Respiratory Physiology. Manuel Otero Lopez Department of Anaesthetics and Intensive Care Hôpital Européen Georges Pompidou, Paris, France

Transcription

1 Respiratory Physiology Manuel Otero Lopez Department of Anaesthetics and Intensive Care Hôpital Européen Georges Pompidou, Paris, France

2 Programme Functional respiratory anatomy Ventilation Mechanics of breathing (compliance & airway resistance) The inefficiency of respiratory gas exchange (Respiratory Dead space and Shunt) Ventilation-perfusion relationship Gas transport Control of ventilation

3 Functional respiratory anatomy Upper respiratory tract (from nostrils to vocal cords) Lower respiratory tract (from vocal cords to alveoli)

4 Functional respiratory anatomy Muscles of respiration Innervation Diaphragm => Phrenic nerves (C3-C5 nerve roots) Intercostal muscles => by their respective thoracic nerves roots Vagus => provide sensory innervation to the tracheobronchial tree (bronchoconstriction, bronchial secretions). Sympathetic activity (T1-T4) causes bronchodilatation and secretions via β2 receptors. α1 receptors cause bronchoconstriction α1 and β2 receptors are also present in the pulmonary vessels.

5 Weibel classification of airways Ewald Weibel Weibel ER. Morphometry of the human lung. Heidelberg: Springer-Verlag, New York: Academic Press; 1963

6 Functional respiratory anatomy Each alveolar sac contains, on average, 17 alveoli. An estimated 300 million alveoli provide a membrane of 50 to 100 m2 for gas exchange. Pulmonary epithelium: - Type 1 pneumocytes - Type 2 pneumocytes (surfactant)

7 Increase in total cross-sectional area of the airways in the respiratory zone

8 Ventilation

9 Total ventilation = tidal volume (V T ) x respiratory frequency Minute volume of ventilation Anatomic dead space = the volume of the conducting airways, which does not take part in gas exchange (V D ) Alveolar ventilation = (V T - V D ) x respiratory frequency the amount of fresh inspired air available for gas exchange Physiologic dead space = the volume of gas that does not eliminate CO 2 V D / V T = 0.3 in spontaneous ventilation V D / V T = 0.5 in mechanical ventilation

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11 Lung volumes and capacities Volume vs. Capacity Volume is the amount of 3D space taken up by an object, e.g. a solid, a liquid or a gas. Capacity is the measure of an object s ability to hold a substance, e.g. a solid, a liquid or a gas. Morgan and Mikhail s Clinical Anesthesiology, 5th ed.

12 Expiratory spirogram Morgan and Mikhail s Clinical Anesthesiology, 5th ed.

13 Peak flowmeter, PEFR Measurement of the peak expiratory flow rate which is sustained for 10 ms

14 Respiratory failure An obstructive pattern (COAD) will show : Low FEV1.0 / FVC ratio, low PEFR, low VC and high RV. A restrictive pattern will show : Normal FEV1.0 / FVC ratio, low VC, low PEFR and low RV. (after resection of the lung, kyphoscoliosis, )

15 Closing capacity The lung volume present after a maximum expiratory effort is called RV (residual volume). At this minimum volume some of the dependent alveoli are close off. The closing capacity is the lung volume at which this closure is first recorded using a marker gas expirogram such as helium. Closing capacity is independent of body position but increases with age. If it exceeds FRC, there is some degree of airway closure during respiration (intrapulmonary shunt). Responsible for the normal age-related decline in arterial O2 tension.

16 Closing capacity (Morgan and Mikhail s Clinical Anesthesiology, 5th ed.)

17 Mechanics of breathing

18 Pressure volume relationship and compliance Compliance is an index of distensibility of elastic organs and defined as the change in volume per unit change in pressure (ΔV/ΔP). Compliance Elastance

19 Factors which modifie compliance Body size Posture Volume history of the lungs Pulmonary blood volume Fibrosis Normal lung compliance ~ L/cm H 2 O (2-3 L/kPa)

20 Surface Tension

21 Surface Tension and Surfactant

22 Surface Tension

23 Type II pneumocyte Start to develop at about 24 weeks of gestation secreting small amounts of surfactant Adequate amounts are not secreted until about 35 weeks of gestation Electron micrograph of type II epithelial cell (x )

24 Pulmonary Surfactant Reduces the surface tension of the alveolar lining layer Increases lung compliance Increases the stability of alveoli Prevents pulmonary edema Has a short half-life Absence Low lung compliance, alveolar atelectasis, tendency to pulmonary edema

25 Airway Resistance Derived from Hagen Poiseuille equation Jean Louis Marie Poiseuille

26 Main site of airway resistance

27 Laminar versus turbulent flow Laminar flow Hagen-Poiseuille equation Turbulent flow In tubulent flow the resistance to flow is greater and increases more rapidly when the flow increases. Reynolds number: V x diameter x gas density gas viscosity

28 The inefficiency of respiratory gas exchange (Respiratory Dead space and Shunt) Ventilation-perfusion relationship

29 John B. West Video Lectures in Respiratory Physiology

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31 Gravitational Distribution of Blood Flow in the Lung The uneven distribution of blood flow can be explained by the hydrostatic pressure differences within the blood vessels («30 cm H2O pressure from top to bottom») On exercise these regional differences become less. John B. West

32 VENTILATION-PERFUSION RELATIONSHIP Distribution of V, Q and V/Q ratio in the normal, upright lung

33 Ventilation /perfusion ratios in an erect subject. (Textbook of Anaesthesia, AR Aitkenhead & G Smith, 2 nd ed. 1990)

34 Dead space The amount of ventilation not taking part in the gas exchange

35 Ventilation-Perfusion Relationship

36 Shunt Cc O2 can be calculated from the alveolar gas equation : Clinically, the alveolar arterial oxygen partial pressure difference is often used as an approximation for «shunt»

37 Hypoxic Pulmonary Vasoconstriction alveolar hypoxia constricts small pulmonary arteries a compensatory mechanism aimed at reducing blood flow in hypoxic lung regions the precise mechanism is not known occurs in excised isolated lung probably a direct effect of the low PO 2 on vascular smooth muscle

38 DIFFUSION

39 DIFFUSION Fick's Law of Diffusion Adolf Fick, 1855

40 DIFFUSION

41 DIFFUSION Diffusion of Oxygen Across the Blood-Gas Barrier At rest PaO 2 virtually reaches PAO 2 after about 1/3 of its time in capillary The diffusion process is challenged by exercise, alveolar hypoxia, and thickening of the blood-gas barrier True diffusion defects that create arterial hypoxemia are rare

42 GAS TRANSPORT O 2 dissolved combined with Hb

43 GAS TRANSPORT Dissolved O 2 For each mmhg of PO ml O ml -1 of blood or vol. % In normal arterial blood PO 2 of 100 mmhg 0.3 ml O ml -1

44 GAS TRANSPORT O 2 capacity: 1 g Hb ml O 2 O 2 saturation of Hb = O 2 combined with Hb X 100 O 2 capacity CO 2 = 1.39 x Hb x SO 2 (%)/ PO 2 Oxygen carrying capacity of Hb = Hüfner s constant (1,39 ml/gr)

45 O 2 dissociation curve

46 Shifts of the O 2 dissociation curve

47 GAS TRANSPORT CO 2 dissolved as bicarbonate in combination with proteins as carbamino compounds

48 CO 2 carriage in the blood

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50 CONTROL OF RESPIRATION

51 Basic elements of the respiratory control system West, John B. Respiratory Physiology: The Essentials, 9th Edition Copyright 2012 Lippincott Williams & Wilkins, a Wolters Kluwer business

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54 Carotid bodies respond to PO 2, PCO 2, ph little response to normoxia very high blood flow respond to arterial, not venous PO 2 response to PCO 2, ph is < important fast response

55 Lung receptors pulmonary stretch receptors (slowly adapting pulmonary stretch receptors) Hering-Breuer inflation reflex irritant receptors (rapidly adapting pulmonary stretch receptors) J receptors (juxtacapillary) bronchial C fibers

56 Mechanisms of hypoxemia Morgan and Mikhail s Clinical Anesthesiology, 5th ed.

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58 NON-RESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM

59 NON-RESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM Protective functions of respiratory tract Non-respiratory functions of pulmonary circulation Metabolic functions of the lung

60 Protective functions of respiratory tract Warming Raises incoming air to 37 Celsius Humidification Filtration > Removal of filtered particles (cough, cilia) Defense mechanisms of terminal respiratory units (macrophages and other relevant cells) Olfaction Raises incoming air to 100% humidity

61 NON-RESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM Non-respiratory functions of pulmonary circulation Reservoir for left ventricle (contains about 500 ml blood) Fluid and electrolyte exchange Filter to protect the systemic circulation including: small fibrin or blood clots, fat cells, bone marrow, detached cancer cells, gas bubbles, agglutinated RBC's, masses of platelets or WBC's, debris in stored blood, particles in i.v. solutions

62 NON-RESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM Metabolic functions of the lung Uptake or conversion of chemical substances by lungs (conversion of angiotensin I to angiotensin II) Formation of chemical substances Pulmonary surfactant Release into blood of substances stored in pulmonary tissues Bradykinin Histamine Serotonin PGE 2, PGF 2 Heparin

63 Many tanks to : Dr. Armen Varosyan Department of Anaesthesiology and Intensive Care Yerevan State Medical University, Yerevan, Armenia

64 Thank you for attention Manuel Otero Lopez