Today we discussed a case of AKI in an elderly patient which was thought to be consistent with normotensive ischemic acute renal failure (a prerenal azotemia in this case leading to an ischemic ATN) with the sedative effect of a recent morphine start leading to relative hypotension. Some learning points:
1) Knowing the patient's previous/baseline blood pressure is key in the diagnosis of normotensive ischemic acute renal failure.
2) The main pathophysiology of the renal response to a drop in perfusion pressure in normotensive ischemic ARF is the limits of autoregulation — maintenance of normal blood flow and glomerular filtration rate at a range of blood pressures. Chronic hypertension shifts the autoregulatroy range to higher levels of mean arterial pressure, so normal range blood pressures lead to renal ischemia as perfusion levels drop, leading to a prerenal AKI. More in this NEJM article.
3) Some of the factors that increase patient's susceptibility to renal hypoperfusion are:
- Structural changes in renal arterioles/small arteries: old age, atherosclerosis, chronic HTN, CKD
- Reduced vasodilatory prostoglandans: NSAIDS, COX-2 inhibitors
- Afferent glomerular arteriolar vasoconstriction: sepsis, hypercalcemia, hepatorenal syndrome, cyclosporine, tacrolimus, radiocontrast agents
- Failure to increase efferent arteriolar resistance: ACEs and ARBs
- Renal artery stenosis
4) The differential for AKI is broad.. Here is what we came up with today: