Septic AKI is where the whole volume converges: the staging of Chapter 1, the perfusion thinking of Chapter 4, the venous-congestion lesson of Chapter 6, and the septic biology of Chapter 7 all meet at the bedside of a shocked, oliguric ICU patient. It is also one of the most trial-tested problems in the field, which means many instincts — push the pressure, start dialysis early, clean the blood — have been formally tested and found wanting. The chapter's job is to translate that evidence into what to do in the first hours and the following days.
Why the septic kidney fails, in the ICU
Chapter 7 established the biology: in sepsis the renal blood flow is often preserved or even increased, yet GFR falls, because the injury is microcirculatory shunting, inflammation, endothelial and glycocalyx damage, and an adaptive tubular cell-cycle arrest rather than global ischaemia. The intensive care unit adds its own insults on top. Positive-pressure ventilation raises intrathoracic pressure, lowering venous return and raising renal venous pressure. Intra-abdominal hypertension — common in the resuscitated, oedematous, or surgical abdomen — compresses the kidney and narrows its perfusion gradient. Fluid overload, so easy to accumulate in days of resuscitation, congests the kidney exactly as in cardiorenal disease. And the ICU is where nephrotoxins cluster: vancomycin, aminoglycosides, contrast. Septic AKI in the ICU is therefore rarely one mechanism; it is the septic injury plus whatever the unit has added.
The first hour is antibiotics and source control
The single most renal-protective intervention in sepsis is not a renal intervention at all: it is killing the infection. Early, appropriate, broad-spectrum antibiotics — with every hour of delay associated with higher mortality — and prompt source control do more for the kidney than any haemodynamic manoeuvre, because they remove the inflammatory driver of the injury. This reframes the bedside priority. The reflex to focus on the kidney — the urine output, the creatinine, the dialysis line — must not crowd out the faster, higher-yield work of treating the sepsis itself. The kidney recovers when the sepsis is controlled, and not reliably before.
Fluids: resuscitate, then de-resuscitate
Sepsis resuscitation has matured from 'fill the tank' to a two-phase discipline. In the first phase, restore perfusion with a balanced crystalloid — an initial volume in the region of 30 mL/kg is a common starting point, but it should be titrated to the individual rather than poured by protocol, assessing fluid responsiveness and tolerance as Chapter 4 described. Balanced solutions are preferred over saline; the trial evidence is not unanimous, but the weight of it favours avoiding the chloride load. The second phase is the harder discipline: stop when responsiveness ends, and de-resuscitate. After the initial resuscitation, restrictive and liberal fluid strategies have performed similarly in large trials, which means there is no benefit to be had from continued liberal fluids — and a persistent positive balance, with its venous congestion, independently harms the kidney and the patient. The modern error is no longer too little early fluid; it is too much late fluid.
The MAP target, and when 65 is not enough
Noradrenaline is the first-line vasopressor, and the default mean arterial pressure target is 65 mmHg. The instinct to aim higher — surely more pressure means more renal perfusion — was tested directly: targeting 80 to 85 mmHg gave no overall benefit and more atrial fibrillation. But the same trial carried a crucial subgroup signal: patients with chronic hypertension, whose renal autoregulation is right-shifted, had less doubling of creatinine and less RRT at the higher target. The practical synthesis is to default to 65 mmHg and to consider a higher target, around 80 to 85, specifically in the chronically hypertensive. This is autoregulation from Chapter 2 made actionable: the pressure a kidney needs depends on the pressure it is used to.
Beyond noradrenaline
When noradrenaline alone is insufficient, vasopressin is the usual next agent. Compared with noradrenaline as a first-line drug it did not change kidney-failure-free days, but it reduced the use of renal replacement therapy and spares catecholamine exposure, so it is a reasonable adjunct. Angiotensin II raises blood pressure in refractory vasodilatory shock and has a place when catecholamines and vasopressin are failing. Throughout, the discipline from Chapter 7 holds: once perfusion is adequate, escalating pressure further does not restore the septic GFR — it is the sepsis, not the blood pressure, that the kidney is waiting on. Vasopressors restore organ perfusion pressure; they do not treat the microcirculatory injury.
Renal replacement: when, and what not to do
Two strong evidence messages govern RRT in septic AKI. First, on timing: starting dialysis early — accelerated, before a conventional indication — does not improve survival and leaves more patients dialysis-dependent than a watchful, standard approach. So do not start RRT simply because a patient has sepsis and AKI; wait for the conventional indications — refractory hyperkalaemia, severe acidosis, diuretic-resistant volume overload, or uraemic complications — unless an emergency forces an immediate start. The detailed timing and prescription belong to Chapters 13 and 14. Second, on the seductive idea of using RRT to 'clean the blood' of inflammatory mediators: high-volume haemofiltration and the various cytokine- and endotoxin-adsorption therapies have not been shown to improve outcomes, and some have been formally negative. Outside a trial, RRT is organ support for the failing kidney, not a treatment for the sepsis itself.
Monitoring, and the prevention bundle
Because septic AKI is multifactorial and the ICU keeps adding insults, monitoring is active. Recognise AKI early by KDIGO criteria, and separate the septic component from the other ICU causes — a pre-renal hit, a nephrotoxic one, an abdominal compartment syndrome, an obstruction. Treat cumulative fluid balance as a vital sign and read venous congestion with point-of-care ultrasound; measure intra-abdominal pressure in the at-risk abdomen. And apply the KDIGO care bundle to every high-risk kidney: discontinue nephrotoxins, optimise volume and perfusion, monitor creatinine and urine output, avoid hyperglycaemia, and avoid contrast where possible. A biomarker-guided version of this bundle has been shown to reduce moderate-to-severe AKI after cardiac surgery, which is the best evidence yet that disciplined supportive prevention — not a drug — is what protects the kidney.
Where the evidence is firm, and where it argues
The negative trials here are unusually clarifying: early RRT, high MAP targets for everyone, liberal late fluids, and blood-purification have all been tested and have not delivered, and that is firm. The positives are softer and more interesting: the chronic-hypertension MAP subgroup, the catecholamine-sparing role of vasopressin, and the balanced-versus-saline question, where trials disagree at the margins. What is not in doubt is the primacy of antibiotics and source control, the harm of fluid overload, and the futility of treating the kidney as if it could be fixed independently of the sepsis. The honest summary is that septic AKI is managed by treating sepsis well and supporting the kidney patiently — and by resisting the interventions that feel active but test negative.
