Hyperchloremia

Know what is actually in the bag - Each 100 mL of 0.9% sodium chloride contains 15.4 mEq of sodium and 15.4 mEq of chloride, with an osmolarity of 308 mOsm/L and a pH of 4.5 to 7. Its chloride concentration of 154 mmol/L is about 50% above serum. Nothing about that fluid is physiologic except the tonicity, and several liters into a case you have manufactured an acidosis.

Balanced crystalloids are the fix - Lactated Ringer's, Plasma-Lyte, and other buffered solutions carry an anion metabolized in vivo to bicarbonate. Lactate and gluconate are cleared hepatically; acetate is metabolized peripherally by skeletal muscle. Two caveats: lactate is a gluconeogenic substrate and excessive lactated crystalloid can push glucose up in a diabetic, and acetate has been implicated in reduced myocardial contractility and blood pressure.

Check the gap before you blame the fluid - Not every intraoperative acidosis is chloride. A low bicarbonate with a normal anion gap (8 to 16 mEq/L) points at chloride; a widened gap points at lactate, ketones, renal failure, or a toxin. Getting that wrong means treating a perfusion problem with a fluid change.

Urine anion gap separates the non-iatrogenic causes - (urine Na + urine K) - urine Cl estimates ammonium excretion. Positive, roughly +20 to +90 mEq/L, means the kidney is not excreting ammonium, which points at distal renal tubular acidosis. Negative, roughly -20 to -50 mEq/L, means ammonium excretion is intact and the bicarbonate is going out somewhere else, classically profuse watery diarrhea. Near zero is indeterminate.

It is a kidney decision, not just an acid-base one - High-volume normal saline causes hyperchloremia-induced constriction of the renal afferent arteriole with a fall in glomerular filtration rate. In a long case with large volume shifts, or in a patient who already has impaired renal function, chloride load should factor into fluid choice from the start rather than after the gas comes back.

Watch the potassium climb - The acidosis produces a transcellular shift of potassium out of cells, so serum potassium rises after large saline volumes. That matters most in renal impairment or alongside a large transfusion.

Take over their compensation - The awake patient compensates with an increased respiratory rate; under anesthesia they cannot. Set minute ventilation to match and confirm with a gas. In long-standing disease that same compensatory work of breathing can fatigue respiratory muscles into failure, which is a reason not to extubate marginal.

The hyperchloremia is silent; the acidosis is not - Patients have no symptoms from the chloride itself. From the acidosis: headache, low energy, nausea and vomiting, and as it worsens, stupor, coma, myocardial instability, or arrest. Exam shows tachycardia, tachypnea, accessory muscle use, altered mental status, and muscle weakness.

Saline is still right sometimes - 0.9% saline is the correct choice when chloride losses equal or exceed sodium losses, and in metabolic alkalosis from fluid loss, meaning the vomiting or high-nasogastric-output patient. Hyperchloremic acidosis is a dose problem, not a reason to abandon the fluid.

Do not swing to a hypotonic fluid - 0.45% saline is a maintenance fluid, not a resuscitation fluid. Overuse causes hyponatremia and cerebral edema by driving water toward higher-sodium compartments.

Volume overload is the other iatrogenic risk - Crystalloid of any composition can produce iatrogenic fluid overload, and the margin is narrow in acute kidney injury, chronic kidney disease, and congestive heart failure, where it can progress to pulmonary edema. Urine output of 0.5 mL/kg/hr is the usual adequacy target, though it is unreliable in renal impairment.

Chloride is the principal extracellular anion, normally 98 to 106 mmol/L, and the kidney does most of the regulating. Perioperative hyperchloremia is usually something we cause: 0.9% saline carries 154 mmol/L of chloride, so any large-volume resuscitation delivers a chloride load the plasma cannot absorb quietly. The rise in chloride narrows the strong ion difference, hydrogen concentration climbs to preserve electroneutrality, bicarbonate shifts intracellularly and is excreted renally, and the result is a normal-anion-gap hyperchloremic metabolic acidosis. Dilution of serum bicarbonate by an unbuffered crystalloid contributes.

Two downstream effects drive perioperative decisions. Chloride constricts the renal afferent arteriole and lowers glomerular filtration rate. And the acidosis shifts potassium out of cells, raising measured serum potassium. Hyperchloremia also arises from bicarbonate loss: diarrhea, pancreatic fistula, nasojejunal suction, renal tubular acidosis, and carbonic anhydrase inhibitors.