Description

1. Introduction

Chronic Obstructive Pulmonary Disease (COPD) presents several physiological, mechanical, and gas-exchange challenges during anesthesia. When such a patient undergoes a laparoscopic anterior resection with hysterectomy, the combination of CO₂ pneumoperitoneum, Trendelenburg positioning, long surgical duration, and general anesthesiamagnifies baseline respiratory limitations.

A careful analysis of the patient’s room-air arterial blood gas (ABG) provides a vital window into her pulmonary reserve, ventilatory control, acid–base status, and expected perioperative risks. The ABG must not be treated merely as a laboratory value but as a physiological map guiding ventilation strategies, anesthetic drug choices, airway planning, and postoperative care.

This chapter rewrites and expands the clinical article into a comprehensive, 4000–7000-word, basic-science anchored textbook resource relevant to anesthesia trainees and practicing anesthesiologists.

2. The Patient and the ABG: A Physiological Window

2.1 Patient Data

Age: 54 years
Diagnosis: COPD (likely mixed phenotype)
Procedure: Laparoscopic anterior resection + hysterectomy
Setting: General anesthesia
Ventilatory status: Spontaneously breathing preoperatively on room air

2.2 Measured Arterial Blood Gas (Room Air)

• pH: 7.39

• PaCO₂: 47 mmHg

• PaO₂: 52 mmHg

• HCO₃⁻: 28.5 mmol/L

• SaO₂: 86%

• Na⁺: 136 mmol/L

• K⁺: 3.5 mmol/L

• Lactate: 0.7 mmol/L

• Hb: 11.5 g/dL

• Hct: 37%

This ABG gives three critical insights:

(1) Chronic Hypercapnic Physiology

Elevated PaCO₂ (47 mmHg) with normal pH and elevated bicarbonate indicates chronic CO₂ retention.
This suggests:

• Long-standing alveolar hypoventilation

• Renal metabolic compensation

• Increased bicarbonate reabsorption and H⁺ secretion (slow process: 3–5 days)

The kidney’s role can be expressed using the Henderson–Hasselbalch equation:
pH = 6.1 + log ([HCO₃⁻] / (0.03 × PaCO₂))

Her pH at 7.39 fits chronic respiratory acidosis physiology perfectly.

(2) Severe Hypoxemia (PaO₂ = 52 mmHg, SaO₂ = 86%)

Using the alveolar gas equation:
PAO₂ = FiO₂ (713) – (PaCO₂ / RQ)

On room air (FiO₂ = 0.21, RQ ≈ 0.8):
PAO₂ ≈ 0.21 × 713 – (47/0.8) ≈ 86 mmHg

Therefore A–a gradient = 86 – 52 = 34 mmHg (elevated).

This is diagnostic of V/Q mismatch, the hallmark of COPD.

Note:

Why “713 mmHg” Appears in the Alveolar Gas Equation

Think of it like this:

• The atmosphere gives us 760 millimetres of mercury.

• The humidifier inside your airway steals 47 millimetres of mercury (water vapor pressure at body temperature).

• What is left for oxygen and nitrogen to share is 713 millimetres of mercury.

So:

760−47=713 mmHg760−47=713 mmHg

This 713 is the effective dry gas pressure used in the alveolar gas equation.

(3) Oxygen Content is Compromised (because Hb = 11.5 g/dL)

Using the oxygen content equation:
CaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂)
CaO₂ ≈ (1.34...