RCSI Bahrain Mechanics of Ventilation-Ⅰ PDF

RCSI Bahrain, Building No. 2441, Road 2835, Busaiteen 228, Kingdom of Bahrain Mechanics of Ventilation- Ⅰ Year Year 1 Course Respiratory Biology Module Code MED104 Lecturer Date Dr Patrick Walsh 30th April 2023 Learning Outcomes 1. Ability to define external and internal respiration 2. Und...

RCSI Bahrain, Building No. 2441, Road 2835, Busaiteen 228, Kingdom of Bahrain Mechanics of Ventilation- Ⅰ Year Year 1 Course Respiratory Biology Module Code MED104 Lecturer Date Dr Patrick Walsh 30th April 2023 Learning Outcomes 1. Ability to define external and internal respiration 2. Understand the basic anatomy relevant for the mechanics of ventilation 3. Describe muscle activities during breathing 4. Ability to describe the causal relationships between forces and pressures in pulmonary ventilation 5. Understand and define lung volumes and capacities and how these are affected in respiratory diseases 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 4 Steps of External Respiration LO: Ability to define external and internal respiration CELLULAR RESPIRATION • Intracellular metabolic process carried out in mitochondria which uses oxygen and produces carbon dioxide while deriving energy (ATP) from nutrient molecules • The nutrient molecules used are generally carbohydrates or fatty acids • This process is called aerobic respiration • (Anaerobic respiration when cells lack oxygen à final product of glycolysis, pyruvate, is converted to lactate) LO: Ability to define external and internal respiration BASIC ANATOMY Extrathoracic Intrathoracic LO: Understand the basic anatomy relevant for the mechanics of ventilation 300 million alveoli in lungs - Max lung volume approx. 4.2 litres (female) 5.7 litres (male) - Surface area for gas exchange approx. 80 sqm - Gas exchange driven by diffusion along concentration gradients (passive) The surface epithelial cells of the alveoli, or pneumocytes, are of two types. The type I pneumocytes form part of the barrier across which gas exchange occurs. They can be identified as thin, squamous cells whose most obvious feature is their nuclei. Type II pneumocytes are larger, cuboidal cells and occur more diffusely than type I cells. They appear foamier than type I cells because of they contain phospholipid multilamellar bodies, the precursor to pulmonary surfactant. LO: Understand the basic anatomy relevant for the mechanics of ventilation • Gas exchange driven by diffusion along concentration gradients (passive) LO: Understand the basic anatomy relevant for the mechanics of ventilation FORCES AND PRESSURES • Ventilation is driven by mechanical forces • Air is moved into and out of the lungs along pressure gradients • Need to understand what “pressure” is Gas pressure = Force that the gas exerts on the walls of its container LO: Ability to describe the causal relationships between forces and pressures in pulmonary ventilation BOYLE’S LAW: “For a fixed mass of enclosed gas at constant temperature, the product of the pressure (P) and volume (V) remains constant.” PV = k or P1V1 = P2V2 Robert Boyle (1627-1691) LO: Ability to describe the causal relationships between forces and pressures in pulmonary ventilation LO: Ability to describe the causal relationships between forces and pressures in pulmonary ventilation • Let’s assume a standard atmospheric pressure of 760 mmHg (=101.3 kPa) • If you relax, open your mouth, and don’t breathe, the pressure in your lung/alveoli = atmospheric pressure • A pressure differential exists only between pulmonary and pleural pressure • Pleural pressure < Pulmonary pressure LO: Ability to describe the causal relationships between forces and pressures in pulmonary ventilation • The pleural sac separates the lung from the thoracic wall • The pleural sac itself is closed/sealed off LO: Ability to describe the causal relationships between forces and pressures in pulmonary ventilation Pleural pressure < Pulmonary pressure • Why? – The lungs are stretched and under tension (pulling inwards) – The chest wall is under tension (pulling outwards) LO: Ability to describe the causal relationships between forces and pressures in pulmonary ventilation • Inspiration and Expiration are two conditions at which we leave the pressure equilibrium • Mechanical forces establish pressure differentials • As a consequence, air is drawn into or (passively) pushed out of the lungs LO: Ability to describe the causal relationships between forces and pressures in pulmonary ventilation EIGHT EVENTS OF INSPIRATION • • • • • • • • Inspiratory muscles contract Thoracic cage diameters increase Intrapleural pressure (PPL) becomes more negative Transmural pressure (PTM) increases and further distends alveoli Intra-alveolar pressure falls < atmospheric pressure Air flows down pressure gradient from atmosphere to alveoli Tidal volume (VT) of about 500 mls is added to resting volume or FRC At end of inspiration – no airflow and intra-alveolar pressure = atmospheric pressure. LO: Ability to describe the causal relationships between forces and pressures in pulmonary ventilation • Pressure profiles during inspiration and expiration LO: Ability to describe the causal relationships between forces and pressures in pulmonary ventilation • Inspiratory Muscles DIAPHRAGM PRIMARY EXTERNAL INTERCOSTALS STERNOMASTOID INSPIRATORY ACCESSORY SCALENE LARYNGEAL AIRWAY PHARYNGEAL GENIOGLOSSUS LO: Describe muscle activities during breathing PRIMARY MUSCLES • Diaphragm: dome shaped; flattens upon contraction and descends (1-10 cm); causing 75% of inspiration • External intercostals: Lift ribs upwards and outwards LO: Describe muscle activities during breathing ACCESSORY MUSCLES • Scalenes: Raise the first 2 ribs • Sternomastoid: Raise the sternum Used during exercise and respiratory disease LO: Describe muscle activities during breathing • Examples for respiratory diseases associated with the requirement of accessory muscle activity • Chronic bronchitis • Asthma • Emphysema à Chronic obstructive pulmonary diseases (COPD) • Bronchiolitis (e.g. respiratory syncytial virus (RSV) in infants) LO: Describe muscle activities during breathing AIRWAY MUSCLES • Laryngeal • Pharyngeal • Genioglossus • à Dilation of the airway à Reduced flow resistance • à Stabilisation of the airway (esp. pharynx) à Preventing collapse LO: Describe muscle activities during breathing • Obstructive sleep apnoea – decreased upper airway muscle activity during sleep – Pharynx can collapse due to the negative pressure in the airway during inspiration – à Patient cannot breath (apnoea) for periods of up to 1 minute in severe cases – Arousal from sleep activates the upper airway muscles and the airway opens – Cycle repeats itself during the night (hundreds of times in severe cases) LO: Describe muscle activities during breathing EXPIRATION • Passive Expiration – Passive process, when at rest – Relaxation of inspiratory muscles sufficient • Active Expiration – Contraction of abdominal muscles (push up diaphragm) – Contraction of internal intercostals When is active expiration required? Coughing, Sneezing, Exercise, Resp. disease, Screaming, Singing, Vomiting… LO: Describe muscle activities during breathing EXPIRATION LO: Describe muscle activities during breathing SUMMARY ANIMATIONS LO: Ability to describe the causal relationships between forces and pressures in pulmonary ventilation 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? FEV1 • Another important measure: – – – – – Forced expiratory volume in one second (FEV1) Typically expressed in % of the (forced) vital capacity ((F)VC) FVC = Max. inhalation followed by fast exhalation FEV1 determined in pulmonary function test FEV1/FVC = 80% for a healthy person FEV1 1 sec TLC 6 FEV 1 Litres RV 0 FVC Peak Flow Meter & Peak Expiratory Flow Meter • PEFR (maximum speed of expiration) – Used as a measure of airway resistance – Cheap and easy to use – Very useful in Asthma LUNG DISEASES • 1. Obstructive (Asthma, COPD) • 2. Restrictive (Lung fibrosis, scoliosis) • FEV1 and FEV1/FVC reduced with obstructive diseases e.g. asthma, COPD • Airflow rate (vol/time) reduced due to airway obstruction • Also RV higher in obstructive diseases • FEV1 decreased, but FEV1/FVC normal or increased in restrictive diseases e.g. lung fibrosis, scoliosis • Due to lungs being less compliant; airways free • RV normal FEV1 & RESPIRATORY DISEASE CATEGORY Obstructive Restrictive XX Obstructive Restrictive Normal Decreased Increased Normal (Forced) Vital Capacity Normal or Slightly Decreased Decreased FEV1 Decreased Decreased FEV1/FVC Decreased Normal Total Lung Capacity XX Residual Volume XX: cannot be measured by spirometry READING All basics are covered in: Sherwood – Human Physiology, 7th ed. Stanfield – Principles of Human Physiology, 4th ed. Berne & Levy 6th Ed ‘Physiology’ – Chapter 21 Guyton & Hall 11th Ed ‘ Medical Physiology’ Netter’s Essential Physiology Optional supplementary video Lectures – See 9 & 10 of https://meded.ucsd.edu/ifp/jwest/resp_phys/