RCSI Work of Ventilation - 1 2023

RCSI Bahrain, Building No. 2441, Road 2835, Busaiteen 228, Kingdom of Bahrain Work of Ventilation - Ⅱ Year Year 1 Course Respiratory module Code MED104 Lecturer Date Dr Patrick Walsh 3rd May 2023 LEARNING OUTCOMES 1. Define non-elastic work of breathing, and explain how airway diameter and...

RCSI Bahrain, Building No. 2441, Road 2835, Busaiteen 228, Kingdom of Bahrain Work of Ventilation - Ⅱ Year Year 1 Course Respiratory module Code MED104 Lecturer Date Dr Patrick Walsh 3rd May 2023 LEARNING OUTCOMES 1. Define non-elastic work of breathing, and explain how airway diameter and flow patterns contribute to this 2. Describe determinants of airway resistance and their relevance in different regions of the airways 3. Understand how respiratory diseases can affect the non-elastic work of breathing Work of Breathing Compliance (Elastic) Work: 1. Force to expand lung against its elastic properties (Elastin Fibres, Alveolar Surface Tension) Frictional/Resistive Work (Non-elastic): 2. Force to overcome viscosity resistance 3. Force to overcome air-flow resistance (force to move air through airways) LO: Define non-elastic work of breathing Non-Elastic Work • Resistance to air-flow – Airway resistance is the impedance of air flow through the tracheobronchial tree as a result of friction of gas molecules – Is present during both inspiration and expiration – Very important contribution to the work of breathing LO: Define non-elastic work of breathing Airway Resistance At rest, airway resistance to flow is low – Normal pressure difference between mouth and alveoli is approximately 1 cmH2O which is sufficient to give normal resting tidal volume of approximately 500 ml – Increased resistance requires greater pressure gradient to drive airflow and result in the same tidal volume of approximately 500 ml LO: Define non-elastic work of breathing Airway Resistance • Two main components determine the airway resistance – Diameter of the airway (cross sectional area) and distance air has to travel • • • • Lung volume Bronchial smooth muscle tone Thickness of mucous lining and submucosa Mucus layer – Flow type/pattern (laminar vs. turbulent flow) LO: Define non-elastic work of breathing AIRWAY RESISTANCE AND AIRWAY DIAMETER à We have to understand how airway diameter is linked to flow resistance • Hagen-Poiseuille Law • Relates flow rate (volume transported per time unit) to airway radius and to distance the air is transported • Applies to a scenario of laminar flow in a straight circular tube. Flow rate (V/t) = ΔP π r4 8ηl ΔP = Driving Pressure; r = Radius; η = viscosity; l = Length • Note the strong influence of the airway radius!!! LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Airway resistance and Airway diameter • Using the Hagen-Poiseuille Law we can also express airway resistance: • Resistance defined as the ratio between the driving pressure and the associated flow rate. Resistance (R) = ΔP = 8ηl V/t πr4 LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Airway resistance and Airway diameter Resistance (R) = ΔP = 8ηl V/t πr4 Resistance is low when a given driving pressure results in a high flow rate • Again, note the influence of the airway radius! • If airway radius reduced to 50%, resistance increases 16-fold! LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Airway resistance and Airway diameter • If airway radius reduced to 50%, resistance increases 16-fold! • Would this not suggest a high resistance in the brionchioles when compared to the trachea? LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Airway Resistance and Cross Sectional Area of Bronchial Tree An individual bronchiole obviously has a far smaller radius than the trachea. However, the large number of bronchioles means that overall the airway widens Therefore, resistance in trachea is higher than in the bronchioles LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Airway Resistance is also affected by Lung Volume Is airway resistance greater at high (inspiration) or low (expiration) lung volume? LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Increasing Lung Volumes Increase Airway Diameter • Therefore – Resistance to air flow decreases with increasing lung volume (inspiration) à airway expanded – Resistance to air flow increases with decreasing lung volume (exhalation) à airway contracts/is compressed LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Airway Resistance • Two main components determining the airway resistance – Diameter of the airway (cross sectional area) and distance air has to travel • • • • Lung volume Bronchial smooth muscle tone Thickness of mucous lining and submucosa Mucus layer But let’s stay with this for a little longer – Flow type/pattern (laminar vs. turbulent flow) LO: Define non-elastic work of breathing Determinants of Airway Diameter • Trachea and main bronchi – Protected against collapse by rings of cartilage (complete, incomplete) • Small bronchi, bronchioles normally offer little resistance but are significant in disease as they are subject to influence by physical, neural and humoral factors – – – – No supporting cartilage Pull of surrounding tissue (radial traction) Innervated smooth muscle Small diameter easily obstructed by external/internal factors LO: Describe determinants of airway resistance and their relevance in different regions of the airways Determinants of Airway Diameter • Outside the airway – Radial traction of elastic tissue – Pressure from lymph nodes • In the wall – Smooth muscle tone – Thickness of mucosa/submucosa • In the lumen – Mucus LO: Describe determinants of airway resistance and their relevance in different regions of the airways Extrinsic Neural & Hormonal Control of Smooth Muscle Tone • Circulating catecholamines cause bronchodilation via β2receptors • Peripheral nervous system causes bronchoconstriction via acetylcholine release and muscarinic receptors (blocked by atropine) • Non-adrenergic non-cholinergic (NANC) is a neurotransmitter that can release dilators (nitric oxide) or constrictors (neurokinin A) LO: Describe determinants of airway resistance and their relevance in different regions of the airways Intrinsic Control of Smooth Muscle Tone by Chemical Mediators • Mast cell degranulation (release of histamine) and inflammatory mediators cause bronchoconstriction • CO2 exerts direct effect on smooth muscle – When raised, bronchodilation – When lowered, bronchoconstriction LO: Describe determinants of airway resistance and their relevance in different regions of the airways Factors Influencing Secretions • Secretions of seromucous glands and goblet cells that line respiratory system – 5-10 μm thick, inner layer more watery to allow ciliary action and upper layer more viscid to trap particles • These secretions are controlled by parasympathetic nervous system reflexes and local chemical stimulation – Decreased by atropine – Increased in bronchitis LO: Describe determinants of airway resistance and their relevance in different regions of the airways Airway Resistance • Two main components determining the airway resistance – Diameter of the airway (cross sectional area) and distance air has to travel • • • • Lung volume Bronchial smooth muscle tone Thickness of mucous lining and submucosa Mucus layer – Flow type/pattern (laminar vs. turbulent flow) LO: Define non-elastic work of breathing Airway Resistance & Pattern of Air Flow • Pattern of airflow linked to velocity • We differentiate between LAMINAR and TURBULENT flow • Laminar flow is a streamline flow (smooth) • Well ordered (not chaotic) LO: Define non-elastic work of breathing Airway Resistance & Pattern of Air Flow • Pattern of airflow linked to velocity • We differentiate between LAMINAR and TURBULENT flow • Turbulent flow is chaotic, not streamline/smooth • Turbulent flow is very inefficient and consumes energy • à Turbulences increase air flow resistance LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Airway Resistance & Pattern of Air Flow LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Reynolds Number and factors determining laminar vs. turbulent flow • Reynolds Number: Dimensionless number to determine whether airflow is turbulent or laminar. • In straight tubes, expressed as: R = 2rvd η • r = Radius; v = Average velocity; d = Fluid density; η = Fluid viscosity • In straight tubes, turbulence occurs if the Reynold’s number is greater than approx. 2000. • High velocities and large diameters promote turbulences LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Turbulences and Airway Resistance • High velocities and large diameters promote turbulences • à e.g. Turbulences in upper airways during exercise • Empty Nose Syndrome/paradoxical obstruction • No turbulences in bronchioles, since velocities low and diameters small LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing Sites of Airway Resistance • Greatest resistance in the large airways, less in the small airways in normal individuals • ½ total airway resistance in the nose, pharynx and larynx • Below larynx – 80% RAW in trachea and main bronchi • Less than 20% RAW in bronchioles <2 mm diameter LO: Explain how airway diameter and flow patterns contribute to non-elastic work of breathing ASTHMA • Respiratory disease characterised by lung inflammation • Treat with bronchodilators and corticosteroids • Bronchodilators relax bronchial smooth muscle and widen airways • Corticosteroids reduce inflammation and reduce secretions LO: Understand how respiratory diseases can affect the non-elastic work of breathing Chronic Obstructive Pulmonary Disease • Chronic Bronchitis – Excess production of mucus which results in cough and production of sputum – Treat by cessation of smoking, antibiotics (if bacterial), bronchodilators • Emphysema – Loss of elastic tissue due to uncontrolled action of proteolytic enzymes – Lung elastase is normally inhibited by antiproteases i.e. α1–anti-trypsin for which 1 in 4000 have genetic deficiency – Treat by cessation of smoking, avoiding irritants, lung transplant LO: Understand how respiratory diseases can affect the non-elastic work of breathing Pulmonary Fibrosis • • • • Formation of excess fibrous connective tissue in the lung. Restrictive lung disease, reduces compliance Causes: – Idiopathic – Inhalation of pollutants (coal dust, asbestos) – Certain medications (bleomycin) – Disease (Sarcoidosis) Treatment – Immune supressive agents (corticosteroids) – Oxygen supplementation (improves QOL) – Lung transplantation LO: Understand how respiratory diseases can affect the non-elastic work of breathing Classification of Respiratory Disease into Obstructive or Restrictive • Obstructive respiratory diseases (COPD, Asthma) – – – – Interfere with the movement of air through airways Increases flow-resistive work No effect on elastic work Decreases FEV1 but not FVC (or only slightly) (-> ratio decreased) • Restrictive respiratory diseases (Pulmonary Fibrosis) – Interfere with ability to expand the lungs (-> compliance is decreased) – Increases elastic work – No effect on flow-resistive work – Decreases FEV1 and FVC (-> ratio remains normal) LO: Understand how respiratory diseases can affect the non-elastic work of breathing Effect of Disease on Lung Compliance & Work of Breathing LO: Understand how respiratory diseases can affect the elastic / non-elastic work of breathing Revision Question • Q. What kind of respiratory disease is pulmonary fibrosis and how does it affect lung compliance, elastic work and flow-resistance work? – – – – – Pulmonary fibrosis is a restrictive respiratory disease FEV1 is decreased and FEV1/FVC is normal or increased It decreases compliance It increases elastic work It has no effect on flow-resistance work • Extra: Treatment – Immune supressive agents (corticosteroids) – Oxygen supplementation (improves QOL) – Lung transplantation Revision Question • Q. What kind of respiratory disease is chronic bronchitis and how does it affect lung compliance, elastic work and flow-resistance work? WORK OF VENTILATION 2 Reading Sherwood – Human Physiology, 7th ed. – Chapter 13 Berne & Levy 7th Ed ‘Physiology’ – Chapter 22 REVISION BODY PLETHYSMOGRAPHY • • • Patients sits in a “body box” (airtight chamber) and breathes through a mouthpiece At FRC, the mouthpiece is closed Patient tries to breathe in • Consequences: - Chest and lungs expand, pressure in lungs drop as volume increases - Expanding chest compresses the air inside the chamber, air volume in chamber decreases, pressure in chamber increases • FRC can be calculated using Boyle’s law https://www.youtube.com/watch?v=OcvIrz1N7-g LO: Understand and describe various methods for lung function and volume measurements Assumption: Pressure in lungs and box identical at time zero Air in box compressed, Pressure in Box increases P1 à P2 2 Closed Systems: Sealed box + Lungs – mouth - valve Lungs expand, Pressure in Lungs drops P1 à P3 The basic physical principle exploited by body plethysmography is the law of Boyle-Mariotte. For a fixed amount of gas in a closed compartment, the relative changes in the compartment’s volume are always equal in magnitude but opposite in sign to the relative changes in pressure. Thus, one can infer relative volume changes from pressure changes. LO: Understand and describe various methods for lung function and volume measurements BODY PLETHYSMOGRAPHY • • • Change in lung volume: FRC à FRC + ∆V Change of air volume in box: V1 à V1 - ∆V ∆V in the box can be determined from the measured change in pressure à P1 x V1 = P2 x (V1 - ∆V) [Boyle’s Law] • • • Knowing ∆V, then allows to calculate FRC P1 x FRC = P3 x (FRC + ∆V) [Boyle’s Law] FRC = P3 x ∆V/(P1 – P3) • • • P1 = Initial pressure in box and airways P2 = Increased pressure in box P3 = Decreased airway/alveolar pressure LO: Understand and describe various methods for lung function and volume measurements LUNG VOLUMES AND CAPACITIES • • • • Readings obtained using a Spirometer (operating principle) Reading = Spirogram Capacity = Sum of volumes The Spirometer can NOT measure RV, FRC, TLC – why? POLL 1 The exchange of gases between blood and tissue cells is called 1. 2. 3. 4. 5. pulmonary ventilation internal respiration external respiration cellular respiration anaerobic respiration BASIC DEFINITIONS • ‘Respiration’ refers to two integrated processes: – External Respiration: • Exchange of oxygen and carbon dioxide between the body and the external environment – Internal, or cellular, respiration: • Uptake, utilisation of oxygen by cells and release of carbon dioxide LO: Ability to define external and internal respiration