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Disturbances of Respiratory Function

Chapter 296 | Part 7: Disorders of the Respiratory System · Part 7 – Respiratory Disorders · Chapter 296

Key Clinical Points

  1. The respiratory system functions to oxygenate blood and eliminate carbon dioxide via diffusion across the alveolar membrane.
  2. The system comprises three independent components: the lung (airways), the neuromuscular system, and the chest wall.
  3. Functional residual capacity (FRC) is the passive resting point where outward chest wall recoil balances inward lung recoil.
  4. Dynamic airflow limitation occurs because bronchial airways are collapsible; high flow velocity can lead to airway collapse via the Bernoulli effect.
  5. Arterial hypoxemia in V/Q heterogeneity is typically responsive to supplemental oxygen, whereas shunt-induced hypoxemia is refractory.
  6. Flow-volume loops distinguish pathologies: 'scooping' indicates obstruction, while reduced volume with normal shape indicates restriction.
  7. The Alveolar Gas Equation ($PAO_2 = FiO_2 imes (P_{bar} - PH_2O) - PACO_2/R$) determines oxygen tension based on inspired gas, pressure, and ventilation.
  8. Severe emphysema leads to dynamic hyperinflation (auto-PEEP), reducing compliance and increasing the work of breathing.
  9. Pulmonary function testing distinguishes restriction (low TLC, normal FEV_1/FVC) from obstruction (low FEV_1, low FEV_1/FVC, elevated RV).
  10. DLCO is a critical differentiator: it is reduced in fibrosis and emphysema but remains normal in asthma and obesity.

DEFINITION & OVERVIEW

• Primary Function: Oxygenate blood and eliminate carbon dioxide. • Mechanism: Requires virtual contact between blood and fresh air to facilitate diffusion across the alveolar membrane. • System Requirements: ◦ Must ventilate lung tidally. ◦ Must provide perfusion proportional to ventilation. ◦ Must allow gas diffusion. ◦ Must accommodate increased oxygen uptake or CO_2 elimination based on metabolic needs or acid-base status. • System Components: ◦ Lung (including airways). ◦ Neuromuscular system. ◦ Chest wall (all non-lung/non-muscle components, e.g., abdomen, heart). • Mechanical Properties: ◦ Volume-Related (Statics): Relate to elastic recoil. ◦ Flow-Related (Dynamics): Relate to airflow resistance and airway collapsibility.

ETIOLOGY & PATHOPHYYSOLOGY

• Gas Exchange Mechanisms: ◦ Diffusion: Oxygen uptake is typically perfusion-limited (limited by blood volume in capillaries) rather than diffusion speed. CO_2 equilibrates rapidly. ◦ V/Q Heterogeneity: Normal lungs have minor heterogeneity; disease widens the distribution of V/Q ratios. ◦ Shunt: Perfusion of nonventilated lung leads to hypoxemia that is refractory to supplemental oxygen. ◦ Dead Space: Anatomic (conducting airways) vs. Functional (areas with no blood flow, e.g., pulmonary embolism). • Alveolar Gas Equation: ◦ Formula: $PAO_2 = FiO_2 imes (P_{bar} - PH_2O) - PACO_2/R$ ◦ Key Variables: ◦ FiO_2: Fraction of inspired oxygen (0.21 for room air). ◦ P_{bar}: Barometric pressure (760 mmHg at sea level). ◦ PH_2O: Vapor pressure of water (47 mmHg at 37°C). ◦ PACO_2: Alveolar CO_2 tension (~40 mmHg). ◦ R: Respiratory quotient (~0.85 for typical American diet). • Ventilatory Restriction: ◦ Definition: Characterized by reduced Total Lung Capacity (TLC) and Forced Vital Capacity (FVC). ◦ Idiopathic Pulmonary Fibrosis: Increased lung recoil → lower TLC, FRC, RV, and FVC; markedly reduced DL_{CO}. ◦ Moderate Obesity: Blunted chest wall outward recoil → lower FRC; TLC usually unchanged; RV normal. ◦ Myasthenia Gravis: Muscle weakness → lower TLC and FRC; RV is elevated. FVC and FEV_1 are reduced as "innocent bystanders." • Airflow Obstruction: ◦ Definition: Characterized by reduced FEV_1 and FEV_1/FVC ratio. ◦ Acute Asthma: Luminal narrowing (muscle contraction, inflammation, thickening) → "scooping" on flow-volume loops; DL_{CO} normal or slightly elevated. ◦ Severe Emphysema: Loss of lung elastic recoil → pulmonary hyperinflation (high TLC, FRC, RV); DL_{CO} reduced due to loss of surface area/capillaries. ◦ Bernoulli Effect: High flow velocity → drop in intraluminal pressure → reduction in transmural pressure → dynamic airway collapse.

Pulmonary Function Abnormalities (Table 1)

• Ventilatory Restriction (Chest Wall): TLC 60%, FRC 65%, RV 100%, FVC 92%, FEV_1 60%, R_{aw} 1.0, DL_{CO} 95%. ◦ Ventilatory Restriction (Obesity): TLC 95%, FRC 60%, RV 100%, FVC 92%, FEV_1 60%, R_{aw} 1.0, DL_{CO} 95%. ◦ Ventilatory Restriction (Myasthenia Gravis): TLC 75%, FRC 100%, RV 120%, FVC 60%, FEV_1 60%, R_{aw} 1.0, DL_{CO} 80%. ◦ Airflow Obstruction (Asthma): TLC 100%, FRC 104%, RV 120%, FVC 90%, FEV_1 35% pre-b.d., R_{aw} 2.5, DL_{CO} 120%. ◦ Airflow Obstruction (Emphysema): TLC 130%, FRC 220%, RV 310%, FVC 60%, FEV_1 38% pre-b.d., R_{aw} 1.5, DL_{CO} 40%.

CLINICAL FEATURES

• Imaging Findings: ◦ Chest Radiograph: Identifies opacities, blunting of costophrenic angles, mass lesions, and volume loss. ◦ Ultrasound: Rapid diagnosis of pneumothorax, pleural effusion, and consolidation; useful in ARDS. ◦ CT Scan: Delineates parenchymal processes, pleural disease, masses, nodules, and large airways; contrast used for pulmonary vasculature/emboli. ◦ PET: Assesses metabolic activity to differentiate malignancy from scar. • Functional Measurements: ◦ Spirometry: Reveals relative volume changes during maneuvers (cannot determine absolute volumes). ◦ Lung Volumes: Determined by inert gas dilution or body plethysmography. ◦ Flow-Volume Loops: Visualize airflow patterns; normal is symmetric, obstruction shows "scooping," restriction shows reduced volume but normal shape.

DIFFERENTIAL DIAGNOSIS

• Restrictive vs. Obstructive: ◦ Restrictive: Low TLC, normal/high FEV_1/FVC (e.g., fibrosis, obesity, neuromuscular weakness). ◦ Obstructive: Low FEV_1, low FEV_1/FVC, elevated RV (e.g., asthma, emphysema, chronic bronchitis). • Shunt vs. V/Q Heterogeneity: ◦ Shunt: Results in hypoxemia that is refractory to supplemental oxygen. ◦ V/Q Heterogeneity: Results in hypoxemia that is responsive to supplemental oxygen.

DIAGNOSTIC APPROACH

  1. Initial Evaluation: Chest ultrasound or plain chest radiograph (posterior-anterior and lateral).
  2. Further Imaging: CT scan with contrast for parenchymal detail and pulmonary emboli; PET for metabolic activity.
  3. Pulmonary Function Testing: Spirometry and lung volume measurement (inert gas dilution or body plethysmography).
  4. Blood Testing: Assessment for hypercoagulable states, serologic testing (infectious/rheumatologic), inflammatory markers, eosinophils, genetic testing.
  5. Advanced Procedures: Bronchoscopy (BAL, biopsy) or surgical lung biopsy if needed.

MANAGEMENT & TREATMENT

  1. Asthma Management: ◦ Acute: Short-acting beta-agonists (SABA) or muscarinic agonists. ◦ Chronic: Longer-acting beta-agonists (LABA), muscarinic antagonists (LAMA), inhaled corticosteroids (ICS), and biologic immunotherapies.
  2. Obesity Management: Weight loss to improve chest wall mechanics.
  3. Neuromuscular Weakness: Respiratory muscle support.

PROGNOSIS & COMPLICATIONS

• Emphysema: May lead to hypoxemia during exercise. ◦ Pulmonary Fibrosis: May lead to severe hypoxemia. ◦ Obesity: May lead to pulmonary arterial hypertension if sleep apnea is present.

SPECIAL CONSIDERATIONS

• Obesity: Moderate obesity blunts chest wall outward recoil → lower FRC; massive obesity may reduce TLC. Sleep apnea may be concurrent. ◦ Neuromuscular Disease: Myasthenia gravis reduces muscle strength → lower TLC and FRC.

KEY PEARLS & CLINICAL TRAPS

• FRC Definition: The passive resting point where chest wall recoil balances lung recoil. ◦ Dynamic Hyperinflation (auto-PEEP): Occurs when expirary flows are insufficient to allow complete exhalation before the next breath; reduces compliance and increases work of breathing. ◦ Oxygen Response: Shunt hypoxemia is refractory to supplemental oxygen; V/Q heterogeneity hypoxemia is responsive. ◦ Flow-Volume Loops: "Scooping" indicates airflow obstruction. ◦ DLCO Differentiation: DL_{CO} is reduced in fibrosis and emphysema but remains normal in asthma and obesity (unless sleep apnea/PAH present). ◦ Bernoulli Effect: High flow velocity → lower intraluminal pressure → reduction in transmural pressure → airway collapse.