Description

Continuation of the COPD Case: Detailed Analysis of the Post-Extubation ABG

(2 Hours After Extubation on 2 L/min Oxygen)

Preoperative Summary of the Patient

The patient is a 54-year-old female with long-standing chronic obstructive pulmonary disease, likely a mixed emphysema–chronic bronchitis phenotype. Her baseline pulmonary physiology demonstrated:

• Chronic hypercapnia: PaCO₂ 47 mmHg, with metabolic compensation (HCO₃⁻ 28.5 mmol/L)

• Severe baseline hypoxemia: PaO₂ 52 mmHg, SaO₂ 86% on room air

• Elevated A–a gradient (~34 mmHg) indicating significant ventilation–perfusion mismatch

• Mild anemia (Hb 11.5 g/dL) but adequate compensatory oxygen extraction

• Increased functional residual capacity and high closing capacity, placing her at high risk of atelectasis during induction

• Prolonged expiratory time constants, making her susceptible to auto-PEEP under positive-pressure ventilation

• Sensitivity to high FiO₂, with theoretical risk of oxygen-induced hypercapnia

She underwent a laparoscopic anterior resection with hysterectomy, a surgery involving pneumoperitoneum, Trendelenburg positioning, and prolonged insufflation—all factors known to worsen pulmonary mechanics, increase PaCO₂, and challenge ventilation in COPD.

After an individualized, lung-protective ventilation strategy, she tolerated extubation well and was placed on 2 L/min oxygen via nasal cannula in the postoperative unit.

Two hours later, an arterial blood gas was obtained to evaluate post-extubation physiologic stability.

For preoperative details of this patient, click the link below

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Post-Extubation Arterial Blood Gas

(On 2 L/min Oxygen, 2 Hours After Extubation)

Measured Values

• pH: 7.36

• PaCO₂: 45 mmHg

• PaO₂: 150 mmHg

• Sodium: 137 mmol/L

• Potassium: 3.5 mmol/L

• Ionized calcium: 1.14 mmol/L

• Glucose: 206 mg/dL

• Lactate: 1.6 mmol/L

• Hematocrit: 42%

Derived Values

• Bicarbonate: 25.4 mmol/L

• Standard bicarbonate: 24.7 mmol/L

• Total CO₂: 26.8 mmol/L

• Base excess: 0 to –0.4

• Oxygen saturation: 99%

• Hemoglobin: 13.0 g/dL

1. Meaning of This ABG at 2 L/min Oxygen: Advanced Interpretation

This ABG must be interpreted in the context of supplemental oxygen, as the patient is breathing an FiO₂ of approximately 0.28–0.32 via nasal cannula.

This influences expected PaO₂ and the alveolar–arterial gradient.

Expected PaO₂ at FiO₂ ~0.30

Using the alveolar gas equation:

With FiO₂ 0.30 and RQ 0.8:

The patient’s measured PaO₂ is 150...