Methods The project will include patients with paroxysmal and persistent AF who, according to standard recommended procedures, are indicated for catheter ablation using pulsed-field energy and will sign informed consent. Patients with spontaneously present hemolysis (hematologic disorders) and patients in a dialysis program will be excluded. The catheterization procedure will be performed routinely as currently practiced at our institution. Specifically, pulmonary vein isolation will be conducted in all patients, and for patients with non-paroxysmal AF, additional additive ablation lesions will also be performed (most commonly ablation of the posterior wall of the left atrium and mitral isthmus, possibly the cavotricuspid isthmus). The ablation will always be performed under moderate sedation or general anesthesia with the participation of an anesthesiologist, as per our institution's practice. A total of 100 patients are planned to be enrolled. Prior to the procedure, patients will receive at least 4 weeks of anticoagulant treatment with NOACs, as recommended, and this treatment will be discontinued on the day of the procedure (i.e., the last tablet will be taken the day before the procedure). NOACs will be reintroduced into their medication regimen the day after ablation. All patients will undergo outpatient baseline blood tests, including renal function (creatinine, urea), as currently standard and required (up to 2 weeks before the procedure). Post-procedure, patients will be adequately hydrated; in addition to the recommended active oral intake post-procedure, all patients will receive an extra 1000 ml of saline solution during the afternoon following the procedure (unless there is a risk of overhydration, for instance, in the case of severe dysfunction).

Blood sampling Blood samples will be collected at two different times. Firstly, at the end of the procedure, 12 ml of blood will be drawn into tubes for hemolysis examination (anticoagulated blood with EDTA and Li-heparin). The degree of hemolysis will be assessed by examining red blood cell fragments (RBC microparticles) using flow cytometry. This examination is highly sensitive and capable of detecting even minimal amounts of damaged erythrocytes. The second blood sample will be collected the morning after the procedure, following the standard protocol, and will also include renal parameters (creatinine, urea) as routinely done. Additionally, on this day, a urine test for the presence of hemoglobinuria will be conducted. Both initial blood samples will be collected and analyzed during the hospitalization. A third sample for a follow-up assessment of kidney function will be required 2-3 days after the procedure. The patient will be asked to come either to our hospital for blood collection or to have it done at a laboratory near their residence. Results of this examination, if conducted outside the hospital, will be obtained from the patient over the phone. If kidney function in this sample is slightly reduced, the patient will be instructed for further monitoring or outpatient examination. In case of significantly elevated values, appropriate action will be advised promptly.

Three months post-procedure, an outpatient check-up will be conducted. Before this evaluation, the effectiveness of the ablation will be assessed using a 24-hour ECG Holter monitor, which is our current standard procedure. Additionally, kidney function will be rechecked. The purpose is to compare the number of applications of pulsed-field energy with the degree of hemolysis observed immediately after the procedure, along with kidney function shortly after. Hemolysis, the damage or breakdown of red blood cells, will be determined using a highly sensitive method and is practically certain to be present at the end of the procedure. However, whether hemolysis might lead to hemoglobinuria the following day in some cases has not been investigated, nor has the potential for temporary worsening of kidney function in certain patients. If this were the case, selecting at-risk patients (those with higher creatinine levels before the procedure) and implementing simple measures (sufficient fluid intake) could effectively prevent this complication. Our primary hypothesis is that while ablation using pulsed-field is associated with intravascular hemolysis, leading to the damage of some red blood cells, this process in only a minimal number of patients will result in hemoglobinuria, and it will not have any significant consequences on kidney function post-procedure.