Abstract The major determinants of success during internal cardioversion of atrial fibrillation (AF

Benedict Glover

Ernest Lau

Ganesh Manoharan

Noel Castro

Omar Escalona

Philip Walsh

Soumya Xavier

Vivek Kodoth

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Linear Variation Analysis of Intracardiac Atrial Impedance During Internal 
Cardioversion using Rectilinear Waveforms and Energy Step Up Protocol 
Omar Escalona1, Vivek Kodoth2, Noel Castro3, Soumya Xavier1, Philip Walsh1, 
Benedict Glover2, Ernest Lau2, Ganesh Manoharan2 
 
1University of Ulster, Newtownabbey, UK, 2BHSCT, Belfast, UK 
3Universidad Simón Bolívar, Caracas, Venezuela 
 
Abstract 
 The major determinants of success during internal 
cardioversion of atrial fibrillation (AF) are voltage, 
duration and intracardiac impedance (ICI). However, 
there is a paucity of published data regarding evidence of 
ICI dynamics during internal cardioversion of AF using 
very-low-tilt rectilinear (VLTR) waveforms with stepped 
energy protocols. In this study, patients with persistent 
AF were internally cardioverted using both biphasic and 
monophasic very-low-tilt rectilinear (B-VLTR and M-
VLTR) waveforms with a step up energy protocol (50V to 
300V). The ICI of patients who had more than 4 shocks 
delivered were retrospectively analyzed from recorded 
voltage and current waveforms.  
 A significant reduction in ICI was noticed after each 
of the first shocks using B-VLTR. The linear change of 
ICI within a shock, as characterized by its slope (Zm), 
increased in algebraic value and presented polarity 
reversal between the positive and negative phase within  
a B-VLTR shock, particularly after the first shock, where 
both B-VLTR and M-VLTR presented positive 2nd 
derivatives.  
 The results obtained provide valuable evidence for 
understanding electrode-tissue interface factors 
depending on VLTR defibrillation waveform amplitude, 
duration and current reversal action. 
 
 
1. Introduction 
     Atrial fibrillation (AF) is one of the most common 
cardiac arrhythmias encountered in medical practise. AF 
occurs in approximately 0.65% of the population between 
the ages of 45 and 64 years and has a prevalence of 9% 
among those over 80 years of age [1,2]. It is currently 
estimated to affect approximately 4.5 million people in 
Europe and over 2.2 million people in the USA.  
     Atrial fibrillation is one of the leading causes of 
stroke, with the associated after care costs identified as 
being almost entirely preventable. Consequently, the need 
for the continued investigations and improvements in AF 
associated therapies remains self evident [3,4]. 
 For chronic AF sufferer's where pharmacological 
intervention is contraindicated or ineffective, internal 
synchronised electrical cardioversion using a step energy 
protocol is most often the only effective treatment 
available. However, optimisation of defibrillation shock 
waveform is still the subject of much debate. Historically, 
defibrillation shock waveforms have been generated 
using standard capacitive discharge circuits. However, it 
has been reported in the literature the successful use of a 
radio-frequency (RF) defibrillator for transcutaneous 
passive atrial defibrillation, which delivers a very low-tilt 
rectilinear (VLTR) power pulse waveform that has been 
found to be both safe and effective [5,6]; thereby enabling 
defibrillation thresholds as low as 1.27 joules in the acute 
AF case. Yet, strategies for the development of sub 1J 
defibrillation threshold protocols are required if patient 
sedation is to be avoided and the low-cost passive 
implantable atrial defibrillator is to become an attractive 
alternative [7].  
 The objective of this work was therefore to investigate 
the dynamic changes of intracardiac atrial impedance 
during internal cardioversion using a step up energy 
protocol with very-low-tilt monophasic and biphasic 
defibrillation waveforms; with a view to understanding 
the likely impact on AF electrical cardioversion protocol 
efficacy. 
 
2. Methods 
2.1. Study design and patients 
 
     Thirty patients with persistent AF, who would 
clinically benefit from electrical cardioversion and with 
previous history of failed transthoracic cardioversion 
were recruited for a comparative study of low tilt biphasic 
and low tilt monophasic waveform for internal 
cardioversion of atrial fibrillation. Each patient was fully 
anticoagulated for 4 weeks prior procedure. The patient 
was adequately sedated prior cardioversion. Internal 
cinc.org Computing in Cardiology 2012; 39:721-724.721
cardioversion was done using a step up protocol after 
adequate sedation. During internal cardioversion current 
and voltage was recorded using a digital oscilloscope 
(Tektronix TDS-3014B) and a current probe (Fluke 80i-
110s). Dynamic intracardiac impedance (ICI) analysis of 
patients, who had more than 4 step up shocks were 
retrospectively analysed from the applied voltage and 
current shock waveforms recorded during cardioversion. 
        The Patients were randomised prior to internal 
cardioversion to 12-ms duration biphasic VLTR {B-
VLTR, chronosymmetric (6ms/6ms), amplitude 
asymmetric (negative phase at 50% amplitude) 
waveform, (Fig. 1a)} and monophasic VLTR {M-VLTR 
waveform (Fig. 1b)} using random cards. Step up voltage 
protocol (50V to 300V) used during the study is as 
presented in Table 1. 
 
 
(a) 
 
 
(b) 
 
Figure 1.  RF defibrillator generated very-low tilt 
waveveforms: (a) biphasic (B-VLTR), 6/6ms, voltage 
waveform, (b) monophasic (M-VLTR), 12ms, waveform. 
 
2.2. Electrodes location 
          Single use, commercially available defibrillation 
catheters (St. Jude 6 French Inquiry™ internal 
cardioversion catheter) was positioned in the right atrium 
and the distal coronary sinus, under fluoroscopic 
guidance (Fig. 2). The internal defibrillation catheter was 
connected to the custom built radiofrequency defibrillator 
using St Judes internal cardioversion junction box. 
Table 1.  Step up protocol with voltage and shock energy 
(referred to a 50Ω heart impedance). 
 
  Study Arm 1
Biphasic, 6/6 (ms) 
Study Arm 2
Monophasic, 12 (ms)
Step  Ph1, Volts Energy (J)  Voltage  Energy (J)
S1  50 0.38  50  0.6
S2  100 1.5  100  2.4
S3  150 3.38  150  5.4
S4  200 6.0  200  9.6
S5  240 8.62  240  13.8
S6  280 11.8  280  18.8
S7  300 18.5  300  21.6
 
 
 
Figure 2.   Position of defibrillation catheters in the right 
atrium (RA) and coronary sinus (CS) for internal 
cardioversion of AF (right anterior oblique view). 
 
2.3. Signal and clinical data acquisition 
               Prior to the commencement of treatment, a low 
voltage test shock (50V) was delivered to a dummy load 
(47Ω) to verify system readiness and synchronisation 
with the patients ventricular activity (R wave).  
Defibrillation was then performed using either a 
monophasic or biphasic (randomised) VLTR electrical 
shock waveform as per the stepp  up protocol (Table 1). 
The maximum voltage level is 300V. Success was 
defined as a return to normal rhythm for a period of 30 
seconds or more. The voltage and current waveforms 
from each shock delivered were recorded using the digital 
oscilloscope (Tektronix TDS 3014B). The minimum 
sampling frequency used by the oscilloscope was 
250kHz.  
 
2.4.    Dynamic impedance analysis 
       The internal cardiac impedance (ICI) across the 
RA and CS electrodes, was considered a dc impedance or 
resistive load, where voltage/current ratio provides a 
722
reasonable estimate of the load into which the wave 
generator is working [8]. For the dc ICI analysis type, 
averaged voltage and current at each waveform phase 
were calculated to get an unique value for each pulse 
wave segment for B-VLTR (positive and negative phase 
segments) and M-VLTR waveforms, thus allowing to 
calculate the dc impedance value (V/I). 
The second type of dynamic ICI analysis was done by 
computing the impedance slope, Z(m), in units of Ω/s, in 
the early waveform segment part (1-5ms, positive phase 
in B-VLTR) and in the late segment part (7-11 ms, 
negative phase in B-VLTR). For this, the standard 
statistical linear regression slope component (m), was 
computed for the impedance sample points along each 
4ms waveform segment (at least 1000 calculated 
impedance points). This robust approach was adopted in 
order to detect subtle ICI linear variations along the 4ms 
time period; without having to use a potentially 
controversial very-low frequency digital filtering based 
approach; due to associated RF defibrillator high 
frequency noise in the voltage and current waveforms.  
 
2.5. Statistical analysis 
      Results are expressed as mean ± standard deviation 
in ohms for all ICI measurements, for both the B-VLTR 
(+ phase), B-VLTR (- phase) and M-VLTR. Student-t test 
was used to determine whether there was any significant 
difference between the groups. A paired, two-tail value of 
p < 0.05 was considered as statistically significant.  
 
 
3. Results 
Two types of dynamic ICI analysis were considered. First 
dc ICI values (average in a particular time-segment) were 
calculated from the recorded V and I waveforms (Table 
2), then on the linear rate of ICI change (Z(m)), within the 
12ms shock waveforms and along the step up of 
defibrillation shock voltages, particularly, progressive 
Z(m) changes in the first three shocks (Table 3).   
 
Table 2.   Results of impedance (Z) change analysis with 
step up shocks and within a B-VLTR shock.  
 
 
Shock Waveform 
      Type/Part  → 
B-VLTR 
(N=14) 
Positive 
B-VLTR 
(N=14) 
Negative 
M-VLTR 
(N=12) 
Mean Z @ S1 
(Ω ± SD) 
76.79 
 ±14.9 
74.07 
 ±14.2 
75.17 
 ±17.8 
Mean Z @ S2 
(Ω ± SD) 
72.64 
 ±13.8 
71.43 
 ±12.2 
73.25 
 ±16.1 
|∆Z| after S1 (Ω) 
(p-value) 
4.15  
(p< 0.00053) 
2.64  
(p< 0.0581) 
1.917  
(p < 0.6284)  
Mean Z @ S3 
(Ω ± SD) 
72.07 
 ±12.8 
72.50 
 ±13.6 
72.00 
 ±15.4 
|∆Z| after S2 (Ω) 
(p-value) 
0.57 
(p < 0.3276)  
1.07 
(p < 0.4242)   
1.25 
(p < 0.1192 ) 
 Mean Z:  S4 → S7 
(Ω ±SD) 
68.93 
±11.9 
70.48 
 ±12.5 
71.00 
 ±14.6 
Table 3.   Results of dynamic impedance slope Z(m) 
analysis within a shock, and with step up shocks. 
 
 
Shock Waveform 
      Type/Part  → 
B-VLTR 
(N=14) 
Positive
B-VLTR 
(N=14)  
Negative 
M-VLTR 
(N=12) 
Early 
M-VLTR 
(N=12) 
Late 
Mean Z(m) @ S1   
(Ω/s  ±SD) 
11.47 
±150.66 
1518.4 
±640.02 
57.93 
±154.97 
55.37 
±153.75 
Mean Z(m) @ S2 
(Ω/s  ±SD) 
-37.00 
±124.5 
1082.7 
±169.36 
-56.81 
±147.12 
-1.13 
±105.86 
|∆Z(m)| after S1 
(Ω/s) 
(p-value) 
48.47 
(p< 0.487) 
435.77 
(p< 0.039) 
114.74 
(p< 0.120) 
56.50 
(p< 0.109) 
Mean Z @ S3 
(Ω/s  ± SD) 
-63.04 
±59.60 
997.92 
±260.74 
-66.90 
±73.91 
-35.98 
±94.99 
|∆Z(m)| after S2 
(Ω/s) 
(p-value) 
26.04 
(p< 0.40)  
84.73 
(p< 0.387)   
10.09 
(p< 0.986) 
34.85 
(p< 0.415) 
Mean Z(m):S4→S7 
(Ω/s  ±SD) 
-164.7 
±116.7 
898.9 
±150.3 
-180.5 
±89.8 
-56.28 
±73.29 
 
      Dynamic ICI slope trends in particular segments of 
the B-VLTR (Positive vs Negative phase) and M-VLTR 
(Early vs Late) waveforms, along the seven increasing 
voltage steps, is graphically presented in Fig. 3. There, 
with B-VLTR waveforms, 14 patient cases were averaged 
at each shock step (S1-S7), and for M-VLTR waveforms, 
12 patients were included and similarly averaged.   
 
 
(a) 
 
 
(b) 
Figure 3. Average dynamic impedance slope (Z(m)) for 
(a) B-VLTR and (b) M-VLTR shock waveforms along 
the step up voltage protocol: S1 → S7. 
723
4. Discussion 
      Significant reduction in dc ICI (∆Z) was noticed after 
the first shock (S1) using B-VLTR waveform, and only in 
the positive phase (4.15Ω, p <0.00053). The difference 
between dc ICI in the positive and in the negative phase 
of B-VLTR was not significant within S1 (2.72Ω, 
p<0.057) and for subsequent shocks (0.71Ω, p<0.293). 
Impedance reduction was not significant with M-VLTR 
waveforms at any shock level at all, and in fact, for both 
waveform types, dc ICI reduction after S2 was minimal 
and practically negligible after S3, which was evidenced 
by smaller dc ICI standard deviations in the last 4 shocks 
(not presented in Table 2). 
     The estimated linear variation of ICI within early 
(positive) and late (negative) segments of a shock, 
characterized by its slope, Z(m), revealed a different and 
consistent behavioral pattern of ICI dynamics, as 
summarised by Table 3 and Fig. 3. From the ICI Z(m)  
analysis, it was noticed that Z(m) increased significantly 
in algebraic value and presented polarity reversal between 
the positive and negative phase within  a B-VLTR shock, 
particularly after the first shock, where both B-VLTR and 
M-VLTR presented positive 2nd derivatives of Z(m). 
Results in Table 3 also indicate an overall insignificant 
variation of Z(m), that is |∆Z(m)|, with step up shocks.  
Significant difference in values of |∆Z(m)| are mainly 
observed between early and late segments (positive and 
negative phase in B-VLTR), within the 12ms of the shock 
waveform. This fact is more clearly evidenced in Fig. 3, 
as the red and blue curves in both Fig. 3(a) and Fig. 3(b), 
diverge from each other, without any crossover point after 
the first shock (S1).  
      Nevertheless, an interesting dynamic feature observed 
in Table 3, is that |∆Z(m)| reached a significant difference 
after S1 for B-VLTR (Negative). An electrophysiological 
interpretation of this particular dynamic behaviour of 
Z(m), could be the possible condition for triggering 
complex dynamic voltage mediated channel ion currents; 
activated by the electric field reversal just at the first 
shock; however, this observation would lead us to ask 
why does this occur only for the first shock?  This is 
something yet to be answered, but could be related to 
some findings that have reported for example that the first 
shock plays the most important role on AF ECG 
organization studies through Sample Entropy [9]. 
     A contrasting difference between intra-shock dc ICI 
and Z(m) dynamic behaviour, is in their trends with step 
up voltage level, for both B-VLTR and M-VLTR 
waveforms. With dc ICI there is no clear tendency of 
change. Whereas with Z(m): Early vs Late segments, 
there is a significant incremental change (ΔZm) tendency 
with step up shock levels. This becomes consistently for 
both B-VLTR and M-VLTR after the S3 and up to the S7; 
(1068Ω/s  ±81.1SD) and (123Ω/s  ±20.3SD) respectively.    
5. Conclusions 
     Significant dc ICI reduction was noted with B-VLTR 
waveforms after first shock at 50V level. ICI reduction 
with biphasic waveforms would correlate to higher 
cardioversion success rate when compared to monophasic 
waveforms. The non-significant effect on dc ICI by M-
VLTR waveforms may suggest a significant dependence 
of ICI and the electrode-tissue interface upon providing a 
temporary current reversal within the shock duration.    
     The dynamic ICI slope (Z(m)) data obtained provides 
valuable evidence for understanding electrode-tissue 
interface factors depending on VLTR defibrillation 
waveform amplitude, duration and current reversal action. 
It also can reveal potential electrophysiological 
implications that could help to explain the evidenced 
superiority of B-VLTR over M-VLTR for AF 
cardioversion [6]. 
 
References 
 
[1] Heart Disease and Stroke Statistics - 2012 Update. 
Circulation, 125, e2-e220. 
[2] Focused updates incorporated into the ACC/AHA/ESC 
guidelines for the management of patients with atrial 
fibrillation  Circulation 2011; 123: e269-e367. 
[3] Guidelines for the management of atrial fibrillation. 
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[4] Atrial fibrillation: national clinical guideline for 
management in primary and secondary care. Royal College 
of Physicians, London, United Kingdom, 2006. 
[5]  Santos JA, Walsh S, Evans NE, Escalona O, Manoharan G, 
Anderson JMcC, Allen JD, Adgey AAJ. Transdermal atrial 
defibrillation with bipolar low-tilt pulses. Electronic Letters 
2003; 39: 1789-1790. 
[6] Kodoth V, Castro NC, Glover BM, Anderson JMcC, 
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[7] Escalona OJ, Velasquez JJ, Waterman N, Chirwa L, 
Anderson J. Transcutaneous dual tuned RF coil system 
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implantable atrial defibrillator. Computing in Cardiology 
2010; 37:569−572. 
[8]  Ellenbogen KA, Wilkoff BL, Kay GN, Lau CP. Clinical 
cardiac pacing, defibrillation and resynchronization 
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[9]  Alcaraz R, Hornero F, Rieta JJ. Organization analysis of 
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Address for correspondence: 
 
Professor OJ Escalona,  
University of Ulster, NIBEC Building,  
Shore Road, Newtownabbey, BT370QB, UK. 
Email: oj.escalona@ulster.ac.uk 
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Linear variation analysis of intracardiac atrial impedance during internal cardioversion using rectilinear waveforms and energy step up protocol

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Linear variation analysis of intracardiac atrial impedance during internal cardioversion using rectilinear waveforms and energy step up protocol

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