RCSI Control of Ventilation 2021 PDF

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Control of Ventilation Class Course Code Title Lecturer Email Date Year 1 Cardiorespiratory Biology CB 42 Control of Ventilation Marc Sturrock [email protected] November 22, 2021 LEARNING OUTCOMES • Describe the re...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Control of Ventilation Class Course Code Title Lecturer Email Date Year 1 Cardiorespiratory Biology CB 42 Control of Ventilation Marc Sturrock [email protected] November 22, 2021 LEARNING OUTCOMES • Describe the respiratory control centres and pathways to the respiratory muscles • Describe the role of the receptors in the lungs and airways • Describe the role of the peripheral and central chemoreceptors in the control of breathing • Describe the general causes of respiratory disorders and acid-base balance INTRODUCTION Normally, the nervous system adjusts the rate of alveolar ventilation to the demands of the body so that arterial blood PO2 and PCO2 are hardly altered even during heavy exercise and most other types of respiratory stress. Ventilation is controlled by the respiratory centre in the brain stem (pons and medulla). RESPIRATORY CONTROL CENTRES & PATHWAYS TO RESPIRATORY MUSCLES Central Controller (pons, medulla & other brain regions) Input Sensors Output Effectors Chemoreceptors Respiratory muscles Lung receptors Other receptors LO: Describe the respiratory control centres and pathways to the respiratory muscles RESPIRATORY CONTROL CENTRES MEDULLARY RESPIRATORY CENTRE • Medullary respiratory centre consists of two clusters of neurons called the dorsal respiratory group and the ventral respiratory group LO: Describe the respiratory control centres and pathways to the respiratory muscles DORSAL RESPIRATORY GROUP (DRG) • Inspiratory neurons • Located in dorsomedial medulla • DRG inspiratory neurons fire inducing muscle contraction and therefore inspiration LO: Describe the respiratory control centres and pathways to the respiratory muscles DORSAL RESPIRATORY GROUP (DRG) & PATHWAYS TO THE RESPIRATORY MUSCLES VENTRAL RESPIRATORY GROUP (VRG) • Both inspiratory and expiratory neurons • Both sets remain inactive during quiet breathing • Utilised when demand for ventilation is increased beyond normal (active expiration) LO: Describe the respiratory control centres and pathways to the respiratory muscles GENERATION OF RESPIRATORY RHYTHM • Neurons in this (medullary) region display pacemaker like activity (i.e. on/off circuit) • Under normal oxygen conditions this region can generate two types of breathing rhythms • 1. normal breathing (fast, low amplitude) • 2. Sighs (slow, large) LO: Describe the respiratory control centres and pathways to the respiratory muscles THE RHYTHM GENERATED IN THE MEDULLA CAN BE MODIFIED BY NEURONS IN THE PONS • Pneumotaxic centre sends signals to the DRG that help silence/inhibit the inspiratory neurons (this mechanism controls the length of each breath) • Apneustic centre conversely stimulates the inspiratory neurons of the dorsal and ventral groups and therefore prevents the inspiratory neurons from being switched off NEURONAL OSCILLATORY CIRCUIT Basic breathing rhythm generated by a neuronal oscillatory (i.e. on/off) circuit in inspiratory area • Inspiratory circuit fires 2 seconds → nerve impulse → diaphragm and external intercostal muscles contract → inspiration • Inspiratory circuit dormant 3 seconds → expiratory area dormant → expiration by default, becomes active in forceful/active breathing • 12 breaths/min quiet breathing, • But, during exercise expiratory area activated. Also, Pneumotaxic area continuously transmits impulses to inspiratory area which have a negative effect and turn off inspiratory signal [prevents over-inflation of lungs]: – Strong impulse = 0.5 sec inspiration → secondary effect to increase breathing to 3040 breaths/min; – Weak impulse = 5 - 7 sec inspiration. • Hering - Breuer Reflex: stretch receptors in lung tissue, bronchi, bronchioles may on activation transmit inhibitory signal via the vagus nerve (X) to the inspiratory area; prevents over-inflation of the lung General causes of respiratory disorders • Any loss of function in any part of this system can cause hypoventilation (=ventilation at an abnormally low rate leading to increased PCO2 in arterial blood) and hypoxaemia (=PO2 less than normal in arterial blood) • Loss of function could be due to – depressant drugs on the respiratory centres – cord injury – damage to motor nerves (trauma, neurological disease) – muscle diseases e.g. dystrophies – dysfunction of the respiratory apparatus i.e. restriction and obstruction Role of the receptors in the lungs and airways The most important inputs to the respiratory centres are from the airways and lungs and from chemoreceptors Stretch receptors Airways and lungs Irritant receptors J receptors Peripheral Chemoreceptors Central STRETCH RECEPTORS • There are nerve endings in the airway smooth muscle which are stimulated by stretch during inspiration • the nerve impulses travel in the vagus nerve to inhibit the inspiratory centre • this is called the Hering-Breuer inflation reflex (- see slide 12) IRRITANT RECEPTORS • there are nerve endings near the airway epithelial cells which are stimulated by noxious gases, cigarette smoke, dust and cold air • the nerve impulses travel in the vagus nerve causing reflex bronchoconstriction or coughing • may be involved in asthma attacks J RECEPTORS • there are nerve endings near the capillaries in the alveolar walls called juxtacapillary receptors which are stimulated by pulmonary congestion and oedema • the nerve impulses travel in the vagus nerve causing reflex apnoea or rapid shallow breathing • may be involved in the rapid shallow breathing and dyspnoea of pulmonary congestion and oedema CHEMORECEPTORS • Specialised cells that respond to changes in the chemical composition of the blood or other fluid • Central Chemoreceptors • Peripheral Chemoreceptors LO: Describe the role of the peripheral and central chemoreceptors in the control of breathing PERIPHERAL CHEMORECEPTORS • Located near the heart in the carotid and aortic bodies • The carotid bodies are located at the bifurcation of the common carotid arteries • The aortic bodies are located above and below the aortic arch Carotid bodies Aortic bodies • Information from the peripheral chemoreceptors is carried via the vagus and glossopharyngeal nerves to the dorsal respiratory group • Region is well supplied with arterial blood, which is logical for an organ expected to sample and analyse it PERIPHERAL CHEMORECEPTORS • • • they are stimulated by a decrease in PaO2 and an increase in PaCO2 and H+ this stimulates the respiratory centre causing an increase in breathing this causes an increase in PaO2 and a decrease in PaCO2 and H+ so there is “negative feedback” control of blood gases CENTRAL CHEMORECEPTORS • located in the medulla separate from the respiratory centres • stimulated by an increase in brain extracellular fluid PCO2 and H+ but not by a decrease in PO2 • responsible for 80% of the ventilatory response to increased PaCO2 • poor response to arterial blood H+ because of the blood-brain barrier Expiratory ce ntr e Inspiratory ce ntr e Medulla Central chemoreceptors Spinal cord Respiratory muscles CENTRAL VS PERIPHERAL CHEMORECEPTORS • All of the ventilatory response to decreased PaO2 is due to the peripheral chemoreceptors • most of the ventilatory response to increased arterial blood H+ is due to the peripheral chemoreceptors • most of the ventilatory response to increased PaCO2 is due to the central chemoreceptors Integration of input from peripheral and central from receptors in the control of breathing • As a result of these multiple inputs to the respiratory centres, the blood gases are normally well controlled, especially the PaCO2 • we can measure the ventilatory response to hypoxia (=PO2 less than normal) and hypercapnia (= PCO2 greater than normal) changing the PaO2 and PaCO2 by inhaling hypoxic and hypercapnic gas mixtures LO: Describe the role of the peripheral and central chemoreceptors in the control of breathing ventilatory response to hypoxia the ventilatory response to hypoxia is not so sensitive with little response until the PaO2 falls below 60 mmHg VE 6 litres/min 60 Pa O 2 100 ventilatory response to hypercapnia the CO2 response line is steep indicating that ventilation is sensitive to a very small change in PaCO2 VE hypocapnia causes increased neuromuscular excitability and tetany and hypercapnia (= PCO2 less than normal) causes depression of the nervous system and coma 6 litres/min 50 40 P a CO 2 CLASSIFICATION OF HYPOXIA • Hypoxic: the PaO2 is < normal • Anaemic: the PaO2 is normal but the O2 content is < normal • Stagnant: the PaO2 and O2 content are normal but O2 delivery to the tissues is reduced due to decreased blood flow • Histotoxic: the PaO2, O2 content and delivery are normal but the tissues cannot use the O2 due to metabolic poisoning e.g. cyanide poisoning GENERAL CAUSES OF RESPIRATORY DISORDERS • Decreased ventilation e.g. neurological damage, muscular disorders, obstructive and restrictive pulmonary diseases etc. • Decreased alveolocapillary diffusion e.g. emphysema, oedema, fibrosis, atelectasis etc. • Decreased transport e.g. anaemia, carbon monoxide poisoning etc. DEFINITIONS • • • • eupnoea = normal quiet breathing hyperpnoea = increased ventilation tachypnoea = increased respiratory rate hyperventilation = overventilation (PaCO2 less than normal) • hypocapnia (PCO2 less than normal) • hypercapnia (PCO2 greater than normal) DEFINITIONS • • • • • • hypoxia = PO2 less than normal hyperoxia = PO2 greater than normal hypoxaemia = PO2 less than normal in blood Asphyxia = hypoxia and hypercapnia dyspnoea = stressful breathing Apnoea = absence of breathing CONTROL OF VENTILATION Reading Sherwood – Human Physiology, 7th ed. – Chapter 13 Berne & Levy 7th Ed ‘Physiology’ – Chapter 22