Discussion
A preliminary study by Vidal et al. reported transcutaneous NIRS
measurements in 24 children after KT [7]. Renal rSrO2 measured by
NIRS significantly increased over time during the first three days after
surgery and a significant correlation was found with serum creatinine,
estimated glomerular filtration rate (eGFR) and with the decrease of
post-operative urine-neutrophilgelatinase-associated lipocaine (u-NGAL),
which is a marker of tubular injury reflecting the ischemic and
reperfusion damage of the kidney. In this series, DUS did not identify
any abnormality in renal vascularisation and no vascular complications
were reported. To date, four patients experienced a delayed graft
function (DGF) without any peculiar modifications of rSO2.
Despite these promising results, Skowno et al. raised some
criticisms [8]. First, DUS, the gold standard for the assessment of
allograft perfusion, was not related to rSrO2. Recently, Malakasiotiet al . identified a significant correlation between renal rSO2
measured by NIRS and resistive index derived from DUS in a series of 29
paediatric patients [9]. Once again, no complications were reported
in the cohort.
Second, the region of interest of NIRS may reach up to 4 cm below the
skin, according to the manufacturers, and its effectiveness in
monitoring somatic perfusion was validated in infants weighing less than
10 kg [10]. This body weight was sensibly lower than those of the
patients undergone KT. However, Skowno et al. confirmed the
effect of body size on transcutaneous NIRS only in a porcine model,
suggesting its in vivo application up to 1 cm of depth [11].
Nevertheless, Pérez Civantos et al. assessed renal rSrO2 in 61
adults undergone KT, using probes with a maximum depth of 2.5 cm
[12]. NIRS readings were significantly correlated to the decreasing
values of serum lactate at eight hours and 24 hours, which is another
marker of ischemic injury. Furthermore, renal rSrO2 was related to
initial diuresis at three hours and to mixed central venous oxygen
saturation. On the other hand, no correlation with DUS was found. In
addition, in this series arterial thrombosis and bleedings were
encountered, showing a maintained decrease in rSrO2 which might
anticipate the vascular events. Moreover, the same study was replicated
in liver transplantations. Even in this scenario, NIRS decreased early
during bleeding or in case of thrombosis [13].
The last concern about NIRS regarded the placement of the probes. The
vascularisation over the surgical wounds may be altered by the healing
process, Skowno et al. speculated that a placement directly over
that area might influence NIRS readings [8]. Therefore, experimental
studies on reliable animal models, such as rats or piglets, should
better clarify the real impact of abdominal wall thickness and of the
healing process on transcutaneous NIRS monitoring.
Furthermore, the normal range of renal rSrO2 values need to be
established by investigating a larger number of patients. Only after
this process, NIRS might help in the prompt identification of acute
adverse events, which is the final aim of real-time NIRS monitoring. At
the moment, NIRS contributed to the early diagnosis of vascular
complications in two children undergone liver transplantation [15].
However, even if renal rSrO2 was not altered among the patients affected
by DGF in the series by Vidal et al. , the Authors suggested that
NIRS might help in discriminating the causes of oligo-anuria in the
early post-operative time after KT, helping in managing the
postoperative fluid balance [7].
In conclusion, preliminary studies showed the correlation between NIRS,
graft function and graft perfusion in paediatric KTs. Further
investigation should also relate rSrO2 to perfusion renal scintigraphy,
which is the most objective modality to assess renal allograft
vascularisation and function [4]. Before large-scale in vivoutilisation, experimentation of NIRS on animal models might investigate
factors influencing the measurements of renal rSrO2, such as the depth
of the region of interest and the correct placement of the probes.
Furthermore, animal models or selected cohorts of patients might help in
defining the normal values of renal rSrO2.