The two current types of energy sources available to create atrial lesions surgically in patients with arrhythmias are radiofrequency (RF) energy and cryothermia. Both are thermal energy sources, with RF ablation depending on tissue heating and cryoablation depending on freezing. RF and cryothermia operate on a temperature spectrum that ranges from 61°C, the critical lethal myocardial temperature of RF-generated heat, , to–30°C, the critical lethal temperature for cryothermia-generated cold ( Fig. 20.1 ).
Temperature scale from the critical lethal temperature using heat-based ablation devices such as radiofrequency (RF) catheters and surgical clamps (61°C) to the critical lethal temperature using cryothermia-based ablation devices such as cryoballoon catheters and surgical cryoprobes (–30°C).
Although there have been no randomized controlled trials (RCTs) to compare the efficacy of one ablation technology with another, these technologies have had proven clinical efficiency over the past two decades. To date, a prospective, multicenter, non-RCT, AtriCure Bipolar Radiofrequency Ablation of Permanent Atrial Fibrillation, has resulted in specific US Food and Drug Administration (FDA) approval for surgical treatment of patients with atrial fibrillation (AF). This device was used on patients with nonparoxysmal AF undergoing concomitant coronary artery bypass graft and/or valve procedures and Maze-IV ablation and resulted in a 76% freedom from AF recurrence off antiarrhythmic drugs at 6 months.
Two factors determine the effectiveness of any surgical procedure for AF, and they are controlled by both the surgeon and the ablation technology: (1) the specific locations where the lesions are created in the atria and (2) the ability of the lesion to create conduction block, which has been shown to require transmural atrial necrosis throughout the entire lesion. It was not challenging to control both of these factors with the cut-and-sew Maze procedures because the same lesion pattern was used within each iteration of the Maze procedure, and surgical incisions precluded any concern about creating lesions with potential gaps or that were not transmural. When the cut-and-sew Maze-III procedure was converted to the minimally invasive CryoMaze procedure in the mid-1990s, the lesion pattern remained the same (see Chapter 14 ). By that time, N 2 O-based cryosurgery had been used for more than 20 years to ablate cardiac arrhythmias clinically (see Chapter 9 ). Therefore each cryolesion in the CryoMaze procedure was believed to be complete, and it was not surprising that the outcomes of the two different surgical techniques for performing a Maze-III lesion pattern proved to be similar.
In 2004, Gaynor et al. were the first to publish their findings on the use of a combination of bipolar RF and cryoablation in performing the Cox-Maze-IV procedure. This technique was introduced after rigorous experimental testing of the bipolar RF clamps, and after two decades, it is the most commonly performed surgical procedure for AF and the only surgical procedure to have an FDA indication to treat patients with AF. Experimental studies have suggested that the Cox-Maze-IV procedure can be performed on the beating heart. ,
Soon after Haissaguerre and colleagues’ 1998 report showing that pulmonary vein isolation (PVI) with RF catheters was effective for the treatment of patients with paroxysmal AF, new atrial lesion patterns were proposed by both surgeons and interventional electrophysiologists, and industry adapted new energy sources for surgical ablation devices. Thus two variables were introduced that directly affected subsequent surgical outcomes, making it difficult to determine whether future failures were due to an inappropriate lesion pattern or to the inadequacy of the ablative energy source. This led a number of groups to go back to the research laboratory and investigate the efficacy of the new ablation technologies. In the first decade after Haissaguerre et al.’s report, most interventional AF procedure failures were attributed to the ablative energy source rather than to the inappropriateness of the lesion pattern. For example, despite PVI alone being an inherently inadequate intervention for AF, its failure in treating persistent and long-standing persistent AF (LSpAF) was commonly attributed to the limitations of the ablative energy source. As a result, several ablative energy sources, such as microwave, laser, and high-intensity ultrasonography, were withdrawn from the market even though they had often been used to create lesion patterns that were likely inadequate for nonparoxysmal AF. However, experimental work did show that many of these energy sources were unable to create reliably transmural lesions. During the past decade, more emphasis has been placed on the importance of creating an adequate lesion pattern for the various types of AF. This gradual change in emphasis was fueled by the dismal results of PVI alone for LSpAF (see Chapters 25 and 31 ) and led to the development of hybrid approaches using both thoracoscopic surgery and catheter ablation for the treatment of complex forms of AF (see Section IV).
Lesion Integrity
The efficacy of a specific lesion pattern depends on each of its individual lesions completely blocking electrical conduction. Because electrical wavefronts propagate through the full thickness of the atrial wall ( Fig. 20.2 ), every lesion must be uniformly transmural and contiguous with no gaps ( Fig. 20.3 ). For example, one of the most common technical errors during endocardial cryosurgery in an arrested heart is the creation of “folds” in the atrial wall that can occur across the line of intended ablation. In this situation, the cryolesion will appear to be contiguous, but the folded portion of the atrial wall will not be ablated ( Fig. 20.4 , upper panel ). When the lesion is allowed to thaw and the atrial wall resumes its normal shape, a gap will exist in the intended line of ablation ( Fig. 20.4 , lower panel ), and the procedure will fail. This is why it is absolutely essential to smooth out all folds in the atrial wall and assure perfect apposition of the cryoprobe and the target atrial tissue without even the smallest of gaps before creating a linear cryolesion.
Diagrammatic sketch of the atrial wall showing that electrical activation (red arrows) and wavefront propagation (dotted black line) occur in all layers of the atrial wall.
The ideal lesion is contiguous (has no gaps) and uniformly transmural. Any lesion that meets these two simple criteria will block the conduction of electrical activity across it.
Upper panel, A common cause of gaps in a lesion, especially cryolesions, occurs when the folds in the atrial wall are not smoothed out before application of the ablation energy. Thus the ablation does not penetrate to the distal portion of the folded wall. Lower panel, When the atrial wall is allowed to unfold, the nonablated portion will appear as a gap in the intended line of ablation.
Similarly, several factors influence the ability of RF energy to create transmural lesions in myocardial tissue, including tissue thickness, overlying epicardial fat, char on the ablation device and tissue, and air pockets in folded tissue. The generators that supply RF energy operate on an impedance algorithm that acts as a surrogate for determining lesion transmurality. Any foreign body that increases impedance may impact the interpretation of lesion transmurality in a negative way. In a study by two of the authors (RJD and SLG), a single bipolar ablation did not result in consistent transmural ablation, with more than one-third of lesions being non-transmural. Performing two successive ablations without unclamping the device produced transmural lesions in 100% of histologic sections in ex vivo human hearts. The technique reliably produced transmural lesions regardless of tissue characteristics (i.e., the presence of epicardial fat, atrial wall thickness) or ablation location. Based on these findings, surgeons performing AF ablation with bipolar RF clamps should use a double-ablation technique to increase the likelihood of achieving conduction block.
Perhaps the most common reason that individual lesions fail is that they are not transmural. Immediately after the creation of an RF lesion or a cryolesion, there are a central area of irreversibly injured cells and a peripheral area around this dead zone where myocardial cells are still viable but temporarily incapable of conducting electrical activity. Immediate intraoperative testing may indicate that the newly created lesion is contiguous and uniformly transmural because it has blocked electrical conduction across the lesion ( Fig. 20.5 , upper panel ). With time, however, the viable peripheral cells can recover from the acute injury and resume their ability to conduct electrical activity. In this case, the lesion that caused complete conduction block intraoperatively will no longer be transmural, and the procedure may fail ( Fig. 20.5 , lower panel ). This highlights the necessity of careful experimental evaluation of each device to define efficacy, in which histologic assessment can be used to definitely define lesion transmurality. Unfortunately, ablation in experimental animals does not always reflect the clinical situation, requiring the development of more realistic models, such as human hearts turned down for transplantation. ,
Upper panel, A lesion created with any ablative energy source has a zone where the cells are killed acutely surrounded by a zone where the cells remain viable but are too damaged temporarily to conduct electrical activity. Lower panel, When enough time has passed for these viable cells to recover, they can resume their normal function and conduct electrical activity. Therefore, it is essential that the initial ablation procedure be sufficient to extend the “dead zone” all the way through the atrial wall.
Unipolar Radiofrequency Ablation
The terminology describing RF devices has evolved, and because it can be confusing, it deserves clarification. RF energy can be delivered to myocardial tissue as either a unipolar or bipolar energy source. Unipolar RF devices are, by definition, unidirectional, meaning that the RF energy can be delivered from only one surface of the atrium at a time, either the epicardium or the endocardium. Unipolar RF devices are the so-called unipolar pens. Bipolar RF devices can be either uni directional or bidirectional. Bipolar unidirectional devices deliver RF via two adjacent electrodes that, like unipolar devices, can deliver ablative energy from either the endocardium or epicardium but not from both simultaneously: that is, they are technically bipolar devices but functionally are unidirectional devices that have to overcome the limitations of unipolar unidirectional devices. Examples of bipolar unidirectional devices are the Atricure Isolator RF pen and the AtriCure Coolrail device (Atricure, Inc., Mason, OH). Bipolar bidirectional devices have the electrodes embedded in the opposite jaws of a clamp, allowing the RF to be delivered from both sides of the target tissue simultaneously but requiring that it be encompassed in the jaws of a clamp. Both the Medtronic Cardioblate clamp and the Atricure Synergy clamp are bipolar bidirectional RF devices, even though the Cardioblate clamp has one electrode in each jaw of the clamp and the Synergy device has two electrodes in each jaw of the clamp. Although catheter techniques are restricted to the use of unipolar RF ablation, both unipolar RF and bipolar RF can be used for the surgical ablation. Unipolar RF devices can measure the electrode interface temperature with or without a suction stabilization device to enhance tissue contact. They can be either irrigated or non-irrigated. Despite the variety of unipolar devices, as mentioned earlier, they have had limited success in creating transmural lesions consistently. , , None has been FDA approved for the surgical treatment of patients with AF. The only FDA-approved unipolar RF device is for hybrid therapy for persistent AF and LSpAF (EPi-Sense Guided Coagulation System with Visitrax, AtriCure) and is described below. The EPi-Sense device is a 3-cm-long, suction-assisted, irrigated unipolar RF device. The lesion transmurality of this device is variable, , but its clinical performance has been validated in the treatment of patients with persistent AF and LSpAF in the setting of the minimally invasive Convergent hybrid procedure (see Chapter 36 ).
Unipolar RF ablation systems deliver RF energy between a single small electrode at the tip of a unipolar catheter or surgical pen and a large grounding patch typically placed on the skin of the abdomen or thigh. When the catheter or surgical pen is placed at the target site, alternating current (500–1000 kHz) is applied to the tip electrode, and RF current passes from the tip of the unipolar device to the grounding patch. Resistive heating occurs in the intervening tissue at a rate proportional to the square of current density, which is highest at the tissue-electrode interface at the tip of the unipolar device. As a result, significant resistive heating occurs only at this interface. Deeper unipolar RF lesions result from conductive heating of myocardium adjacent to the site of resistive heating.
The efficacy of unipolar RF ablation varies depending on the state of the heart and whether the unipolar RF is applied epicardially or endocardially. The use of unipolar RF can be problematic in totally thoracoscopic procedures and in hybrid procedures for AF because of the “cooling-sink” effect of circulating intracavitary blood. When the patient is off pump, the cavitary blood is not only normothermic but also flowing through the atrium at 4 to 5 L/min ( Fig. 20.6 , upper panel ). Continuous cooling of the endocardial surface of the atrial wall by the intracavitary blood flow will prevent the subendocardium from reaching a critical lethal temperature, and the lesion will be non-transmural ( Fig. 20.6 , lower panel ). Most transmural gaps that remain after the initial thoracoscopic step in hybrid procedures are located in the subendocardium, emphasizing the importance of performing the follow-up endocardial catheter ablation to close any remaining gaps. The problem of residual endocardial gaps also favors the theoretical advantage of delaying the follow-up catheter ablation long enough to allow the thoracoscopic lesions to heal. In extensive experimental evaluation, all unipolar RF devices have been shown to be incapable of creating reliable transmural lesions from the epicardial surface in the beating heart. The same is true for cryoablation, which is also a unidirectional ablative energy source that relies on thermal cooling.
Unipolar radiofrequency (RF) ablation: epicardial, off pump. Upper panel, When unipolar RF energy is applied epicardially to create a lesion in the atrial wall and the patient is not on cardiopulmonary bypass (CPB), the cavitary blood acts as a “cooling sink,” making it difficult for the epicardial RF device to heat the subendocardium of the atrial wall to a lethal temperature. The “cooling-sink” effect results not only from the blood being normothermic but also because it is circulating through the atrium at 4 to 5 L/min, thereby enhancing its ability to keep the endocardium cool. Lower panel, If the epicardial unipolar RF device is unable to heat the subendocardium to a lethal temperature (61°C), the permanent lesion will not be transmural, and the lesion will fail to block electrical conduction.
The ability to create transmural lesions with unipolar RF devices from the epicardium is enhanced by cardiopulmonary bypass (CPB) because the “cooling-sink” created by circulating cavitary blood is eliminated ( Fig. 20.7 ). However, both epicardial and endocardial unipolar RF is unpredictable even during normothermic CPB because there is no visual or quantitative feedback that confirms transmurality during the creation of a lesion. In a normothermic heart on or off CPB, RF ablative energy must increase the temperature of myocardial cells from 37°C to more than 61°C to ensure cell death. However, when unipolar RF ablation is performed endocardially during cardioplegic arrest when the targeted myocardial temperature is only 10° to 15°C, the magnitude of change in the myocardial temperature to ensure cell death is even greater ( Fig. 20.8 ). Although unipolar RF can be capable of doing this in thin atrial tissue, its success depends on the persistence and patience of the surgeon to attain transmurality. Because unipolar linear lesions on the endocardium have to be created much like those with endocardial catheters (i.e., by creating a line of overlapping transmural “dots”), the use of endocardial unipolar RF in the arrested heart can be laborious and time consuming ( Fig. 20.9 ). Using unipolar RF pens to simply “paint a line” on the endocardium in the desired location of a lesion is usually ineffective because the depth of such “painted lines” is only a few millimeters. Although irrigation can increase the depth of unipolar RF ablation, it still does not reliably penetrate thick tissue or fat. Thus, these devices have been largely abandoned in clinical practice and have never received an indication to treat AF during concomitant surgery. It is preferable to avoid the use of unipolar RF surgical ablation devices in favor of bipolar RF clamps or cryosurgery whenever possible, as proposed in the Cox Maze-III and Cox Maze-IV procedures.
Unipolar radiofrequency (RF) ablation epicardial, on normothermic cardiopulmonary bypass. Upper panel, Because there is no atrial cavitary blood on pump, unipolar RF applied to the epicardium is more likely to result in a transmural lesion than the same energy applied at the same site off pump. However, there is still no way to confirm in the operating room if the lesion is transmural or not. In this case, transmurality depends primarily on the thickness of the atrial wall in the target area. Lower panel, If the subendocardium does not reach the lethal temperature of 61°C, the permanent lesion will be non-transmural. It is not well established how large this remaining bridge of subendocardium has to be to conduct electrical activity and cause the lesion to fail. However, the accessory atrio-ventricular connections (which are residual strands of atrial muscle) responsible for bidirectional electrical conduction in the Wolfe-Parkinson-White syndrome are approximately the size of a human hair (see Chapter 1, Fig. 1.6 ).
Unipolar radiofrequency (RF) ablation: endocardial, cardioplegic arrest. Upper panel, The temperature of the atrial wall should be approximately 15°C during cardioplegic arrest, and there should be no warm blood circulating through the capillaries of the atrial wall. Therefore, either epicardial or endocardial RF ablation should be more successful under cardioplegic arrest than when the heart is normothermic on or off pump. Lower panel, The lesions are more likely to be transmural if they are performed under cardioplegic arrest, but they can still fail depending on the surgical technique and the thickness of the targeted atrial wall .
Unipolar radiofrequency (RF) ablation: endocardial, cardioplegic arrest. Upper panel, To create a linear lesion using unipolar RF endocardially, multiple individual applications are required, and each of them must be transmural and contiguous with lesions on both sides. Each of these lesions has a zone of instant cell death and a surrounding zone of viable but nonconducting tissue. Lower panel, With time some of the viable but non-conducting tissue can heal and resume electrical conduction. Therefore, it is preferable to use bipolar RF clamps or cryoprobes to create the lesions when the heart is arrested.
Currently, the most frequently used surgical ablation devices for the treatment of AF are manufactured by either AtriCure or Medtronic ( Fig. 20.10 ). All of the AtriCure and Medtronic unipolar RF devices can be used to create epicardial and endocardial lesions in the atrium off-pump during normothermic CPB or under cardioplegic arrest. AtriCure bipolar unidirectional RF ablation devices include a pen and the Coolrail linear ablation pen as well as the EPi-Sense unidirectional device (see later). Medtronic unipolar RF ablation devices include a unipolar ablation pen.
The most commonly used unipolar and bipolar unidirectional devices. The Medtronic and Covidien unipolar pens both have single electrodes and the AtriCure Coolrail and unidirectional pens have adjacent and parallel dual electrodes. See text for further discussion.
RF energy can be delivered by either dry or irrigated electrodes. Irrigation helps deliver RF energy uniformly and to prevent char formation by keeping temperatures cooler at the tissue interface. Unipolar RF ablation works by delivering RF energy from the probe directly to the tissue. The unipolar devices do not provide surgeons with transmurality indicators. In contrast, bidirectional bipolar RF (bipolar clamps) can be either directional or constrained, and transmurality can be implied by the manufacturer’s dose-response algorithms. The unidirectional bipolar devices have two side-by-side electrodes that are applied to the target tissue surface, with the energy passing through the tissue between them.
Unlike bidirectional bipolar RF devices, unipolar and unidirectional bipolar devices have failed to consistently create transmural lesions and have a risk of forming endocardial char or thrombus when applied endocardially. , , , Both unipolar and unidirectional bipolar RF energy sources have had difficulty creating transmural lesions when used from the epicardial surface on the beating heart. This difficulty is due to the circulating intracavitary blood flow, as stated earlier. To overcome this problem, devices have used suction to pull the atrial tissue into apposition, thus partially ameliorating the circulating heat sink.
The EPi-sense unipolar RF ablation device ( Fig. 20.11 ) is specifically designed to be used in the Convergent hybrid procedure described in Chapter 36 . It differs from other unipolar RF ablation devices by incorporating sensing electrodes and a vacuum system that holds the epicardium of the atrium in position against the ablating electrodes to enhance ablation efficacy. Because it is a unidirectional unipolar device, it does not always create transmural lesions when used off pump, but because it is designed for use solely in the Convergent hybrid procedure, initial transmurality is less important. Since it is applied from the epicardial surface, virtually all remaining gaps are at the subendocardial level and can be easily closed during the obligatory endocardial catheter ablation portion of the hybrid procedure.
The AtriCure EPi-Sense Device is specially designed for use in the Convergent hybrid procedure. It incorporates a vacuum to hold the atrial wall tightly against the sensing and ablation electrodes. The EPi-Sense device and the Convergent procedure are described in greater detail Chapter 36 .
Bipolar Radiofrequency Ablation
Bidirectional Bipolar Radiofrequency
As mentioned above, bipolar devices can be either unidirectional (Coolrail device) or bidirectional (bipolar clamps). The constrained bidirectional bipolar devices consist of a clamp with two jaws, which are applied on opposite sides of the targeted tissue. The RF energy passes through the tissue between the two jaws. RF ablation with constrained bipolar devices has provided faster and more efficient ablation because of direct contact with the tissue, with ablation time measured in seconds, as opposed to minutes. Because the tissue is ablated between the jaws of a clamp, the energy delivery is focused and isolated from the surrounding circulating intracavitary blood, allowing for more effective creation of lesions on both the beating and arrested heart.
Factors that affect lesion size and depth are power, impedance, ablation duration, temperature, and contact force. , The generators of the irrigated and nonirrigated bipolar RF clamps produce power transmitted to the electrodes, and these data are used to predict the transmurality of the lesion. The generators of irrigated clamps do this by measuring the impedance between electrodes, varying the power according to the impedance, and terminating power delivery once the feedback program detects a steady-state plateau. On the other hand, the generators of nonirrigated clamps measure conductance and continue ablation until a stable low conductance is reached. Voltage is varied according to the conductance, resulting in a safe delivery of energy to the tissue. Care should be taken to clean the electrodes after every two to three ablations with the nonirrigated clamps because char decreases conductance, which can result in nontransmural lesions.
Importantly, in a human heart ex vivo model, a double ablation without unclamping significantly improved lesion transmurality. Epicardial fat and muscle thickness can also decrease conductance and limit ablation depth. The ablation duration affects the tissue temperature profile. Cardiac muscle exposed to temperature of 50°C or higher for more than a few seconds will show irreversible coagulation necrosis. Last, adequate but not excessive contact force is needed to achieve a reliable transmural lesion. ,
Bipolar RF clamps can be used to treat patients with AF regardless of whether the heart is off pump, on normothermic CPB, or under cardioplegic arrest. A bipolar RF clamp can be used to create a lesion in the atrial free wall off pump by either completely surrounding a structure such as the pulmonary veins (PVs) or left atrial appendage or by placing one arm of the clamp inside and the other arm outside the atrium. The latter approach is not recommended in patients who are not on CPB because of bleeding and embolic risks. The endocardial arm of the clamp excludes the cavitary blood from the targeted tissue thus negating the “cooling-sink” effect that complicates epicardial unipolar RF ( Fig. 20.12 , upper panel ). It also effectively prevents damage to collateral tissue. Bipolar RF ablation during surgery is commonly performed on CPB.
