Near‐field impedance accurately distinguishes among pericardial, intracavitary, and anterior mediastinal position

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Percutaneous epicardial access is a useful adjunct for catheter ablation of complex cardiac arrhythmias. Increasingly used for epicardial catheter ablation, this technique is also being used for left atrial appendage ligation and epicardial lead placement.1
Whereas accessing the pericardial space is straightforward in patients with pericardial effusion, accessing the normal pericardium can be challenging. In its physiologic state, the pericardial space contains approximately 20–25 mL of fluid, separating the parietal and visceral pericardia by about 2.0–2.5 mm.3
Even with modern techniques and equipment, epicardial access still poses risks. Major complications include pericardial effusion due to ventricular puncture, cardiac tamponade, pleuro‐pericardial fistula, coronary vessel laceration, and visceral injury.4 Right ventricular myocardial puncture reportedly occurs in up to 17% of patients when the subxiphoid approach is used.5 Intrapericardial bleeding after puncture is often self‐limited if the needle and wire are retracted; however, if the sheath is inadvertently advanced into the myocardium, bleeding can be more severe and may even require surgical repair. Major intrapericardial bleeding is reported in 4.5% of cases.5
Percutaneous epicardial access is a complex procedure associated with a high rate of complications; few significant strides have yet been made in minimizing complications. Clearly, a more definitive method is needed to confirm appropriate positioning within the pericardium. Toward this end, we explored the use of bioimpedance to facilitate percutaneous epicardial access. Given that pericardial fluid, like other serosal fluids, is a plasma ultrafiltrate, we hypothesized that the electrical properties of the fluid, including impedance, would be measurably distinct as well.
Bioimpedance has been used in various medical applications, including radiofrequency (RF) catheter ablation. Real‐time impedance monitoring is useful in this procedure because a sudden increase in impedance can indicate the adherence of tissue or coagulum to the catheter tip or predict steam pops. Commercially available impedance monitoring systems generally use unipolar measurement in which the vector is from the ablation tip to the grounding patch, which is often placed on the patient's back. Because this measurement system is unipolar, the impedance values are heavily influenced by the fluid‐filled tissue between the ablation tip and the grounding patch.
To address this problem, we attempted to use bipolar near field impedance measurements to more accurately measure the impedance of the tissue of interest. We sought to develop a system that utilized the relative differences in bipolar impedance among three different compartments—the pericardial space, anterior mediastinum, and right ventricle (RV)—to guide clinicians accessing the pericardial space. After developing this system, we tested it in vivo on sheep.
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