Cardiopulmonary Physiotherapy - V/Q Review

Cardiopulmonary Physiotherapy - Review of Respiratory Physiology II: Ventilation and Perfusion

• The lecture covers respiratory physiology, specifically ventilation and perfusion.
• Learning objectives include defining the ventilation/perfusion (V/Q) ratio, explaining regional differences in ventilation and perfusion of the lungs, illustrating V/Q matching and mismatch, describing regulatory mechanisms responding to disturbed V/Q ratios, applying the concept of ventilation and perfusion to clinical decision-making for adults and pediatric patients with respiratory dysfunction, and understanding the different aspects of V/Q ratio.
• Gas exchange is crucial and happens at the alveoli level. Oxygen and carbon dioxide diffuse across the respiratory membrane. Key factors are the partial pressures of oxygen and carbon dioxide in the alveoli and pulmonary capillaries.
• Ventilation is the movement of air in and out of the lungs. Minute ventilation (VE) is the volume of air that enters or leaves the lungs per minute. Tidal volume (VT) is the volume of air inhaled or exhaled in a single breath. Respiratory rate (f) is the number of breaths per minute.
• The formula for minute ventilation (VE) is: VE = f x VT
• Typical values for adults at rest are f = 12 breaths/minute and VT = 500 mL. Children's values vary from 3 to 5 mL/kg.
• Lung volumes and capacities include inspiratory reserve volume (IRV), tidal volume (VT), expiratory reserve volume (ERV), residual volume (RV), vital capacity (VC), functional residual capacity (FRC), and total lung capacity (TLC).
• Anatomical dead space is the air in the conducting passages that does not reach the alveoli. Alveolar dead space is air reaching alveoli but not participating in gas exchange due to insufficient blood supply. Physiological dead space is the combined anatomical and alveolar dead space.
• Alveolar ventilation is the amount of minute ventilation reaching the respiratory zone used for gas exchange.
• The formula for alveolar ventilation is: Alveolar ventilation = (Tidal volume – anatomical dead space) x Respiratory rate
• Hyperventilation and hypoventilation are deviations from normal ventilation.
• Hypoventilation results in poor CO2 excretion, leading to respiratory acidosis. Causes include respiratory depression, respiratory muscle weakness, infection, consolidation, or atelectasis, and COPD.
• Hyperventilation leads to excessive CO2 excretion and respiratory alkalosis. Possible causes include anxiety, metabolic disease (like hyperthyroidism), and high altitude.
• The V/Q ratio describes the ratio of ventilation to perfusion. Normal value in healthy people is 0.8 representing 4 liters of ventilation per minute, and 5 liters of perfusion.
• V/Q matching is effective ventilation matched with blood supply from pulmonary capillaries.
• V/Q mismatch is an imbalance between ventilation and perfusion. This occurs when ventilation or perfusion is inadequate or uneven in different parts of the lungs.
• Consequences of V/Q mismatch include inadequate oxygenation of blood and CO2 removal.
• Hypoxic pulmonary vasoconstriction (HPV) is an automatic response to low oxygen, to redirect blood flow to areas of the lung with good ventilation.
• The mechanism is controlled by smooth muscle cells in pulmonary arterioles and is sensitive to alveolar PO2.
• Autoregulating homeostatic mechanisms in the lungs adjust pulmonary arterioles and bronchioles to match ventilation and perfusion. Hypoxia is a key stimulus.
• Clinical implications of ventilation-perfusion imbalances in patient care, including positioning for adults and infants, are crucial for optimal treatment outcomes. COPD patients necessitate special care.