From the Departments of Pharmacology (A.C.S., R.M., O.B., J.M.D., J.J.)
and Pediatrics (Cardiology) (R.M.), SUNY Health Science Center at Syracuse,
Syracuse, NY.
Correspondence to Dr J. Jalife, MD, Department of Pharmacology, SUNY Health Science Center, Irving Ave, Syracuse, NY. E-mail jalife{at}vax.cs.hscsyr.edu
Methods and Results—We used a combination of high-resolution
video imaging, ECG recordings, and spectral analysis to
identify sequential wave fronts with temporal periodicity and similar
spatial patterns of propagation during 20 episodes of AF in 6
Langendorff-perfused sheep hearts. Spectral analysis of AF
demonstrated multiple narrow-band peaks with a single dominant peak in
all cases (mean, 9.4±2.6 Hz; cycle length, 112±26 ms). Evidence of
spatiotemporal periodicity was found in 12 of 20 optical
recordings of the right atrium (RA) and in all (n=19)
recordings of the left atrium (LA). The cycle length of
spatiotemporal periodic waves correlated with the dominant frequency of
their respective optical pseudo-ECGs (LA:
R2=0.99, slope=0.94 [95% CI, 0.88 to
0.99]; RA: R2=0.97, slope=0.92 [95% CI,
0.80 to 1.03]). The dominant frequency of the LA pseudo-ECG alone
correlated with the global bipolar atrial EG
(R2=0.76, slope=0.75 [95% CI, 0.52 to
0.99]). In specific examples, sources of periodic activity were seen
as rotors in the epicardial sheet or as periodic breakthroughs that
most likely represented transmural pectinate muscle
reentry. However, in the majority of cases, periodic waves were seen to
enter the mapping area from the edge of the field of view.
Conclusions—Reentry in anatomically or functionally determined
circuits forms the basis of spatiotemporal periodic activity during AF.
The cycle length of sources in the LA determines the dominant peak in
the frequency spectra in this experimental model of AF.
Although some data suggest underlying spatial
organization,10 11 12 13 little work has been done on the
temporal characterization of AF. Analyses of recordings
of the arrhythmia in the frequency domain have demonstrated
multiple narrow-band peaks, often with a single dominant
peak.10 11 12 The presence of a dominant peak
suggests that a substantial portion of the atria is activated
at that frequency, potentially in a spatially ordered manner. However,
the spatial patterns of wave propagation that correspond to the
dominant peak of the spectral analysis remain unknown. We used
high-resolution video imaging in combination with ECG techniques and
frequency analyses to simultaneously study
electrical wave propagation on the surface of both atria in an acute
model of AF in isolated Langendorff-perfused sheep hearts. We had 2
principal aims: (1) to identify patterns of wave propagation that
demonstrate spatial and temporal periodicity and (2) to use spectral
analysis to determine the contribution of the periodic activity
to the frequency content of global measures of atrial electrical
activity, especially in relation to the dominant frequency.
High-Resolution Optical Mapping
Isochrone Maps
Pseudo-ECGs
Bipolar EG
Definition of AF
Analyses
Periodicity Analysis
Signal Analysis
Statistical Analysis
Spectral Analysis of Global Measures of Activity
Spectral Analysis of Pseudo-ECGs
Periodic Activity in Optical Recordings
The data in Figure 2
To assess quantitatively the contribution of the frequency of the local
periodic activation to that of the global measures of electrical
activity in all analyzed episodes of AF, the cycle length of
the periodic activations (in ms) was correlated with the inverse
frequency (1/f in ms) of the dominant peaks of the RA and LA pseudo-EGs
and the recorded bipolar EG. Figure 3A
To determine the contribution of the individual atria to the dominant
peak of the bipolar EG, we correlated the frequency of the dominant
peaks of the pseudo-ECGs with the dominant peak from the spectral
analysis of the bipolar EG. These data for the LA are shown in
Figure 3C
Periodic Activity in Transmembrane Signal of Optical
Recordings
In Figure 4
Results that were qualitatively similar to those in Figure 4
In summary, in all analyzed episodes, at least 1 segment of AF
demonstrated spatiotemporal periodicity. Furthermore, the frequency of
the spatiotemporal periodic activity in the LA was
represented as the dominant peak in the frequency content
of measures of global atrial electrical activity (ie, bipolar EG and LA
pseudo-ECG). Complex patterns of wave propagation away from the source
of periodic activity resulted in complex electrical activity, as seen
in the optical recordings of the LA. Activation patterns seen
from the optical recordings of the RA were highly complex, with
incomplete reentry, breakthroughs, and wave collisions. However,
despite the spatial complexity, spectral analysis suggested
that the overall temporal pattern of activation of the RA resulted from
simple ratios of the frequency of activation from the source. Our
interpretation of the above data is that the AF activity in this model
was maintained in the majority of cases by a source of periodic
activity located beyond the mapped regions, most likely in the LA.
Because the dominant frequency of the source was probably too high to
be able to activate the RA in a 1:1 manner, it is likely that
the additional frequency components, and thus activation ratios
contained in the RA pseudo-ECG, were the result of a spectral
transformation imposed by intermittent propagation from the LA through
interatrial pathways (eg, Bachmann's bundle and pectinate muscle
network) into the RA free wall.
Sources of Periodic Activity
Sources of periodic activity could also be seen as breakthroughs of
activation, which were probably a result of endocardial activation via
a pectinate muscle, with transmural propagation to the epicardium and
reentry into the pectinate muscle.14 An
example is shown in Figure 7
Duration of Spatiotemporal Patterns
In most analyzed episodes, periodic activation was transient,
consisting of 4 to 14 consecutive activations. In all cases, periodic
activation from a specific region was interrupted by wave fronts
propagating from a different region in a different pattern. The former
pattern of spatiotemporal periodicity returned in all cases. That is,
these recordings were characterized by segments in which the
same periodic wave appeared for 4 to 14 sequential activations,
separated by segments in which no periodic activity was seen or
periodic activation occurred from another location and direction. An
example is shown in the 4 isochrone maps in Figure 8
Frequently, transient sources were seen to border on the edge of the
mapping region (see Figure 8
Number of Apparent Sources
Irrespective of the mechanism, duration, and number of the sources of
periodic activity in the LA, their cycle length correlated well with
the dominant frequency of spectral analysis of the bipolar EG.
As seen in Figure 9
Previous Work on Organization
Mechanism Underlying the Periodicity
Reentry in the Plane of the Epicardium
Transmural Reentry
Other Possible Mechanisms
Transient Nature of the Source
LA Versus RA
Implications for the Mechanism of AF
Limitations
1. Temporal resolution: Optical recordings were acquired at 120
Hz for
2. Bipolar EG: Because spectral analysis of the atrial
electrical signal was an important tool in this study, an atrial signal
minimally contaminated by ventricular signal was required.
On rewarming, VF often occurred. Therefore, to minimize contamination
by VF signal, we chose a bipolar EG over a volume-conducted ECG. The
frequency content of the bipolar EG correlated well with an
intracavitary ECG, a recording method that theoretically should
sample from all regions of the atria (see Methods and Results).
Moreover, a separate peak corresponding to VF could be identified in
all experiments surveyed.
3. Experimental model: The experimental studies were carried out in an
acute animal model of AF under the artificial conditions of isolation
and crystalloid perfusion. Clearly, although previous studies in some
patients with AF have shown that the ECGs of these patients have a
narrow-band frequency spectrum, suggesting some degree of
organization,10 11 12 the relevance of these data
to human AF remains to be studied.
4. Voltage-sensitive dye: The limitations of this technique resulting
from the usage of a voltage-sensitive dye and mechanical uncoupler have
been discussed repeatedly and in detail
elsewhere.21 24 25
Received October 15, 1997;
revision received March 31, 1998;
accepted April 20, 1998.
© 1998 American Heart Association, Inc.
Basic Science Reports
Spatiotemporal Periodicity During Atrial Fibrillation in the Isolated Sheep Heart
Abstract
Top
Abstract
Introduction
Methods
Results
Discussion
References
Background—The activation patterns
that underlie the irregular electrical activity during atrial
fibrillation (AF) have traditionally been described as disorganized or
random. Recent studies, based predominantly on statistical methods,
have provided evidence that AF is spatially organized. The objective of
this study was to demonstrate the presence of spatial and temporal
periodicity during AF.
Key Words: atrium • arrhythmia • mapping • imaging • Fourier analysis
Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
The activation
patterns that underlie the irregular electrical activity during atrial
fibrillation (AF) have traditionally been described as disorganized or
random.1 2 3 4 5 6 7 Recent studies, based predominantly
on statistical methods, have provided evidence that AF is not entirely
random.8 9 Evidence for preferential routes of
wave propagation during AF has been suggested by the demonstration of
transient linking.9 Furthermore, the
anatomy and electrophysiological
characteristics of the atria are likely to constrain the patterns of
wave propagation, resulting in some degree of underlying order. In
fact, spatial organization has been demonstrated over short distances
by statistical analysis of signals recorded by bipolar
electrograms (EGs) during AF.8 However,
little is known of the spatial characteristics of waves that might
contribute to the underlying order.
Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
Experimental Protocol
Langendorff-Perfused Sheep Heart Preparation
Young sheep (18 to 25 kg) were anesthetized with sodium
pentobarbital (35 mg/kg). The heart was rapidly removed, placed in cold
cardioplegic solution (mmol/L: glucose 280, KCl 13.44,
NaHCO3 12.6, mannitol 34, 4°C) for
transportation, then connected to a Langendorff apparatus.
This method has been described elsewhere in
detail.14 Briefly, the coronary arteries
were continuously perfused via a cannula in the aortic root with warm
Tyrode's solution under a constant flow of 115 to 140 mL/min and
bubbled with 95% O2/5%
CO2. We ensured that the heart was in sinus
rhythm and contracting forcefully and rhythmically at the initiation of
the experiment. In most cases, ventricular fibrillation
(VF) occurred on rewarming shortly after the heart was connected to the
Langendorff apparatus. If this occurred, the VF was allowed
to continue throughout the experiment. Methoxyverapamil
(D600, 2x10-6 mol/L)15
was added to the Tyrode's solution as an electromechanical uncoupler
and maintained throughout the experiment. A bolus injection of 5 to 10
mL of the potentiometric dye di-4-ANEPPS (10 mg/mL) enabled us to
simultaneously image the fluorescence resulting
from changes in transmembrane potentials from the epicardium.
The video imaging approach used for these studies is a
modification of that described elsewhere in
detail.14 We recorded
simultaneously from 20 000 sites in the right atrial (RA)
free wall and 10 000 sites in the left atrial (LA) appendage using 2
identical cameras. A diagram of the experimental protocol is
presented in Figure 1
. Briefly,
quasi-monochromatic light (535 nm) was directed onto the epicardial
surface of the atria. The emitted fluorescence was transmitted
through a 645-nm filter and projected onto CCD video cameras (Cohu
6500) and acquired at a rate of 120 frames per second (sampling at
8.33-ms intervals). Both video cameras were triggered
simultaneously by a delivered pulse. The areas of the
mapped regions were as follows: 3x5 cm of the RA free wall and
3.5x3.5 cm of the LA appendage. This represents 
40% of the
total surface area of the sheep atrium, including the septum.
View larger version (43K):
[in a new window]
Figure 1. Diagram of experimental setup. For details, see
text. SVC indicates superior vena cava; IVC, inferior vena
cava; ST, sulcus terminalis; RAA, right atrial appendage; and LAA, left
atrial appendage.
Isochrone maps were generated from the sequence of the video
image of electrical activity on the heart surface by analysis
of the value of each pixel over time in a manner previously
described.14 16 17 18 Briefly, by a threshold
technique, points for each pixel over time were labeled as part of the
wave front if they were part of the fastest part of the upstroke of the
action potential (maximum first derivative). In this way, 8.33-ms
isochrone bands were formed.16
Pseudo-ECGs were constructed from optical recordings by
integrating the transmembrane fluorescence signal over the left
and right halves of the mapped region and taking the
difference.14 17 18 This was done both for the
entire mapped regions of both atria and for smaller areas when
spatiotemporal periodicity was observed. Although the pseudo-ECG is
different from the traditional ECG, it captures the important global
aspects of a true ECG. The surface ECG leads record the
extracellular potential arising from transmembrane potentials closest
to the electrode. By integrating the transmembrane fluorescence
signal from 10 000 to 20 000 sites on the atrial epicardium and
calculating the difference between left and right halves, the
pseudo-ECG gives a comparable measure of electrical activity.
A continuous atrial bipolar EG was recorded as the
difference between 2 epicardial leads, 1 located on the RA and the
other on the LA. This method was chosen over a volume-conducted
ECG14 to obtain an atrial signal minimally
contaminated by ventricular signal (VF or sinus rhythm).
The electrodes were connected to a Gould amplification system and
filtered at 0.1 to 300 Hz. To ensure that the recorded bipolar EG
was a true reflection of global activity within the atria, in 2
experiments (6 episodes), an intracavitary atrial EG was also
recorded. This EG recorded the difference of 2 leads surrounded
by cotton wicks and allowed to float freely in the Tyrode's
solution–filled cavities of the LA and RA away from their walls.
Theoretically, the intracavitary EG should sample all areas of the
atria with similar weighting. Correlation between the dominant peaks of
frequency of the intracavitary EG and bipolar EG was strong
(R2=0.94, slope=0.91; 95% CI, 0.74 to
1.08). Therefore, the bipolar EG was considered to contain most
frequency components of the electrical activity of the atria; it was
considered unlikely that large regions of high-frequency components
were not represented in the bipolar EG.
AF was induced by burst rapid atrial pacing from the epicardial
surface of either the RA or LA after the addition of acetylcholine
(ACh; 0.1 to 0.5x10-6 mol/L) to the
perfusate. We defined AF as follows: (1) the bipolar EG was
required to demonstrate a rapid sustained irregular rhythm with
variability of morphology and/or timing of EGs on a beat-to-beat basis
and (2) the optical recordings of 1 or both atrial movies were
required to demonstrate wave propagation and lines of block, which
changed on a beat-to-beat basis. AF was considered sustained if it
lasted 2 to 5 minutes. In most experiments, once initiated, AF lasted
>20 minutes.
In 6 experiments, 20 optical recordings were made; in 5
experiments, recordings were made from both atria
simultaneously; and in 1 experiment, only the RA was
recorded. For each experiment, episodes of AF obtained several
minutes apart were selected for analysis on the basis of
signal quality.
To identify sequential wave fronts that might demonstrate
spatial and temporal periodicity, optical recordings were
analyzed by recordings of both the timing and spatial
direction of propagation of all new waves entering into the mapping
area. Sequential wave fronts demonstrated spatiotemporal periodicity if
a minimum of 4 sequential waves entered as new wave fronts or emerged
as breakthroughs (1) from the same location (edge or breakthrough) and
direction or (2) with a timing that varied by no more than ±1 frame
(8.33 ms) from a mean period. As it turned out, no region was
activated by 4 sequential periodic wave fronts alone without
the return of the same spatiotemporal pattern of activation at a later
time during the recording.
Spectral analysis was performed with fast Fourier
transforms (FFTs) on measures of global atrial electrical activity
(bipolar EGs and pseudo-ECGs of both atria) and the pseudo-ECGs of
discrete regions activated by periodic waves, as well as
single-pixel recordings. The content in the 0.4- to 60-Hz band
was analyzed, and peak frequencies from these sources were
compared. The peak frequencies were also compared with the frequency of
the periodic activity as determined visually from the optical
recordings (see above under Periodicity Analysis). In
each FFT, the total power varied according to the intensity of
fluorescence. As such, the magnitudes of various peaks were not
compared from one FFT to another. Rather, the relative amplitudes of
peaks in each FFT were compared to determine the dominant peak. The
power axes for each FFT were therefore not labeled. The bipolar EG was
acquired at 1713 Hz for 10 seconds and filtered (bandpass, 0.1 to 300
Hz). This provided a spectral resolution of 0.1 Hz over the range of
0.4 to 60 Hz. Optical recordings were acquired at 120 Hz (8.33
ms) for 360 to 480 frames (3.0 to 4.0 seconds). This provided a
spectral resolution of 0.33 to 0.25 Hz.
Correlation of frequencies was performed by simple linear
regression analysis (Statview 4.53, Abacus Concepts). Slopes
are presented with 95% CIs. Correlation coefficients
(R2) are also presented, with
associated P values.
Results
Top
Abstract
Introduction
Methods
Results
Discussion
References
Twenty episodes of AF were analyzed from 6 experiments; 1
episode was recorded from the RA only. In all cases, patterns of
wave propagation were complex, with evidence of incomplete and complete
reentry, epicardial breakthrough patterns, and wave collisions as in
our previous work.14
To evaluate the frequency content of episodes of AF in our model,
we first performed spectral analysis of global measures of
atrial activity, ie, the bipolar EG. A representative
example is shown in Figure 2. The bipolar
EG (Figure 2A) demonstrates irregular electrical activity
characteristic of AF. Its corresponding FFT (2B) demonstrates multiple
discrete narrow-band peaks. This is similar to the findings of previous
reports of spectral analyses of AF.10 11 12
A dominant narrow peak is seen at 8.3 Hz. A peak corresponding to a
harmonic frequency is also seen at 16.7 Hz. Other smaller-amplitude
narrow-band peaks were also found, including a narrow peak at 13.9 Hz
that corresponded to ongoing VF. In all episodes of AF, spectral
analysis of the bipolar EG demonstrated multiple narrow-band
peaks with a single dominant peak. The dominant frequency of the
bipolar EG ranged between 6.4 and 16.7 Hz, with a mean of 9.4±2.6 Hz
(cycle length, 112±26 ms). The presence of a dominant peak suggests
that a substantial portion of the atria is activated at that
frequency, potentially in a spatially ordered manner.
View larger version (48K):
[in a new window]
Figure 2. EGs with corresponding spectral analyses
and isochrone maps from a single episode of AF. A and B, Bipolar EG
and its FFT. C through F, LA pseudo-EG and its FFT and RA pseudo-EG and
its FFT, respectively. G, Four sequential LA isochrone maps during
AF. H, Three sequential RA isochrone maps corresponding in timing
to LA. Timing of isochrone maps is indicated by horizontal bars
over EGs.
To investigate the origin of the various peaks, especially the
dominant peak seen in the narrow-band power spectrum of the bipolar EG,
spectral analysis was performed on the pseudo-ECG constructed
from the optical recording of each atrium. In Figure 2
, the
pseudo-ECG of the LA (Figure 2C) appears more regular compared with the
bipolar EG (2A) and the pseudo-EG of the RA (2E). Spectral
analysis of the LA pseudo-ECG demonstrates a discrete
narrow-band dominant peak at 8.2 Hz. This corresponds to the dominant
peak in the spectrum of the bipolar EG (8.3 Hz). The pseudo-ECG of the
RA (2E) is characterized by more irregular but less rapid activation.
This is corroborated by a narrow-band spectrum that demonstrates
prominent peaks at 6.3 and 4.0 Hz, which are also seen on FFTs of the
bipolar EG (2B). A small peak is also seen in the RA spectrum at 8.0
Hz.
Having identified the major peaks in the frequency content of the
optical recordings from both LA and RA, we proceeded to locate
the specific sources of activity at these frequencies. Figure 2G shows
4 sequential color isochrone maps of activation recorded from
the LA over a period of 490 ms (see horizontal bar in 2A). It is clear
from these maps that, during that interval, the LA was repetitively
activated via the same spatially oriented wave front (lower
right). The wave front was periodic throughout the entire 3 seconds of
recording (2C). The cycle length of periodic waves was 120 ms
(8.3 Hz), which corresponded to the dominant frequency peak seen in the
bipolar EG. In fact, the frequency of activation in this region of the
LA corresponded to the dominant peak in the power spectra of both the
pseudo-ECG of the LA (2D) and the bipolar EG (2B). Nevertheless,
analysis of the 4 consecutive LA isochrone maps (2G)
revealed some variability in the pattern of wave front propagation from
1 map to the next, which translated into the somewhat variable
morphology of complexes seen in the LA pseudo-ECG (2C) and probably
resulted in additional minor peaks in the corresponding power spectrum
(2D). Figure 2H shows three 8.3-ms color isochrone maps
of the RA, which correspond in time to those in 2G. Despite
spatiotemporal periodicity seen in the LA, no such periodicity was seen
in the RA. As previously reported,14
isochrone maps of the RA showed incomplete reentry, breakthrough
patterns, and ever-changing lines of conduction block over the entire
mapping region. This complex pattern of activation resulted in marked
variability in the RA pseudo-ECG and contributed significantly to the
irregular activity seen in the bipolar EG (2A). are representative of 19 episodes
of spatiotemporal periodicity originating in the LA. Indeed,
analysis of all episodes of AF demonstrated spatiotemporal
periodic activity more often in the LA; spatiotemporal periodicity was
seen in all 19 recordings of the LA and 12 of 20 (60%) of the
recordings of the RA. Moreover, in recordings in which
spatiotemporal periodicity was seen in both atria during the same
episode of AF, the frequency of the LA sources of periodic activity was
greater than or equal to the RA in all cases (see below and Figure 5).
View larger version (28K):
[in a new window]
Figure 5. Ratios of dominant frequencies from RA and LA
pseudo-EGs (RA:LA). A, Ratio of RA dominant frequency vs LA dominant
frequency is plotted (bars) as an integer ratio for all 19 episodes for
which both LA and RA optical recordings were analyzed.
Ratios vary from 2:5 to 1:1. B, Same data plotted as a histogram. shows the correlation of the frequency
of the periodic activations (n=12) found in the RA and the inverse of
the dominant frequency from the RA pseudo-ECG. These were strongly
correlated (R2=0.97, P<0.0001).
The slope of the regression line was 0.92 (95% CI, 0.80 to 1.03). A
similarly strong correlation is shown in Figure 3B for the 19 LA
recordings of AF (R2=0.99,
P<0.0001, slope=0.94; 95% CI, 0.88 to 0.99). The strong
correlation between the cycle length of periodic activation and the
dominant frequency of the pseudo-ECG of the respective atrium suggests
that a significant portion of the mapped area was activated in
a 1:1 pattern by these periodic activations.
View larger version (21K):
[in a new window]
Figure 3. Correlation of cycle length of periodic activity
with inverse dominant frequency of respective pseudo-EG and bipolar EG.
A, RA periodic activity vs dominant frequency of RA pseudo-EG. B, LA
periodic activity vs dominant frequency of LA pseudo-EG. C, Dominant
frequency of LA pseudo-EG vs dominant frequency of bipolar EG. Dashed
line is line of unity. D, Dominant frequency of LA pseudo-EG vs
dominant frequency of bipolar EG without 3 outliers. (R2=0.76, P<0.0001,
slope=0.75; 95% CI, 0.52 to 0.99). As can be seen in Figure 3C, there
are 3 clear outliers in the data. Two points lie above the unity line;
the dominant frequency of the LA pseudo-ECGs was higher than that of
the bipolar EG. In these cases, the dominant frequency of the LA
pseudo-ECG was represented in the FFT of the bipolar EG,
not as the dominant peak but as a smaller-amplitude peak with a higher
frequency. It is conceivable that an LA source might generate
repetitive impulses that propagate with varying degrees of block (see
below). If the region of 1:1 conduction from the source is sufficiently
small compared with other regions, the frequency of the source will not
be the dominant peak in the bipolar EG. The third outlier lies below
the unity line; the dominant frequency of the bipolar EG is higher than
that of the LA pseudo-ECG. In this case, the bipolar EG may be sampling
activity beyond the mapping region, which is more rapid than that
recorded from the LA appendage. As shown in Figure 3D, when these
outliers were excluded, the correlation was higher:
R2=0.98, P<0.0001, slope=1.03;
95% CI, 0.94 to 1.12. No such correlation was seen for the RA
(R2=0.17, P=0.11). This occurred
because the bipolar EG in 12 of 16 episodes had a peak that was higher
in frequency than that of the corresponding RA, whereas the remaining 4
were similar (data not shown).
Detailed analysis of local activation in the individual
movies revealed important information about wave propagation and
frequency relations between neighboring sites within a given atrium and
between the 2 atria. In Figure 4, we
present recorded transmembrane fluorescent signals from
selected regions of both LA and RA for the same episode of AF. Each
recording represents the integrated signal from a
region 16x8 pixels in size, located as indicated by the numbers in the
respective LA and RA maps. In Figure 4A, site 1 corresponds to the
region of spatiotemporal periodicity at the lower right corner of the
LA appendage (see also Figure 2A). The transmembrane signal was
periodic and relatively uniform in amplitude throughout the 3.0 seconds
of this AF movie (25 activations). The power spectrum of this signal
had a single narrow peak at 8.2 Hz, which matches the dominant peaks of
both the LA pseudo-ECG (8.2 Hz) and the bipolar EG (8.3 Hz). Site 2 was
located at some distance away from the source of periodic activity in
the LA (4A). Both the isochrone maps (Figure 2) and the
transmembrane signal of site 2 (Figure 4) demonstrated an interval in
time at which the frequency of activation was half of that at site 1
(ie, 2:1). The transmembrane signal recorded from regions at a
greater distance from the source of periodic activity (marked 3 and 4)
demonstrated increasing variability in period and amplitude of activity
compared with that seen at site 1. The variability in the transmembrane
signal activation seen in panels 3 and 4 resulted from beat-to-beat
fluctuations in propagation patterns in these regions as seen from the
isochrone maps in Figure 2. In Figure 4B, transmembrane signal is
shown for 4 regions (marked 1 to 4) of the RA during the same AF
episode. Marked variability in the signal is seen at all sites, which
correlates with the complex patterns of activation seen in the color
isochrone maps in Figure 2. Note that despite the complex
activation patterns seen in the RA, spectral analysis of the RA
pseudo-ECG showed dominant peaks at 4.0 and 6.3 Hz, compared with 8.2
Hz in the LA.
View larger version (49K):
[in a new window]
Figure 4. Transmembrane signal recorded from 4 sites
(16x8 pixels) each from LA and RA. Isochrone maps in A and B are
first maps from sequence shown in Figure 2 . Transmembrane signal from
LA site 1 demonstrates periodicity. FFT of this signal is shown; a
single peak is seen at 8.2 Hz. This corresponds to dominant peak of LA
pseudo-EG and bipolar EG (see Figure 2). Signals at sites 2 through 4
show increasing variability in signal. B, Transmembrane signals from 4
sites show marked variability vs LA site 1. See text for details., when one relates the number of activations in the RA
(Figure 4B) to the number of activations in the LA site 1 (4A), a
pattern of 4:3 (8:6; LA1:RAn) is seen at sites RA1 to RA3 over the 2
seconds of recorded signal. Similarly, a relation of 2:1 (8:4) is
seen for site RA4; hence, the overall pattern of activation in each of
these regions is a simple ratio of the frequency of the source of
periodic activity in the LA. These simple ratios also correspond to the
ratios of peaks in the power spectra of the pseudo-ECGs; ie, 8.2:6.3 Hz
and 8.2:4.0 Hz. It is important to point out that whereas the spatial
pattern of wave propagation in the LA that corresponded to the dominant
source at the frequency of 8.2 Hz was only slightly irregular from 1
activation to the next (see Figure 2G), the spatial pattern of RA
activation was highly irregular (Figure 2H). Furthermore, a given area
in the RA need not have a fixed ratio over time; intervals of simple
ratios could coexist in the same recording. This is illustrated
in Figure 4B by the pattern at site RA4, where the last 4 complexes
occurred at a ratio of 5:4 in relation to LA1. were
obtained in all 20 AF episodes when the LA-to-RA dominant frequency
relations were analyzed. Quantitatively, there were some
differences from 1 episode to the next. In Figure 5A, we have plotted the ratio of the
dominant frequency of RA pseudo-ECG over the dominant frequency of LA
pseudo-ECG (RA/LA) for each episode; the numbers adjacent to the bars
are ratios rounded off to the closest integer ratio. In 5B, the data
are plotted as a histogram. The following simple ratios were seen: 1:1
(n=5), 4:5 (n=4), 3:4 (n=4), 2:3 (n=1), 1:2 (n=4), and 2:5 (n=1).
Similar frequency ratios were seen when the frequencies of sources of
periodic activity within a given atrium were compared with those of
sites distant from the source but within the same atrium (see Figures 4 and 7).
View larger version (48K):
[in a new window]
Figure 7. Periodic breakthroughs of activity. A, Three
sequential LA isochrone maps demonstrate 2 phase-locked
breakthroughs of activation (*). B, Transmembrane signal from
breakthrough on right (site 1, 16x8 pixels) with its FFT.
Spatiotemporal activity is seen at this site. C and D, Transmembrane
signal at sites 2 and 3 (away from site 1). E, LA pseudo-EG.
To gain insight into the underlying mechanism of spatiotemporal
organization in AF, we attempted to identify sources of periodic
activity in the 20 initial episodes surveyed. In most cases, periodic
activity emerged from 1 of the edges into the mapping field. In several
episodes, however, the specific source of the periodicity could be
identified. Sources of spatiotemporally periodic activity could be seen
from the epicardium as stationary rotors on the anterior wall of the LA
appendage, as seen in Figure 6. Figure 6A
shows on the left an 8.33-ms isochrone map for a single rotation.
The corresponding pseudo-EG is shown in 6B. Spectral analysis
of these signals (not shown) revealed a direct correspondence between
the frequency of rotation of this source (9.6 Hz) and the dominant
frequency in the global bipolar EG (6E). Yet, despite the highly
periodic activity in the LA, the RA was activated in an
apparently aperiodic fashion, as seen in the 2 sequential RA
isochrone maps in 6C and by the highly irregular activity in the RA
pseudo-ECG (6D). The global bipolar EG taken during the same period
(6E) is consistent with AF, and the FFT (not shown) revealed a
dominant peak at 9.6 Hz. Although in this specific example the entire
mapping region of the LA was activated periodically, more
commonly, regions demonstrating dominant periodic activity were smaller
than that shown here.
View larger version (57K):
[in a new window]
Figure 6. Isochrone maps and pseudo-EGs constructed from
LA and RA optical recordings of episode of AF. A, Isochrone
map of single rotation of stable rotor located in LA. B, Pseudo-EG of
this signal, which is monomorphic. C, Two sequential RA isochrone
maps during AF. RA pseudo-EG is shown in D. Despite uniform activation
in LA, RA is highly heterogeneous in both activation
sequence (see isochrone maps) and electrical activity (see
pseudo-EG). Bipolar EG seen in E is consistent with AF. , in which 2
breakthroughs of activity demonstrate continuous spatiotemporal
periodicity. 7A shows 3 sequential isochrone maps; the
breakthroughs are indicated by asterisks. In each map, the location of
the breakthroughs is maintained for many cycles. Furthermore, the color
of the breakthroughs is constant, demonstrating the temporal
periodicity. This is confirmed in 7B by the single-pixel transmembrane
signal at site 1, corresponding to 1 of the breakthroughs. Its FFT is
shown on the right; a dominant single peak is seen at 20.3 Hz. In
contrast, as shown in 7C, the transmembrane signal of region 2 (same
size) located at some distance below that breakthrough shows more
complex variability in signal amplitude. Such variability is due to
cycle-to-cycle changes in wave propagation away from the breakthrough
and is manifested in the FFT as an additional narrow peak at 10.1 Hz.
The dominant peak at 20.3 Hz is maintained. For comparison, the LA
pseudo-ECG and its FFT are presented in Figure 7E. Note that
the dominant peak in both was 20.4 Hz, which was practically the same
as that in sites 1 and 2 of the LA. When a single-pixel
recording was obtained from site 3 (7D), an even more complex
activation pattern was seen. In this case, the FFT presented a
dominant large peak at 15 Hz, which demonstrated that propagation to
this region from the breakthrough site was not 1:1 but occurred at a
ratio of 4:3. Thus, global activation of the LA in this AF episode
was the result of an LA source that manifested as highly periodic
epicardial breakthroughs.
In some cases, spatiotemporal periodic activity could be seen on a
continuous basis; ie, throughout the 3.0- to 4.0-second
recording of AF (see Figures 2 and 6). These regions could be
seen in the RA or LA. In general, the size of these regions varied from
cycle to cycle, depending on propagation patterns from their source and
in relation to their interaction with preexisting waves. . Initially, a wave front of activation
near the left edge activated a portion of the LA appendage
periodically for 8 sequential cycles at 71-ms intervals (only the last
2 are shown in 8A and 8B). After this, a wave front from the opposite
direction propagated from the edge of the mapping area (indicated by an
asterisk in 8C). This wave front propagated into the refractory tail of
previous waves, resulting in the formation of a
rotor.19 20 Because the formation of the rotor
was out of phase with the source of periodic activity at the left edge,
waves emanating from the rotor activated the entire mapping
region, thus interfering with the original source of periodic activity.
This rotor continued for 3 rotations (the second complete rotation is
shown in 8D) at a period similar to that of the source of periodic
activity (within 8 ms, 1 frame) and acted as the dominant source of
periodic activity in the field of view for 240 ms. Subsequently, the
rotor was terminated by collision with another wave front propagating
from the right edge of the field of view (not shown). At a later time,
the periodic activity from the left edge returned. Hence, this example
demonstrates the simultaneous occurrence of 2 independent
but transient sources of periodic activity, occurring through
potentially different mechanisms during 1 second of AF. Figure 8E shows
the pseudo-ECG during this episode. Despite the presence of 2 sources
of spatiotemporal periodicity, complex dynamics are produced by the
transient interaction of the 2 sources of periodic activity, which
results in complex electrical activity as seen in the pseudo-ECG.
View larger version (63K):
[in a new window]
Figure 8. Isochrone maps demonstrating source of
spatiotemporally periodic activity from edge of field of view and
formation of rotor. Four sequential isochrone maps of LA are shown.
A and B, Source of periodic activity (green, *) activated LA
mapping region from left edge. C, Wave from right edge (red, *) enters
field of view, blocks in superior direction with formation of a rotor.
D, Second of 3 complete rotations (third rotation not shown). E, LA
pseudo-ECG; timing of isochrone maps is indicated by horizontal
bar. ). In these instances, it was impossible to
know the fate of the underlying source of periodic activity, ie,
whether the underlying source itself was transient in nature (similar
to rotor in Figure 8) or whether propagation pathways to the mapping
region from the source had transiently been altered by the interaction
with other propagating waves.
It was also noted that multiple discrete sources of spatiotemporal
periodicity with the same approximate cycle length could coexist. The
number of sources of spatiotemporal periodicity found in the same
atrium ranged from 1 to 3. These sources were found to be
simultaneous and phase-locked (see breakthroughs, Figure 7)
in specific cases, whereas in others, sources were intermittent,
sequential, and not phase-locked (see Figure 8). In either case, these
phenomena might result from the activation of these epicardial sites
from a single source via multiple select routes of ongoing 1:1
propagation in the case of continuous phase-locked periodicity or in
complex patterns of propagation, resulting in the sequential use of
different routes of propagation in the case of intermittent,
sequential non–phase-locked sources. , R2=0.94 (P<0.0001) and
slope=0.88 (95% CI, 0.73 to 1.02). These data strongly suggest
that the sources of periodic activity in the LA are the dominant source
of activity in this model of AF.
View larger version (12K):
[in a new window]
Figure 9. Correlation of cycle length of LA periodic
activity in milliseconds with inverse dominant frequency of bipolar
EG.
Discussion
Top
Abstract
Introduction
Methods
Results
Discussion
References
The most important result of this study is the demonstration of
specific sequences of spatially similar temporally periodic activity
(ie, spatiotemporal periodicity) that were identified during complex
patterns of activation seen in AF. Furthermore, the frequency of the
spatiotemporally periodic activity found in the LA recordings
was well correlated with the frequency of the dominant peak, as
measured by spectral analysis performed on global atrial
electrical activity in the bipolar EG. These data strongly suggest that
the source or sources of these spatiotemporally periodic activations
are the dominant source of activity maintaining AF in this model.
In the original description of the multiple-wavelet hypothesis of
AF as put forward by Moe et al1 and later
substantiated by Allessie et al,2 5 the wavelets
were thought to move randomly throughout the atria. However, more
recent studies that have applied rigid statistical methods to long
episodes of endocardial recordings have provided evidence that
AF is not random.8 9 Botteron and
Smith8 showed that closely spaced bipolar
recordings of human AF could be cross-correlated over distances
of 1.5 to 6 cm (mean, 2.6 cm), and hence, spatial organization was
demonstrated. Gerstenfeld et al9 demonstrated
transient "linking" during 1-minute episodes of human AF by showing
that the direction of activation of 6 successive EGs varied by <30°
(ie, linked). The number of successive linked EGs ranged from 6 to 14.
Schuessler et al21 used multiple epicardial EGs
in isolated canine RA to study the activation patterns emerging after
single extrastimuli at increasing concentrations of ACh. Their maps
revealed that in the presence of large concentrations of ACh, rapid
repetitive responses were characterized by multiple reentrant circuits.
However, such multiple wavelets failed to perpetuate, and with time,
sustained fibrillation was the result of a relatively stable single
reentrant circuit. Moreover, 2 recent surgical
studies22 23 using a small number of
multielectrode recordings suggested that regular repetitive
activations could be seen in the LA during AF in patients with isolated
mitral valve disease undergoing valve replacement. Taken together,
these studies provide strong evidence that AF is not an entirely random
phenomenon. Given the anatomic and
electrophysiological substrate of the
atria, it is not unexpected that repetitive activations seen during AF
are constrained to preferred routes of propagation. However, our study
is the first to clearly demonstrate that wavelets with similar spatial
patterns of propagation can activate regions of the atrium with
temporal periodicity during AF.
Commonly, the periodic activity that was identified bordered on
the edge of the mapping region. For this reason, it was difficult to
know the underlying mechanism of their periodicity. In several
episodes, however, the mechanism of the underlying periodicity could be
identified from the optical recordings. Both rotors in the
plane of the epicardial sheet and breakthrough patterns were
identified.
Rotors in the plane of the epicardium could be transient, in
general lasting 3 to 5 rotations before termination (see Figure 8), or
continuous throughout the entire recording (see Figure 6). This
latter phenomenon has been reported by Schuessler et
al24 in a canine model of AF in which a stable,
anatomically based flutter circuit rotated around 1 pulmonary
vein while the recorded activity in the RA changed moment to
moment. The recorded ECG was consistent with AF.
Several episodes of AF demonstrated transient or continuous
breakthroughs of periodic activity (see Figure 7). These sources
probably represented reentry in specific pectinate bundles.
This is supported by the work of Gray et al,14
who demonstrated breakthroughs of activation in optical
recordings of the RA free wall during AF. Furthermore, computer
simulations performed in our laboratory have also provided evidence
that the natural asymmetries found in the pectinate muscle network can
provide the substrate for unidirectional conduction block and pectinate
bundle reentry (data not shown).
Most commonly, the regions that demonstrated periodic activity
bordered on the edge of the mapping region and propagated into the
field of view. These regions most likely represented
preferential routes of propagation (1:1, or in some cases 6:5, 5:4,
etc) from a source of periodic activity beyond the mapping region. It
is difficult to infer the nature of the underlying source. Clearly,
because rotors in the plane of the epicardial sheet and breakthroughs
of activation have been identified within the mapped areas, these
phenomena are also likely to underlie periodic activity beyond the
mapping field. Another possible explanation for the underlying
periodicity comes from recent observations in humans in which a single,
repetitive focal source of activity propagated from an individual
pulmonary vein to the remainder of the atrium as fibrillatory
waves.25 However, because of the continuous
presence of ACh in our experiments, it seems unlikely that if such
focal activity was present, it was the result of spontaneous or
triggered activity. Therefore, we favor functional or anatomically
based reentry as the underlying mechanism of these sources. In cases in
which periodic activity was seen to propagate from the edge of the
mapping area, the source could be located at sites not mapped by our
technique, such as the pulmonary vein region or interatrial
septum. Regardless of the underlying mechanism of the source, periodic
waves might propagate to our mapping field in a spatially similar
fashion, depending on the interaction with other intervening wave
fronts.
Periodic activity that was seen to border the edge of the mapping
area was often transient but recurrent, ie, these recordings
were characterized by segments in which the same periodic wave appeared
for 4 to 14 sequential activations separated by segments in which no
periodic activity was seen. The transient nature of these regions is
interesting. It is possible that the underlying source of these
activations was itself transient, or it is just as likely that the
route of propagation from a constant stable source changed because of
complex interactions with other wave fronts. Because we have been able
to identify both transient and continuous rotors in the epicardial
sheet as well as transient and continuous breakthroughs of activity
during AF, both explanations for the transient nature of some periodic
activity seem plausible.
Several studies in different models have demonstrated that
activity in the LA is more rapid than in the RA during
AF.22 23 26 Indeed, this is the case in our
study. Moreover, periodic activity was seen more often in the LA than
the RA, and the frequency of activation of LA periodic activity was
greater than or equal to that found in the RA in all cases. Our data
strongly support the contention that a source of periodic activity was
located in the LA during AF and that activation in the RA resulted from
complex patterns of propagation through interatrial pathways. In 5
cases in which the dominant frequency of the RA pseudo-EG was similar
to the dominant frequency of the LA pseudo-EG, it is possible that the
origin of periodic activity could have been located in either atrium.
As such, sources of periodic activity can undergo spectral
transformation through the fractionation of periodic waves into
multiple wavelets. Although complex, it appears that this process is
restrained by relatively simple deterministic principles; simple
patterns of wave propagation change over space and time, resulting in a
complex global picture that characterizes AF. The detailed mechanisms
by which this occurs require further study.
Clearly, the demonstration of spatiotemporal periodicity during AF
has implications for the underlying mechanism. We have seen transient
and continuous sources of periodic activity spatially
represented as rotors in the epicardial plane and
breakthroughs representative of transmural reentry. As
such, the level of organization during AF may lie on a spectrum
depending on the number of sources and the degree of fractionation of
the periodic activity into independent wavelets, as well as their
subsequent fate. A single rapid source of activity (see Figure 6) that
undergoes substantial fractionation will manifest as AF. This was
recognized as early as 1925 by Sir Thomas
Lewis,27 who used the term "impure" flutter.
However, if the newly created wavelets, once formed, initiate rotors of
activity, new sources of periodic activity will be created secondary to
the ongoing primary source.19 20 The life span of
the secondary rotors may be short, in which case the
maintenance of the AF will continue to depend on the primary
source, the wavelets serving only to add to the overall complexity of
activation. However, if the life span of the newly formed rotors is
sufficiently long, the primary source of periodic activity need not
continue for the maintenance of the arrhythmia. In the
latter scenario, the rate of creation and termination and the life span
of the individual wavelets may be critical for the degree of complexity
and maintenance of AF.
The high spatial resolution and large field of view of the video
camera provide an invaluable tool for the study of cardiac
arrhythmias in the isolated heart. However, our study has some
limitations that must be considered and may be summarized as
follows. 3 to 4 seconds with a resolution of 0.25 to 0.33 Hz. As such,
we were unable to distinguish absolutely periodic sources of activity
from those that might be quasi-periodic, ie, 1 source frequency
modulated by another frequency. Long recordings of AF at high
sampling frequencies have demonstrated
quasi-periodicity.28 To confirm or disprove that
various spatiotemporal sources in this study were quasi-periodic rather
than periodic was beyond the capabilities of the system. Furthermore,
it is also possible that higher-frequency sources of periodic activity
beyond our mapping region propagated with some degree of block (ie, 8:7
or 6:5). The resultant beat-to-beat changes in cycle length might be
beyond the temporal resolution of our method.
Acknowledgments
This work was supported in part by grant PO1-HL-39707 from
the National Heart, Lung, and Blood Institute, NIH; a grant from
InControl Inc; a NASPE Fellowship awarded to Dr Skanes; and an AHA, New
York State Affiliate, Fellowship awarded to Dr Berenfeld. We
would like to thank Jiang Jiang, Megan Flanagan, Laverne Gilbert,
Tatiana Yuzyuk, and Fan Yang for their technical assistance.
Footnotes
1 Dr Skanes and Dr Mandapati contributed equally to this work. 
References
Top
Abstract
Introduction
Methods
Results
Discussion
References
This article has been cited by other articles:
 |
|
 |
|
  S. Weinberg, S. Iravanian, and L. Tung Representation of Collective Electrical Behavior of Cardiac Cell Sheets Biophys. J., August 1, 2008; 95(3): 1138 - 1150. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. Oral, A. Chugh, E. Good, T. Crawford, J. F. Sarrazin, M. Kuhne, N. Chalfoun, D. Wells, W. Boonyapisit, N. Gadeela, et al. Randomized Evaluation of Right Atrial Ablation After Left Atrial Ablation of Complex Fractionated Atrial Electrograms for Long-Lasting Persistent Atrial Fibrillation Circ Arrhythmia Electrophysiol, April 1, 2008; 1(1): 6 - 13. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. Tanaka, S. Zlochiver, Karen. L. Vikstrom, M. Yamazaki, J. Moreno, M. Klos, Alexey. V. Zaitsev, R. Vaidyanathan, D. S. Auerbach, S. Landas, et al. Spatial Distribution of Fibrosis Governs Fibrillation Wave Dynamics in the Posterior Left Atrium During Heart Failure Circ. Res., October 12, 2007; 101(8): 839 - 847. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S.-L. Chang, C.-T. Tai, Y.-J. Lin, W. Wongcharoen, L.-W. Lo, K.-T. Lee, S.-H. Chang, T.-C. Tuan, Y.-J. Chen, M.-H. Hsieh, et al. The Role of Left Atrial Muscular Bundles in Catheter Ablation of Atrial Fibrillation J. Am. Coll. Cardiol., September 4, 2007; 50(10): 964 - 973. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. Sanders, M. Hocini, P. Jais, F. Sacher, L.-F. Hsu, Y. Takahashi, M. Rotter, T. Rostock, C. J. Nalliah, J. Clementy, et al. Complete isolation of the pulmonary veins and posterior left atrium in chronic atrial fibrillation. Long-term clinical outcome Eur. Heart J., August 1, 2007; 28(15): 1862 - 1871. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. Calkins, J. Brugada, D. L. Packer, R. Cappato, S.-A. Chen, H. J.G. Crijns, R. J. Damiano Jr, D. W. Davies, D. E. Haines, M. Haissaguerre, et al. HRS/EHRA/ECAS Expert Consensus Statement on Catheter and Surgical Ablation of Atrial Fibrillation: Recommendations for Personnel, Policy, Procedures and Follow-Up: A report of the Heart Rhythm Society (HRS) Task Force on Catheter and Surgical Ablation of Atrial Fibrillation Developed in partnership with the European Heart Rhythm Association (EHRA) and the European Cardiac Arrhythmia Society (ECAS); in collaboration with the American College of Cardiology (ACC), American Heart Association (AHA), and the Society of Thoracic Surgeons (STS). Endorsed and Approved by the governing bodies of the American College of Cardiology, the American Heart Association, the European Cardiac Arrhythmia Society, the European Heart Rhythm Association, the Society of Thoracic Surgeons, and the Heart Rhythm Society. Europace, June 1, 2007; 9(6): 335 - 379. [Full Text] [PDF]
|
|