Paradoxical generator impedance behavior during catheter ablation in a patient with severe polycythemia

An extremely high generator impedance in the blood pool can be observed in a patient with severe polycythemia. However, ablation can be performed safely as long as the generator impedance during contact with the myocardial tissue is within acceptable limits. A 28-year-old woman underwent catheter ablation of atrial flutter. The patient had Ebstein's anomaly with pulmonary stenosis. By the age of 6 years, the patient had undergone six operations, including a bidirectional Glenn procedure with proximal RPA banding, tricuspid valve closure, ASD creation, and central shunt creation. However, the Fontan procedure could not be performed eventually. The schema of the circulatory pattern after the last operation is shown in Figure 1. The patient presented with chronic hypoxia (SpO2 70%–90% under 2 L/min O2) and compensatory polycythemia (hemoglobin concentration 23.9 g/dL, hematocrit 74.9%) because of decreased pulmonary blood flow. The patient's data are shown in the Table 1. The ablation was started using the RHYTHMIA HDx™ system (Boston Scientific, Marlborough, MA, USA) under sinus rhythm. In order to minimize the risk of intraoperative thrombosis, warfarin was uninterrupted, and the activated clotting time was kept at >300 seconds by unfractionated heparin infusion. We initially attempted to acquire high-resolution mapping with the ORION™ (Boston Scientific) catheter. However, the splines of the ORION catheter were not navigated while the catheter shaft was accurately navigated. Since we were unable to identify the cause of the phenomenon, we decided to switch to the CARTO3® system (Biosense Webster, Diamond Bar, CA, USA). However, we encountered a similar issue with the splines of the OCTARAY™ catheter (Biosense Webster) (Movie S1). As the shafts of the mapping catheters were accurately navigated, the magnetic navigation of each system was considered to be working correctly. We therefore mapped induced atrial flutter using a magnetically navigated ablation catheter (THERMOCOOL SMARTTOUCH® SF, Biosense Webster). During mapping, the generator impedance (GI) of the ablation catheter exhibited paradoxical behavior. The GI in the blood pool was greater than 250 ohms, even though the dispersive electrode was positioned just behind the heart. On the other hand, the GI paradoxically decreased to approximately 160 ohms during contact with the myocardium of the cavotricuspid isthmus (CTI) area (Figure 2A). The activation map was consistent with counter-clockwise atrial flutter (Figure 2B,C) rotating around the closed tricuspid valve. As the GI during contact with the CTI was within acceptable limits, we decided to perform CTI ablation. The relationship between GI and contact force during CTI ablation also showed a paradoxical behavior; the higher the contact force, the lower the GI (Figure 3). CTI ablation with up to 30 W of power terminated the AT without complications. After the procedure, we confirmed that the splines of the ORION and OCTARAY catheters used during the session could be accurately navigated in the testing water reserver. This case presented paradoxical phenomena regarding GI. The GI is a composite resistance (impedance) of the ablation system, myocardial tissue, extracardiac tissue, and blood pool.1 Normally, the GI in the blood pool is lower than during contact with the myocardial tissue because the impedance of the blood is lower than that of the myocardial tissue. However, the patient's GI in the blood pool was extremely high, while the GI was only slightly higher than normal when the ablation catheter was in contact with the myocardial tissue of the CTI. Therefore, the high GI in the blood pool could be attributed to the patient's high blood impedance, not to the impedance of the myocardial or extracardiac tissue. Given the blood test results and other conditions, the most likely cause of high blood impedance in this patient was severe polycythemia. Polycythemia causes high blood impedance because the red blood cell membrane is less conductive than the plasma.2 Other factors such as temperature, current frequency, and fibrinogen and plasma electrolyte concentrations have also been reported to be associated with blood impedance.2-4 We also suspect that the high blood impedance was the cause of the unavailability of atrial mapping with the multi-electrode catheters. The splines of the ORION and OCTARAY catheters are navigated using impedance and current data: The folding state of the ORION splines is calculated by the impedance field generated and measured by ORION itself, and the OCTARAY splines are navigated by measuring the current between the catheter and the body surface patches. As the shafts of both catheters were accurately navigated by magnetic navigation and no catheter defects were found during the postprocedure check, we suspected that there were problems making impedance-based navigation impossible in the patient's body, and the most likely cause was considered to be a high blood impedance of the patient. However, further investigation would be required as the blood impedance thresholds are not available in either mapping system. In conclusion, this case presented an extremely high GI in the blood pool in a patient with severe polycythemia. Compensatory polycythemia is a common condition in patients with reduced pulmonary blood flow after the surgeries of congenital heart disease. Even if the GI in the blood pool is extremely high, ablation can be performed safely as long as the GI during contact with the myocardial tissue is within acceptable limits. In such a situation, point-by-point mapping with a magnetically navigated ablation catheter might be needed in place of multipoint electroanatomical mapping. Takayuki Sekihara was involved in investigation, original draft writing, and visualization. Tomoaki Nakano, Akira Yoshida, Takafumi Oka, and Yasushi Sakata were involved in review and editing. We greatly appreciate our hospital's clinical engineers. Authors declare no conflict of interests for this article. Available upon reasonable request. Movie S1. The movie during the mapping with the OCTARAY catheter (green shaft). Normally, the splines of the OCTARAY catheter can be displayed in the calibrated area (green-colored boxes), where the magnetic location and the current data detected by the body surface patches are acquired. However, the splines of the OCTARAY catheter could not be displayed, even after the calibration by THERMOCOOL SMARTTOUCH SF (white shaft). The calibration by the OCTARAY itself was also unavailable. A similar phenomenon was observed when attempting to map with the ORION catheter. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.