The rationale behind the surgery for atrial fibrillation (AF) is in the creation of conduction block between the pulmonary veins and left atrium to block abnormal activation arising in the pulmonary veins and lines of block on the atria to prevent macro- and micro-reentrant activations on the atria.1,2 The cut-and-sew technique has been the most reliable method to create a conduction block. A variety of ablation devices have emerged to replace the cut-and-sew technique,3–7 resulting in shorter ischemic and cardiopulmonary bypass times and prevention of major complications, such as bleeding.8
The drawback of ablation devices is potential incompleteness in creating a transmural and continuous necrosis, so the activation may conduct across the lines of block and result in recurrence of AF or reentrant tachycardia.9–11 Intraoperative testing detects the site of incomplete ablation and directs additional ablation and may prevent the postoperative atrial tachyarrhythmias.12–14 Pacing the pulmonary vein can be easily performed to verify complete pulmonary vein isolation. We have shown that recording the coronary sinus electrograms detects the residual conduction in the coronary sinus.12 However, testing of an ablation line on the atrial free wall is not as easy as the pulmonary vein isolation or coronary sinus.15
Activation mapping may be used to detect a residual conduction in the ablation line on the atrial free wall,16 but it requires a specialized mapping system and considerable time for the analysis to draw isochronal maps. Double potentials recorded by bipolar electrodes that straddle the ablation line represent the local activations at both sides across the ablation line.15,17,18 The site of incomplete ablation may be localized as a conduction gap by analyzing the activation sequence of the double potentials recorded at a few locations along the ablation line. The purpose of this study was to examine the feasibility and efficacy of double potential mapping for locating the site of conduction gap in the ablation line created on the atrial free wall.
All animals received humane care as outlined by the US Public Health Service Policy on Humane Care and Use of Laboratory Animals and the Guide for the Care and Use of Laboratory Animals (National Institutes of Health Publication, revised 2002). In addition, this study protocol was approved by the Animal Ethics Committee of Nippon Medical School.
Adult mongrel canines weighting 15 to 25 kg (n = 7) were anesthetized with intravenous thiamylal sodium (10 mg/kg), intubated with a cuffed endotracheal tube, and mechanically ventilated with a volume-controlled respirator. An adequate level of anesthesia was maintained by a continuous infusion of 5 to 20 mg/h propofol. A limb-lead electrocardiogram was monitored continuously.
After the pericardium was opened through a median sternatomy, a linear ablation was made on the lateral wall of the right atrium using a bipolar radiofrequency ablation device (AtriCure Isolator Surgical Ablation System [OLL1]; AtriCure Inc, West Chester, OH USA). A 3-mm-wide tape was placed on both jaws 10 mm from the tip of the ablation electrode to intentionally create an incomplete ablation lesion (Fig. 1). One of the jaws was inserted into the atrium through a purse-string suture with 4–0 prolene. The lateral right atrium was clamped with the device and radiofrequency energy was delivered until the conductance between the electrodes reached the defined value. Another line of ablation, toward the opposite direction, was made without making an incomplete ablation, to block the stimulated activation propagated around the cranial side of the block line during the following mapping study.
The lateral right atrium was mapped using a custom-made composite electrode patch composed of 11 pairs of bipolar electrodes and 45 target-type bipolar electrodes. The electrode patch was placed in the same position on the lateral right atrium throughout the data acquisition. The 11 pairs of bipolar electrodes were designed for recoding double potentials across the ablation line, serving as the double potential mapping. The electrodes had an intraelectrode distance of 10 mm and an interelectrode distance of 3 mm. The composite electrode patch was placed on the right atrial epicardium in such a way as to have the 11 pairs of bipolar electrodes straddle the line of ablation (Fig. 2). The 45 target-type bipolar electrodes served as activation mapping to examine the activation around the ablation line. The electrodes had an intraelectrode distance of 0.1 mm and an interelectrode distance of 5 mm.
All the electrograms were recorded simultaneously 5, 10, 20, and 30 minutes after ablation during continuous pacing at a cycle length of 300, 250, 200, and 150 milliseconds. Two pairs of bipolar electrodes were sutured on the right atrial epicardium close to the cranial corners of the electrode patch and used for pacing. Stimulation was performed using a programmable pulse generator (Cardiac Stimulator, Fukuda Denshi Corp, Tokyo, Japan), and the stimulus output was set at twice the pacing threshold. At each time set, the electrograms were recorded during pacing from each pair of stimulation electrodes at each pacing cycle length.
The electrograms for the double potential mapping, along with the electrocardiogram, were displayed continuously on the monitor throughout the study to locate the site of conduction gap in the ablation line. All the bipolar electrograms were recorded at a gain of 500, with a frequency response of 5 to 500 Hz. Each channel was digitized at 5000 Hz with a 12-bit resolution and the data were saved in a hard disk. The offline analysis was performed using a 512-channel computerized data-acquisition and data-analysis system to examine the accuracy of the double potential mapping.
The site of conduction gap in the ablation line was determined by the double potential mapping. The double potentials recorded by the 11 pairs of bipolar electrodes that straddled the ablation line were continuously displayed on the computer along with the electrocardiogram. The initial deflections of the double potentials after the stimulation represent the activation in the region proximal to the ablation line, and the second deflections represent the activation distal to the line. The site of conduction gap was determined at the electrode location with the earliest deflection in the second potentials of the double potentials. The time of local activation was defined as the steepest deflection in both the initial and second deflections of the double potentials. The time of the steepest deflection was accurately determined by the derivative of the potentials.
The site of conduction gap in the ablation line was also determined by activation mapping of each animal. In activation mapping, the activation time was defined as the time of maximum absolute amplitude at the 45 bipolar electrograms, and the map was constructed on a three-dimensional surface model of the right atrium displayed on the computer. The site of conduction gap was presumed by the pattern of atrial activation and the location with the earliest activation in the region distal to the ablation line.
The stability of the double potential mapping in locating the conduction gap was assessed by the variance of the located site in each measurement. Because conduction gap was not a single point and there was an interelectrode distance of 3 mm, the site of conduction gap was assumed to be located at the location averaged from all the measurements in each study. Then, the variance of the located site was determined as the distance from the averaged location. The stability of the double potential mapping was assessed in relation to different times of measurement after ablation and different pacing cycle lengths.
All continuous variables were expressed as mean ± SD. Statistical analysis was performed by JMP (version 10; SAS Institute Inc, Cary, NC USA). Repeated measurement of analysis of variance was used to compare the distances with a correlation with the time after ablation and the different cycle lengths of pacing. A P value of less than 0.05 was considered statistically significant.
Localization of Conduction Gap
The site of conduction gap was clearly determined by the double potential mapping on a real-time basis in all animals. The bipolar electrograms with double potentials were continuously displayed on the computer, and the sequences of both initial and second deflections of the double potentials were easily identified. The site of conduction gap was localized at the electrode location with the earliest activation in the second potentials of the double potentials. The site of conduction gap was more precisely localized by derivation of the double potentials (Fig. 3).
Comparison With Activation Mapping
The activation mapping was also performed to localize the conduction gap in each animal. After the offline analysis of the data and the activation map was constructed, the conduction gap was localized by the pattern of atrial activation and the earliest activation in the region distal to the ablation line (Fig. 4). Because the interelectrode distance of the activation mapping was larger than that of the double potential, the accuracy in locating the conduction gap was lower than the double potential mapping. There was a 3- to 5-mm distance between the sites localized by the double potential mapping and activation mapping methods in each animal.
Stability of the Double Potential Mapping
The stability of the double potential mapping in locating the conduction gap was assessed by the variance of the located site in each measurement in relation to the time of measurements after ablation and the pacing cycle length. The variance of the located site from the average during pacing at a cycle length of 150 milliseconds was 1.3 ± 0.5 mm, 1.5 ± 0.4 mm, 1.2 ± 0.4 mm, and 1.3 ± 0.4 mm measured at 5, 10, 20, and 30 minutes after ablation, respectively. The variance of the located site from the average during pacing at a cycle length of 200 milliseconds was 1.5 ± 0.5 mm, 1.4 ± 0.4 mm, 1.2 ± 0.4 mm, and 1.5 ± 0.4 mm measured at 5, 10, 20, and 30 minutes after ablation, respectively. The variance of the located site from the average during pacing at a cycle length of 250 milliseconds was 1.5 ± 0.5 mm, 1.3 ± 0.4 mm, 1.2 ± 0.4 mm, and 1.5 ± 0.4 mm measured at 5, 10, 20, and 30 minutes after ablation, respectively. The variance of the located site from the average during pacing at a cycle length of 300 milliseconds was 1.3 ± 0.5 mm, 1.6 ± 0.4 mm, 1.2 ± 0.4 mm, and 1.5 ± 0.4 mm measured at 5, 10, 20, and 30 minutes after ablation, respectively. The variance remained less than the interelectrode distance (3 mm) throughout the study. There was no significant change in the variance between the different times after ablation and the different pacing cycle lengths (Fig. 5).
The present animal study has demonstrated that double potential mapping is capable of locating the conduction gap in linear ablation, on a real-time basis with a small number of electrodes and with sufficient accuracy, without drawing any activation maps. This mapping technique may be applied in beating-heart epicardial ablation to reduce the incidence of postoperative atrial tachycardia and fibrillation by directing the surgeon to perform additional applications of ablation to the incomplete ablation site intraoperatively.
Prevention of Postoperative Atrial Tachycardias
Surgery for AF may fail and result in postoperative atrial tachyarrhythmias because of the following causes: incomplete lesion sets may leave macro-reentrant circuits for atrial tachycardia. More frequently, incomplete ablation due to imperfect ablation technology or inappropriate technique may create a nontransmural or noncontinuous necrosis and leave a conduction gap in the ablation lesions and subsequently result in atrial tachycardia.9–11,19 Intraoperative verification of conduction block is useful in detecting the residual conduction across the lines of block.12–14 Pulmonary vein isolation can be tested by examining whether the atria are driven by the pacing at each of the pulmonary veins. Conduction block within the coronary sinus can also be tested intraoperatively by analyzing the activation sequence of the electrograms recorded from the catheter electrodes inserted in the coronary sinus.12 However, testing of the ablation line on the atrial free wall is not as easy or simple as in the pulmonary vein isolation and the coronary sinus.15,16
Locating the site of conduction gap in the ablation line on the atrial free wall by activation mapping requires a specialized mapping system, multiple electrodes, and a considerable amount of time to draw the isochronal activation maps and the analysis of the data.16 On the other hand, double potential mapping does not require such a system and is capable of locating the conduction gap without drawing any activation maps. The double potential mapping is quick, and its result is shown on a real-time basis with fewer electrodes distributed along the ablation line. These two mapping methods are compared in Table 1.
Mechanism of Double Potentials
Bipolar electrograms represent the difference in the voltage between the two electrodes, whereas unipolar electrograms represent the absolute voltage, assuming the indifferent electrodes as zero volt.20 In bipolar electrograms, the intraelectrode distance is usually set sufficiently small to reduce distant potentials and to generate a more distinct waveform for accurate determination of the local activation time between the electrodes. The waveform of the electrograms is affected by the spatial correlation between the activation propagation and the electrode orientation as well as the fiber orientation of the myocardium.21
The electrograms recorded by bipolar electrodes with a wide intraelectrode distance may contain the activations of the myocardium between and around the electrodes, making the distinct determination of the activation time at a single point difficult. If the activation propagates perpendicular to the electrode orientation, the difference in voltage between the electrodes is small, and subsequently, the electrogram is low voltage and fractionated. If the activation propagates parallel to the orientation of the electrodes, the electrograms form double potentials. As the wavefront of activation approaches closer to one of the bipolar electrodes, the voltage gradient between the electrodes increases and makes a slope. At the moment when the wavefront passes across one of the electrodes, the polarity of the potential is abruptly inverted and a steep deflection is made. The deflection of the electrogram is the steepest at the moment when the wavefront is across the electrode. This moment is accurately determined by derivation of the potential. During the time when the wavefront traverses in between the electrodes, the potential represents the difference in the voltage between the electrodes. At the moment when the wavefront passes across another electrode, the potential makes another steep deflection with an inverted polarity.
In the present study, the bipolar electrodes were placed on the atrial epicardium in a way that the electrodes straddle the ablation line and the electrograms were recorded during pacing from either corner of the electrode patch. This setting of the electrodes allowed the activation wavefront to propagate parallel to the orientation of the bipolar electrodes that straddle the conduction gap and subsequently formed clearly separated double potentials that represent the activations at the regions proximal and distal to the ablation line. The site of conduction gap in the linear ablation should be located at the electrode with the earliest activation across the ablation line.
Double potential mapping should be useful in detecting the site of incomplete ablation particularly in the beating-heart epicardial ablation in the off-pump setting.22,23 Linear ablation using a radiofrequency device or cryothermia can leave residual conduction in the ablation line. If the region is localized by the double potential mapping, an additional ablation may be applied to the region and the mapping repeated until the complete line of conduction block is confirmed. A sequence of activation starting from the edge of the ablation line indicates the complete line of conduction block. Mounting the line of bipolar electrodes on both sides of ablation devices allows a sequence of ablation and testing without displacing the ablation device from the target region.
The conduction gap can be wide or multiple in patients with incomplete linear ablation. In such cases, early activation of the second deflections of the double potentials may occur at multiple electrode locations. The additional ablation at these electrode locations will reduce the number of early activation in a stepwise fashion before a completely continuous and transmural line of conduction block is made and the activation across the ablation line starts from the edge of ablation line.
A conduction gap with slow conduction may not be identified through the double potential mapping or any other mapping modalities. The double potentials may not be clearly separated and continuous activities may be present between the potentials at the conduction gap with slow conduction. In addition, the site of conduction gap may not be detected when the wavefront rotates around the edge of the block line and activates the distal region earlier than that through the gap. Even by precise activation mapping with hundreds of electrodes, the activation through the slow conduction may be concealed during sinus rhythm or during pacing. Furthermore, an acutely proven conduction block does not necessarily guarantee conduction block in the long term after ablation.24
In conclusion, the double potential mapping locates the site of conduction gap in the linear ablation made on the free wall, on a real-time basis, with a small number of electrodes and sufficient accuracy, without drawing any activation maps. This technique may be applied in the off-pump maze procedure by combining it with epicardial ablation devices.
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This is a fascinating experimental study from Dr Takashi Nitta and his colleagues at Nippon Medical School in Tokyo, Japan. His group has extensive experience with arrhythmia surgery. In a canine model, they examined whether double potential mapping using bipolar electrodes straddling the ablation line could locate conduction gaps on a real-time basis. They found that this simple technique was able to precisely locate gaps in ablation lines, without the need for time-consuming and difficult-to-perform epicardial mapping. This is a very practical and useful technique for surgeons to verify the integrity of their ablation lines particularly in less-invasive beating-heart epicardial ablation performed without cardiopulmonary bypass.