*Csaba Kopitkó1, László Medve1, Tibor Gondos2
Renal blood supply and fluid therapy
1Dr. Kenessey Albert Hospital, Department of Anaesthesiology and Intensive Care Medicine, Balassagyarmat, Hungary
Head of the Faculty: Szabó Gèza, MD
2Department of Clinical Studies, Faculty of Health Sciences, Semmelweis University, Budapest, Hungary
Head of the Faculty: Zoltán Zsolt Nagy, MD, PhD
There are different underlying mechanisms of acute kidney injury (AKI) in various type of shock, but restoration of renal blood flow (RBF) is crucial in prevention of AKI. The first 24-48 hours of shock seem to be critical. Monitoring of global RBF and its intrarenal distribution is not possible in current clinical practice. The only way for optimization of renal blood supply is optimization of macrohemodynamics. In volume-responsive AKI, fluid therapy restores kidney function. Many clinical signs and parameters can be of use in determining the volume status. The accuracy of the assessment may be improved with the help of tools quantifying the clinical parameters (e.g. hypovolemic index – HVI). The basis of intravenous fluid therapy are crystalloids, and their effect is reported to be shorter than 120 min. Every form of hydroxyethyl starch has been shown to be harmful for patients at risk of impaired renal function. In sepsis, the boundary between volume-responsive and volume-unresponsive AKI is blurred. Fluid responsiveness can be lost in the course of AKI as early as on the first day of sepsis. According to the results of the ARDS Network study, the conservative approach in fluid therapy resulted in a shorter time of mechanical ventilation and did not affect the renal function, except for a slight increase of the serum creatinine level. Fluid overload is to be avoided, as renal venous and lymphatic congestion can limit the urine filtration rate, further worsening edema.
Despite the development of renal replacement techniques, acute kidney injury (AKI) remains a poor prognostic factor in critically ill patients. AKI is diagnosed based on serum creatinine and urine output (table 1) (1). The optimization of renal blood supply can improve the outcome in patients suffering from AKI (2). The role of venous and lymphatic flow has been underlined in multiple studies on animal and human subjects concerning the pathophysiology of sepsis and cardiovascular disorders (3-8). Despite the availability of an increasing number of biomarkers for the early diagnosis of acute kidney injury, serum creatinine remains the widely available gold standard. In clinical setting, avoiding hypo- and hypervolemia and preserving renal blood flow remain the best preventive measures against AKI (9). The aim of this paper was to review the diagnostic and therapeutic options for AKI.
Tab. 1. Definition of acute kidney injury
|Class||Serum creatinine||Urine output|
|I||Increased to ≥ 0.3 mg/dl (≥ 26.4 μmol/l) or 1.5-2 times baseline||< 0.5 ml/kg/hour in > 6 hours|
|II||Increased to 2-3 times baseline||< 0.5 ml/kg/hour in > 12 hours|
|III||Increased to > 3 times baseline, or serum creatinine ≥ 4.0 mg/dl (≥ 354 μmol/l), or an acute rise ≥ 0.5 mg/dl (44 μmol/l)||< 0.3 ml/kg/hour over 24 hours or anuria lasting > 12 hours|
Pathophysiology of AKI
The renal blood flow (RBF) depends on cardiac output and renal vascular resistance. The underlying mechanism of AKI in different types of shock varies. The available data is based primarily on animal studies, as the continuous measurement of RBF in human is difficult to perform. In animal models, glomerular filtration rate (GFR) remains constant until RBF has decreased to 1-10% of the baseline, and the near-total occlusion of the renal artery for 2 hours results in a transient decline of renal function after the restoration of blood circulation (9-12). AKI is very rare in human survivors of cardiac arrest without shock, therefore, the coexistence of another disorder is necessary for the development of renal dysfunction (12).
Septic AKI can develop even in a hyperdynamic circulatory pattern, but low systemic blood flow aggravates this condition (13). A decrease in GFR in sepsis can develop as a result of the constriction of afferent arterioles, which lowers the filtration pressure, however, as recent studies underline, it is mainly caused by the dilation of efferent arterioles. Studies suggest that the first 24-48 hours of shock are critical for the renal function (13).
Monitoring and optimization of the renal blood flow
Arterial blood supply
In an ideal setting, the global and regional RBF of high-risk patients would be continuously monitored, preferably in a noninvasive way. There are several methods for measuring renal blood supply: non-imaging procedures (microsphere deposition, para-aminohippurate clearance, renal vein thermodilution, xenon washout technique, intravascular Doppler), nuclear techniques (scintigraphy, renal extraction of 51chromium-ethylenediaminetetraacetic acid, positron emission tomography), magnetic resonance imaging, and ultrasound imaging (Doppler and contrast-enhanced ultrasound). Unfortunately, these methods are either difficult (para-aminohippurate clearance), risky (renal vein thermodilution), inappropriate for human examination (intravascular Doppler), unavailable in critical care units (nuclear techniques and MRI), or do not provide the possibility of continuous observation of individual patients (15, 16).
Monitoring global RBF and its intrarenal distribution is not possible in current clinical practice. The decreased RBF may be either a cause or a consequence of AKI, e.g. due to venous congestion and elevated interstitial and intracapsular pressure. Dynamic parameters and tests, such as pulse pressure variation (PPV), systolic pressure variation (SPV), stroke volume variation (SVV) and the passive leg raising test are considered the most appropriate for assessing volume responsiveness, and therefore, intravascular hypovolemia (17-20). However, these methods have several limitations. For example, multiple conditions, including arrhythmia, respiratory effort due to an inadequate sedation of a mechanically ventilated patient, and a low tidal volume (< 6 ml/kg) may all result in erroneous results (21, 22).
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