# Clinical Cases: Pulmonary Gas Exchange

## Case 1: Carbon Monoxide Poisoning

### Clinical Image
![Pathophysiology of pulmonary embolism](case_01_image.jpg)
*Source: [Wikimedia Commons - Pulmonary Embolism](https://commons.wikimedia.org/wiki/File:Pulmonary_embolism.png) - Public Domain*

### Case Presentation
A 32-year-old man is brought to the emergency department by paramedics after being found unresponsive in his garage with his car engine running. His wife called 911 when he did not answer his phone for several hours. On arrival, he is confused and complaining of severe headache. Vital signs show heart rate 110 bpm, blood pressure 145/88 mmHg, respiratory rate 24/min, and pulse oximetry reading 98% on room air. Physical examination reveals cherry-red skin coloration and no focal neurological deficits.

Arterial blood gas on room air shows pH 7.32, PaCO2 32 mmHg, PaO2 95 mmHg, calculated SaO2 97%. However, co-oximetry reveals carboxyhemoglobin (COHb) level of 35% (normal <3%). The calculated A-a gradient is normal at 12 mmHg.

This case illustrates a critical limitation of standard pulse oximetry and ABG analysis. The patient's PaO2 is normal because dissolved oxygen in plasma is unaffected by carbon monoxide. However, CO binds hemoglobin with 200-250 times greater affinity than oxygen, forming carboxyhemoglobin that cannot carry oxygen. Additionally, CO shifts the oxygen-hemoglobin dissociation curve leftward, impairing oxygen release to tissues. The pulse oximeter reads falsely normal because it cannot distinguish oxyhemoglobin from carboxyhemoglobin.

The patient is immediately placed on 100% oxygen via non-rebreather mask to accelerate CO displacement from hemoglobin (reducing half-life from 4-5 hours on room air to 60-90 minutes). Given his altered mental status, he is transferred for hyperbaric oxygen therapy, which further reduces CO half-life to 20-30 minutes and increases dissolved oxygen delivery. His symptoms resolve over 24 hours, and follow-up COHb levels normalize.

### Key Learning Points
- Carbon monoxide binds hemoglobin with 200-250 times greater affinity than oxygen, forming carboxyhemoglobin
- CO causes a leftward shift of the oxygen-hemoglobin dissociation curve, further impairing tissue oxygen delivery
- Standard pulse oximetry and ABG-calculated oxygen saturation are falsely normal in CO poisoning; co-oximetry is required
- PaO2 reflects dissolved oxygen only and may be normal despite severely reduced oxygen content
- Oxygen content equation: CaO2 = (1.34 x Hgb x SaO2) + (0.003 x PaO2) - CO reduces the first term dramatically

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## Case 2: High Altitude Pulmonary Edema

### Case Presentation
A 28-year-old previously healthy man presents to a mountain clinic at 4,200 meters (13,800 feet) in the Himalayas. He ascended rapidly from sea level over 3 days. He reports progressive dyspnea at rest for the past 12 hours, dry cough, and fatigue. He notes his symptoms worsened significantly overnight. On examination, he appears dyspneic and mildly cyanotic. Vital signs show respiratory rate 32/min, heart rate 120 bpm, blood pressure 100/70 mmHg, temperature 37.8C, and oxygen saturation 72% on room air. Lung examination reveals bilateral crackles. He has no peripheral edema.

The alveolar gas equation explains his hypoxemia. At 4,200 meters, barometric pressure is approximately 450 mmHg. Alveolar PO2 = FiO2 x (Patm - PH2O) - (PaCO2/RQ) = 0.21 x (450 - 47) - (30/0.8) = 47 mmHg. This profound alveolar hypoxia triggers hypoxic pulmonary vasoconstriction (HPV), which is normally a protective mechanism to optimize V/Q matching. However, in susceptible individuals, exaggerated HPV leads to uneven pulmonary vasoconstriction, regional overperfusion of areas with less vasoconstriction, capillary stress failure, and high-altitude pulmonary edema (HAPE).

Arterial blood gas shows pH 7.48, PaCO2 30 mmHg, PaO2 38 mmHg. The A-a gradient is elevated (47 - 38 = 9 mmHg, which is significant at altitude where the normal gradient is smaller). Chest X-ray shows patchy bilateral alveolar infiltrates.

The patient is treated with supplemental oxygen (improving saturation to 92%), nifedipine (to reduce pulmonary artery pressure), and immediate descent to lower altitude. Within 24 hours of descent to 2,500 meters, his symptoms resolve and chest X-ray normalizes.

### Key Learning Points
- The alveolar gas equation shows how reduced barometric pressure at altitude decreases PAO2
- Hypoxic pulmonary vasoconstriction is normally protective but can cause HAPE in susceptible individuals
- HAPE is a form of non-cardiogenic pulmonary edema from pulmonary hypertension and capillary leak
- The A-a gradient is elevated in HAPE due to V/Q mismatch and shunt from alveolar flooding
- Treatment includes supplemental oxygen, descent, and pulmonary vasodilators (nifedipine)

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## Case 3: Methemoglobinemia from Dapsone

### Case Presentation
A 55-year-old woman with Pneumocystis jirovecii pneumonia prophylaxis on dapsone presents with progressive dyspnea, fatigue, and cyanosis over 3 days. Her HIV viral load is undetectable on antiretroviral therapy, and her CD4 count is 180 cells/microliter. On examination, she has central cyanosis with brownish discoloration of her nail beds, lips, and mucous membranes. Vital signs show respiratory rate 22/min, heart rate 100 bpm, blood pressure 118/72 mmHg, and pulse oximetry reading 85% that does not improve with supplemental oxygen.

The arterial blood gas on 100% FiO2 shows pH 7.38, PaCO2 38 mmHg, PaO2 380 mmHg, calculated SaO2 99%. This striking discrepancy between pulse oximetry (85%) and calculated saturation (99%) with an elevated PaO2 is characteristic of methemoglobinemia. Co-oximetry confirms methemoglobin level of 28% (normal <1%).

Methemoglobin forms when hemoglobin iron is oxidized from the ferrous (Fe2+) to ferric (Fe3+) state, which cannot bind oxygen. Dapsone is a known oxidizing agent that can cause methemoglobinemia. Like carboxyhemoglobin, methemoglobin also shifts the oxygen-hemoglobin dissociation curve leftward, impairing oxygen release from the remaining functional hemoglobin. Pulse oximetry characteristically reads approximately 85% regardless of true saturation because methemoglobin absorbs light equally at both wavelengths used by pulse oximeters.

The patient is treated with intravenous methylene blue 1 mg/kg, which acts as an electron donor to reduce methemoglobin back to functional hemoglobin. Within 30 minutes, her cyanosis resolves and pulse oximetry improves to 98%. Dapsone is discontinued and alternative PCP prophylaxis is prescribed. G6PD deficiency is ruled out, as methylene blue is contraindicated in G6PD-deficient patients.

### Key Learning Points
- Methemoglobin contains oxidized iron (Fe3+) that cannot bind oxygen
- Methemoglobin shifts the O2-Hgb dissociation curve leftward, impairing O2 unloading to tissues
- Pulse oximetry characteristically reads approximately 85% in methemoglobinemia, regardless of true saturation
- Co-oximetry is required for diagnosis; PaO2 is typically normal or elevated
- Methylene blue is the treatment of choice (contraindicated in G6PD deficiency)