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

What is the primary mechanism for maintaining acid-base balance during prolonged vomiting?

Increased bicarbonate excretion

Increased chloride reabsorption

A 22-year-old female college student presents to the emergency department due to severe pain in her stomach after an evening of heavy drinking with her friends. The pain is located in the upper half of the abdomen, is severe in intensity, and has an acute onset. She claims to have consumed a dozen alcoholic drinks. Her past medical history is unremarkable. She has recently completed an extremely low-calorie diet which resulted in her losing 10 kg (22 lb) of body weight. Her pulse is 130/min, respirations are 26/min, and blood pressure is 130/86 mm Hg. Examination reveals a visibly distressed young female with periumbilical tenderness. Her BMI is 23 kg/m2. Laboratory tests show: Arterial blood gas analysis pH 7.54 Po2 100 mm Hg Pco2 23 mm Hg HCO3- 22 mEq/L Serum Sodium 140 mEq/L Potassium 3.9 mEq/L Chloride 100 mEq/L Which of the following most likely caused her elevated pH?

Anxiety-induced hyperventilation

Renal failure-induced electrolyte imbalance

Weight loss-induced electrolyte imbalance

Alcohol-induced respiratory depression

A 25-year-old man is in the middle of an ascent up a mountain, at an elevation of about 4,500 meters. This is the 4th day of his expedition. His friend notices that in the last few hours, he has been coughing frequently and appears to be short of breath. He has used his albuterol inhaler twice in the past 4 hours, but it does not seem to help. Within the past hour, he has coughed up some frothy, slightly pink sputum and is now complaining of nausea and headache. Other than his asthma, which has been well-controlled on a steroid inhaler, he is healthy. Which of the following is the most likely cause of this man’s symptoms?

Non-cardiogenic pulmonary edema

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 27-year-old man presents to the emergency department with his family because of abdominal pain, excessive urination, and drowsiness since the day before. He has had type 1 diabetes mellitus for 2 years. He ran out of insulin 2 days ago. The vital signs at admission include: temperature 36.8°C (98.2°F), blood pressure 102/69 mm Hg, and pulse 121/min. On physical examination, he is lethargic and his breathing is rapid and deep. There is a mild generalized abdominal tenderness without rebound tenderness or guarding. His serum glucose is 480 mg/dL. Arterial blood gas of this patient will most likely show which of the following?

↓ pH, ↓ bicarbonate and ↑ anion gap

↑ pH, ↑ bicarbonate, and normal pCO2

↑ pH, normal bicarbonate and ↓ pCO2

↓ pH, ↓ bicarbonate and normal anion gap

↓ pH, normal bicarbonate and ↑ pCO2

A 57-year-old woman comes to the emergency department because of dizziness, nausea, and vomiting for 4 days. Her temperature is 37.3°C (99.1°F), pulse is 100/min, respirations are 20/min, and blood pressure is 110/70 mm Hg. Physical examination shows no abnormalities. Arterial blood gas analysis on room air shows: pH 7.58 PCO2 43 mm Hg PO2 96 mm Hg HCO3- 32 mEq/L The most appropriate next step in diagnosis is measurement of which of the following?

Urine albumin to creatinine ratio

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

Respiratory Alkalosis - The Big Gasp

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Pathophysiology - How We Blow It Off

Primary Defect: Excessive elimination of $CO_2$ from the lungs due to alveolar hyperventilation, causing hypocapnia.
Mechanism: A drop in arterial $PCO_2$ (< 35 mmHg) shifts the carbonic acid-bicarbonate buffer equation to the left: $H^+ + HCO_3^- \leftarrow H_2CO_3 \leftarrow CO_2 + H_2O$. This consumes free $H^+$ ions, directly raising the blood pH (> 7.45).

Compensation (Metabolic): Kidneys respond by decreasing $H^+$ secretion and reducing $HCO_3^-$ reabsorption, effectively excreting bicarbonate. This is a slow process, taking 2-3 days to maximize.

⭐ Clinical Pearl: Alkalosis increases albumin's binding to calcium, causing a drop in ionized calcium. This can lead to perioral numbness, paresthesias, and even carpopedal spasm, mimicking hypocalcemia.

Respiratory Alkalosis: Causes, Effects, and Compensation

Compensation - Kidneys to the Rescue

Renal compensation is a slow process, taking 2-3 days to manifest. The goal is to decrease serum bicarbonate to correct the pH.

Primary Mechanism: Triggered by low pCO₂, leading to intracellular alkalosis in renal tubule cells.
- Reduced activity of carbonic anhydrase.
- Decreased H⁺ secretion into the tubular lumen.
- Results in ↓ HCO₃⁻ reabsorption and ↑ HCO₃⁻ excretion.
- Urine becomes alkaline (pH > 6.0).

Renal bicarbonate reabsorption and titratable acid formation

Expected Change:
- Acute: For every 10 mmHg ↓ in pCO₂, [HCO₃⁻] ↓ by 2 mEq/L.
- Chronic: For every 10 mmHg ↓ in pCO₂, [HCO₃⁻] ↓ by 4-5 mEq/L.

⭐ In a fully compensated chronic state, for every 10 mmHg ↓ in pCO₂, the [HCO₃⁻] will decrease by approximately 4-5 mEq/L. This demonstrates appropriate renal compensation.

Clinical Features & Dx - Spotting the Signs

Key symptoms stem from cerebral vasoconstriction (neurologic signs) & hypocalcemia (↓ ionized $Ca^{2+}$, neuromuscular irritability):
- Neurologic: Lightheadedness, dizziness, confusion, syncope, due to ↓ cerebral blood flow.
- Neuromuscular: Perioral/digital paresthesias, muscle cramps, carpopedal spasm (Trousseau's), facial twitching (Chvostek's), tetany.
- Cardiovascular: Tachycardia, palpitations, arrhythmias.
Diagnostic cornerstone is Arterial Blood Gas (ABG):
- Primary finding: ↑ pH > 7.45 with a primary ↓ PaCO₂ < 35 mmHg.
- Check serum electrolytes & anion gap to narrow the differential diagnosis.

⭐ Alkalosis increases albumin's affinity for calcium, causing a drop in its ionized form. This leads to neuromuscular excitability (tetany, Chvostek's sign) even when total serum calcium is normal.

Trousseau's and Chvostek's signs of hypocalcemia

High‑Yield Points - ⚡ Biggest Takeaways

Respiratory alkalosis is driven by hyperventilation, causing a primary ↓ in PaCO₂.

Key triggers include hypoxemia (e.g., high altitude, PE), anxiety/panic attacks, and salicylate toxicity.

Acute compensation is minimal, relying on intracellular H⁺ buffering.

Chronic renal compensation (2-3 days) involves ↓ H⁺ excretion and ↑ HCO₃⁻ excretion, markedly lowering serum HCO₃⁻.

Clinical signs like tetany and paresthesias result from decreased ionized calcium.

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