Acid–Base Disorders, Electrolytes & Renal Replacement
Key Takeaways
Anion gap equals sodium minus chloride minus bicarbonate; low albumin can conceal an increased gap.
For metabolic acidosis, expected PaCO2 is approximately 1.5 times bicarbonate plus 8, with a range of about 2 mm Hg.
Hyperkalemic ECG changes require immediate membrane stabilization and potassium-lowering treatment.
KDIGO suggests CRRT rather than intermittent treatment in AKI with acute brain injury or increased ICP.
Read the Gas and Chemistry Together
A blood gas provides pH and a respiratory component; serum chemistry adds bicarbonate, electrolytes, kidney function, and context. Determine whether the predominant process is metabolic or respiratory, then assess whether the compensatory response is appropriate. Compensation does not normally overcorrect the pH past normal. A near-normal pH can conceal two severe opposing processes, such as lactic acidosis plus respiratory alkalosis in sepsis.
For a metabolic acidosis, calculate the anion gap using sodium minus chloride minus bicarbonate. Potassium is conventionally omitted in this formulation. Reference ranges depend on the laboratory. Albumin contributes substantially to the normal gap; low albumin can hide unmeasured anions, so consider an albumin-adjusted interpretation. An increased gap prompts evaluation for lactate, ketones, renal retention of acids, and selected toxins. Hyperchloremic acidosis can follow large chloride loads, gastrointestinal bicarbonate loss, or renal tubular processes.
Check Respiratory Compensation
Winter's formula estimates the expected PaCO2 in a metabolic acidosis: 1.5 multiplied by serum bicarbonate, plus 8, with an approximate range of plus or minus 2 mm Hg. If bicarbonate is 12 mEq/L, expected PaCO2 is 26 mm Hg, approximately 24–28. A measured PaCO2 of 40 indicates an additional respiratory acidosis rather than adequate compensation. The same measured CO2 that seems normal in another patient can therefore represent severe ventilatory failure here.
This matters during intubation. A patient with profound metabolic acidosis may maintain pH through very high spontaneous minute ventilation. Apnea or a much lower ventilator minute ventilation can produce abrupt acidemia and cardiovascular collapse. Prepare a ventilation and hemodynamic plan before induction when possible. In concomitant brain injury, monitor the conflict between compensatory hypocapnia and cerebral perfusion instead of blindly applying a single CO2 target without acknowledging the acid–base disorder.
Electrolyte Emergencies
| Disorder | Neurologic or systemic risk | Initial clinical priorities |
|---|---|---|
| Hyperkalemia | Conduction failure and cardiac arrest | ECG assessment; IV calcium for membrane stabilization when indicated; intracellular shift and definitive potassium removal |
| Severe hypokalemia | Weakness, arrhythmia, rhabdomyolysis | Controlled replacement, magnesium assessment, cardiac monitoring |
| Hypomagnesemia | Arrhythmia and seizures | Replace magnesium and investigate ongoing losses |
| Hypocalcemia | Tetany, seizures, prolonged QT | Assess ionized calcium and treat symptomatic severe deficiency |
| Severe hypophosphatemia | Respiratory muscle weakness and impaired energy metabolism | Replacement with renal-function and electrolyte monitoring |
Calcium stabilizes the myocardium in hyperkalemia but does not remove potassium. Insulin with appropriate glucose shifts potassium intracellularly and requires glucose monitoring; inhaled beta-agonists can be adjunctive. Persistent or recurrent hyperkalemia may require renal replacement. Assess exogenous potassium, acidosis, renal failure, tissue breakdown, and medications. A reassuring initial ECG does not exclude dangerous hyperkalemia.
Differentiate a total calcium change due to albumin from an ionized calcium abnormality. Acid–base shifts influence protein binding, so ionized calcium is especially useful in critical illness and major transfusion. Replace phosphate or magnesium carefully in renal dysfunction because the same treatment that corrects deficiency can accumulate when clearance falls.
When Renal Replacement Is Needed
Consider urgent treatment for refractory electrolyte abnormalities, severe acid–base derangement, clinically important fluid overload, selected poisonings, or uremic complications. Evaluate the overall trajectory and treatable cause rather than waiting for a universal BUN or creatinine threshold. The modality, rate, anticoagulation, and fluid-removal goals must match the patient's physiology.
Intermittent hemodialysis can lower solute concentrations rapidly and cause hypotension. Both reduced MAP and osmotic brain water shifts can compromise an injured brain. Continuous renal replacement therapy (CRRT) generally allows slower fluid and solute changes and is suggested by KDIGO for AKI with acute brain injury, increased ICP, or generalized cerebral edema. This preference does not make CRRT risk-free; close monitoring remains necessary.
If the neurocritical care team has prescribed hypernatremia for a specific indication, ordinary replacement or dialysate sodium can reduce the serum concentration unexpectedly. Coordinate the prescription and monitor the rate of change. Account for citrate and calcium when regional citrate anticoagulation is used; accumulation and electrolyte abnormalities require specialist interpretation, particularly in severe shock or impaired metabolism. Circuit interruption also changes fluid balance and medication clearance.
Avoid the Osmotic and Medication Traps
Dialysis disequilibrium reflects rapid osmotic change with potential cerebral edema and neurologic deterioration. Risk assessment and a gradual initial prescription are especially important with marked azotemia or intracranial disease. If consciousness worsens during therapy, evaluate systemic pressure, oxygenation, glucose, sodium, seizures, and structural causes rather than assuming every event is disequilibrium.
Renal replacement removes some antibiotics and antiseizure drugs, sometimes requiring different maintenance doses or supplementary doses. Changes in effluent rate, residual renal function, and downtime matter. A free drug concentration may help when protein binding is altered. Document the modality and prescription in the medication plan so that a renal dose adjustment does not inadvertently become undertreatment after CRRT begins.
Source checked October 17, 2026: KDIGO AKI guideline, renal replacement recommendations.
A patient has metabolic acidosis with bicarbonate 12 mEq/L and PaCO2 40 mm Hg. Using Winter’s formula, what additional disorder is present?
Appropriate compensation
Additional respiratory acidosis
Isolated respiratory alkalosis
No acid–base disorder
Why is CRRT often preferred for AKI with increased ICP?
It guarantees no neurologic complications
It eliminates the need to monitor sodium
It allows slower fluid and solute changes and generally less hemodynamic disturbance
It removes every drug at an identical rate
Sections you finish are checked off in the contents.