(Circulation. 1997;95:1487-1496.)
© 1997 American Heart Association, Inc.
Articles |
Internal Atrial Defibrillation in Humans
Improved Efficacy of Biphasic Waveforms and the Importance of Phase Duration
From the Department of Medicine, Duke University Medical Center, Durham, NC.
Correspondence to Randolph A.S. Cooper, MD, Room B-140, Volker Hall, 1670 University Blvd, University of Alabama at Birmingham, Birmingham, AL 35294-0019. E-mail rac{at}crml.uab.edu.
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Abstract |
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Background The optimal waveform for internal atrial defibrillation (IAD) in humans is unknown. This study tested the effect of waveform duration and phase duration on the efficacy of biphasic waveforms for IAD.
Methods and Results Electrodes were positioned in the right atrial appendage and coronary sinus in 13 patients. In part 1, the atrial defibrillation thresholds (ADFTs) for 5 monophasic waveforms (2, 4, 6, 10, and 20 ms) and 5 symmetrical biphasic waveforms (1/1, 2/2, 3/3, 5/5, and 10/10 ms) were compared in 6 patients. In part 2, the ADFTs for two asymmetrical biphasic waveforms (7.5/2.5 and 2.5/7.5 ms) were compared with those for a symmetrical biphasic waveform (5/5 ms) and a monophasic waveform (10 ms) in 7 patients. In part 1, biphasics with total durations of 4 to 20 ms had significantly lower ADFTs than monophasic waveforms of the same total duration. For a total duration of 2 ms, there was no significant difference in ADFTs between the biphasic and the monophasic waveforms. There was no difference between symmetrical biphasic waveforms of 4 to 20 ms. In part 2, the 7.5/2.5 ms asymmetrical biphasic had significantly lower ADFTs than the three other waveforms tested. Both the 7.5/2.5 ms asymmetrical and the 5/5 ms symmetrical biphasic waveform had significantly lower ADFTs than the 2.5/7.5 ms asymmetrical biphasic and the 10 ms monophasic waveforms.
Conclusions For IAD in humans, biphasic waveforms were more efficacious than monophasic waveforms. This improved efficacy is related to the total duration of the biphasic waveform and each individual phase duration of the biphasic waveform.
Key Words: defibrillation • atrium • arrhythmia
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Introduction |
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Atrial fibrillation is one of the most common arrhythmias encountered in clinical medicine.1 2 3 Therapy includes pharmacological and/or electrical therapy to restore sinus rhythm. Transthoracic electrical cardioversion has been shown to be an effective treatment for atrial fibrillation4 ; however, the relatively high energy shocks (25 to 400 J) that are required can be detrimental to the heart.5 6 7 8 9 10 They also can be painful to the patient, which may result in refusal to undergo repeated cardioversions after unsuccessful attempts to restore sinus rhythm.
Electrical cardioversion of atrial fibrillation has changed little over the past 30 years. However, several groups have shown recently that internal defibrillation of atrial fibrillation in animals and humans is safe as well as effective with the use of transvenous electrodes with certain biphasic waveforms.11 12 13 14 15 However, this method still requires energy levels that are usually painful in nonsedated patients.13 With more efficient defibrillation waveforms, energy requirements would be reduced and discomfort to the patient would be minimized.
For internal ventricular defibrillation in animals and humans, certain biphasic waveforms consisting of two phases of opposite polarity decrease the shock strength required for defibrillation compared with equal duration monophasic waveforms.16 17 18 19 20 21 22 23 24 In animal studies the relative phase durations of these biphasic waveforms have been shown to be an important determinant of ventricular defibrillation efficacy.18 19 25 The effect of total waveform duration and the duration of each phase of a biphasic waveform on internal atrial defibrillation efficacy in humans is unknown. This study investigated the effect of phase duration on the atrial defibrillation efficacy of biphasic waveforms. In part 1, the atrial defibrillation thresholds (ADFT) of multiple monophasic and equal duration biphasic waveforms ranging in total duration of 2 to 20 ms were compared. In part 2, equal duration symmetrical biphasic, asymmetrical biphasic, and monophasic waveforms were compared to assess the importance of phase duration on atrial defibrillation efficacy.
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Methods |
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Patients
Internal cardioversion was performed in 11 patients undergoing clinically indicated electrophysiological procedures and in 2 patients when external cardioversion was unsuccessful. Informed consent was obtained in all cases, and the study protocol was approved by the Institutional Review Board of Duke University Medical Center. The patients were brought to the clinical electrophysiology laboratory in a fasting, nonsedated state. The patients were then moderately sedated with intravenous midazolam, meperidine, and promethazine. Local anesthesia at the sites used for catheter insertion was achieved by subcutaneous infiltration of a mixture of 0.25% bupivacaine and 2% procaine. A 7F quadripolar catheter was advanced from the right femoral vein to the right ventricular apex for synchronization of the atrial defibrillation shocks to ventricular activation and for postshock pacing if necessary for treatment of bradycardia. A 7F hexapolar catheter was advanced to the low right atrium for induction of atrial fibrillation with rapid atrial burst pacing and bipolar recording of atrial activation.
Defibrillation Electrodes
Two 6F nonapolar catheters (Electro-Catheter Corp) were used as
the defibrillation anode and cathode. Each catheter had nine 5-mm
electrode rings with 2-mm interelectrode spacing. All nine electrodes
were combined for a total surface area of 
140 mm2.
The anode was advanced from the right femoral vein so that the distal
tip was positioned in the anteromedial right atrial appendage with the
body of the electrodes positioned along the posterolateral right atrium
(Fig 1
). The cathode was advanced from the
right internal jugular or subclavian vein so that the distal tip was
positioned as anteriorly as possible in the coronary sinus, usually
with the body of the electrodes positioned in the lateral margin of the
heart (Fig 1). Once all catheters were in position, an anticoagulation
protocol was followed in the clinical electrophysiology laboratory,
consisting of 5000 units of intravenous heparin sulfate as an initial
bolus followed by 1000 units intravenously every hour during the entire
procedure.
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Defibrillator and Waveforms
All defibrillation waveforms were delivered from a Ventritex
HVS-02 programmable cardioverter/defibrillator (Ventritex Inc), which
has a capacitance of 150 µF. This device has two programmable
capacitor outputs, each capable of delivering a truncated exponential
monophasic waveform. The pulse widths and polarities of both outputs
are programmable. To mimic a biphasic shock from a single capacitor
defibrillator, the leading edge voltage of the second phase was set
equal to the trailing edge voltage (VT) of the first phase.
VT was calculated by using the equation
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where VL is the leading edge voltage of the first phase, t is the duration of the first phase, R is the resistance estimated by the resistance from the previous shock, and C is the capacitance (150 µF). The polarity of the second phase (second output) was set opposite to the first phase (first output) with an interphase delay of 0.2 ms.
In part 1, five monophasic waveforms were compared with five biphasic
waveforms with equal-duration first and second phases. The biphasic
waveforms had phase durations of 1/1, 2/2, 3/3, 5/5, and 10/10 ms (Fig 2A). The five monophasic waveforms had single
phase durations of 2, 4, 6, 10, and 20 ms (Fig 2B). Thus, each biphasic
waveform was compared with a monophasic waveform with the same total
duration to allow comparison of total waveform duration on
defibrillation efficacy.
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In part 2, all waveforms had the same total duration of 10 ms. One
monophasic (10 ms) and one symmetrical biphasic (5/5 ms) waveform were
compared with two asymmetrical biphasic waveforms (7.5/2.5 and 2.5/7.5
ms) with unequal first and second phase durations (Fig 2
, A, B, C, and
D). This allowed determination of the effect of relative phase duration
on the efficacy of the biphasic waveform.
Defibrillation Protocol
If the patient was not already in atrial fibrillation, then
atrial fibrillation was induced by rapid atrial burst pacing. Informed
consent was obtained after initial screening of 15 patients, of which
13 had at least inducible sustained atrial fibrillation. Atrial
fibrillation was considered sustained if it lasted for at least 5
minutes, and the protocol was not performed on patients unless atrial
fibrillation was sustained. The 13 patients with sustained atrial
fibrillation were divided into two groups. The first group consisted of
the 6 patients enrolled in part 1 of the study and the second group
consisted of the 7 patients enrolled in part 2 of the study. There was
no crossover of patients between parts 1 and 2. After a successful
defibrillation, a period of at least 1 minute was used before
reinduction of atrial fibrillation. The next test shock was given at
least 1 minute after atrial fibrillation was reinduced. The
defibrillation shocks were synchronized to right ventricular
activation.
In part 1, a step-up protocol was used starting with a shock strength of 0.5 J and increasing to 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, up to a maximum of 10 J. In part 2, a step-up protocol was used starting with a shock strength of 0.5 J, increasing to 1, 2, 3, 4, 5, 6, 7, 8, 9, up to a maximum of 10 J. In part 2, the 1.5-J shock strength was not used for two reasons. First, the 1.5-J step in part 1 did not appear to significantly add to the resolution of the ADFT. Second, the number of shocks per patient was reduced and minimized the amount of time added to the clinical electrophysiological procedure time. The ADFT for each waveform was defined as the lowest energy shock that defibrillated the atria.
In part 1, randomization was achieved by drawing chits for each total waveform duration. A coin flip then was performed to determine whether the monophasic or the biphasic shock of the same total duration was delivered first. In part 2, randomization was achieved by drawing chits for each waveform. These randomization procedures were repeated for each patient.
For each delivered shock, the leading edge voltage and the shock
waveform phase durations were programmed on the HVS-02 defibrillator.
The trailing edge voltage, the delivered energy, and shock impedance
were measured by the HVS-02 defibrillator. From the set values of
leading edge voltage and pulse width, the leading edge current was
calculated using the measured shock impedance. For the initial
defibrillation shocks in each patient, the interelectrode impedance was
assumed to be 50 
. Measured impedances from the HVS-02 were then
used to determine subsequent shock leading edge voltage
requirements.
Statistical Methods
Results are expressed as mean±SD. Comparisons between the two
groups were made by using a two-tailed Student's t test for
paired data and the 
2 test for unpaired data. In
part 1, comparisons between monophasic and biphasic waveforms of the
same total duration were made by paired t test
analysis.26 In parts 1 and 2, ANOVA multivariate analysis
with repeated measures was used to compare the mean values of the
atrial defibrillation thresholds among waveforms and
patients.26 For all statistical tests performed, a value
of P
.05 was considered significant.
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Results |
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Patient Characteristics
The clinical characteristics of the study population for parts 1 and 2 are summarized in Table 1
. There was no
statistical difference between the two groups of patients in terms of
age, height, or weight. The left atrial size was measured by
transthoracic echocardiogram. Ejection fraction was determined by
either transthoracic echocardiography or angiographic left
ventriculography. The reported echocardiographic data were obtained
during the admission when the defibrillation protocol was performed.
Two patients did not have echocardiograms performed during the
admission when the study protocol was performed. However, both patients
had undergone echocardiographic evaluation previously at outside
hospitals. The study population for parts 1 and 2 are also shown in
regard to their electrophysiological diagnosis and therapy (Table 2). Seven of the 13 patients were in atrial
fibrillation at the time of admission to the electrophysiology
laboratory. In part 1, three patients (50%) had a history of atrial
fibrillation, and in part 2, five patients (71%) had a history of
atrial fibrillation. Six of the eight patients with a history of atrial
fibrillation were on warfarin therapy for at least 3 weeks before the
study protocol. Postprocedure warfarin therapy was restarted. Patients
No. 6 in part 1 and No. 6 in part 2 were diagnosed with lone atrial
fibrillation and were treated with aspirin before the study protocol.
Postprocedure aspirin therapy was continued. Although many of the
patients were on antiarrhythmic medications before their study, all of
the patients except for three in part 2 were in the drug-free state
(Table 2), defined as no medication for at least five half-lives of the
drug. After ADFT determination, 10 of the 13 patients underwent some
type of radiofrequency ablation procedure. Of the three remaining
patients, two had failed attempts at external cardioversion and
underwent successful internal cardioversion procedures and one had a
diagnostic electrophysiological study for wide-complex tachycardia with
presyncope and was shown to be noninducible.
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Atrial Defibrillation Threshold Part 1
For most waveform durations, the mean and standard deviations in
terms of total delivered energy at the ADFT were lower for the
symmetric biphasic waveforms compared with the equal-duration
monophasic waveforms (Table 3). For total
waveform durations of 4 to 20 ms, the symmetrical biphasic waveforms
had significantly lower ADFTs than the monophasic waveforms. However,
for the total duration of 2 ms, there was no significant difference
between the biphasic and monophasic waveforms. The 1/1-ms biphasic
waveform had a significantly higher ADFT in terms of delivered energy
than the other 4 biphasic waveforms tested, but there was no
significant difference between the symmetrical biphasic waveforms of 4-
to 20-ms total duration. Similar findings were observed for the mean
leading edge voltage and current at the ADFT (Table 3). Thus,
prolonging the biphasic waveform duration for longer than 4 ms did not
further enhance the defibrillation efficacy, and the strength-duration
curves in terms of leading edge voltage as well as total energy were
flat for biphasic waveform durations of 4 to 20 ms (Fig 3).
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P
The strength-duration characteristics for atrial defibrillation were
different for the monophasic waveform compared with the biphasic
waveform. Unlike the biphasic waveform, the shortest-duration
monophasic waveform with 2-ms duration had a defibrillation efficacy
similar to the longer monophasic waveforms (Table 3). In terms of
leading edge voltage at the ADFT, there was not a statistically
significant difference for monophasic waveforms throughout the
durations tested of 2 to 20 ms. Thus, the strength-duration curve in
terms of voltage was relatively flat, although voltage requirements
tended to increase for the 20-ms monophasic waveform (Fig 3). Similar
findings were seen in terms of leading edge current (Table 3).
Reflecting the increase in both voltage and current requirements for
the longest monophasic waveform, the 20-ms–duration monophasic
waveform had a significantly higher energy ADFT compared with the other
monophasic waveforms, suggesting that durations of >10 ms have an
adverse effect on the defibrillation efficacy of monophasic waveforms.
Thus, unlike the biphasic energy strength-duration curve, the
monophasic curve for total energy was flat for durations of 2 to 10 ms
but sloped significantly upward at a duration of 20 ms (Fig 3). There
were no significant differences in mean impedance for all the waveforms
tested (Table 3). The delivered charge for each waveform was calculated
from the mean leading edge current, pulse width, and mean impedance.
These values are shown in Table 3 for each waveform in part 1. The
minimal delivered charge was at a total pulse duration of 2 ms and
increased with longer pulse durations up to 20 ms. This occurred for
both the monophasic and biphasic charge curves; however, the monophasic
curve was steeper than the biphasic curve.
Atrial Defibrillation Threshold Part 2
The 7.5/2.5-ms asymmetrical biphasic waveform had a significantly
lower leading edge voltage and total delivered energy at the ADFT than
the other three waveforms tested in part 2 (Fig 4). The 5/5-ms symmetrical biphasic waveform
had a significantly lower ADFT than the 2.5/7.5-ms asymmetrical
biphasic waveform and the 10-ms monophasic waveform. There was no
significant difference between the 2.5/7.5-ms asymmetrical biphasic
waveform and the 10-ms monophasic waveform. Similar findings were seen
with the mean leading current at the ADFT, and there was no significant
difference in mean impedance for all waveforms tested (Table 3).
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Postshock Data
The total number of shocks that had adequate electrogram data for
analysis, the longest post–successful shock pause to first P wave, and
the longest post–successful shock pause to first R wave are shown in
Table 4 for each waveform. The combined mean
and standard deviation for the longest post–successful shock pause to
first P wave and the longest post–successful shock pause to first R
wave for biphasic and monophasic waveforms are also shown in Table 4.
The average number of shocks delivered to each patient was 50±18
shocks in part 1 and 26±10 shocks in part 2. There was no significant
difference between biphasic and monophasic waveforms in terms of
longest pause after a successful shock to the first P or R wave. The
duration of the pauses appeared to be patient dependent and
shock-strength independent. Several of the patients had postshock
pauses that required temporary ventricular pacing for a maximum of 10
seconds. Occasionally, postshock first-degree (38 cases), second-degree
(25 cases), or third-degree heart block (8 cases) was observed;
however, this was transient and no permanent heart block was induced.
The longest episode of any type of atrioventricular conduction block
was 6.4 seconds.
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No sustained ventricular arrhythmias were induced with shocks
appropriately synchronized to the R wave. However, two episodes of
ventricular fibrillation were induced when shocks were inadvertently
delivered during the ventricular vulnerable period (Fig 5) as the
result of sensing of electrical noise in the external sensing
circuit. Both of these episodes of
ventricular fibrillation were converted to sinus rhythm with a single
200-J transthoracic countershock.
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Combined Data From Parts 1 and 2
The patient population consisted of eight patients with a
history of clinical atrial fibrillation and five patients without a
history of clinical atrial fibrillation. For both groups there was a
significant increase in the atrial defibrillation threshold for the
10-ms monophasic waveform compared with the 5/5-ms biphasic waveform.
Only one patient had a decrease in threshold with the 10-ms monophasic
waveform compared with the 5/5-ms biphasic waveform. The atrial
defibrillation threshold with the 5/5-ms biphasic was 2.9±1.3 J for
patients with a history of atrial fibrillation and 1.7±1.5 J for
patients without a history of atrial fibrillation. This trend toward a
higher ADFT for the biphasic waveform in the patients with a history of
atrial fibrillation was not statistically significant
(P=.08). The atrial defibrillation threshold of the 10-ms
monophasic waveform was 5.2±2.5 J for patients with a history of
atrial fibrillation and 4.7±2.7 J for patients without a history of
atrial fibrillation. There was no significant difference between these
two groups (P=.55).
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Discussion |
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Improved Efficacy of Biphasic Waveforms (Part 1)
Certain biphasic waveforms have been shown to be more effective than certain monophasic waveforms in both animal and human studies involving ventricular defibrillation16 17 18 19 20 21 22 23 24 as well as atrial defibrillation.11 12 13 14 15 For human atrial defibrillation, Johnson and coworkers12 directly compared a biphasic with a monophasic waveform of the same total duration for transcatheter internal cardioversion of atrial fibrillation in humans. They found that for a total duration of 6 ms, a symmetrical biphasic waveform (3/3 ms) had a significantly lower atrial defibrillation threshold than a monophasic waveform (6 ms) in 18 patients undergoing electrophysiological procedures. However, this study did not evaluate the effect of total waveform duration on the efficacy of the biphasic waveform for atrial defibrillation. In the present study, the symmetrical biphasic waveforms with total pulse durations of 4, 6, 10, or 20 ms were more efficacious than monophasic waveforms of the same total duration. However, shorter symmetrical biphasic waveforms of 2-ms total duration were not more efficacious than a 2-ms monophasic waveform. Thus, while symmetrical biphasic waveforms are generally more efficacious than equal-duration monophasic waveforms, this effect is clearly waveform-duration dependent.
Several studies have evaluated the effect of waveform duration on ventricular defibrillation. Schuder and coworkers27 found that the transthoracic defibrillation efficacy of several types of monophasic waveforms in dogs with ventricular fibrillation was critically dependent on pulse duration, with shorter (1 to 4 ms) and longer (64 to 256 ms) pulse durations having lower probabilities of successful defibrillation. Similarly, for ventricular defibrillation in dogs, Chapman et al,28 using nonthoracotomy leads, showed that shorter (2.5 ms) and longer (20 ms) monophasic waveforms had higher defibrillation threshold voltages than waveforms with durations in the middle of this spectrum. Hahn and colleagues29 showed a more complex relationship for ventricular defibrillation in pigs with biphasic waveforms with a total duration of 3 to 20 ms requiring higher leading edge voltage but less total energy compared with a longer biphasic waveform with a total duration of 30 ms. Like monophasic waveforms for ventricular defibrillation, the strength-duration curve in terms of voltage was steep with short biphasic waveform durations. However, unlike monophasic waveforms for ventricular defibrillation, the strength-duration curve in terms of voltage was flat with longer biphasic waveform durations.
Less is known about the effect of waveform duration on atrial defibrillation with either monophasic or biphasic waveforms. Cooper et al11 demonstrated in a sheep model of atrial fibrillation that monophasic waveform durations of 1.5 to 6 ms were not significantly different in terms of threshold voltage; thus, the strength-duration curve was flat for durations of 1.5 to 6 ms. For symmetrical biphasic waveforms, shorter (3 ms) and longer (12 ms) total pulse durations were associated with higher threshold voltages than with a total pulse width in the middle of this spectrum (6 ms). Thus, the strength-duration curve for biphasic waveforms for atrial defibrillation in sheep showed a relatively steep increase at shorter (3 ms) and longer (12 ms) total pulse durations, with a distinct nadir at 6 ms.
In the present study, the strength-duration curve for the monophasic
waveform in humans was flat from 2 to 10 ms and thus similar to that
described for atrial defibrillation in sheep. However, the 20-ms
monophasic waveform had a significantly higher ADFT in terms of total
energy compared with the other monophasic waveforms. This suggests that
duration 
20 ms has a negative effect on monophasic waveform
defibrillation efficacy and is similar to the adverse effect of
excessively long monophasic waveform durations seen for ventricular
defibrillation.27 28 29 Thus, the strength-duration curve for
symmetrical biphasic waveforms in human atrial defibrillation appears
to be steep for total waveform durations <4 ms and flat for total
pulse durations of 4 to 20 ms. The strength-duration curve for the
symmetrical biphasic waveforms is shifted downward compared with the
curve for the monophasic waveforms with 4- to 20-ms total duration. The
sharp increase in ADFT at 2 ms for the biphasic waveform nullified any
enhancement of efficacy compared with the monophasic waveform. No
distinctly unique advantage was seen with the 3/3-ms biphasic waveform
as seen in sheep. This study did not demonstrate that the threshold was
higher for longer biphasic waveform durations. Whether symmetrical
biphasic waveforms with total durations >20 ms would have
significantly higher ADFTs is not known. As is shown in this study,
waveform duration may have divergent effects on the efficacy of
monophasic waveforms and symmetrical biphasic waveforms for atrial
defibrillation in humans, especially at relatively shorter and longer
durations.
Importance of Phase Duration (Part 2)
Several groups have demonstrated with ventricular defibrillation
studies in animals that the efficacy of a biphasic waveform is
dependent on the duration as well as the amplitude of both phases of
the waveform.18 19 25 Dixon et al18
demonstrated that asymmetrical biphasic waveforms with the second phase
shorter than the first phase are more efficacious than biphasic
waveforms with a longer second phase than first phase for electrically
induced ventricular fibrillation in a canine model. They also found
that there was not a significant difference between the 5/5-ms
symmetrical (mean voltage at ventricular defibrillation threshold of
116±19.4 V) and 7.5/2.5-ms asymmetrical biphasic (mean voltage at
ventricular defibrillation threshold of 114±19.7 V) waveforms with the
second phase shorter than the first phase for ventricular
defibrillation. The present study demonstrated a similar phase
dependence for atrial defibrillation in humans. The 7.5/2.5-ms
asymmetrical biphasic waveform with the first phase longer than the
second phase was more effective than either the 5/5-ms symmetrical
biphasic waveform with equal phase durations or the 2.5/7.5-ms
asymmetrical biphasic waveform with a longer second phase than first
phase duration. The symmetrical biphasic waveform was significantly
better than the unfavorable 2.5/7.5-ms asymmetrical biphasic waveform,
but the unfavorable asymmetrical biphasic waveform was similar in
defibrillation efficacy to the 10-ms monophasic waveform of the same
total duration. Again, these results are very similar to the findings
with similar asymmetrical biphasic waveforms for ventricular
defibrillation in dogs, suggesting a similar mechanism of enhanced
efficacy with biphasic waveforms in atrial and ventricular
defibrillation. However, the one major difference is that the
7.5/2.5-ms asymmetrical biphasic waveform with the second phase shorter
than the first phase was more effective than the 5/5-ms symmetrical
biphasic waveform in this study. This difference between atrial and
ventricular defibrillation may be explained in part by differences in
species, myocardial properties, electrode configuration, hemodynamic
state, autonomic tone, and type of anesthesia/sedation.
Safety
This method of atrial defibrillation appears to be safe and
effective when appropriate R-wave sensing occurs. There were no
long-term complications, and temporary pacing was required in a
minority of the patients for postshock conduction delays. No permanent
sinus or atrioventricular nodal dysfunction occurred as a complication
of the defibrillation protocol. However, since postshock bradycardia
does occur, backup bradycardia pacing will be needed with any
implantable atrial defibrillator. The only serious complication was
induction of ventricular fibrillation due to inappropriate
synchronization from equipment error and delivery of the shock during
the ventricular vulnerable period. This emphasizes the critical
importance of ensuring accurate synchronization of the shock to the R
wave to avoid potentially lethal postshock ventricular arrhythmias in
future implantable devices; otherwise, backup ventricular
defibrillation will be needed.
Study Limitations and Combined Data From Parts 1 and 2
The limitations to this study are the fact that not all of the
patients had a history of atrial fibrillation and several of them had
relatively normal atria by clinical evaluations. Also, in all cases,
subsequent episodes of atrial fibrillation were induced electrically,
and this may not be the same as naturally occurring atrial
fibrillation. However, all of these atria were able to sustain atrial
fibrillation and the majority of the patients did have a clinical
history of paroxysmal atrial fibrillation. Another limitation is that
the ADFT can vary with time within a patient. The defibrillation
threshold determination is not suggesting that a sharp cutoff point
exists below which all attempts to defibrillate fail and above which
all attempts succeed. A single "threshold" value is used to
represent the efficacy of a waveform because it could be measured
easily and limited the number of shocks to the patient. It is hoped
that by using the defibrillation threshold method, marked differences
in the defibrillation requirements would be discovered and the efficacy
of multiple waveforms could be compared. Furthermore, the ADFT was
determined for each waveform in each patient, which should at least in
part control for the variability in ADFT. Support of this method is
demonstrated by the finding that all but one of the total 13 patients
had at least a small increase in defibrillation requirements with the
10-ms monophasic versus the 5/5-ms symmetrical biphasic waveform. Also,
in part 2, the 1.5-J step size in determining the ADFT was not used,
and this could have biased part 1 results toward lower defibrillation
thresholds. The differences in atrial defibrillation thresholds for the
5/5-ms symmetrical biphasic and the 10-ms monophasic waveforms in parts
1 and 2 can be explained in part by the inherent variability in
defibrillation threshold determination, the differences in the way the
atrial defibrillation threshold was determined in each part, and the
differences in patient populations between the two groups.
A total of 13 patients were tested with both the 5/5-ms biphasic and the 10-ms monophasic waveforms. However, these patients were from combined data from the two parts of the study, and these two groups were not treated identically in terms of waveforms tested. Patients with a history of atrial fibrillation did not have a significantly higher ADFT than patients without a history of atrial fibrillation for either the biphasic or monophasic waveform; however, there did appear to be a trend with the biphasic waveform. There was no difference in the 10-ms monophasic waveform between the two groups. The reason for this is probably that the sample size was not quite large enough to detect a statistical difference. Johnson et al30 reported on another group of 18 patients in whom a 3/3-ms biphasic waveform and 6-ms monophasic waveform were tested. They found that patients with a history of atrial fibrillation (n=9) had a significantly higher ADFT for both the biphasic and monophasic waveforms. These findings, along with the findings of the present study with a trend toward a lower biphasic threshold in patients without a history of atrial fibrillation, support the findings of Levy et al.31 They reported that the ADFT for a 3/3-ms biphasic waveform was statistically higher in patients with a history of atrial fibrillation (n=120).
Most human internal atrial defibrillation studies have involved heavy
sedation of the patients, and the pain associated with the shocks could
not be quantified. Murgatroyd et al13 demonstrated that
more than 1 J of energy was associated with intolerable pain in
patients undergoing internal atrial defibrillation shocks without
sedation. Although pain to the patients was not measured in this study,
the mean ADFT energy for all waveforms tested was >1 J. However,
waveforms with total pulse durations >2 ms had significantly lower
ADFTs, and it is hoped that future research in this area will be
focused toward the development of even more efficient waveforms and
lead systems to help lower the threshold and minimize discomfort to the
patient.
Clinical Implications
There are several clinical applications for this type of
cardioversion system. A considerable amount of interest has developed
over the past 4 years for an implantable atrial defibrillator, and
clinical trials assessing this technology are presently in progress.
This device will be especially useful in the medically refractory
patients with infrequent paroxysms of atrial fibrillation. This type of
atrial cardioversion system is useful in the clinical
electrophysiological laboratory by providing a means to quickly
cardiovert atrial fibrillation induced during an electrophysiological
study. Furthermore, this type of system may be useful in patients who
have failed external cardioversion.32 33 Last, addition of
atrial defibrillation capacity to a ventricular defibrillator would
potentially allow for better arrhythmia detection and discrimination as
well as provide a more complete arrhythmia treatment system.
Summary
Certain biphasic waveforms are more effective than certain
monophasic waveforms for internal atrial defibrillation in humans.
Biphasic waveforms with the first phase longer than the second phase
appear to be more effective than biphasic waveforms with both phases of
the same duration as well as biphasic waveforms with the first phase
duration shorter than the second phase duration. Although some of the
biphasic waveforms evaluated in this study were more efficient for
internal atrial defibrillation, the threshold levels were still in the
range that most patients would probably feel significant discomfort
without sedation. It is hoped that continued research into this area
will result in even more efficient defibrillation waveforms and lead
systems to help further minimize the discomfort to the patient.
|
Acknowledgments |
|---|
This study was supported in part by National Institutes of Health grant HL-17670.
Received May 2, 1996; revision received October 24, 1996; accepted November 12, 1996.
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