Everything You Need to Know About Respiratory acidosis & alkalosis for Step 1

Respiratory acid–base disorders are one of those Step 1 “you either see it instantly or you don’t” topics—until you learn the shortcuts. The key is simple: if the lungs are the primary problem, the primary change is in $\mathrm{PaCO_2}$ , and the kidneys respond (slowly) by adjusting $\mathrm{HCO_3^-}$ . This post will make respiratory acidosis and alkalosis feel automatic: definitions, mechanisms, classic presentations, ABG patterns, compensation rules, and the high-yield disease associations you’ll actually be tested on.

Big-Picture Framework (What Step Questions Are Really Testing)

The Henderson–Hasselbalch relationship (conceptual)

Blood pH tracks the ratio of bicarbonate to carbon dioxide:

$\mathrm{pH} \propto \frac{[\mathrm{HCO_3^-}]}{\mathrm{PaCO_2}}$

$\uparrow \mathrm{PaCO_2}$ (hypoventilation) pushes pH down → respiratory acidosis
$\downarrow \mathrm{PaCO_2}$ (hyperventilation) pushes pH up → respiratory alkalosis
Kidneys compensate by changing $\mathrm{HCO_3^-}$ , but that takes time (hours to days)

Time course (very testable)

Acute respiratory disorder: minimal renal compensation (minutes–hours)
Chronic respiratory disorder: substantial renal compensation (≥ 2–3 days)

First Aid cross-reference: Renal—Acid-base disorders; Respiratory—COPD; Biochem—Carbonic anhydrase inhibitors (acetazolamide).

Quick Pattern Recognition: ABG “Look and Lock”

Disorder	Primary change	pH	$\mathrm{PaCO_2}$	$\mathrm{HCO_3^-}$ (compensatory)
Respiratory acidosis	Hypoventilation	↓	↑	↑ (if chronic)
Respiratory alkalosis	Hyperventilation	↑	↓	↓ (if chronic)

Step trick: If pH and $\mathrm{PaCO_2}$ move in opposite directions → respiratory.

pH ↓ with CO₂ ↑ → respiratory acidosis
pH ↑ with CO₂ ↓ → respiratory alkalosis

Respiratory Acidosis (Hypoventilation → CO₂ Retention)

Definition

A primary increase in $\mathrm{PaCO_2}$ leading to decreased pH, due to alveolar hypoventilation.

Core pathophysiology (why CO₂ matters)

CO₂ diffuses into blood and forms carbonic acid:

$\mathrm{CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-}$

So CO₂ retention increases $\mathrm{H^+}$ , dropping pH.
Kidneys compensate by:

Increasing H⁺ secretion (α-intercalated cells)
Increasing $\mathrm{HCO_3^-}$ reabsorption and generation
Increasing ammoniagenesis ( $\mathrm{NH_3}$ buffers H⁺ → $\mathrm{NH_4^+}$ excretion)

First Aid cross-reference: Renal tubular physiology—α-intercalated cells, ammonium trapping.

High-yield causes (think: “can’t ventilate”)

Group them by mechanism:

1) CNS respiratory depression

Opioids, benzodiazepines, barbiturates
Brainstem injury

2) Neuromuscular weakness / impaired respiratory mechanics

Guillain-Barré, myasthenia gravis, ALS
High cervical cord injury
Severe kyphoscoliosis

3) Airway disease / obstructive lung disease

COPD exacerbation (classic board favorite)
Severe asthma (late/failing ventilation)

4) Reduced ventilation due to obesity or sleep

Obesity hypoventilation syndrome
Severe OSA (esp when hypoventilation persists)

5) Iatrogenic ventilation issues

Inadequate ventilator settings (low minute ventilation)

Step association: COPD + chronic CO₂ retention → renal compensation → elevated bicarbonate baseline.

Clinical presentation

Depends on acuity:

Acute respiratory acidosis

Headache, confusion, somnolence
Asterixis can occur in severe hypercapnia
Warm, flushed skin (hypercapnia causes vasodilation)

Chronic respiratory acidosis (compensated)

Milder symptoms
Signs of underlying disease (barrel chest, wheeze, etc.)
Possible polycythemia if chronic hypoxemia (e.g., COPD)

High-yield pearl: Hypercapnia increases cerebral blood flow → headache, increased intracranial pressure symptoms.

Diagnosis (ABG interpretation + context)

ABG hallmark: pH low, $\mathrm{PaCO_2}$ high.

Then decide acute vs chronic using compensation.

Compensation rules (board-friendly)

For respiratory acidosis, expected bicarbonate increases:

Acute: $\Delta \mathrm{HCO_3^-} \approx +1$ mEq/L for every +10 mmHg $\Delta \mathrm{PaCO_2}$
Chronic: $\Delta \mathrm{HCO_3^-} \approx +4$ mEq/L for every +10 mmHg $\Delta \mathrm{PaCO_2}$

If measured $\mathrm{HCO_3^-}$ is higher than expected, suspect a concurrent metabolic alkalosis.
If it’s lower than expected, suspect concurrent metabolic acidosis.

Treatment (fix ventilation—don’t chase the number)

Principles:

Treat the cause
- Naloxone for opioid overdose
- Bronchodilators + steroids ± antibiotics for COPD exacerbation
- IVIG/plasmapheresis for GBS; acetylcholinesterase inhibitors/immunotherapy for MG
Support ventilation
- Noninvasive ventilation (BiPAP) often for COPD hypercapnic exacerbation
- Intubation if failure to protect airway, worsening mental status, fatigue, severe acidosis

High-yield caution: In chronic CO₂ retainers (e.g., severe COPD), giving high-flow oxygen can worsen hypercapnia via:

V/Q mismatch (reversal of hypoxic vasoconstriction)
Haldane effect (oxygenated hemoglobin carries less CO₂) Hypoxic drive is a simplified explanation—know it exists, but V/Q mismatch is the more testable physiology.

Respiratory Alkalosis (Hyperventilation → CO₂ Blown Off)

Definition

A primary decrease in $\mathrm{PaCO_2}$ leading to increased pH, due to alveolar hyperventilation.

Core pathophysiology

Dropping CO₂ shifts the equilibrium left:

$\mathrm{H^+ + HCO_3^- \rightarrow H_2CO_3 \rightarrow CO_2 + H_2O}$

So $\mathrm{H^+}$ decreases, raising pH.
Kidneys compensate (chronically) by:

Decreasing H⁺ secretion
Decreasing $\mathrm{HCO_3^-}$ reabsorption → bicarbonate “wasting”

First Aid cross-reference: Renal—Compensation for respiratory alkalosis; Pulm—PE, high altitude.

High-yield causes (think: “too much ventilation”)

1) Hypoxemia-driven hyperventilation

Pulmonary embolism (very classic: sudden dyspnea + respiratory alkalosis early)
Pneumonia, pulmonary edema
High altitude

2) Central stimulation

Anxiety/panic attack
Pain
Fever/sepsis (early)
Pregnancy (progesterone stimulates respiratory center)

3) Iatrogenic

Overventilation on mechanical ventilation

4) Salicylate poisoning (sneaky Step favorite)

Early aspirin toxicity → respiratory alkalosis (direct stimulation of respiratory center)
Later → anion gap metabolic acidosis (mixed disorder)

First Aid cross-reference: Tox—Salicylates cause early respiratory alkalosis + later metabolic acidosis.

Clinical presentation

Lightheadedness, dizziness
Perioral numbness, tingling
Carpopedal spasm / tetany (from decreased ionized calcium)

Why tetany happens (high-yield physiology): Alkalosis increases albumin binding of calcium → decreases ionized Ca²⁺ → neuromuscular irritability.

Diagnosis (ABG + compensation)

ABG hallmark: pH high, $\mathrm{PaCO_2}$ low.

Compensation rules

For respiratory alkalosis, expected bicarbonate decreases:

Acute: $\Delta \mathrm{HCO_3^-} \approx -2$ mEq/L for every -10 mmHg $\Delta \mathrm{PaCO_2}$
Chronic: $\Delta \mathrm{HCO_3^-} \approx -5$ mEq/L for every -10 mmHg $\Delta \mathrm{PaCO_2}$

Mismatch → suspect a mixed disorder.

Treatment

Treat the trigger (PE workup and anticoagulation, treat sepsis, adjust ventilator settings, manage pain/anxiety)
If anxiety-induced and severe: coaching breathing; short-term anxiolysis may be used clinically, but Step exams prefer recognize cause > symptomatic paper-bagging (generally discouraged medically).

How to Tell Acute vs Chronic in 10 Seconds

Step-by-step approach

Identify primary disorder from pH and $\mathrm{PaCO_2}$
Calculate expected $\mathrm{HCO_3^-}$ compensation using acute vs chronic rule
Pick the one that matches → that’s the time course
If neither matches → mixed acid–base disorder

Quick reference table (memorize-worthy)

Primary disorder	Acute renal response	Chronic renal response
Respiratory acidosis	$\mathrm{HCO_3^-} +1$ per +10 CO₂	$\mathrm{HCO_3^-} +4$ per +10 CO₂
Respiratory alkalosis	$\mathrm{HCO_3^-} -2$ per −10 CO₂	$\mathrm{HCO_3^-} -5$ per −10 CO₂

Classic Vignettes You Should Instantly Recognize

Respiratory acidosis

COPD patient with somnolence after being placed on high-flow O₂
Opioid overdose with hypoventilation
Neuromuscular disease with rising CO₂ (fatiguing respiratory muscles)

Respiratory alkalosis

Young patient with chest pain/shortness of breath after surgery → likely PE → respiratory alkalosis
First day at high altitude → hyperventilation → respiratory alkalosis
Early salicylate toxicity (tinnitus + respiratory alkalosis)

High-Yield “Gotchas” and Associations

1) Mixed disorders: salicylates and severe asthma

Salicylates: respiratory alkalosis (early) + anion gap metabolic acidosis (late)
Severe asthma: early respiratory alkalosis (hyperventilation), then respiratory acidosis when tiring/failing

2) Compensation never “overcorrects”

If pH is completely normal, think:

Very well-compensated chronic disorder or
Two primary processes offsetting each other (mixed disorder)

3) CO₂ retention and bicarbonate baseline

Chronic COPD can have:

High $\mathrm{PaCO_2}$
High $\mathrm{HCO_3^-}$ (renal compensation)
pH near normal

That “elevated baseline bicarbonate” is a huge clue in Step questions.

Rapid Review: What to Memorize for Test Day

Respiratory acidosis = hypoventilation = $\uparrow \mathrm{PaCO_2}$
- Causes: COPD, opioids, neuromuscular weakness
- Compensation: kidneys retain/generate $\mathrm{HCO_3^-}$
- Rules: +1 (acute) or +4 (chronic) per +10 CO₂
Respiratory alkalosis = hyperventilation = $\downarrow \mathrm{PaCO_2}$
- Causes: PE, panic, pregnancy, altitude, early salicylates
- Symptoms: lightheadedness, paresthesias, tetany (↓ ionized Ca²⁺)
- Rules: −2 (acute) or −5 (chronic) per −10 CO₂