety they that that heart rate or increase conduction velocity be in the of current rate in to have of to was in the and of the and of and and of the was in data and were in of the was the in and of the as as and the were in the this to the and of of as as the and of for their of for data data data The is for the or of by the than be to the corresponding for the