Metabolic Acidosis and Alkalosis

METABOLIC ACIDOSIS Mechanisms

• Bicarbonate loss pathway: The kidneys normally reabsorb filtered bicarbonate (normally <5 mEq/day losses). Type II (proximal) renal tubular acidosis causes impaired HCO3− reabsorption in the proximal convoluted tubule due to reduced proximal tubule H+ secretion or dysfunction of Na+/H+ exchangers (NHE3). This results in bicarbonate "wasting" into urine; as plasma [HCO3−] falls below the renal threshold (~15 mEq/L), urinary bicarbonate losses eventually cease but plasma pH remains depressed. Gastrointestinal bicarbonate loss (diarrhea) removes the alkaline small bowel secretions, directly depleting the body's bicarbonate pool and shifting the Henderson-Hasselbalch equation toward acidemia.

• Unmeasured anion accumulation (high anion gap metabolic acidosis): When organic or inorganic acids accumulate faster than renal excretion capacity, the anion gap—calculated as Na+ − (Cl− + HCO3−), normally 8–16 mEq/L—widens because bicarbonate is consumed buffering these anions while the responsible anions accumulate. For example, in lactic acidosis, lactate (normally <1 mEq/L) accumulates to pathological levels (>4 mEq/L in severe cases) from tissue hypoperfusion or mitochondrial dysfunction, consuming bicarbonate via the reaction: H+ + lactate− + HCO3− → H2CO3 → CO2 + H2O. Each lactate anion that accumulates causes loss of one bicarbonate molecule, widening the anion gap. Similarly, β-hydroxybutyrate and acetoacetate in diabetic ketoacidosis (DKA) accumulate to levels exceeding 10 mEq/L, with each ketone consuming bicarbonate.

• Renal acid excretion failure (Type I and IV RTA): Type I (distal) RTA results from impaired distal H+ secretion or increased collecting duct permeability to H+ (back-diffusion), preventing the kidneys from achieving a urine pH <5.5 even during systemic acidosis. Normally, the distal tubule and collecting duct secrete H+ ions to acidify urine to pH 4.5–5.0; failure to do so means unmeasured acids (phosphate, sulfate from protein metabolism) accumulate systemically as anions while sodium is reabsorbed without H+ (hyperchloremic acidosis). Type IV RTA involves aldosterone deficiency or resistance, impairing the electrogenic secretion of H+ and K+ in the collecting duct, leading to concurrent hyperkalemia and normal anion gap metabolic acidosis. Type III RTA (rare combined form) shows features of both proximal and distal dysfunction.

• Respiratory compensation limitations: The respiratory system normally compensates for metabolic acidosis through increased minute ventilation, decreasing PaCO2 according to Winter's formula (expected PaCO2 = 1.5 × [HCO3−] + 8 ± 2). This is mediated by chemoreceptor activation of the dorsal respiratory group in the medulla. If observed PaCO2 is higher than predicted, concurrent primary respiratory acidosis exists; if lower, concurrent primary respiratory alkalosis exists. In severe acidosis (pH <7.1), respiratory muscles fatigue or mechanical limitations may prevent adequate hyperventilation.

METABOLIC ALKALOSIS Mechanisms

• Chloride-responsive (saline-sensitive) alkalosis: Represents ~80% of metabolic alkalosis cases. The initiating mechanism is typically gastric H+ loss (vomiting, nasogastric suction) or loop diuretic use. With gastric acid loss, H+ is removed directly from the ECF, increasing pH. Critically, the kidneys attempt to reabsorb filtered HCO3−, but volume depletion activates the renin-angiotensin-aldosterone system (RAAS), which simultaneously:
• Increases proximal tubule NaCl and HCO3− reabsorption via angiotensin II stimulation
• Increases aldosterone secretion, promoting distal Na+ reabsorption and H+/K+ secretion
• Activates sympathetic nervous system, reducing renal perfusion pressure sensing

• Chloride-resistant (saline-resistant) alkalosis: Represents ~20% of cases, characterized by hypokalemic, hypochloremic, hypertensive alkalosis (from primary hyperaldosteronism, Cushing's syndrome, or other volume-expanded states). Here, volume expansion is either absolute (primary hyperaldosteronism) or perceived due to abnormal RAAS activation. The kidneys cannot excrete HCO3− because distal H+ secretion is constitutively activated by mineralocorticoid excess or hypokalemia. Distal H+ secretion is directly driven by the H+-ATPase in the intercalated cell, and hypokalemia paradoxically increases this due to H+/K+ exchanger regulation—intracellular K+ depletion favors H+ secretion. Giving saline does not correct this because the primary problem is not volume depletion but rather ongoing H+ secretion exceeding reabsorption, perpetuated by hypokalemia and/or mineralocorticoid excess.

• Contraction alkalosis: Develops when volume depletion occurs in the context of HCO3− retention. For example, if a patient loses isotonic or hypotonic fluid, the remaining HCO3− becomes concentrated, raising plasma [HCO3−]. Concurrent volume depletion prevents renal HCO3− wasting, perpetuating the alkalosis.

METABOLIC ACIDOSIS

• High anion gap metabolic acidosis (anion gap >12 mEq/L):
• Lactic acidosis: Develops in tissue hypoperfusion states (shock, sepsis, severe heart failure), mitochondrial diseases, intense exercise, thiamine deficiency, alcoholism, liver disease, nucleoside reverse transcriptase inhibitors (NRTIs), and metformin (especially in renal failure). Type A lactic acidosis (tissue hypoxia) has worse prognosis than Type B (no tissue hypoperfusion). Lactate >4 mEq/L is considered significant; >10 mEq/L indicates severe disease.
• Diabetic ketoacidosis (DKA): Results from absolute or relative insulin deficiency combined with glucagon excess, seen in new-onset or poorly controlled type 1 diabetes, illness-precipitated DKA in known diabetics, and rarely euglycemic DKA (normal glucose despite acidosis). Uncontrolled gluconeogenesis and lipolysis produce excessive ketone bodies.
• Uremia: Chronic kidney disease with GFR <15 mL/min/1.73m² causes accumulation of unmeasured organic anions (phosphate, sulfate, organic acids). Acute kidney injury (AKI) can precipitate rapid metabolic acidosis if accompanied by hypercatabolic states.
• Alcoholic ketoacidosis: Occurs in chronic alcoholics with recent alcohol cessation, characterized by ketosis without significant hyperglycemia, from impaired NAD+ regeneration and glycogen depletion.
• Medication-induced: Aspirin (salicylates directly uncouple oxidative phosphorylation and cause hyperventilation but also respiratory alkalosis, creating mixed disorder), metformin-associated lactic acidosis (MALA) in renal dysfunction, NRTIs, linezolid, topiramate.
• Ingestions: Ethylene glycol, methanol (producing formate), isoniazid, propylene glycol.

• Normal anion gap (hyperchloremic) metabolic acidosis (anion gap ≤10 mEq/L, chloride >110 mEq/L):
• Gastrointestinal HCO3− loss: Diarrhea (especially small bowel/colonic diarrhea), ileostomy, pancreatic fistula, cholestyramine use (binds bicarbonate in colon).
• Type I RTA: Impaired distal H+ secretion, seen in systemic lupus erythematosus (SLE), Sjögren's syndrome, sickle cell disease, medullary sponge kidney, amphotericin B toxicity, NSAIDs. Urine pH >5.5 despite systemic acidosis is diagnostic.
• Type II RTA: Impaired proximal HCO3− reabsorption, associated with myeloma, Fanconi syndrome (heavy metals, cisplatin), acetazolamide use, topiramate. Urine pH can be appropriately low but bicarbonate is wasted.
• Type IV RTA: Aldosterone deficiency (adrenal insufficiency, heparin, ACE inhibitors, angiotensin receptor blockers) or resistance (NSAIDs, potassium-sparing diuretics, trimethoprim). Concurrent hyperkalemia is characteristic. Can be seen with urinary obstruction or renal transplantation.
• Rapid normal saline administration: Dilutional acidosis from large volumes of 0.9% saline (which contains 154 mEq/L each of Na+ and Cl−, raising serum chloride and driving down HCO3−).
• Ureteral diversions: Ileal conduit, ureterosigmoidostomy allow urine to contact bowel epithelium, causing net acid loss and HCO3− consumption.

METABOLIC ALKALOSIS

• Chloride-responsive (volume-depleted) (~80% of cases):
• Gastric acid loss: Vomiting, nasogastric suction in patients with gastric outlet obstruction or on mechanical ventilation. Loss of HCl directly raises serum pH; volume depletion prevents renal HCO3− excretion.
• Loop or thiazide diuretics: Cause both volume depletion and hypokalemia, the latter perpetuating alkalosis through increased distal H+ secretion.
• Mineralocorticoid excess: Primary aldosteronism may present with chloride-responsive features if volume-depleted at presentation.

• Chloride-resistant (volume-expanded or normal) (~20% of cases):
• Primary hyperaldosteronism: Autonomous aldosterone secretion (aldosterone-producing adenoma, bilateral hyperplasia) drives distal H+ and K+ secretion in the setting of sodium retention and volume expansion. Often hypertensive with hypokalemia.
• Cushing's syndrome: Cortisol excess activates mineralocorticoid receptors in distal tubule, mimicking aldosterone effect; associated with hypertension and hypokalemia.
• Renovascular hypertension: Unilateral renal artery stenosis activates RAAS chronically, leading to mineralocorticoid-mediated alkalosis despite volume expansion.
• Hypokalemia per se: Any cause of hypokalemia (diuretics, vomiting, diarrhea) perpetuates alkalosis by stimulating distal H+ secretion and proximal HCO3− reabsorption. Must be corrected for alkalosis to resolve.
• Severe hypomagnesemia: Impairs distal H+ secretion regulation; often coexists with hypokalemia.
• Exogenous alkali administration: Sodium bicarbonate infusion, excessive antacid use (milk-alkali syndrome when combined with high calcium intake).

METABOLIC ACIDOSIS - Acute Manifestations

• Dyspnea and Kussmaul respirations: The hallmark of moderate-to-severe metabolic acidosis is respiratory compensation manifesting as rapid, deep breathing (Kussmaul respirations). This occurs when pH <7.20 and represents medullary chemoreceptor activation by peripheral and central acidosis. Patients describe breathlessness despite normal lung mechanics. In DKA, this may be accompanied by fruity-smelling breath (acetone from decarboxylation of ketones).

• Altered mental status: Severe metabolic acidosis (pH <7.1) causes confusion, lethargy, or coma through direct effects on neuronal membrane potential and neurotransmitter function. In DKA, altered sensorium may also reflect hyperosmolarity or hypoglycemia. Metabolic acidosis impairs catecholamine responsiveness, worsening hemodynamic decompensation in shock states.

• Cardiovascular manifestations: Metabolic acidosis causes decreased myocardial contractility, increased systemic vascular resistance (initially), and eventually vasodilation if severe. In the setting of lactic acidosis from septic shock, this creates a paradoxical picture of high-output shock with persistent acidosis. Acidosis impairs beta-adrenergic responsiveness, making catecholamine-refractory shock more likely. Electrolyte disturbances accompanying acidosis (hyperkalemia in AKI, hypokalemia in DKA) cause arrhythmias.

• Hyperkalemia (when GFR reduced): In acute metabolic acidosis with renal dysfunction, extracellular acidosis shifts K+ out of cells via H+/K+ exchange, raising serum potassium despite total body depletion (especially in DKA where total body K+ is severely depleted). This increases ECG abnormalities: peaked T waves, prolonged PR interval, widened QRS.

• Nausea and vomiting: Acidosis directly stimulates chemoreceptor trigger zone; accompanied by dehydration worsens symptoms.

METABOLIC ACIDOSIS - Chronic Manifestations (especially RTA)

• Nephrolithiasis: Chronic metabolic acidosis from Type I RTA promotes uric acid and calcium phosphate stone formation due to persistently alkaline urine. Type IV RTA with hyperkalemia predisposes to potassium urate stones.

• Bone disease: Chronic acidosis causes secondary hyperparathyroidism and increased bone resorption. Prolonged renal tubular acidosis may lead to renal osteodystrophy.

• Growth retardation: In children with chronic RTA, persistent acidosis impairs linear growth through effects on growth hormone signaling.

METABOLIC ALKALOSIS - Acute Manifestations

• Neuromuscular irritability: Metabolic alkalosis causes hypocalcemia through decreased ionized calcium (more calcium binds to albumin at higher pH), leading to perioral paresthesias, acral paresthesias, tetany, and positive Chvostek's and Trousseau's signs. These occur typically at pH >7.55.

• Seizures: Severe alkalosis (pH >7.65) directly lowers seizure threshold through membrane hyperpolarization and reduced neuronal excitability paradoxically making neurons more prone to synchronous firing.

• Cardiac arrhythmias: Alkalosis prolongs the QT interval and increases digitalis sensitivity, predisposing to atrial fibrillation and other supraventricular arrhythmias. Hypokalemia (which often accompanies chloride-responsive alkalosis) worsens this through its own mechanism (peaked U waves, flattened T waves, prolonged QT).

• Decreased ventilation: Paradoxically, metabolic alkalosis suppresses respiratory drive through chem

Step 1 — confirm the primary disorder

• Arterial (or venous) blood gas with a simultaneous basic metabolic panel: the initial test. pH and PaCO2 from the gas, HCO3− and electrolytes from the chemistry panel; a venous gas is acceptable for pH/HCO3− trending, but arterial sampling is preferred when PaO2 or precise PaCO2 matters. A low measured HCO3− with low pH confirms metabolic acidosis; the reverse confirms alkalosis.
• Check compensation: for metabolic acidosis apply Winter's formula (expected PaCO2 = 1.5 × HCO3− + 8 ± 2). In metabolic alkalosis, PaCO2 rises roughly 0.7 mmHg per 1 mEq/L rise in HCO3−. Deviation from predicted defines a superimposed respiratory disorder.

Step 2 — anion gap and its refinements

• Serum anion gap: Na+ − (Cl− + HCO3−). Correct upward for hypoalbuminemia (add roughly 2.5 mEq/L per 1 g/dL fall in albumin) — otherwise an ICU patient's gap acidosis is missed.
• Delta-delta ratio: (Δanion gap / ΔHCO3−). A ratio well below 1 suggests a concurrent normal-gap acidosis; well above 1 suggests a coexisting metabolic alkalosis.
• Osmolar gap: measured minus calculated osmolality; a widened gap with a wide anion gap points to toxic alcohol ingestion (methanol, ethylene glycol) and mandates specific levels.

Step 3 — identify the anion or the loss

• Wide gap: lactate, beta-hydroxybutyrate (preferred over nitroprusside serum/urine ketones, which miss beta-hydroxybutyrate), creatinine/BUN, salicylate level.
• Normal gap: urine anion gap (UNa + UK − UCl). Negative ("*neGUTive*") indicates intact ammoniagenesis and points to GI bicarbonate loss; positive indicates impaired renal acid excretion (RTA). Then urine pH >5.5 despite acidemia with hypokalemia suggests type I; fractional bicarbonate excretion rising markedly during alkali loading suggests type II; hyperkalemia with acid urine suggests type IV.
• Metabolic alkalosis: urine chloride is the discriminator — low values indicate chloride-responsive (vomiting, prior diuretic, contraction), higher values indicate chloride-resistant states (mineralocorticoid excess, active diuretic, Bartter/Gitelman). Endocrine Society guidance directs aldosterone/renin ratio screening when hypertension with hypokalemic alkalosis is present.

Immediate stabilization

• Airway, perfusion, potassium: severe acidemia depresses contractility and blunts catecholamine response. Restore perfusion first; treat hyperkalemic ECG changes (calcium gluconate for membrane stabilization, then insulin/dextrose). If mechanical ventilation is required in severe metabolic acidosis, preserve the compensatory minute ventilation — sedating and normalizing PaCO2 can precipitate lethal acidemia (classic in salicylate poisoning).

First-line: treat the generator, not the number.

• Volume resuscitation and source control for lactic acidosis, per the Surviving Sepsis Campaign; SSC recommends against sodium bicarbonate for hypoperfusion-induced lactic acidemia at pH ≥7.15.
• Insulin infusion plus IV fluids and potassium repletion for DKA, per the ADA Standards of Care; hold insulin until K+ is above roughly 3.3 mEq/L, and reserve bicarbonate for extreme acidemia (pH <6.9).
• Fomepizole (alcohol dehydrogenase inhibitor) for methanol/ethylene glycol, with cofactors (folate for methanol, thiamine/pyridoxine for ethylene glycol) and hemodialysis for severe acidemia, high levels, or end-organ injury (EXTRIP/AACT guidance).
• Urinary alkalinization with sodium bicarbonate infusion plus hemodialysis for salicylate toxicity.

Chronic and tubular disease

• Oral alkali (sodium bicarbonate or sodium citrate) for CKD-associated metabolic acidosis; KDIGO advises alkali supplementation to keep serum bicarbonate in the normal range.
• Potassium citrate for type I RTA (also reduces calcium phosphate stones); large-dose alkali with a thiazide for type II; dietary potassium restriction, loop diuretic, and fludrocortisone where mineralocorticoid deficiency drives type IV.

Metabolic alkalosis

• Isotonic saline plus KCl for chloride-responsive alkalosis — chloride, not volume alone, is the missing ingredient. Add a proton pump inhibitor when nasogastric suction is ongoing.
• Acetazolamide (carbonic anhydrase inhibitor) when the patient is volume-overloaded and diuretic-associated.
• Mineralocorticoid receptor antagonist (spironolactone) or adrenalectomy for aldosterone-producing adenoma in chloride-resistant disease; saline will not correct these.

Complications of metabolic acidosis

• Hyperkalemic cardiac arrest (emergency): extracellular H+ drives K+ out of cells, most dramatically in inorganic acidoses and AKI. Signal: peaked T waves progressing to a sine-wave QRS. Organic acidoses (lactate, ketoacids) shift K+ less than the classic teaching implies.
• Catecholamine-refractory shock (emergency): acidemia below roughly pH 7.1 depresses myocardial contractility and uncouples beta-adrenergic signaling, so pressors fail while lactate keeps rising.
• Cerebral edema in pediatric DKA (emergency): osmolar shifts with overly rapid fluid or glucose correction; signal is headache, bradycardia with hypertension, and declining mental status during apparent biochemical improvement — the ADA and pediatric protocols emphasize controlled fluid rates and avoidance of bicarbonate for this reason.
• End-organ damage from the causative toxin: calcium oxalate crystalluria and AKI in ethylene glycol; optic neuropathy and putaminal necrosis in methanol.
• Chronic acidosis sequelae: bone demineralization from skeletal buffering, muscle wasting from accelerated proteolysis, and accelerated CKD progression.

Complications of treating acidosis

• Bicarbonate-induced hypokalemia and hypocalcemia: alkalinization drives K+ intracellularly and increases albumin binding of calcium — signals are new ectopy, perioral paresthesias, or tetany.
• Sodium and volume overload/hypernatremia: hypertonic bicarbonate ampules carry a large sodium load, precipitating pulmonary edema in CKD or heart failure.
• Paradoxical intracellular and CSF acidosis: bicarbonate generates CO2, which crosses membranes faster than HCO3−; if minute ventilation is fixed, cerebral pH falls despite a rising serum bicarbonate.
• Overshoot alkalosis: after the organic anion is metabolized back to bicarbonate, exogenous alkali leaves the patient alkalemic.

Complications of metabolic alkalosis and its therapy

• Seizures, tetany, arrhythmias (emergency at severe alkalemia): reduced ionized calcium plus hypokalemia lengthens the QT and potentiates digoxin toxicity.
• Hypoventilation and hypoxemia: compensatory CO2 retention plus a leftward oxyhemoglobin dissociation shift (Bohr effect) impairs tissue O2 unloading and may delay ventilator weaning.
• Saline therapy complications: volume overload, and hyperchloremic normal-gap acidosis from large-volume 0.9% saline.

• Always calculate three numbers: anion gap (albumin-corrected), Winter's formula, and the delta-delta. A "normal" bicarbonate with a wide gap and a large delta ratio hides a mixed gap acidosis plus metabolic alkalosis — the classic vomiting-plus-DKA stem.
• Wide gap + wide osmolar gap = toxic alcohol until proven otherwise: the single best next step is fomepizole (plus dialysis for severe acidemia or end-organ injury), not bicarbonate. Calcium oxalate crystals and AKI point to ethylene glycol; vision loss points to methanol.
• Normal-gap acidosis: get a urine anion gap. "*neGUTive*" (negative gap) = GI bicarbonate loss such as diarrhea; positive = renal tubular acidosis. This one calculation resolves most Step 2 CK normal-gap vignettes.
• Type IV RTA is the hyperkalemic RTA with an appropriately acid urine — think diabetic nephropathy, ACE inhibitor/ARB, trimethoprim, or heparin. Type I is hypokalemic with urine pH persistently above 5.5 and calcium phosphate stones; type II is hypokalemic with proximal wasting and Fanconi syndrome.
• Urine chloride, not urine sodium, sorts metabolic alkalosis. In active vomiting, urine sodium can be misleadingly high because it accompanies excreted bicarbonate; chloride stays low. Low urine chloride → give saline plus KCl; high urine chloride with hypertension → screen for primary aldosteronism (Endocrine Society aldosterone-to-renin ratio).
• Aspirin overdose is a mixed disorder: primary respiratory alkalosis (direct medullary stimulation) plus a primary anion-gap metabolic acidosis. The trap is intubating and normalizing PaCO2 — this abolishes compensation and can be fatal.
• Bicarbonate is rarely the answer. ADA reserves it for DKA at pH below 6.9, and the Surviving Sepsis Campaign advises against it for hypoperfusion-induced lactic acidosis at higher pH. Correct potassium before starting insulin in DKA.
• Common distractor: attributing hyperchloremic acidosis after massive 0.9% saline to "dilution of bicarbonate by water." The mechanism is the chloride load itself; balanced crystalloids avoid it.