This is a single-arm cohort study aimed to evaluate change of arteriovenous fistula volume blood flow / cardiac output ratio in patients with chronic heart failure (NYHA I-II classes) and with preserved ejection fraction as a result of a hemodialysis session after a "long" interdialysis interval.

Inclusion Criteria:

• signed informed consent,
• > 18 years
• maintenance ("chronic") hemodialysis,
• native arteriovenous fistula,
• arteriovenous fistula volume blood flow ≥1 l/min,
• arteriovenous fistula volume blood flow / cardiac output ratio < 0.3,
• interdialysis weight gain <5% of "dry" body weight,
• sufficient hemodialysis dose (eKt/V > 1.2),
• chronic heart failure (NYHA I-II classes only),
• preserved left ventricular ejection fraction (≥50%),
• absence of arrhythmias (except first-degree atrioventricular block block),
• absence of heart valve disease (except mitral regurgitation grade I-II),
• absence of intradialysis hypo/hypertension
• "long" interdialysis interval (3 days).

Exclusion Criteria:

• hypo/hypertension during or after hemodialysis,
• arrhythmias (except first-degree atrioventricular block) after hemodialysis.

Stable adult patients on maintenance hemodialysis.