Metabolic Acidosis and Metabolic Alkalosis

Clinical Orientation

Ms. Williams, 28, has been vomiting for four days from a viral illness. She cannot keep anything down—not even sips of water. She is lightheaded when she stands. Her respirations are slow and shallow. Her ABG shows: pH 7.52, PaCO2 48, HCO3 36. Her urine chloride is less than 10 mEq/L. She has metabolic alkalosis from loss of gastric acid, and her lungs are compensating—barely—by retaining CO2. But her compensation is limited because she still needs to oxygenate her blood. Meanwhile, her potassium is 3.0 mEq/L because her kidneys, stimulated by aldosterone from volume depletion, are exchanging potassium for sodium. This case has it all: acid loss, volume contraction, hypokalemia.

Governing Question: What mechanism links metabolic acidosis and metabolic alkalosis to its required bedside findings, tests, red flags, and nursing priorities?

What Is Normal?

Renal Bicarbonate Conservation and Acid Excretion: The kidneys filter approximately 4000 mEq of bicarbonate daily. Nearly all of it is reabsorbed—about 80% in the proximal tubule, 15% in the thick ascending limb, and 5% in the distal tubule and collecting duct. The kidneys also secrete about 50-100 mEq of hydrogen ions per day (the daily fixed acid load), which are excreted buffered by phosphate (titratable acidity) and ammonia (NH3, which binds H+ to form NH4+). The kidneys also regenerate the bicarbonate consumed by buffering the daily acid load. Without renal acid excretion, metabolic acidosis develops within days.

GI Bicarbonate: The GI tract is both a source and a sink for acid and base. The stomach secretes HCl (acid) into the lumen. The pancreas and duodenum secrete bicarbonate-rich fluid to neutralize gastric acid. Normally, these secretions balance—the bicarbonate secreted by the pancreas neutralizes the acid from the stomach. When gastric contents are lost (vomiting, NG suction), the acid is lost and the pancreatic bicarbonate enters the bloodstream unopposed → metabolic alkalosis. When lower GI contents are lost (diarrhea), bicarbonate is lost → metabolic acidosis.

Metabolic Acid Production: Normal metabolism generates fixed (non-volatile) acids: sulfuric acid from metabolism of sulfur-containing amino acids (cysteine, methionine), phosphoric acid from phospholipids, and organic acids (lactic acid from anaerobic glycolysis, ketoacids from fatty acid metabolism). These are buffered by bicarbonate and excreted by the kidneys. When production exceeds renal capacity (lactic acidosis, ketoacidosis) or renal excretion is impaired (renal failure), metabolic acidosis develops.

Ventilatory Response: The respiratory center in the brainstem responds to changes in pH and PaCO2. In metabolic acidosis, the falling pH stimulates peripheral chemoreceptors (carotid and aortic bodies) and central chemoreceptors (medulla). This increases tidal volume and respiratory rate, lowering PaCO2. This is the Kussmaul breathing pattern—deep, rapid, regular respirations. In metabolic alkalosis, the rising pH suppresses the respiratory drive, causing hypoventilation and CO2 retention. But this compensation is limited by the need to oxygenate—the patient cannot hypoventilate enough to fully correct pH without becoming hypoxic.

What Goes Wrong?

Metabolic Acidosis Is Primary Bicarbonate Fall from Acid Gain or Base Loss: When acids accumulate faster than the kidneys can excrete them, or when bicarbonate is lost from the GI tract or kidneys, bicarbonate concentration falls. The [HCO3-]/[CO2] ratio decreases, driving pH down. The respiratory system responds by increasing ventilation to lower PaCO2, partially correcting pH (but never fully). The underlying mechanism divides into two categories based on the anion gap.

High-Anion-Gap Metabolic Acidosis: Unmeasured acids accumulate and consume bicarbonate. The added acid's anion (lactate, ketone bodies, formate from methanol, glycolate/oxalate from ethylene glycol, salicylate, or uremic acids) is "unmeasured" in the anion gap calculation, so the gap widens. Causes (mnemonic MUDPILES or GOLDMARK): Methanol, Uremia, Diabetic ketoacidosis (and alcoholic and starvation), Paraldehyde (rare now), Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. In practice, the most common causes are lactic acidosis, ketoacidosis, renal failure, and toxic alcohols.

Normal-Anion-Gap (Hyperchloremic) Metabolic Acidosis: Bicarbonate is lost and replaced by chloride, keeping the anion gap normal. Causes: diarrhea (bicarbonate-rich fluid loss), renal tubular acidosis (impaired bicarbonate reabsorption or acid secretion), ureteral diversion (urine in contact with bowel → chloride-bicarbonate exchange), saline infusion (large volumes of normal saline dilute bicarbonate and add chloride), and carbonic anhydrase inhibitors (acetazolamide).

Metabolic Alkalosis Is Primary Bicarbonate Rise from Acid Loss, Base Gain, or Maintenance Factors: For metabolic alkalosis to persist, two things are generally required: a generation phase that raises bicarbonate (loss of gastric acid, diuretic-induced contraction, exogenous base) and a maintenance phase that prevents the kidneys from simply excreting the excess bicarbonate. The most common maintenance factor is volume depletion with chloride deficiency—when the kidneys are avidly retaining sodium, they cannot excrete bicarbonate effectively because sodium must be reabsorbed with either chloride or bicarbonate. Low chloride + volume depletion = the kidneys retain bicarbonate to accompany sodium reabsorption.

Causes, Risk Factors, and Triggers

Metabolic Acidosis

• Lactic acidosis: Type A (tissue hypoxia—shock, cardiac arrest, severe hypoxemia, regional ischemia). Type B (no overt hypoxia—sepsis, liver failure, malignancy, medications like metformin, inherited mitochondrial disorders).
• Ketoacidosis: Diabetic ketoacidosis (insulin deficiency → lipolysis → ketone body production), alcoholic ketoacidosis (poor intake + alcohol metabolism), starvation ketosis.
• Renal acid retention: Acute kidney injury, chronic kidney disease (especially when GFR falls below ~20-30 mL/min), renal tubular acidosis.
• Diarrhea: Loss of bicarbonate-rich intestinal and pancreatic secretions.
• Toxins: Methanol → formic acid. Ethylene glycol → glycolic and oxalic acid. Salicylates → mixed metabolic acidosis and respiratory alkalosis.
• Other: Pancreatic fistula, ureteral diversion procedures.

Metabolic Alkalosis

• Vomiting or gastric suction: Loss of gastric HCl. The alkaline tide from gastric acid secretion is not neutralized by pancreatic bicarbonate, so net bicarbonate rises.
• Diuretics: Loop and thiazide diuretics cause volume contraction, chloride loss, and increased distal sodium delivery (which promotes hydrogen and potassium secretion). This is "contraction alkalosis."
• Mineralocorticoid excess: Primary hyperaldosteronism, Cushing's syndrome, licorice ingestion (glycyrrhizic acid has mineralocorticoid effects). Aldosterone promotes hydrogen and potassium secretion.
• Chloride/volume depletion: Any cause of volume depletion with chloride loss can maintain metabolic alkalosis because the kidneys retain bicarbonate to accompany sodium reabsorption.
• Excessive base: Bicarbonate administration, citrate from massive blood transfusion (citrate is metabolized to bicarbonate), milk-alkali syndrome (calcium carbonate antacids).
• Severe hypokalemia: Potassium depletion drives hydrogen into cells and promotes renal hydrogen secretion.

What Happens Inside the Body?

Causal Chain 1: Metabolic Acidosis

Acid gain or bicarbonate loss → Bicarbonate falls → pH falls → Ventilation increases to lower CO2

Fixed acids (lactate, ketones, uremic toxins) accumulate in the blood. These acids dissociate, releasing hydrogen ions. Bicarbonate buffers the hydrogen: H+ + HCO3- → H2CO3 → CO2 + H2O. Bicarbonate is consumed. The CO2 produced stimulates the respiratory center through peripheral and central chemoreceptors. The patient hyperventilates—Kussmaul respirations: deep, rapid, regular breaths. PaCO2 falls. This shifts the equilibrium, consuming hydrogen and partially raising pH. But if acid production continues (ongoing shock, untreated DKA) or renal function is impaired, bicarbonate continues to fall and pH worsens. Key finding: Kussmaul respirations are a classic, often dramatic finding in severe metabolic acidosis. If you see them, think metabolic acidosis until proven otherwise. The patient may not report dyspnea—the breathing is deep but usually not labored in the sense of "air hunger."

Causal Chain 2: Metabolic Alkalosis

Hydrogen loss or bicarbonate gain plus renal maintenance → Bicarbonate rises → pH rises → Ventilation retains some CO2 but is limited by oxygen need

Gastric acid is lost through vomiting or NG suction. The stomach normally secretes HCl into the lumen. This acid secretion is accompanied by bicarbonate movement into the blood (the "alkaline tide"). Normally, when gastric acid reaches the duodenum, it stimulates pancreatic bicarbonate secretion, which neutralizes the acid and consumes the alkaline tide. When gastric contents are lost externally, the pancreatic bicarbonate is secreted but there is no acid to neutralize it. Net bicarbonate rises. Volume depletion from fluid loss activates RAAS → aldosterone promotes sodium reabsorption and hydrogen/potassium secretion, further raising bicarbonate. Hypokalemia develops as potassium is lost in urine (and shifts into cells as hydrogen shifts out). The respiratory center is suppressed by the rising pH → hypoventilation → PaCO2 rises, partially lowering pH. But hypoventilation is limited—the patient must still oxygenate. Key finding: The combination of vomiting/NG suction + slow/shallow respirations + hypokalemia + volume depletion is the classic metabolic alkalosis picture. Check urine chloride: low (<20 mEq/L) suggests chloride-responsive alkalosis (volume/chloride depletion driving the process).

What the Nurse May See

Metabolic Acidosis

• Deep, rapid breathing (Kussmaul): The hallmark. Tidal volume is large, rate is fast, the pattern is regular. Patients rarely complain of dyspnea—the drive is chemical, not from airway obstruction or pulmonary disease.
• Nausea and vomiting: Common, especially in DKA and uremia.
• Weakness and lethargy: CNS depression from acidemia.
• Confusion progressing to coma: As pH drops, neuronal function deteriorates.
• Dysrhythmia: Acidosis + associated potassium shifts (hyperkalemia from transcellular shift) create electrical instability.
• Warm, flushed skin with bounding pulses: In some cases (e.g., early sepsis with lactic acidosis), vasodilation from acidosis produces a hyperdynamic state. In late shock, vasoconstriction predominates.
• Signs of underlying cause: Fruity breath (DKA), uremic fetor (renal failure), signs of sepsis, signs of toxin ingestion.

Metabolic Alkalosis

• Slow, shallow respirations: The compensatory hypoventilation. This may be subtle.
• Confusion, dizziness, and irritability: Alkalemia increases CNS excitability.
• Muscle cramps and tetany: Alkalosis increases calcium binding to albumin → less ionized calcium → neuromuscular irritability. Also, alkalosis directly increases nerve excitability.
• Dysrhythmia: Alkalosis shifts potassium into cells → hypokalemia → prolonged QT, U waves, risk of ventricular arrhythmias.
• Hypoventilatory compensation is limited: The patient cannot hypoventilate enough to normalize pH because oxygenation requirements set a floor on ventilation. This is why severe metabolic alkalosis (pH >7.55) is dangerous—compensation is inadequate.
• Signs of volume depletion: Orthostasis, tachycardia, dry mucosa, concentrated urine (if vomiting or diuretic-induced).

Tests, Labs, and Monitoring

pH, PaCO2, Bicarbonate: The primary data. Metabolic acidosis: low pH, low bicarbonate, low PaCO2 (compensation). Metabolic alkalosis: high pH, high bicarbonate, high PaCO2 (compensation). Compare the actual compensation to what is expected. If compensation is inappropriate, consider a mixed disorder.

Electrolytes: Sodium, potassium, chloride, bicarbonate. Calculate the anion gap: Na - (Cl + HCO3). Normal ~8-12 mEq/L. High gap = unmeasured acids. Normal gap = hyperchloremic (chloride rises as bicarbonate falls). In metabolic alkalosis, chloride is often low (from vomiting, diuretics).

Anion Gap: This is the key to classifying metabolic acidosis. Check the gap on every BMP. A normal gap + low bicarbonate suggests diarrhea, RTA, or saline administration. A high gap + low bicarbonate suggests lactic acidosis, ketoacidosis, renal failure, or toxin. Trend the gap—a falling gap suggests the acid is being cleared.

Lactate: The most common cause of elevated anion gap metabolic acidosis in hospitalized patients. Rising lactate indicates worsening tissue hypoperfusion or unresolved sepsis. Serial lactate measurements gauge response to resuscitation.

Glucose/Ketones: For DKA: elevated glucose, positive serum/urine ketones, elevated beta-hydroxybutyrate, low bicarbonate, elevated anion gap. For alcoholic ketoacidosis: glucose is often normal or low despite ketosis.

Renal Function: BUN and creatinine determine whether renal failure is contributing to acidosis (reduced acid excretion) or whether the kidneys are responding appropriately to metabolic alkalosis. Serum potassium—hypokalemia is common in metabolic alkalosis and DKA (total-body depletion).

Chloride and Urine Studies: Urine chloride is useful in metabolic alkalosis. Low urine Cl (<20 mEq/L) = chloride-responsive alkalosis (volume/chloride depletion—will respond to saline). High urine Cl (>20 mEq/L) = chloride-resistant (mineralocorticoid excess, severe hypokalemia, base loading). Urine sodium, osmolality, and pH provide additional context.

Nursing Priorities

Assess Perfusion, Breathing, Neurologic Status, Rhythm, Losses, Medications, and Trend: In metabolic acidosis: is the patient perfusing (lactate trend, vitals, urine output)? Are they breathing adequately, or are they tiring? In metabolic alkalosis: what is the source of acid loss (vomiting, NG output, diuresis)? Is the patient volume-depleted? Is potassium low? The assessment determines the urgency.

Recognize Compensation Versus Failure: A patient with metabolic acidosis who is hyperventilating is compensating. If the respiratory rate drops or breathing becomes shallow, compensation is failing—the pH is about to crash. In metabolic alkalosis, severe alkalemia (pH >7.55) with inadequate respiratory compensation is dangerous.

Follow Cause-Directed Orders/Protocols: Metabolic acidosis: treat the underlying cause—fluids and insulin for DKA, resuscitation for shock/sepsis, dialysis for renal failure or toxins, bicarbonate administration in selected cases (controversial, ordered by provider). Metabolic alkalosis: volume repletion with normal saline (chloride-responsive), potassium replacement, addressing the source of acid loss (anti-emetics, reducing NG suction as ordered), acetazolamide in selected cases, and rarely dilute hydrochloric acid for severe, refractory alkalosis.

Complications and Red Flags

Patient and Family Teaching

One-Minute Mechanism: "Your body produces acid constantly, especially when your tissues are not getting enough oxygen or when you burn fat for fuel too quickly. Your kidneys and your breathing normally keep everything balanced. When acid builds up too fast—from serious infection, uncontrolled diabetes, or kidney failure—your body tries to breathe it off. That deep, heavy breathing is a sign of a dangerous acid buildup. When acid is lost instead—from vomiting a lot—your body becomes too basic, and you breathe more slowly to compensate. Both situations are serious and need medical attention."

Key Points: If you have diabetes, monitor your blood sugar closely during illness and know the warning signs of DKA: deep rapid breathing, nausea, fruity breath, confusion. Seek emergency care. If you have kidney disease, follow dietary and medication plans—your kidneys cannot manage the daily acid load effectively. If you experience prolonged vomiting or diarrhea, you are losing acids or bases—seek medical care if it lasts more than 24 hours or you cannot keep fluids down. Do not take large amounts of baking soda (bicarbonate) or antacids without medical guidance—you can cause metabolic alkalosis.

Key takeaways and summary

Summary

Normal → Change → Consequence → Finding → Priority: Metabolic acidosis (bicarbonate falls from acid gain or base loss → pH falls → Kussmaul respirations compensate) threatens perfusion and consciousness as pH drops. Metabolic alkalosis (bicarbonate rises from acid loss or base gain → pH rises → limited hypoventilation compensates) threatens cardiac rhythm and neuromuscular stability. The nursing priority is to assess breathing pattern, perfusion, and neurologic status; classify by anion gap; recognize when compensation is failing; and address the underlying cause through ordered interventions.

Causal Chain 1: Acid gain/base loss → bicarbonate falls → pH falls → ventilation increases to lower CO2. Causal Chain 2: Hydrogen loss/base gain + renal maintenance → bicarbonate rises → pH rises → ventilation retains CO2 (limited by oxygen need).

If You Remember Nothing Else:

1. Metabolic acidosis means bicarbonate is low—from acid gain (high gap) or base loss (normal gap).
2. Kussmaul respirations are compensation for metabolic acidosis, not a primary lung problem.
3. Metabolic alkalosis requires both a cause and a maintenance factor (usually volume/chloride depletion).
4. Red flag: Respiratory fatigue in metabolic acidosis signals impending decompensation.
5. Test limitation: The anion gap classifies metabolic acidosis—always calculate it when bicarbonate is low.

One-Minute Teach-Back: "Explain the difference between high-gap and normal-gap metabolic acidosis, and why vomiting causes the opposite problem from diarrhea."

Common Student Mistakes

Mistake: "Rapid breathing is the cause rather than compensation in metabolic acidosis." Wrong. The deep, rapid breathing (Kussmaul) is the body's compensatory response to metabolic acidosis—it is lowering CO2 to raise pH. Treating the hyperventilation (e.g., sedating the patient) without addressing the underlying acidosis would cause the pH to crash. The breathing is a compensatory mechanism, not the problem.

Mistake: "Vomiting simply removes water and electrolytes, not acid." Wrong. Gastric fluid contains hydrochloric acid (HCl). When it is lost, hydrogen ions are lost, and the pancreatic bicarbonate that would have neutralized it enters the bloodstream unopposed. This is why vomiting and NG suction cause metabolic alkalosis—it is acid loss, not just volume loss.