The Relation Between the Renal Resistive Index and Glomerular Hyper Filtration

Apart from acute kiddney injury (AKI), critically ill patients with sepsis or trauma can also exhibit glomerular hyperfiltration (2-4). Glomerular hyperfiltration is not easily recognized, because the decrease in serum creatinine is a late manifestation and generally interpreted as normal renal function. Glomerular hyperfiltration may have clinical consequences, because it leads to augmented renal clearance of water soluble drugs. This is especially relevant for antibiotics, because augmented clearance can lead to underdosing and therapeutic failure (5-9). Patients with glomerular hyperfiltration are generally younger patients with less severe disease (3) and often exhibit a hyperdynamic circulation. The mechanism of glomerular hyperfiltration is poorly understood. High catecholamine release with increased renal blood flow could play a role. Direct measurement of renal blood flow is not available in daily clinical practice.

Nowadays, the investigators can measure Renal Resistive Index (RRI) using renal Doppler ultrasound. The RRI is a sonographic index assessing resistance of the intrarenal arcuate or interlobar arteries and is normally used to assess renal arterial disease. The method has now become available at the bedside in the intensive care unit. RRI is calculated as: (peak systolic velocity - end diastolic velocity)/peak systolic velocity. Normal values are between 0.60 and 0.70. A mean value of 0.72 has been found in critically ill patients admitted to the intensive care unit (personal data).

The investigators hypothesize that high glomerular filtration rate as measured with creatinine clearance is associated with a low renal resistive index and accelerated microvascular blood flow.

To prove or reject this hypothesis, the following study measurements will be performed in critically ill patients with sepsis or trauma:

1. Renal ultrasound to measure renal resistive index (RRI) After visualising the kidney in ultrasound mode, checking for (chronic) renal damage, an arcuate or interlobar artery will be localized and three successive Doppler measurements at different positions in the kidney (high, middle and low) will be performed. This will be repeated 3 times in each kidney. So a total number of 9 RRI values will be obtained in each kidney.
2. Sublingual microcirculation using Sidestream Dark Field imaging (SDF) After removal of secretions with a gauze, the device will be applied below the tongue and three sequences of about 20 seconds from adjacent areas will be recorded and stored. The investigators will measure the perfused vessel density (PVD), the proportion of perfused vessels (PPV) and the microvascular flow index (MFI) for small vessels. Each image will be divided into four quadrants, and the predominant type of flow (0 = absent, 1 = intermittent, 2 = sluggish, 3 = normal, 4 = high) will be evaluated in each quadrant. The mean of the four quadrants will be used for analysis.
3. To assess fluid status, Bioelectrical impedance analysis (BIA) will be performedusing the Akern BIA 101 device.

BIA measures Resistance (R) and Reactance (Xc) reflecting extracellulair (R) and cellular (Xc) resistance to an alternating current of 400 μA with afrequency of 50 kHz. In previous studies the investigators found that (changes in) R are highely correlation with (changes in) fluid status.