Respiratory Disorders

Introduction

Introduction to Respiratory Disorders: Pathophysiology and Physiology

Respiratory disorders encompass a broad spectrum of conditions that impair gas exchange, ventilation, or lung mechanics. These disorders arise from structural, inflammatory, infectious, or neoplastic processes affecting the airways, alveoli, or pulmonary vasculature. Understanding the pathophysiology requires integration of pulmonary physiology, including ventilation-perfusion matching, diffusion capacity, and respiratory control mechanisms. This topic explores the foundational principles underlying common respiratory disorders and their impact on lung function.

Scope of Respiratory Pathophysiology

Respiratory pathophysiology examines how diseases disrupt normal lung function, leading to hypoxemia, hypercapnia, or respiratory failure. Key areas include obstructive diseases (e.g., asthma, COPD), restrictive diseases (e.g., pulmonary fibrosis), vascular disorders (e.g., pulmonary hypertension), and infectious processes (e.g., pneumonia). Each disorder alters specific physiological parameters, such as airway resistance, lung compliance, or diffusion capacity, which are critical for diagnosing and managing these conditions.

Study

Ventilation-Perfusion Mismatch in Respiratory Disorders

Ventilation-perfusion (V/Q) mismatch is a central mechanism of hypoxemia in respiratory disorders. In normal lungs, ventilation and perfusion are closely matched to optimize gas exchange. Obstructive diseases, such as chronic bronchitis, reduce ventilation in affected lung regions, creating low V/Q units. Conversely, pulmonary embolism obstructs blood flow, producing high V/Q units or dead space. Shunt physiology occurs when blood bypasses ventilated alveoli, as seen in atelectasis or severe pneumonia, leading to refractory hypoxemia. Understanding V/Q mismatch is essential for interpreting arterial blood gases and selecting appropriate therapeutic interventions.

Airway Resistance and Obstructive Lung Diseases

Obstructive lung diseases, including asthma and chronic obstructive pulmonary disease (COPD), are characterized by increased airway resistance due to inflammation, mucus hypersecretion, or structural remodeling. In asthma, bronchoconstriction and airway hyperresponsiveness result from mast cell degranulation and eosinophilic inflammation. COPD involves chronic bronchitis and emphysema, where airway narrowing and loss of elastic recoil impair expiratory flow. These changes increase the work of breathing and lead to air trapping, hyperinflation, and dynamic compression of airways during expiration. Spirometry is the primary tool for assessing airway obstruction, with a reduced FEV1/FVC ratio serving as a diagnostic hallmark.

Lung Compliance and Restrictive Lung Diseases

Restrictive lung diseases, such as idiopathic pulmonary fibrosis (IPF) and sarcoidosis, are defined by reduced lung compliance, which impairs lung expansion and decreases total lung capacity. Fibrosis of the lung parenchyma increases stiffness, requiring greater transpulmonary pressure to achieve normal tidal volumes. This leads to rapid, shallow breathing and increased work of breathing. Diffusion capacity is also impaired due to thickening of the alveolar-capillary membrane, resulting in hypoxemia, particularly during exercise. Pulmonary function tests reveal a reduced FVC and normal or elevated FEV1/FVC ratio, distinguishing restrictive from obstructive patterns.

Pulmonary Hypertension: Pathophysiology and Hemodynamic Consequences

Pulmonary hypertension (PH) is a progressive disorder characterized by elevated pulmonary arterial pressure, leading to right ventricular strain and eventual cor pulmonale. PH can result from chronic hypoxemia (e.g., COPD, interstitial lung disease), pulmonary vascular remodeling, or left heart disease. Hypoxic vasoconstriction, endothelial dysfunction, and smooth muscle proliferation contribute to increased pulmonary vascular resistance. Over time, the right ventricle hypertrophies and dilates, impairing cardiac output. Clinical manifestations include dyspnea, fatigue, and syncope, with diagnosis confirmed by right heart catheterization showing a mean pulmonary arterial pressure ≥25 mmHg at rest.

Diffusion Impairment and Alveolar-Capillary Membrane Disorders

Diffusion impairment occurs when the alveolar-capillary membrane thickens or its surface area decreases, limiting oxygen transfer. Conditions such as pulmonary edema, interstitial lung disease, and emphysema disrupt the membrane's integrity. In pulmonary edema, fluid accumulation in the interstitium and alveoli increases the diffusion distance for oxygen. Emphysema reduces the surface area for gas exchange by destroying alveolar septa. Diffusion capacity of the lung for carbon monoxide (DLCO) is a key test for assessing diffusion impairment, with reduced values indicating membrane dysfunction or loss of alveolar units.

Summary

Key Takeaways

Respiratory disorders disrupt normal lung physiology through mechanisms such as V/Q mismatch, increased airway resistance, reduced lung compliance, pulmonary hypertension, and diffusion impairment. Obstructive diseases primarily affect airflow, while restrictive diseases impair lung expansion. Pulmonary vascular disorders lead to right heart strain, and diffusion abnormalities result in hypoxemia. Recognizing these pathophysiological changes is critical for accurate diagnosis, interpretation of pulmonary function tests, and targeted therapeutic strategies.

Clinical Correlate: Arterial Blood Gas Interpretation

Arterial blood gas (ABG) analysis is a cornerstone of evaluating respiratory disorders. Hypoxemia with a normal or low PaCO2 suggests V/Q mismatch or diffusion impairment, while hypoxemia with elevated PaCO2 indicates hypoventilation or severe V/Q mismatch. In COPD, chronic hypercapnia and hypoxemia lead to compensatory metabolic alkalosis. In acute respiratory distress syndrome (ARDS), severe hypoxemia refractory to oxygen therapy reflects shunt physiology. ABG results guide oxygen therapy, ventilatory support, and monitoring of disease progression.

Therapeutic Implications

Treatment of respiratory disorders targets underlying pathophysiological mechanisms. Bronchodilators and anti-inflammatory agents reduce airway resistance in asthma and COPD. Oxygen therapy corrects hypoxemia in V/Q mismatch, while positive pressure ventilation supports ventilation in restrictive diseases. Pulmonary vasodilators, such as prostacyclin analogs, are used in pulmonary hypertension to reduce afterload on the right ventricle. Lung transplantation may be considered for end-stage restrictive or obstructive diseases. Understanding pathophysiology ensures rational selection of therapies to improve patient outcomes.