Acid Base Disorders (Respiratory and Mixed) - Le

normal panel

• pH | pCO₂ | pO₂ | HCO₃

7.35 - 7.45 | 35 - 45 | 80 - 100 | 22 - 26

respiratory acidosis

• pH

• PaCO₂

• pH < 7.35

• PaCO₂ > 45 mmHg

causes of respiratory acidosis (5)

• central hypoventilation

  • sedatives, opioids, anesthetics, CNS injury

• neuromuscular disorders

  • MG, Guillan-Barre Syndrome

• pulmonary diseases

  • COPD, asthma, pneumonia, pulmonary edema

• chest/pleural restriction

• ventilatory hypoventilation

respiratory acidosis: clinical presentation (6)

• altered mental status, confusion, seizures, coma

• neurological damage and symptoms similar to stroke/CNS mass

• cerebral vasodilation → increased intracranial pressure → HA, etc

• blunted respiratory drive

• can ↑ CO and pulmonary pressure in mild/mod (PaCO₂ 50 -55)

• ↓ contractility and CO in severe/prolonged (PaCO₂ > 80)

respiratory acidosis: effects (3)

• increased CO₂ retention

• compensation by HCO₃ reabsorption

  • every 10 mmHg ↑ pCO₂ above 40 mmHg, HCO₃ ↑ ~1 mEq/L [acute]

  • every 10 mmHg ↑ pCO₂ above 40 mmHg, HCO₃ ↑ ~4 mEq/L [chronic]

• increased K+ from extracellular shift

  • H+ shifts inward to compensate for increased acidity — Hyperkalemia

acute respiratory acidosis treatment

restore oxygenation and ventilation

• might need mechanical ventilation

• treat underlying cause

acute respiratory acidosis treatment: treat underlying cause

• bronchospasm

• sedation

• infections

• bicarbonate

• bronchospasm → bronchodilators

• sedation → naloxone/flumazenil (seizure risk)

• infections → antibiotics

• bicarbonate → avoid overcorrection of it (may suppress respiratory drive)

chronic respiratory acidosis treatment

• treat underlying causes (of hypoventilation)

• do not treat chronic compensation unless acute exacerbation occurs

• titrate O₂ to SpO₂ 88 - 92% (excess O₂ worsens V/Q mismatch → ↑ PaCO₂

respiratory alkalosis

• pH

• PaCO₂

• pH > 7.45

• PaCO₂ < 35 mmHg

respiratory alkalosis: causes (5)

• central hyperventilation

  • anxiety, pain, fever, sepsis, pregnancy, stroke, trauma)

• drug-induced

  • salicylates, progesterone, catecholamine, nicotine, theophyline)

• hypoexemia or tissue hypoxia

• mechanical ventilation too fast

• liver failure or CNS disorders

respiratory alkalosis: clinical presentation

• usually asymptomatic

• lightheadedness, confusion, syncope, seizures

• decreased cerebral blood flow

• ↑ catecholamine sensitivity → arrhythmias

• n/v due to cerebral hypoxia/alkalemia

respiratory alkalosis: effects (3)

• increased CO₂ loss

• compensation by increased HCO₃ excretion

  • every 10 mmHg ↓, pCO₂ below 40 mmHg, HCO₃ ↓ ~ 2 mEq/L [acute]

  • every 10 mmHg ↓, pCO₂ below 40 mmHg, HCO₃ ↓ ~ 4 - 5 mEq/L [chronic]

• decreased HCO₃ (excretion) and K+ (inward shift)

  • H+ shifts outwards to compensate for lack of acid in circulation — Hypokalemia

acute respiratory alkalosis: treatment

• pH > 7.5

• focus on cause of hyperventilation

  • O₂ tx for hypoxemia

  • rebreathing for anxiety/stress-induced hyperventilation

  • mechanical ventilation, sedation, paralysis (if severe) - arrhythmias/seizures

  • in ventilated pts, reduce set respiratory rate or tidal volume to correct iatrogenic alkalosis

chronic respiratory alkalosis: treatment

• pH ≤ 7.5

• treat underlying cause

• mild, asymptomatic → tx unnecessary

mixed acid-base disorders: diagnostic approach

• pH levels

• primary cause (metabolic or respiratory)

• determine expected compensation

• compare expected to observed compensation

  • compensation NOT at expected level → likely a mixed disorder

• if metabolic acidosis → calculate AG

mixed-acid base disorders occur —

when 2+ primary acid-base disturbances are present at the same time

mixed-acid base disorders: clues

• pH or HCO₃ or PaCO₂ not in expected compensatory range

• clinical hx suggests > 1 process

compensation equations

compensation should be — of expected calculation for metabolic disorders

• within 2

  • if compensation of PaCO₂ < expected, then respiratory alkalosis (not enough CO₂

  • if compensation of PaCO₂ > expected, then respiratory acidosis (too much CO₂

mixed metabolic acidosis and respiratory acidosis

• MOA

• overall

• clinical scenarios

• management

• MOA

  • concurrent accumulation of acid

    Failure of compensation d/t limiting corrective responses

    • Respiratory acidosis prevents expected ↓ PaCO₂ 

    • Metabolic acidosis prevents renal buffering (↑ HCO₃⁻)

• overall

  • decreased pH

• clinical scenarios

  • Cardiorespiratory arrest

  • Chronic lung disease + shock/sepsis

  • Metabolic acidosis + respiratory failure 

• management

  • treat BOTH concurrently

    → oxygen delivery/ventilation

    → correct acidosis with alkali if SEVERE

mixed metabolic alkalosis and respiratory alkalosis

• MOA

• overall

• clinical scenarios

• management

• MOA

  • Concurrent loss of acid and retention of base

    • Respiratory alkalosis prevents compensatory ↑ PaCO₂ 

    • Metabolic alkalosis prevents renal excretion of HCO₃⁻

• overall

  • increased pH

• clinical scenarios

  • ICU or post-op patients on mechanical ventilation, sepsis, pain, CNS injury, or medications

  • Vomiting, nasogastric suctioning, or massive blood transfusions

  • Cirrhosis or chronic respiratory acidosis patients over-ventilated or on diuretics

• management

  • treat BOTH simultaneously

    → Adjust ventilator or correct cause of hyperventilation

    → Replace NaCl and KCl to correct metabolic alkalosis

mixed metabolic acidosis and respiratory alkalosis

• MOA

• overall

• clinical scenarios

• management

• MOA

  • ↓ HCO₃⁻ + excessive ↓CO₂

    • Respiratory alkalosis lowers PaCO₂ 

    • Metabolic acidosis decreases HCO₃⁻

• overall

  • pH appears overall normal

• clinical scenarios

  • Advanced liver disease or sepsis

  • Salicylate intoxication

  • Pulmonary–renal syndromes or hypoxemia

• management

  • Identify and treat the underlying cause 

  • Support oxygenation and ventilation as needed

  • Avoid overcorrection

mixed metabolic alkalosis and respiratory acidosis

• MOA

• overall

• clinical scenarios

• management

• MOA

  • Concurrent HCO₃⁻ retention and CO₂ retention

    • Metabolic alkalosis ↑ HCO₃⁻, blunting respiratory drive

    • Respiratory acidosis ↑ PaCO₂, worsening alkalosis compensation

• overall

  • pH may appear near normal

• clinical scenarios

  • COPD or chronic respiratory acidosis on diuretics or salt restriction

  • Glucocorticoid therapy

  • Chronic heart failure with volume depletion

• management

  • → Discontinue diuretics if possible

    → Administer NaCl and KCl to enhance bicarbonate excretion

    → Use cautiously in heart failure to avoid fluid overload