assessment of the p wave morphology is the first step in the mapping and ablation of both focal atrial tachycardia ( fat ) and atrial flutter ( afl ) .
fat is a relatively uncommon mechanism of svt , characterized by a pattern of activation from a point source in the atrium .
the underlying mechanism of the focal firing can be abnormal automaticity , micro - reentry or triggered activity .
afl e.g. typical right atrial flutter involves macro re - entry around one or more fixed or functional areas of block , with the circuit involving the majority or the whole of the chamber .
both can be ablated with high rates of success , but an understanding of whether the tachycardia is focal or macro - reentrant is necessary for ablation to succeed .
this may occur fortuitously during recording of the ecg , but often the p wave is partially obscured by the qrs complex or t - wave . to obtain an  unencumbered view of atrial activity , one can give adenosine or perform carotid sinus massage .
however , an effective method in the ep lab is to pace the ventricle during svt in order to reveal the p waves in the compensatory pause that follows . in order to accurately assess the p wave morphology
, one must determine the onset , and the initial positive or negative deflection may be preceded by an isoelectric interval that may vary from lead to lead .
therefore , it is best to examine p wave morphology in all 12 ecg leads simultaneously with adequately increased gains .
use of p wave morphology to localize atrial activation has been studied largely in patients with normal atria ; hence in diseased or previously ablated atria it may not be as reliable .
atrial activation from a focal source in the atrium can propagate radially from myocyte to myocyte , but more rapid activation typically occurs along the direction of fiber orientation and through myocardial bundles or tracts .
connections exist between the right and left atria , the best known of which is bachmann s bundle superiorly .
there are further connections described a ) posteriorly joining the inter - caval area of the right atrium to the left atrium , b ) in the region surrounding the fossa ovalis , and c ) inferiorly from the left atrium running into the wall of the coronary sinus [ 3 , 4 ] . as a consequence even in sinus rhythm
upright p waves in the right pre - cordial leads appear to be associated with conduction , not only via bachmann s bundle but also via other connections in the posterior atrium , thus resulting in posterior to anterior propagation of sinus beat in both right and left atria . in early intra - operative pacing studies by waldo et al .
, during right atrial pacing from low endocardial sites , depending on the site paced the p wave in inferior leads could be either negative , biphasic , or positive .
more recently , non - contact mapping studies have confirmed variation in the preferential routes of atrial activation in vivo both during sinus rhythm and atrial tachycardia [ 7 - 9 ] .   due to the large circuit , afls
. there can be exceptions to this general rule , particularly in scarred or previously ablated atria , such that afl can have the appearance of a focal circuit , and fat can appear 
1 ) . variation in p wave cycle length is more suggestive of a focal mechanism . in equivocal cases , entrainment or detailed electro - anatomic activation mapping
if a focal mechanism is suspected , the first step is to determine the chamber of origin .
one of the first algorithms to localize the origin of a fat was produced by tang and colleagues from a series of 31 patients , where the most useful leads for discriminating left and right atrial origin were v1 and avl .
the sensitivity and specificity of a positive or biphasic p wave in lead avl to predict a right atrial focus were 88% and 79% respectively .
the sensitivity and specificity of a positive p wave in lead v1 in predicting a left atrial focus were 93% and 88% respectively .  
2 ) based on a series of 126 patients to determine the likely site of origin of fat .
the algorithm was then prospectively tested in further 30 patients and found to have accuracy of 93% .
a negative or biphasic ( positive , then negative ) p - wave in lead v1 was associated with a 100% specificity and ppv for a focus from the right atrium .
a positive or biphasic ( negative , then positive ) p - wave in lead v1 was associated with a 100% sensitivity and npv for a focus originating in the left atrium .   a potential disadvantage of many electrophysiology diagnostic 
algorithms is that if the first step of assessment of the algorithm is incorrect , this can lead the clinician down the wrong pathway .
this can be exacerbated by inter - individual variation of the atrial anatomic position , relative to each other and to the ecg leads .
qian and colleagues have observed that using multiple ecg leads for each algorithm decision step yields greater accuracy in locating the focus to a region of the atrium ( see fig .
positive p waves in inferior leads and a negative p wave in lead avr indicated high atrial origin with a sensitivity of 95% and a specificity of 90% .
negative p waves in inferior leads and a positive p wave in lead avr suggested right low septal origins , with sensitivity and specificity 88% and 89% , respectively .
it is important to remember that fat does not occur randomly throughout the atrium and tends to cluster around certain anatomic regions .
common sites of origin in the right atrium include the crista terminalis , tricuspid annulus , coronary sinus os , perinodal region , and more rarely the right atrial appendage ( see fig . ( 3 ) for examples ) . within the right atrium ,
a negative p wave in avr has high sensitivity and specificity for the crista terminalis .
fat can arise from anywhere around the tricuspid annulus , but a p wave that includes negative polarity in v1 and is positive or isoelectric in avl is associated with a very high specificity for a focus at the tricuspid annulus .
tricuspid fat often has a characteristically bifid negative morphology in lead v1 , and the polarity of leads ii and iii is deeply negative for an inferior location , and low amplitude , positive , or biphasic for a superior location .
coronary sinus os p wave morphology is deeply negative in the inferior leads , positive in avl and biphasic negative or isoelectric to positive in lead v1 , then progressively negative across the precordium .
fat from the right atrial appendage has p waves that are negative in lead v1 , becoming progressively positive across the precordial leads and in the inferior leads these p waves are low amplitude positive in the majority of patients .  
left sided fat occurs in the pulmonary veins , mitral annulus and more rarely the left atrial appendage ( see fig .
pulmonary vein fat tends to have positive p waves across the precordium , with negative p waves in avl and avr .
left sided veins have a broader , notched p wave compared to right side veins , and superior veins a taller p wave in the inferior leads .
yamane and colleagues reported that positive p waves in i and avl had high specificity , but moderate sensitivity for right pulmonary vein origin .
superior pulmonary vein origin could be distinguished from inferior vein by amplitude in lead ii of > 100 v .
mitral annular fat tends to cluster superiorly around the aortomitral continuity with a p wave that is negative - positive biphasic in v1 , low amplitude in the limb leads and negative or isoelectric in i and avl .
the p wave in left atrial appendage ( laa ) fat is similar to that from a left - sided pulmonary vein , but the presence of a deeply negative p - wave in lead i suggests an origin in the laa .
it is important to remember that the overall spatial resolution of the p wave may be not more than 1.7 cm . therefore locations where p wave morphology is less useful are areas of close proximity of left and right atrial structures e.g. right upper pulmonary vein and crista terminalis . in this case , it can help to compare sinus and fat p waves in v1  right upper pulmonary vein foci show a change in configuration from biphasic in sr to upright in at , a change not observed for right atrial tachycardias .
the mode of induction of tachycardia can also be helpful in distinguishing these  cristal fat often requiring programmed stimulation , whilst pulmonary vein fat tends to be spontaneous or requires isoprenaline for induction .
5 ) , where an isoelectric or low amplitude p wave in v1 points towards origin in the right septal or perinodal region , while a biphasic negative - positive p wave in v1 raises the possibility of a left septal focus .
mapping and ablation of septal fat from the aortic non coronary cusp are also recognized and in the series of ouyang et al .
this too was associated with a biphasic negative - positive p wave in v1 and v2 .
some afls such as typical right atrial flutter have a readily recognizable morphology that allows their identification with confidence . however , due to the fact that afl can arise due to scar that is of variable location , the ecg can be similarly variable .
if one is trying to decide if the chamber involved is right or left , the most useful lead is v1 .
a broad - based upright p wave in v1 is predictive of left - sided flutter , but when v1 has an initial isoelectric ( or inverted ) component followed by an upright component ; this is consistent with a right afl .
when v1 is deeply inverted , this strongly suggests a right - sided afl , but when v1 is biphasic or isoelectric , it is not helpful in predicting the chamber of origin .  
right atrial flutters can be divided into cavo - tricuspid isthmus dependent and other independent circuits .
the former can be treated by linear cavo - tricuspid ablation so predicting the circuit from the p wave morphology could be useful in selecting an ablation target .
typical atrial flutter is the commonest afl , involving a counterclockwise circuit around the right atrium when viewed from the left anterior oblique perspective .
the anatomic circuit has been well characterized with the crista terminalis , eustachian ridge , and tricuspid annulus identified as barriers to conduction [ 25 , 26 ] .
the p wave morphology of typical flutter is well known as having an inferior lead 
sawtooth pattern , but closer inspection reveals an initial gradual downward segment followed by a sharp descent , then a sharp ascent with a low amplitude terminal positive component that merges into the next flutter wave ( see fig .
the negative p wave in the inferior leads is because left atrial activation dominates the p wave and in typical flutter starts at the coronary sinus and proceeds in a superior direction . in the precordial leads , v1 is biphasic isoelectric then upright , and moving across the precordium the initial component becomes negative and the second component isoelectric .
this typical appearance can be modified in the presence of previous surgery or ablation in the atria , including af ablation .  
reverse typical flutter involves the same circuit as typical flutter but the direction of rotation is now clockwise .
the ecg appearance is more variable but in some respects is the opposite of typical counterclockwise flutter ( see fig .
the positive p wave in the inferior leads is due to left atrial activation via bachmann s bundle that then proceeds in an inferior direction .
there is usually a downward component preceding the upward notched component , but the appearance can vary from continuous undulation without a predominant upward or downward component , or it may even appear that the downward component is dominant almost resembling counterclockwise flutter .
the p wave in v1 is broad negative and usually notched with transition across the precordium to an upright deflection in v6 .
lower loop re - entry is another cavo - tricuspid dependent circuit , which can rotate counterclockwise or clockwise around the inferior vena cava with break through the conduction barrier at the crista terminalis .
a potential clue to the presence of lower loop re - entry or another atypical ra flutter is a cycle length that is faster than the usual approximately 200ms seen with typical flutter .
p wave morphology is dependent on the caudo - cranial level at which the wavefront breakthrough occurs in the crista terminalis .
as the la and septum are activated in a similar sequence to counterclockwise typical afl , the ecg appearance is similar .
however when breakthrough occurs at the low crista , an ascending wavefront collides with the counterclockwise wavefront that is heading towards the lateral wall from the interatrial septum and roof of the right atrium .
loss of the late inferiorly directed forces is seen on the ecg as attenuation in the late positive deflection of the p wave normally seen in counterclockwise typical flutter .
if there are multiple breakthroughs in the crista , then the p wave morphology can vary during tachycardia .
upper loop reentry rotates around the superior vena cava through a break in the crista terminalis .
the ecg morphology is often similar to clockwise typical flutter due to similar activation pattern of the left atrium .
however , it has been reported that a negative or isoelectric flutter wave with amplitude < or = 0.07 mv in lead i can differentiate upper loop reentry from clockwise typical flutter .
atypical flutter circuits involving the free wall of the right atrium usually arise as a result of prior surgical incisions or less commonly due to spontaneous scar .
spontaneous right atrial free wall afl is uncommon and has been described in a couple of small series , in which the p wave morphology was quite varied [ 36 , 37 ] . due to the variable location of scar and other factors including direction of rotation and whether the typical flutter circuit is also involved in a 
double loop configuration , the ecg is not of great value in defining the circuit . a feature that can be useful in indicating a right atrial
ablation of these circuits is targeted at the low lateral right atrium with linear ablation from the scar to the inferior vena cava or tricuspid annulus .
demonstration of bidirectional block across the linear ablation rather than just acute termination of flutter is important to ensure long - term success .  
left sided afl circuits can also arise as a result of prior surgery or ablation or due to spontaneous scar 
described a series of 22 patients with left atrial afl arising as a result of spontaneous scar .
circuits observed included peri - mitral , roof dependent or around an area of scar , and in minority dual or three loops . in all patients ,
the surface ecg showed a regular monomorphic p wave predominantly positive in lead v1 and distinct in the limb leads from the pattern described for right atrial flutter ( see fig .
linear ablation to the roof or between the mitral annulus and another anchor point were employed to interrupt the circuit . in the series by ouyang et al .
the majority of circuits were dual loop and once again linear ablation across the critical isthmus was successful . in the presence of such complex circuits
the p wave morphology may be of limited use apart from localizing the problem to the left atrium .
the ablation target in fat is elimination of the focal source of activity , while in afl the aim is to create a line of block between two electrically inert atrial structures in order to interrupt the macro - reentrant circuit . in practice
one would tend to use some form of activation mapping or entrainment to define the focus or circuit before ablation .
nonetheless , careful evaluation of the p wave morphology is often extremely valuable in directing where to focus efforts at mapping and in planning the ablation strategy .
 focal atrial tachycardias arise preferentially from specific locations within the atria . careful analysis of the p wave can provide useful information about the chamber and likely site of origin within that chamber . 
 macro - reentrant atrial flutter also tends to occur over a limited number of potential circuits . in this case 
 , the ecg usually gives a guide to the chamber of origin , but unless it shows a specific morphology it is less useful in delineating the circuit involved . 
 nonetheless , prior knowledge of the likely chamber of origin helps to plan the ablation strategy . 
INTRODUCTION 
PRACTICAL SUGGESTIONS FOR ASSESSING THE P WAVE 
CONTRIBUTION TO THE P WAVE
FOCAL ATRIAL TACHYCARDIA
ATRIAL FLUTTER
HOW TO INTEGRATE THE P WAVE INTO THE ABLATION STRATEGY
CONFLICT OF INTEREST