Resistance index measured by doppler ultrasound as a predictor of graft function after kidney transplantation

Background / Aim. As an optimal treatment of choice for the patients with the
 latest stage of chronic renal failure (CKD), renal transplantation is
 performed. The resistance index (RI)of renal artery is measured by Doppler
 ultrasonography routinely at certain time intervals to show the condition of
 the renal graft. The value of RI > 0,75 is considered abnormal. Goal to
 determine the correlation between the values of the RI index and the
 function of the transplanted kidney. Methods. in whom the Doppler of renal
 blood vessels were made in the first month after the renal transplant. In
 addition to standard demographic data, all patients were analyzed the RI We
 analyzed retrospectively 63 patients at the Clinic for Nephrology and
 Clinical Immunology in KC Vojvodina in the period from 2013 to 2017. In the
 study were included all patients index and its relationship to the function
 of the transplanted kidney. Results. Out of 63 patients, men were 63.5%, and
 women 26.5%, with an average age of 47.67 years. +/- 13.62. The primary
 diseases in patients which led to the terminal CKD stage were hypertension
 (HTA) at 33.3% and different forms of glomerulonephritis (GN); while other
 diseases (diabetes mellitus (DM), chronic pyelonephritis (chr PN),
 eclampsia, polycystic kidneys, kidney agenesia and unknown cause) were
 present in a smaller percentage. RI <0.75 was present in 73% of patients,
 and RI> 0.75 in 27%. According to the data, there is no statistically
 significant association between the resistance index and serum creatinine or
 creatinine clearance at a given time, and there is no connection to gender
 and length on HD. There is statistically significant association between RI
 and age of kidney recipient, as well as Tx type. Conclusion. In the observed
 group of patients, the resistance index of renal arteries did not prove to
 be a good predictor of the function of the transplanted kidney either in the
 early or later post-transplant periods. RI might have greater predictive
 significance if it were determined on or immediately after the
 transplantation procedure.

pyelonephritis (chr PN), eclampsia, polycystic kidneys, kidney agenesia and unknown cause) were present in a smaller percentage. RI <0.75 was present in 73% of patients, and RI> 0.75 in 27%. According to the data, there is no statistically significant association between the resistance index and serum creatinine or creatinine clearance at a given time, and there is no connection to gender and length on HD. There is statistically significant association between RI and age of kidney recipient, as well as Tx type. Conclusion. In the observed group of patients, the resistance index of renal arteries did not prove to be a good predictor of the function of the transplanted kidney either in the early or later posttransplant periods. RI might have greater predictive significance if it were determined on or immediately after the transplantation procedure. Tx is the method of choice for the treatment of patients in stage five CKD regardless of its etiology, because in addition to excretory, it replaces all other function that a healthy 5 kidney has. In addition to improving health, kidney transplantation reduces mortality, improves patient quality of life, and increases survival rates relative to hemodialysis and retroperitoneal dialysis methods. [1][2][3] Tx is complex surgical procedure which replaces nonfunctional organ with a new one in order to compensate tissue or organ function.
During kidney transplantation, organ is usually placed ileocecaly. The donor/donor is the person who gives the spool or transplant. Donors can be living related donors, living unrelated donors and a cadaveric donors, that is, a person who has been diagnosed with brain death with the consent of the family. [2] There is a spectrum of complications that may occur after Tx. Vascular complications include hematoma, hemorrhage, renal vein and artery thrombosis, lymphocele, pseudoaneurysm, renal artery stenosis. Urological complications are urine leakage and hydronephrosis. [3] The diagnostic method by which the occurrence of complications after renal transplantation can be determined in the most rapid and non-invasive manner is Doppler ultrasonography.
It is an imaging method for monitoring the condition after renal transplantation. [4,5] By calculating RI at certain time intervals, the function of the renal graft can be monitored.
The first examination is performed shortly after the transplant, and then after examination according to the appropriate protocols. [6] Arterial Resistance RI is a measure of pulse blood flow that shows resistance to blood flow caused by a microvascular bed distal to the site of measurement. It is usually measured in three places: the upper, middle and lower poles of the kidney.
Doppler ultrasonography measures maximum systolic value (Vmax) and minimum diastolic value (Vmin), so the resistance index is measured as 100x [1-(Vmin / Vmax)]. [7] The physiological value of RI, that is, the upper limit is taken to be 0.7 while the resistance index greater than 0.75 is interpreted as pathological peripheral resistance. Values> 0.7 and

General demographic data
The study included 63 patients. Patients were followed for 18 to 48 months after kidney transplantation. Median time of follow up was 24 months. Main demographic characteristics of our patients were shown in table 1.
The main cause that lead to end-stage of chronic kidney disease, also to the need for kidney transplantation, were hypertension in 21 patients (33,3%) and glomerulonephritis in 21 (33,3%). Other causes were shown in chart 1.

Function of transplanted kidney
Mean values of serum creatinine and creatinine clearance in the 1 st , 12 th , 18 th , 24 th and 48 th posttransplantation month are shown in table 2. Last control of kidney function was within 3 months before analysis. As insufficient transplant function we considered creatinine values greater than 200 µmol/l) and this was found in the last control in 6 patients (9.5%).
Creatinine values in this group were from 206 to 583 µmol / l, median value was 216,5 µmol / l. Median value of creatinine clearance in this group was 29,4 ml/min/1,73m 2 .
In the group of patients with creatinine values less than 200 µmol/l, range of creatinine was from 68 to 199 µmol/l, median value was 116,0 µmol/l. Median value of creatinine clearance in this group was 54,2 ml/min/1,73m 2 . In the observed group of 63 patients, one (1.6%) required active replacement of renal transplant function by hemodialysis method, 4 years after kidney transplantation.

Resistance index
Doppler ultrasound and RI measurement time rages from 1 to 24 days after transplantation.

Correlation of the pathological resistance index
We examined the correlation of the value of RI with the serum creatinine and creatinine clearance value in the 1 st , 12 th , 18 th , 24 th and 48 th posttransplantation month, as well as the correlation with gender and age of kidney recipients, type of transplantation and previous dialysis duration time. Results were shown in table 3.

Disscusion
Kidney transplantation is a method that replaces not only excretory but also all other kidney functions. Therefore, it is of great importance to maintain adequate function of the transplanted organ for as long as possible. Numerous complications can occur after Tx kidney. Over the years, research has focused on the detection of non-invasive diagnostic techniques that could allow early detection of complications and graft rejection. [9] RI is useful for showing different types of graft dysfunction which can be: acute tubular necrosis (ATN); acute graft rejection; renal vein thrombosis; ureteral obstruction; and pyelonephritis but cannot differentiate between diseases. [4] Measuring resistance index over a longer period is a predictor for the early detection of chronic nephropathy. [1,4] Our retrospective study included 63 patients treated with kidney transplantation in Cc Vojvodina from 2013 to 2017. Gender and age of patients were consistent with the literature. [1,6,9].
The most common underlying disease leading to CKD was hypertension and some form of glomerulonephritis, which is similar to finding in developed countries. [12] Cessation of graft function, ie the transfer of patients to another form of active treatment in our study sample was determined in one patient four years after transplantation. Similar results were obtained by Nessens et al. [6]. Therefore, according to the literature, we took a creatinine value greater than 200 µmol / l as a value indicating inadequate graft function. In our study, these creatinine levels at the last control of the nephrologist were observed in 9.5% of patients, which is slightly better than the literature, where the incidence of inadequate renal transplant function was 23%. [1] A possible explanation for these results would be the rigorous selection of recipients due to the relatively small number of 9 transplants in our center, as well as the fact that patients who had a permanent loss of graft function or death due to surgical complications in the immediate postoperative course were not included in the trial. patients. RI in our patients is most commonly measured at the first outpatient check-up of the nephrologist during the first month after kidney transplantation, or in patients with delayed graft function, during hospitalization, also during the first month after transplantation. A pathological value of RI higher than 0.75 was present in 27% of our subjects. According to the literature data, 20% of patients had pathological resistance index values, that is, RI> 0.75. [3][4][5] In our study, we found no statistically significant association of RI with serum creatinine values at all time intervals tested. It is possible that such data are due to the time of measurement of the RI, that is, the RI might have greater predictive significance if it were determined on or immediately after the transplantation procedure. In our center, we do not have a standard protocol which include doppler ultrasound of transplanted kidney on the day of the transplantation.This procedure is performed bya radiologist, specialized in this field, so measurements were done when radiologist was available.Data from the work of Cano et al., Show an association when measuring RI in the early period after transplantation as a valid marker for determining renal graft function, whereas in other literature this association has not been established. [1,3,4] According to other literature, we can conclude that a statistically significant association between RI and creatinine is shown over a period of 12 to 18 months. [1,4] We did not demonstrate an association between RI and gender of patients, whereas in other literature we found no correlation, which is consistent with the literature data. [1,3,4,5,6,9,12] According to the literature data, a statistically significant correlation of RI was found with recipient years, confirming that RI depends on the vascular characteristics of the recipient.
We have reached the same conclusions in our research. [1,4] A significant statistical correlation was shown between RI and type of transplantation, which coincides with research in papers already published. [1,11] In the study, we proved that there is no statistically significant correlation between RI and the length of previous dialysis treatment, as confirmed by the available literature data. [11]

Conclusion
In the observed group of patients, resistance index of renal arteries was not proven to be a good predictor of renal transplant function in the early or later post-transplant periods.
RI might have greater predictive significance if it were determined on or immediately after the transplantation procedure. Immunosupressive drugs (corticosteroids) 100