A 60-year-old woman is brought to the emergency department by her husband because of worsening shortness of breath over the past 2 days. Last week, she had a sore throat and a low-grade fever. She has coughed up white sputum each morning for the past 2 years. She has hypertension and type 2 diabetes mellitus. She has smoked 2 packs of cigarettes daily for 35 years. Current medications include metformin and lisinopril. On examination, she occasionally has to catch her breath between sentences. Her temperature is 38.1°C (100.6°F), pulse is 85/min, respirations are 16/min, and blood pressure is 140/70 mm Hg. Expiratory wheezes with a prolonged expiratory phase are heard over both lung fields. Arterial blood gas analysis on room air shows: pH 7.33 PCO2 53 mm Hg PO2 68 mm Hg An x-ray of the chest shows hyperinflation of bilateral lung fields and flattening of the diaphragm. Which of the following additional findings is most likely in this patient?

Decreased urinary bicarbonate excretion

Decreased urinary chloride concentration

A 58-year-old man presents to the emergency department with a chief complaint of ringing in his ears that started several hours previously that has progressed to confusion. The patient denies any history of medical problems except for bilateral knee arthritis. He was recently seen by an orthopedic surgeon to evaluate his bilateral knee arthritis but has opted to not undergo knee replacement and prefers medical management. His wife noted that prior to them going on a hike today, he seemed confused and not himself. They decided to stay home, and roughly 14 hours later, he was no longer making any sense. Physical exam is notable for a confused man. The patient's vitals are being performed and his labs are being drawn. Which of the following is most likely to be seen on blood gas analysis?

pH: 7.30, PaCO2: 15 mmHg, HCO3-: 16 mEq/L

pH: 7.37, PaCO2: 41 mmHg, HCO3-: 12 mEq/L

pH: 7.41, PaCO2: 65 mmHg, HCO3-: 34 mEq/L

pH: 7.47, PaCO2: 11 mmHg, HCO3-: 24 mEq/L

pH: 7.31, PaCO2: 31 mmHg, HCO3-: 15 mEq/L

A 24-year-old woman presents to the emergency department after she was found agitated and screaming for help in the middle of the street. She says she also has dizziness and tingling in the lips and hands. Her past medical history is relevant for general anxiety disorder, managed medically with paroxetine. At admission, her pulse is 125/min, respiratory rate is 25/min, and body temperature is 36.5°C (97.7°F). Physical examination is unremarkable. An arterial blood gas sample is taken. Which of the following results would you most likely expect to see in this patient?

pH: increased, HCO3-: decreased, Pco2: decreased

pH: increased, HCO3-: increased, Pco2: increased

pH: decreased, HCO3-: decreased, Pco2: decreased

pH: decreased, HCO3-: increased, Pco2: increased

pH: normal, HCO3-: increased, Pco2: increased

A 52-year-old man with a history of Type 1 diabetes mellitus presents to the emergency room with increasing fatigue. Two days ago, he ran out of insulin and has not had time to obtain a new prescription. He denies fevers or chills. His temperature is 37.2 degrees Celsius, blood pressure 84/56 mmHg, heart rate 100/min, respiratory rate 20/min, and SpO2 97% on room air. His physical exam is otherwise within normal limits. An arterial blood gas analysis shows the following: pH 7.25, PCO2 29, PO2 95, HCO3- 15. Which of the following acid-base disorders is present?

Metabolic acidosis with appropriate respiratory compensation

Respiratory alkalosis with appropriate metabolic compensation

Respiratory acidosis with appropriate metabolic compensation

Mixed metabolic and respiratory acidosis

Metabolic alkalosis with appropriate respiratory compensation

A 70-year-old woman is brought to the emergency department due to worsening lethargy. She lives with her husband who says she has had severe diarrhea for the past few days. Examination shows a blood pressure of 85/60 mm Hg, pulse of 100/min, and temperature of 37.8°C (100.0°F). The patient is stuporous, while her skin appears dry and lacks turgor. Laboratory tests reveal: Serum electrolytes Sodium 144 mEq/L Potassium 3.5 mEq/L Chloride 115 mEq/L Bicarbonate 19 mEq/L Serum pH 7.3 PaO2 80 mm Hg Pco2 38 mm Hg This patient has which of the following acid-base disturbances?

Non-anion gap metabolic acidosis with respiratory compensation

Anion gap metabolic acidosis with respiratory compensation

Non-anion gap metabolic acidosis

Respiratory acidosis mechanisms and compensation for USMLE Step 3: High-Yield Physiology Lessons

Pathophysiology - The CO₂ Traffic Jam

Foundation: Henderson-Hasselbalch Equation: $pH = pKa + log([HCO₃⁻] / [0.03 * PCO₂])$
Definition: A primary acid-base disorder characterized by:
- ↓ pH < 7.35
- ↑ PaCO₂ > 45 mmHg
Primary Disturbance: Alveolar hypoventilation is the root cause, impairing CO₂ elimination from the lungs. This leads to CO₂ accumulation in the blood (hypercapnia).
Mechanism: As PaCO₂ rises, the equilibrium of the bicarbonate buffer system shifts right, increasing H⁺ concentration and decreasing pH.
- $CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻$

⭐ In acute respiratory acidosis, plasma [HCO₃⁻] increases by ~1 mEq/L for every 10 mmHg increase in PaCO₂ above 40 mmHg.

Etiologies - Why the Lungs Loaf

⭐ In opioid overdose, the earliest and most specific sign of respiratory compromise is a decreased respiratory rate (bradypnea), a direct effect on the brainstem's respiratory centers.

The fundamental issue is alveolar hypoventilation, leading to inadequate $CO_2$ clearance. Causes can be remembered with the mnemonic 📌 DEPRESS:

Category	Causes (Hypoventilation → ↑Pa$CO_2$)
Drugs / CNS Depression	* Opioids, Sedatives, Anesthetics: Suppress respiratory drive. - Brainstem Injury: Stroke, trauma, infection. - Central Sleep Apnea.
Edema / Airway Obstruction	* COPD/Asthma Exacerbation: Air trapping & obstruction. - Laryngospasm, Foreign Body. - Pulmonary Edema (late stage).
Pneumonia / Pulmonary	* Severe Pneumonia/ARDS: V/Q mismatch & ↑ dead space. - Interstitial Lung Disease.
Respiratory Muscle Weakness	* Neuromuscular: Myasthenia Gravis, Guillain-Barré, ALS. - Toxins: Botulism. - Muscular Dystrophy.
Emboli / Chest Wall	* Chest Wall Restriction: Kyphoscoliosis, flail chest, obesity. - Massive Pulmonary Embolism (can cause late respiratory failure).
Spinal Cord / Secretions	* High Spinal Cord Injury (above C3-C5). - Obesity Hypoventilation Syndrome (Pickwickian).

Compensation - The Kidney Cleanup Crew

The body employs two main strategies to counteract respiratory acidosis:

Acute (Cellular Buffering): An immediate, but limited, first line of defense.
- Intracellular proteins and hemoglobin bind $H⁺$.
- Formula: For every 10 mmHg ↑ in PCO₂, $HCO₃⁻$ ↑ by 1 mEq/L.
Chronic (Renal Compensation): A slower, more powerful response taking 3-5 days.
- The kidneys enhance the excretion of acid ($H⁺$, primarily as $NH₄⁺$) and increase the reabsorption of base ($HCO₃⁻$).
- Formula: For every 10 mmHg ↑ in PCO₂, $HCO₃⁻$ ↑ by 3-4 mEq/L.

⭐ In chronic respiratory acidosis, the resulting increase in serum bicarbonate is a direct compensatory action by the kidneys, not a separate metabolic alkalosis. Evaluating the anion gap can help differentiate complex disorders.

Renal Compensation for Respiratory Acidosis

Diagnosis - Cracking the ABG Code

Clinical Features: Headache, anxiety, blurred vision, & restlessness, which can progress to confusion, somnolence, asterixis, and coma.
Arterial Blood Gas (ABG): Key findings are a ↓ pH (< 7.35), ↑ PaCO₂ (> 45 mmHg), and a compensatory ↑ HCO₃⁻.
Compensation Check: Calculate the expected HCO₃⁻ to determine if a mixed disorder is present.

⭐ In chronic respiratory acidosis, for every 10 mmHg increase in PaCO₂, the HCO₃⁻ is expected to increase by 3.5-4 mEq/L. A value outside this range points to a mixed disorder.

High-Yield Points - ⚡ Biggest Takeaways

Respiratory acidosis is driven by hypoventilation, leading to CO₂ retention (↑ PaCO₂) and subsequent acidemia (↓ pH).

Key causes include CNS depression (e.g., opioids), airway obstruction (COPD), and neuromuscular disease (e.g., Guillain-Barré).

Acute compensation is minimal; for every 10 mmHg ↑ in PaCO₂, HCO₃⁻ rises by ~1 mEq/L.

Chronic compensation is renal; kidneys increase HCO₃⁻ reabsorption, raising it by ~4 mEq/L per 10 mmHg ↑ in PaCO₂.

Always look for a primary ↑ in PaCO₂ as the initiating event.

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