#2879 Residual kidney function is more than urinary output alone in PD patients: insights derived from tubular functional Na MRI of the kidney

Abstract

Abstract Background and Aims Maintenance of a cortico-medullary concentration gradient (CMG) is required for urine concentration and is one of most important tubular functions. We have previously demonstrated that the CMG can be quantified in both healthy controls and patients receiving HD, using 23NaMRI of the kidney. Peritoneal dialysis (PD) patients, in general, are considered to have better preservation of residual renal function (RKF), than comparative haemodialysis (HD) patients. We aimed to investigate if this was evident with respect to functional imaging of the CMG, to explore the concept that glomerular and tubular function may be dichotomized in patients with chronic kidney disease (CKD) requiring maintenance dialysis. Method We conducted an exploratory pilot study for 8 PD patients and 21 HD patients. Patients were instructed to fast from midnight before scanning. Urines samples were collected to measure osmolarity after fasting and a first MRI scan- performed to acquire baseline anatomical and sodium images. All MR imaging was carried out on a GE MR750 3T (GE Healthcare, WI) instrument. A custom-built two-loop (18cm in diameter) butterfly radiofrequency surface coil tuned for 23Na frequency (33.786 MHz) was used to acquire renal 23Na images. Results PD participants were 60 ± 10 years old, 50% male, mean body mass index (BMI) was 26.7 ± 4.6 kg/m2 and dialysis vintage was 20 ± 13 months, mean creatinine and urea clearance was 12 ± 7 mL/min/1.73 m2; HD participants were well matched, 60 ±18 years old, 52% men, BMI 27 ± 6 kg/m2 and dialysis vintage was 33 ± 25 months respectively. Urinary osmolarity was 284 ± 66 mosm/L and 239 ± 72, p = 0.1; mean medulla to cortex ratio was 1.2 ± 0.2 and 1.2 ± 0.1, p = 0.2 between PD and HD participants respectively. 7/8 PD participants were able to perform 24 hrs urine collection. Mean urine volume was substantial at 1525 ± 594 mL. We found no correlation between urine volume and corticomedullary gradient (Fig. 1A). Fig. 2C illustrates a 75 year old male, with medulla to cortex ratio at 1, urine volume 2100 mL, CLcrUr 18 mL/min/1.73 m2 and Fig. 1C showed a 55 years old patients, male, medulla to cortex ratio at 1.4, urine volume 2100 mL, CLcrUr 18 mL/min/1.73 m2. Conclusion These results underscore residual kidney function currently estimated by urine volume provides an incomplete assessment. Despite PD patients retaining higher degrees of RKF than HD patients, as assessed by urinary volume and small solute clearance, both patient groups have very similar degrees of directly imaged tubular function. The clinical importance of a focus on functional tubular mass to outcomes in dialysis patients is currently under further investigation.