PHSI N303F Cardiopulmonary Physiotherapy

Cardiopulmonary Physiotherapy - Review of Respiratory Physiology I

• Course: PHSI N303F Cardiopulmonary Physiotherapy
• Topic: Review of Respiratory Physiology I: Mechanics of Breathing
• Lecturer: Ms Eva Chan, Senior Lecturer, Department of Physiotherapy, HKMU
• Date: 3 Sept 2024

Learning Objectives

• Recognize the properties of pulmonary structures and their functions
• Illustrate factors contributing to airflow resistance
• Describe inspiration and expiration, explaining pressure changes during the respiratory cycle
• Explain regional differences in lung volume
• Apply breathing mechanics to pulmonary physiotherapy practice, including airflow assessment, decision-making regarding interventions (positioning, breathing exercises, secretion clearance)

Revision on Breathing

• A video demonstrating pulmonary ventilation (breathing) is available.

Properties of the Lung

• Include: airway resistance, collateral ventilation, compliance, elastic recoil, surfactant

• Key terms: respiratory mechanics; clinical implications for physiotherapists

Airway Resistance

• Resistance of the respiratory tract to airflow during inhalation and exhalation
• Calibre of the airways is inversely proportional to the 4th power of the radius (resistance is 16 times greater if radius is halved).
• Narrowed airways increase resistance
• Factors affecting calibre: bronchial smooth muscle (parasympathetic causes constriction, sympathetic causes dilation), secretion, mucosal oedema, tumour/mass, inhaled foreign body, loss of radial traction.

Bronchial Smooth Muscle

• Balanced activity between parasympathetic nervous system (bronchoconstriction) and sympathetic nervous system (bronchodilation)

Collateral Ventilation

• Airflow through interalveolar pores (Kohn), bronchiole-alveolar canals (Lambert), and interbronchial channels (Martin).
• Microscopic pathways for collateral air distribution.

Clinical Importance

• Gas can bypass obstructed airways through collateral pathways.
• Increasing lung volume aids in recruiting collateral ventilation and re-expanding collapsed alveoli.
• This helps propel sputum more proximally.
• Interventions include deep breathing with holds and secretion removal.

Compliance

• Distensibility (flexibility) of an elastic structure
• Measured as change in pulmonary volume over change in pressure
• Less compliant (stiff) at extremes of lung volume.

Elastic Recoil of the Lung

• Elastic fibers of elastin and collagen at alveolar walls and around vessels and bronchi help with lung recoil.
• Recoil helps to maintain lung shape and return to resting volume after inspiration.

Pulmonary Surfactant

• Mixture of phospholipids and apoproteins

• Synthesized and secreted by type II alveolar cells between 23 and 24 weeks gestation.

• Reduces surface tension of fluid lining alveoli, preventing their collapse.

• Clinical significance: insufficient surfactant reduces compliance leading to alveolar atelectasis and risk of pulmonary edema; neonatal Respiratory Distress Syndrome (RDS) is common in premature infants due to insufficient surfactant.

Reduced Compliance

• High pressure required to increase volume, due to: increased fibrous tissue (pulmonary fibrosis/alveolar edema), lung not being ventilated for long (atelectasis), reduced pulmonary surfactant.

Increased Compliance

• Pulmonary emphysema, aging,

Extrapulmonary Structures - Reduced Chest Wall Compliance

• Thoracic deformity, raised intra-abdominal pressure

Work of Breathing

• Work is done to move the lung and chest wall to overcome elastic and resistive forces in the airways, lungs, and chest wall.
• Approximately 80% of the work of breathing is elastic resistance.
• Approximately 20% of the work of breathing is airflow resistance.

How Do Patients Present Clinically If There Is Airflow Limitation?

• Increased work of breathing
• Shortness of Breath (SOB)
• Wheezing
• Lowered oxygen saturation (SpO2)

How Does Ventilation Happen?

• Ventilation involves a sequence of volume change -> pressure change -> gas flow.

Human Lung Model

• A model demonstrating the structure of the lungs is referenced.

Boyle's Law

• States that pressure and volume of a gas have an inverse relationship (P1V1 = P2V2).

Pulmonary Ventilation

• Movement of air into and out of the lungs.

Understanding the Terms

• Atmospheric pressure: pressure exerted by gases in the air
• Intrapulmonary pressure: pressure within the alveoli
• Intrapleural pressure: pressure within the pleural cavity
• Transpulmonary pressure: difference between intrapulmonary and intrapleural pressures

Clinical Significance–Pneumothorax

• Presence of air in the pleural space due to abnormal communication between the lung and pleural space or the atmosphere.
• Leads to lung collapse.
• Intervention: insertion of drains to restore negative pressure.

Forced Expiration Technique - Huffing

• Active Cycle Breathing Technique (ACBT) component
• Clearing peripheral airway secretions using mid to low lung volumes
• Clearing upper airway secretions with high lung volumes using coughing technique

Closing Volume

• Inspiration falls below a lower inflection point on the pressure/volume curve when closing volume is reached
• Large pressure needed to open airways/alveoli
• Closing volume increases with age, smoking, lung disease, and position (supine > upright).

Positioning

• Enhance lung volume, improve ventilation/perfusion matching, and achieve secretion drainage by manipulating patient position.

Forced Expiration at Mid to Low Lung Volumes

• Equal pressure point (EPP) shifts distally into smaller peripheral airways as lung volumes decrease.

Using More Effort

• Dynamic compression of airways due to increased effort for expiration.

Measurement of Expiratory Flow Rate

• Spirometric measurements like FEV1, FVC (forced vital capacity) are used.
• Differentiate between restrictive and obstructive lung diseases.

Positions of Equal Pressure Points

• Forced expiration at volumes above functional residual capacity (FRC) or at decreasing capacities involving lower volumes in the smaller segmental and peripheral airways.

Pre-inspiration

• Negative intrapleural pressure results from the chest wall tendency to pull outward versus the lungs tendency to pull inward.