Case Study: ABG and PFT Interpretation in Emphysema

Patient Presentation

A 67-year-old male with a 40-pack-year smoking history presents with progressive shortness of breath, chronic cough, and unintentional weight loss. He reports difficulty climbing a flight of stairs and increased use of accessory muscles when breathing. On exam, he has diminished breath sounds, a barrel chest, and pursed-lip breathing.

Arterial Blood Gas (ABG) Results

• pH: 7.32
• PaCO₂: 58 mmHg
• HCO₃⁻: 30 mEq/L
• PaO₂: 55 mmHg
• SaO₂: 86% on room air

Stepwise Interpretation (Compensatory Rules)

1. Assess pH: 7.32 → Acidemia.
2. Assess PaCO₂: Elevated at 58 → respiratory component.
3. Assess HCO₃⁻: Elevated at 30 (above normal 22–26) → metabolic ompensation.
4. Primary disorder: Respiratory acidosis.
5. Compensation check (chronic respiratory acidosis rule — barrel chest implies chronic):

• Expected ↑ HCO₃⁻ = 3.5 mEq/L for every 10 mmHg ↑ PaCO₂ above 40.
• PaCO₂ rise = 58–40 = 18 → expected HCO₃⁻ ↑ ≈ 6.3 → HCO₃⁻ ≈ 30.3.
• Patient’s HCO₃⁻ = 30 → compensation appropriate → chronic respiratory acidosis.
• Interpretation: partialy compensated chronic respiratory acidosis with hypoxemia.

Pulmonary Function Test (PFT) Results

Interpretation

• Obstructive pattern: FEV₁ ↓, FEV₁/FVC < 0.70.
• Severity: FEV₁ < 50% predicted → severe obstruction.
• Hyperinflation & air trapping: Elevated TLC and RV.
• Gas exchange abnormality: Severely reduced DLCO, consistent with emphysema.

Integrated Diagnosis

• Chronic partially (pH is not normal) compensated respiratory acidosis due to chronic hypercapnia.
• Severe obstructive lung disease with emphysematous changes (hyperinflation, air trapping, ↓ DLCO).
• Findings are consistent with GOLD stage III COPD (emphysema-predominant).

Teaching Points

1. Chronic respiratory acidosis shows near-normal pH with elevated HCO₃⁻ due to renal compensation.
2. Compensation rules help differentiate acute vs chronic disturbances. See this article for rules: https://medium.com/@tranquilityhealth/list/abg-and-acidbase-42ab9c1ed522
3. In emphysema, PFTs reveal obstruction, hyperinflation, and reduced DLCO, reflecting alveolar destruction.
4. ABG and PFTs together provide a functional and gas-exchange profile, guiding treatment (e.g., oxygen, pulmonary rehab, smoking cessation, possible non-invasive ventilation).

Glossary of Terms and Acronyms

ABG (Arterial Blood Gas)

A blood test measuring pH, PaCO₂, PaO₂, HCO₃⁻, and SaO₂ to evaluate acid-base balance and gas exchange.

pH

Represents the hydrogen ion concentration of blood.

• Normal: 7.35–7.45
• <7.35 = acidemia
• 7.45 = alkalemia

PaCO₂ (Partial Pressure of Carbon Dioxide)

Reflects adequacy of alveolar ventilation.

• Normal: 35–45 mmHg
• ↑ PaCO₂ = hypoventilation → respiratory acidosis
• ↓ PaCO₂ = hyperventilation → respiratory alkalosis

HCO₃⁻ (Bicarbonate)

A base regulated by the kidneys, reflects metabolic component of acid-base balance.

• Normal: 22–26 mEq/L
• ↑ HCO₃⁻ = metabolic alkalosis or compensation for chronic respiratory acidosis
• ↓ HCO₃⁻ = metabolic acidosis

PaO₂ (Partial Pressure of Oxygen)

Measures dissolved oxygen in arterial blood.

• Normal: 80–100 mmHg

SaO₂ (Oxygen Saturation)

Percentage of hemoglobin bound to oxygen.

• Normal: 95–100%

Compensation (ABG Rule)

Physiologic attempt to restore normal pH.

• Respiratory compensation: rapid, by lungs.
• Metabolic (renal) compensation: slower, by kidneys.

PFT (Pulmonary Function Test)

Tests that measure lung mechanics, airflow, lung volumes, and gas exchange.

FEV₁ (Forced Expiratory Volume in 1 Second)

Volume of air exhaled in the first second of a forced breath.

• ↓ FEV₁ = obstruction severity.

FVC (Forced Vital Capacity)

Total volume of air forcibly exhaled after maximal inhalation.

FEV₁/FVC Ratio

Key to differentiating obstruction vs restriction.

• <0.70 = obstruction
• Normal/high ratio with ↓ volumes = restriction

TLC (Total Lung Capacity)

Maximum volume of air lungs can hold.

• ↑ TLC = hyperinflation (seen in emphysema, asthma)
• ↓ TLC = restriction (seen in fibrosis)

RV (Residual Volume)

Volume of air left after maximal exhalation.

• ↑ RV = air trapping (seen in obstructive diseases).

DLCO (Diffusing Capacity for Carbon Monoxide)

Measures gas exchange efficiency across alveolar-capillary membrane.

• ↓ DLCO = emphysema, interstitial lung disease, pulmonary hypertension.

COPD (Chronic Obstructive Pulmonary Disease)

A group of progressive lung diseases (emphysema, chronic bronchitis, asthma, bronchiectasis, cystic fibrosis) characterized by airflow obstruction and impaired gas exchange.

GOLD Staging (Global Initiative for Obstructive Lung Disease)

Standard classification for COPD severity, based on FEV₁ % predicted.

References

1. Global Initiative for Chronic Obstructive Lung Disease (GOLD). Global Strategy for the Diagnosis, Management, and Prevention of COPD. 2024 Report.
2. Celli BR, Wedzicha JA. Update on clinical aspects of chronic obstructive pulmonary disease. N Engl J Med. 2019;381(13):1257–1266. doi:10.1056/NEJMra1900500
3. MacIntyre N, Crapo RO, Viegi G, et al. Standardisation of the single-breath determination of carbon monoxide uptake in the lung. Eur Respir J. 2022;59(2):2102430. doi:10.1183/13993003.02430–2021
4. Bhatt SP, et al. Clinical and physiological predictors of chronic hypercapnia in COPD. Chest. 2020;158(4):1256–1266. doi:10.1016/j.chest.2020.03.043
5. Stanojevic S, Kaminsky DA, Miller MR, et al. ERS/ATS technical standard on interpretive strategies for routine lung function tests. Eur Respir J. 2022;60(1):2101499. doi:10.1183/13993003.01499–2021