Dynamic Mechanics of Breathing

Transcription

1 Dynamic Mechanics of Breathing สรช ย ศร ส มะ พบ., Ph.D. ภาคว ชาสร รว ทยา คณะแพทยศาสตร ศ ร ราชพยาบาล มหาว ทยาล ยมห ดล Learning Objectives Define the flow, resistance, velocity and their changes in their magnitude in airway during airflow Describe the factors that contribute to airway resistance in the lungs Define the concept of waterfall phenomeonon, starling resistor, flow limitation and dynamic compression of airway during expiration including their principles leading to the phenomenon Integrate the principles that are necessary to understand flow rate measurement, lung volume change and flow-volume loop

2 Outline ครอบคล มสร รว ทยา 2 หน า Resistance, poiseuille s law Airflow: Laminar flow, Turbulent flow, Reynold number Velocity: Bernoulli s principle Factors determining airway cross sectional area Waterfall phenomenon Flow limitation Dynamic compression of airway Equal pressure point Forced Expiratory Spirography Work of breathing Driving Pressure Provides Energy to Move Air to the Lung 0 cmh 2 O Atmosphere pressure gradient Mouth/Nose Upper Airway Lower Airway Alveoli

3 Airway Resistance in Respiratory System Oral Breathing Pharynx-Larynx Airway > 2 mm diameter Airway < 2 mm diameter Total airway resistance Airway Resistance (cmh 2 O/Litre/sec) Normal Percentage (%) Normal Laminar Airflow Airflow is proportional to driving pressure or pressure gradient (ΔP) but inversely proportional to airway resistance (R) (Ohm s law) V = ΔP R = PB -Palv R V is airflow, measured in liters/second The dot above indicates the time derivative of volume

4 Resistance and Poiseuille s Law When airflow is laminar, air molecules move smoothly in the same direction Poiseuille s law states that airway resistance is proportional to the viscosity of the gas (η) and the length of the tube (l), but inversely proportional to the fourth power of radius (r) R = 8ηl πr 4 Turbulent Flow High flow rates, particularly through branched or irregular tubes Turbulent flow is present when high resistance to airflow exists ΔP α ( V ) 2

5 Turbulent Flow If the Reynolds number (Re) > 2000, turbulent flow develops Re = 2 r υ ρ η r is the radius of tube υ is gas velocity ρ is gas density η is viscosity Airflow in the Lung Laminar: small airways distal to terminal bronchioles Transitional: mostly throughout tracheobronchial tree Turbulent: only in trachea, where radius is large and linear air velocity is high during exercise and coughing

6 Gas velocity and Airflow in the Lung V = volume time = Total area x length time = Total area x velocity Resistance in the Lung

7 The Bernoulli s Principle Velocity 140 cm/sec Flow 1 liter/sec Total Energy Velocity 280 cm/sec Potential (Pressure) Energy Kinetic Energy Velocity 140 cm/sec 1 liter/sec Distance along tube When a fluid moves through a tube at a constant flow rate, the total energy of fluid (potential + kinetic energy) decreases because frictional losses convert some of the energy into heat Increase in fluid velocity occurs where the tube narrows, causing an increase in kinetic energy component at the expense of potential energy, that is pressure decreases Bernoulli s Principle and Airplane Higher velocity, lower pressure Wing cross-section Lower velocity, high pressure The flow of gas above and beneath the wing must be similar Velocity top > velocity bottom Pressure top < pressure bottom The difference in pressure exerted on the wing lifts the airplane off the runway

8 Bernoulli s Principle and Flow Through Tubes P4 P4 P4 P1 υ1 υ2 P2 υ1 P3 Constant flow (liter/min) υ1 < υ2 P1 > P3 > P2 The greater the increase in velocity, the greater the decrease in pressure The small tube will be compressed if P4 > P2 Remember transmural pressure = pressure inside pressure outside/ surrounding tissue Bernoulli Effect and Airflow During expiration, gas velocity must increase dramatically as flow travels toward the trachea, because the total airway area decreases This causes the fall in the airway pressure (Paw)

9 Determinants of the Cross-Sectional Area of the Airway Airway Structure Bronchial Smooth Muscle Tone Lung Volume Elastic Recoil of the Lung Determinants of the Cross-Sectional Area of the Airway: Airway Structure

10 Determinants of the Cross-Sectional Area of the Airway: Bronchial Smooth Muscle Tone Autonomic Nervous System Parasympathetic (vagus nerve) stimulation via Muscarinic 3 receptor bronchoconstriction Sympathetic stimulation via β2 Adrenergic receptor bronchodilation Determinants of the Cross-Sectional Area of the Airway: Bronchial Smooth Muscle Tone Autonomic Nervous System Non-adrenergic non-cholinergic system Bronchodilator nervous pathway Nitric oxide, vasoactive intestinal peptide Bronchoconstrictor nervous pathway Substance P and neurokinin A from non-myelinated C fibers

11 Determinants of the Cross-Sectional Area of the Airway: Bronchial Smooth Muscle Tone Pharmacological substances Parasympathomimetics β2 agonist, epinephrine Anticholinergics Reflex bronchoconstriction from irritants in airway Chemicals: histamine, PAco 2 bronchoconstriction Determinants of the Cross-Sectional Area of the Airway: Lung Volume pleural space resting chest wall alveoli airway inspiration chest wall pleural space alveoli airway

12 Determinants of the Cross-Sectional Area of the Airway: Lung Volume Determinants of the Cross-Sectional Area of the Airway: Lung Elastic Recoil End of inspiration chest wall pleural cavity Expiration alveoli pleural cavity airway alveoli airway

13 Waterfall Phenomenon and Respiratory System P origin P TM P destination If PTM > Porigin, no flow occurs If PTM < Pdestination, driving pressure = Porigin Pdestination If PTM > Pdestination, driving pressure = Porigin PTM Waterfall Phenomenon and Respiratory System P origin P TM P destination Pressure in the airways decreases during air flows from alveolus to the mouth Transmural pressure and force on the airway determine the airflow

14 Equal Pressure Point andstarling Resistor Equal Pressure Point andstarling Resistor

15 Equal Pressure Point andstarling Resistor Equal Pressure Point andstarling Resistor

16 Dynamic Compression of Airway and Equal Pressure Point Dynamic Compression of Airway and Equal Pressure Point

17 Dynamic Compression of Airway and Equal Pressure Point Dynamic Compression of Airway and Equal Pressure Point

18 Dynamic Compression of Airway and Equal Pressure Point Dynamic Compression of Airway and Equal Pressure Point

19 Expiratory Flow Limitation Expiratory Flow Limitation

20 Transmural Pressure and Flow Limitation during Forced Expiration alveoli P TM = Pel= Palv Ppl Ppl airway P TM = Paw Ppl P TM = Pel Ppl P TM Palv Paw Transmural Pressure and Flow Limitation during Forced Expiration P TM = Pel Ppl Ppl P TM airway P TM = Paw Ppl P TM = Pel = Palv Ppl Palv Paw Vmax = ΔP R = Palv -Paw R = (Ppl + Pel) (Ppl + PTM ) R Vmax = Pel PTM R

21 Factors Determining Flow Limitation during Forced Expiration Vmax = Pel PTM R Reduced Pel Vmax Increased PTM Vmax Increased R Vmax Measurement of Airway Resistance Body plethysmograph: to determine Palv -- ไม เร ยน R = ΔP V = PB -Palv V Closing volume -- ไม เร ยน Determine the decrease in expiratory airflow among population V = Δvolume Δtime Forced expiratory spirography Lab Nov9,10 Flow-volume loop/curve KSA Nov 8