RCSI Work of Ventilation - I Past Paper May 2023

RCSI Bahrain, Building No. 2441, Road 2835, Busaiteen 228, Kingdom of Bahrain Work of Ventilation - Ⅰ Year Year 1 Course Respiratory module Code MED104 Lecturer Date Dr Patrick Walsh 2nd May 2023 Learning Outcomes 1. Understand and describe Volume/Pressure loops 2. Describe contributions t...

RCSI Bahrain, Building No. 2441, Road 2835, Busaiteen 228, Kingdom of Bahrain Work of Ventilation - Ⅰ Year Year 1 Course Respiratory module Code MED104 Lecturer Date Dr Patrick Walsh 2nd May 2023 Learning Outcomes 1. Understand and describe Volume/Pressure loops 2. Describe contributions to the elastic work of breathing 3. Define compliance work and explain the contribution of tissue elasticity and surface tension 4. Explain the role of surfactant in compliance work and the biochemistry of surfactant 5. Understand how respiratory diseases can affect the elastic work of breathing Work of Breathing • Definitions – Work required to move the lung & chest wall (W = P x ΔV) – Physics definition of work – work is done on an object when energy is transferred to that object • In normal ‘quiet’ breathing, inspiratory muscles do all the work as expiration is passive – Elastic work represent about 70% total work – Non-elastic work represent about 30% total work Is Breathing Hard Work? • Oxygen cost of breathing in healthy individuals is low – Respiratory muscles consume only 2-5% of total oxygen consumption for minute volumes of about 50 L/min • Increases with higher minute volumes – Severe exercise • Increases in disease (breathing demands more energy) – Limits exercise tolerance – Results in breathlessness or dyspnoea Optimising Work • Minute volume or minute ventilation (L/min) = Respiratory rate per minute x tidal volume • Any given minute ventilation can be achieved by either – a high tidal volume & lower rate – a lower tidal volume but higher rate • The higher the tidal volume, the greater the elastic work • The higher the rate, the higher the flow & resistive (nonelastic) work There is an optimum value of respiratory rate (15/min) to minimise work There is an optimum value of tidal volume (500ml) to minimise work Resistive Elastic TIDAL VOLUME Disease alters the optimal respiratory rate/tidal volume for minimising work Work of Breathing Compliance (Elastic) Work: 1. Force to expand lung against its elastic properties (Elastin Fibres, Alveolar Surface Tension) Frictional/Resistive Work: 2. Force to overcome air-flow resistance (force to move air through airways) Definition of Compliance • Compliance is a measure of the ease with which the lungs can be stretched or inflated. CL = ΔV / ΔP ΔV : change in lung volume ΔP: change in transmural pressure Normal adult Lung compliance 0.1-0.4 L/cmH2O (Chest wall compliance CT = 0.2 L/cmH2O) • Total compliance of the respiratory system is less than that of the compliance of the lung or the chest wall alone – When acting together in vivo more force is needed to produce a given volume change Pressure – Volume relationship and compliance Graphical representation of relationship between volume change (ΔV) and pressure change (ΔP) during quiet breathing à Hysteresis Pressure – Volume relationship and compliance B A Steepness between points A and B represent a measure for compliance Area in the loop = Work of breathing Disease Alters Compliance • Increased compliance – Loss of elastin fibres/elastic tissue in early emphysema or ageing • Decreased compliance – Chest wall compliance: scoliosis, ankylosing spondylitis – Pulmonary fibrosis COMPLIANCE AND ELASTANCE • Compliance describes distensibillity of system – How easily an object stretches • Elastance is inversely related to compliance – Measure of "snap back” or elastic recoil force • Emphysema --> increased compliance, decreased elastance • Fibrosis --> decreased compliance, increased elastance ELASTIC WORK HAS TWO MAIN CONTRIBUTORS • Tissue elasticity – Energy (ATP) is required to deform elastic tissues (stretch elastin fibers and overcome surface tension) – This work is then stored as potential energy (recovered during passive expiration) • Surface tension (“air-water” interface in the lungs) – Water molecules are more attracted to each other than to air, creating a surface tension. – Surface tension contributes to minimising the surface area of alveoli LO: Define compliance work and explain the contribution of tissue elasticity and surface tension Surface Tension can be alleviated by surfactant • Surface tension of water would lead to alveolar/lung collapse • High surface tension à low compliance • Water surface tension reduced by pulmonary surfactant LO: Explain the role of surfactant in compliance work and biochemistry of surfactant What is Surfactant? • Complex mixture of proteins and lipids – 10% surfactant specific proteins (SPA, SPB, SPC, SPD) – 90% lipids including phospholipids mainly DPPC (Disaturated palmitoyl phosphatidylcholine) • Synthesis and storage – Synthesised by type II pneumocytes (alveolar cells) between 2232 weeks gestation – Stored in cytoplasmic lamellar bodies until released to surface of alveolus and made available at air-liquid interface Function of Surfactant? - Reduces surface tension by interfering with water molecule interactions - à increases compliance of the lung - Important role also in stabilising alveoli of different sizes This is not trivial! Role of surfactant in stabilising alveoli of different sizes - Assume a scenario of communicating alveoli in absence of surfactant: • Laplace’s Law: Relates pressure to (surface) tension and radius Pressure = 2T/r • • à small radius = High Pressure Without surfactant small alveoli would empty into bigger alveoli and collapse • In presence of surfactant, surface tension is no longer a constant! • Surface tension in small alveoli lower than in large alveoli • Small and large alveoli can co-exist Surface Tension Lowering Effect of Surfactant • Surfactant molecules position at air-liquid interface with hydrophobic fatty acid chains projecting into alveolar air and hydrophyllic end into the fluid lining of the alveolus. • Surface tension lowering effect is greatest as alveoli become smaller in expiration as concentration of surfactant increases at air-liquid interface. • Surfactant differentially reduces surface tension in alveoli, more at lower volumes and less at higher volumes leading to alveolar stability and co-existence of large and small alveoli Copyright Dr Kevin McGuigan, Dr Markus Rehm, RCSI, 2005, 2010 Newborn Respiratory Distress Syndrome • Previously known as hyaline membrane disease • Developing foetal lungs do not normally synthesise surfactant until late in pregnacy. • Therefore, premature infants may not have enough pulmonary surfactant and struggle to breathe. • Treatment: – Stimulated by corticosteroids given to mother prior to delivery of premature infant – Oxygen through continuous positive airway pressure – Survanta (surfactant) Summary of Surfactant Functions • Lowers surface tension of fluid lining alveoli – Increasing compliance and reducing work of breathing – Preventing collapse at low lung volumes (atelectasis) • Allows small and large alveoli to co-exist – Influencing regional distribution of ventilation and relationship of ventilation to perfusion • Contributes to defence mechanisms in the lung – Enhancing macrophage activity (SPA & SPD) Disease Alters Compliance • Increased compliance – Loss of elastin fibres/elastic tissue in early emphysema or ageing • Decreased compliance – Chest wall compliance: scoliosis, ankylosing spondylitis – Pulmonary fibrosis • See figures 14.10 A & B in Nettler’s Essential Physiology Alveoli Interdependence has an additional stabilising function • When an alveolus in a group of interconnected alveoli begin to collapse, the surrounding alveoli are stretched. • Neighbouring alveoli recoil in response to being stretched, pulling outwards on the collapsing alveoli and keeping it open. Copyright Dr Kevin McGuigan, Dr Markus Rehm, RCSI, 2005, 2010 EXAMPLE MCQ • Q. A baby born prematurely may have difficulty in breathing due to a deficiency in surfactant. What cells produce surfactant? – – – – – A. Alveolar macrophages B. Endothelial cells C. Mast cells D. Type I pneumocytes E. Type II pneumocytes EXAMPLE MCQ According to the Law of Laplace, air should flow from the smaller alveoli to the larger, collapsing them. In the lungs, several factors counter that tendency, and stabilize the alveolar structures. Which of the following is NOT one of them? -- A. Surfactant lowers surface tension to a greater degree when it is on a smaller surface area, allowing the smaller alveoli to stay open. -- B. Mechanical stability is given by surrounding alveoli. -- C. Transpulmonary pressure is lower for smaller alveoli, allowing them to stabilise in comparison to the bigger ones. -- D. Surface tension at the gas-liquid interface increases as alveolar surface area increases. QUESTIONS RELATING TO LECTURES ON MECHANICAL VENTILATION RESPIRATORY GASES COMPOSITION ROOM / EXHALED AIR / ALVEOLAR kPa Atmospheric Air Exhaled Air Alveolar Air pO2 21 15.9 14.7 pCO2 0 4.2 5.3