Renal failure on intensive care
Intensive care exam revision on renal failure and filtration for medical student finals, PLAB exams and MRCP PACES
- This section covers renal replacement therapy that can be offered on ICU.
- Acute kidney injury (AKI) is common in critical illness and is seen regularly on the medical take. Patients suffer from acute kidney injury for many reasons. One of the commonest reasons is due to renal hypoperfusion which can be secondary to shock. It is a common occurrence to see a patient with acute kidney injury secondary to sepsis or diarrhoea and vomiting. It is especially common in the elderly, isolated population who may have been unwell for several days yet continued to take their medications including antihypertensives. To compound matters these patients may have also fallen and had a long lie on the floor releasing creatine kinase which is also toxic to the kidneys.
- When assessing a patient with AKI it is important to undertake some basic investigations. All patients should have a full set of U&Es sent, a venous or arterial blood sample is also important and can give a quicker assessment of potassium and acid-base status. Patients should ideally be catheterised and a fluid balance assessment is imperative. Patients should be treated with appropriate IV fluid resuscitation and urine output measured hourly. If potassium is high this should be treated with insulin and dextrose infusions. The underlying cause of the hypoperfusion should be identified and treated. Even if the cause of the AKI is thought to be pre-renal an USS of the renal tract is also advised. Reno-toxic drugs should be stopped.
- Patients will need reassessment at least twice a day, if not more frequently. Blood tests will need to be repeated at least 12 hourly. If a patient with AKI fails to improve with therapy then they may need to be considered for filtration. Do not be tricked by high output renal failure, where the urea and creatinine continue to rise despite good urine output.
- Renally excreted medications need to be reviewed in AKI and during renal replacement therapy. With a fall in eGFR many renally excreted drugs will not be metabolised as quickly and so accumulate. It is important to seek advice from a pharmacist as the prescription of opiates, antibiotics and many more drugs can be affected by renal failure. In addition nephrotoxic drugs need to be stopped e.g. ACE inhibitors. NSAIDs, aminoglycosides and caution is needed with contrast media and the use of chemotherapy agents.
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- Persistently raised potassium >6.5 despite medical management
- Persistent acidosis: PH <7.2
- Fluid Overload and pulmonary oedema
- Within 24 hours:
- Urea >40-50mmol/l and rising
- Creatinine > 400 and rising
- Severe sepsis not improving despite treatment
Continuous Venovenous Haemofiltration (CVVHF):
Blood from the patient is passed through a filter which allows plasma water, electrolytes and small molecular weight molecules to pass through a filter down a pressure gradient.
Continuous Venovenous Haemodialysis (CVVHD):
In CVVHF the clearance of small molecules and solutes is inefficient. In CVVHD, dialysis fluid is passed over the filter in a countercurent manner. Fluids, electrolytes and small molecules can move in both directions depending on hydrostatic pressure, ionic binding and osmotic pressure. Creatinine clearance is much better than in filtration alone. Fluid balance can also be manipulated by altering the quantity of diasylate passed into the patient at the end. By allowing more diasylate to pass out, fluid can be effectively removed from the patient up to 200ml/hour.
In order for renal replacement therapy to occur, a large bore double lumen, vascular access is required. This is known as a “vascath” and is inserted using Seldinger technique. Most commonly the femoral vein is used, but the internal jugular can also be used (the right is better than the left as the right internal jugular vein is straighter). In addition to requiring large bore access, anticoagulation is needed as passing blood into an extracorporeal circuit can lead to clotting. Unless the patient has a severe coagulopathy, they tend to be loaded on heparin and then have a heparin infusion run into the dialysis circuit. The APTT and activated clotting time must be monitored.
Complications of renal replacement therapy:
- Platelet consumption
- Disturbances in fluid balance
- Air embolism
Most cases of AKI caused by acute tubular necrosis tend to recover, however there are a proportion of patients who fail to regain adequate renal function or are left with severe renal impairment. These patients should be referred to the renal team whilst they are on ICU so they can be further assessed and further management can be planned. Patients may need to be transferred to a renal unit for permanent dialysis.
Plasma exchange is used as a therapy in some acute immune mediated conditions. The aim is to exchange a patient’s plasma, thus removing the immunologically active proteins.
Indications for plasma exchange (PEX):
- Guillain-Barre syndrome
- Myasthenia Gravis
- Systemic Lupus erythematosus
- Vasculitis e.g. Wegner’s and good pastures
- Thrombotic thrombocytopenic purpura (TTP)
Plasma exchange requires large bore vascular access and is associated with coagulopathy and hypotension secondary to fluid shifts. There are many other conditions where PEX can be considered but is not currently standardised therapy.