respiratory physiology - euroanaesthesia...
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Respiratory Physiology
Manuel Otero Lopez Department of Anaesthetics and Intensive Care Hôpital Européen Georges Pompidou, Paris, France
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Conflict of interest declaration
I do not have conflict of interest
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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
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Functional respiratory anatomy
• Upper respiratory tract (from nostrils to vocal cords)
• Lower respiratory tract (from vocal cords to alveoli)
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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.
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Weibel classification of airways
Weibel ER. Morphometry of the human lung. Heidelberg: Springer-Verlag, New York: Academic Press; 1963
Ewald Weibel
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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)
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Increase in total cross-sectional area of the airways in the respiratory zone
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Ventilation
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Total ventilation = tidal volume (VT) x respiratory frequency • Minute volume of ventilation
Anatomic dead space = the volume of the conducting airways, which does not take part in gas exchange (VD)
Alveolar ventilation = (VT - VD) x respiratory frequency
• the amount of fresh inspired air available for gas exchange
Physiologic dead space = the volume of gas that does not eliminate CO2
• VD / VT = 0.3 in spontaneous ventilation
• VD / VT = 0.5 in mechanical ventilation
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Lung volumes and capacities
Morgan and Mikhail’s Clinical Anesthesiology, 5th ed.
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.
Volume vs. Capacity
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Expiratory spirogram
Morgan and Mikhail’s Clinical Anesthesiology, 5th ed.
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Peak flowmeter, PEFR
Measurement of the peak
expiratory flow rate which is
sustained for 10 ms
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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,…)
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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.
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Closing capacity
(Morgan and Mikhail’s Clinical Anesthesiology, 5th ed.)
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Mechanics of breathing
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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
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Normal lung compliance ~ 0.2-0.3 L/cm H2O (2-3 L/kPa)
Factors which modifie compliance • Body size • Posture • Volume history of the lungs • Pulmonary blood volume • Fibrosis
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Surface Tension
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Surface Tension and Surfactant
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Surface Tension
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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 10 000)
Type II pneumocyte
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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
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Airway Resistance
Derived from Hagen–Poiseuille equation
Jean Louis Marie Poiseuille
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Main site of airway resistance
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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
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The inefficiency of respiratory gas exchange (Respiratory Dead space
and Shunt) Ventilation-perfusion relationship
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John B. West
Video Lectures in Respiratory Physiology
http://meded.ucsd.edu/ifp/jwest/resp_phys/index.html
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Gravitational Distribution of Blood Flow in the Lung
John B. West
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.
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VENTILATION-PERFUSION RELATIONSHIP
Distribution of V, Q and V/Q ratio in the normal, upright lung
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Ventilation /perfusion ratios in an erect subject. (Textbook of Anaesthesia, AR Aitkenhead & G Smith, 2nd ed. 1990)
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Dead space
The amount of ventilation not taking part in the gas exchange
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Ventilation-Perfusion Relationship
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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 »
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• 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 PO2 on vascular smooth muscle
Hypoxic Pulmonary Vasoconstriction
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DIFFUSION
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DIFFUSION
Adolf Fick, 1855
Fick's Law of Diffusion
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DIFFUSION
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• At rest PaO2 virtually reaches PAO2 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
Diffusion of Oxygen Across the Blood-Gas Barrier
DIFFUSION
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GAS TRANSPORT
•dissolved
• combined with Hb
O2
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GAS TRANSPORT
Dissolved O2
For each mmHg of PO2 0.003 ml O2 · 100 ml-1 of blood
or
0.003 vol. %
In normal arterial blood PO2 of 100 mmHg 0.3 ml O2 · 100 ml-1
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GAS TRANSPORT
O2 capacity: 1 g Hb - 1.39 ml O2
O2 saturation of Hb = O2 combined with Hb X 100 O2 capacity
CO2 = 1.39 x Hb x SO2 (%)/100 + 0.003 PO2
Oxygen carrying capacity of Hb = Hüfner’s constant (1,39 ml/gr)
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O2 dissociation curve
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Shifts of the O2 dissociation curve
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GAS TRANSPORT
CO2
• dissolved
• as bicarbonate
• in combination with proteins as carbamino compounds
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CO2 carriage in the blood
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CONTROL OF RESPIRATION
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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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• respond to PO2, PCO2, pH
• little response to normoxia
• very high blood flow
• respond to arterial, not venous PO2
• response to PCO2, pH is < important
• fast response
Carotid bodies
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• 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
Lung receptors
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Mechanisms of hypoxemia
Morgan and Mikhail’s Clinical Anesthesiology, 5th ed.
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NON-RESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM
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NON-RESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM
• Protective functions of respiratory tract
• Non-respiratory functions of pulmonary circulation
• Metabolic functions of the lung
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• Protective functions of respiratory tract
• Warming
• 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
Raises incoming air to 37 Celsius
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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
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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
• PGE2, PGF2
• Heparin
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Thank you for attention
Manuel Otero Lopez [email protected]
Many tanks to : Dr. Armen Varosyan Department of Anaesthesiology and Intensive Care Yerevan State Medical University, Yerevan, Armenia