This is a prospective randomised control trial 1:1 in patients at risk of developing Acute Kidney Injury after cardiac surgery comparing the RenalGuard® System to current medical treatment. 110 patients will be recruited for each group. The aim of the study is to assess whether the RenalGuard® system reduces Acute Kidney Injury (AKI) after cardiac surgery as compared to current practice.

Patients undergoing elective or in-house urgent cardiac surgery will be approached for the KIDNEY study if they fulfil the inclusion criteria. Upon consent pre-operative blood tests will be as per the usual practice. Moreover, 2ml of the serum will be stored for NGAL analysis.

Following the consenting process, patients will be randomised (envelop- based) into either having the RenalGuard® System (Study group) or current medical management (Control group).

Study group:

Patients in the study group will have the RenalGuard® System started in anaesthetic room once the peripheral line and arterial lines are sited. The RenalGuard® System will continue to run throughout the cardiac procedure in the operating room and up to 6 hours post-op after the patient has been transferred to the unit. Patients will be managed at a zero balance i.e. volume of the urine output will be matched to the volume of Hartmann's fluid infusion. Forced diuresis is then initiated.

Control group:

Patients in the control group will be managed as per current medical practice which will include no forced diuresis in operating room and the use of inotropes for maintenance of mean arterial pressure and intravenous furosemide for diuresis.

An additional blood test (NGAL) at six hours post-op will be taken and would require 2mls of blood. The NGAL specimens will be dealt with by the biochemistry lab (centrifuged & stored) so that the specimens can be analysed in batches.

Patients kidney function (U&Es) will be tested at the time of post-operatively as per usual practice (Day1, pre-discharge and as clinically indicated) and at the post-operative surgical out-patient visit.

Pre-operative, intra-operative and post-operative data will be collected for each patient group.

Inclusion Criteria:

1. Patients undergoing surgery (elective or in-house urgent)

2. Patient 18 years old and over

3. Patient able to give written consent

4. Patient at risk of developing AKI after cardiac surgery (at least one factor)

   1. Diabetics (IDDM or NIDDM) with normal kidney function pre-op
   2. Patients with eGFR 20-60
   3. Patients undergoing combined cardiac procedures when the CPB time is likely to exceed 120 minutes
   4. Patients with Hb of 12.5 g/dl or below
   5. Logistic Euroscore of 5 and above

5. Patient not involved with another study

Exclusion Criteria:

1. Emergency surgery
2. Patient unable to give written consent
3. Patient already dialysis dependent or eGFR <20
4. Patient partaking in another study
5. Pregnant patients