Respiratory anatomy and physiology in relation to anaesthesia

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respiratory anatomy and physiology in relation to anesthesia.


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DNB Teaching Programme-2012. Department Of Anaesthesia: 

DNB Teaching Programme-2012. Department Of Anaesthesia Manipal Hospitals ,old airport road Bengalure. 25-04-2012

By Dr.Liyakhath Ali. : 

By Dr.Liyakhath Ali. 25-04-2012 3

Overview …!: 

1.Respiratory anatomy Upper respiratory system ; Nose, pharynx. Lower respiratory system ; Larynx, tracheobronchial tree. 2.Respiratory physiology. Ventilation –lung volumes, Dead space. Blood flow & metabolism-pulmonary circulation. Ventilation-Perfusion Relationships:shunt,V/Q ratio, water's zone of lung Mechanics of Breathing ; muscle’s of respiration , compliance, airway-resistance ,work of breathing , Airway closure & Closing volume . Overview …! 25-04-2012 4

1. Respiratory system Anatomy: 

Upper RS  The mouth ,nose ,paranasal sinuses pharynx, NOSE  Natural humidifier. Important in humidification, trapping of FB,dust,defence against infection by mucous secretion and sophisticated ciliary activity Pharynx-membrano-muscular tube ,can be divided into nasopahrynx,oropharynx,laryngopharynx. 1. Respiratory system Anatomy 25-04-2012 5


Pharynx 25-04-2012 6

Anatomic axis of mouth, pharynx, trachea for endotracheal intubation.: 

Anatomic axis of mouth, pharynx, trachea for endotracheal intubation. 25-04-2012 7

Relative humidity of gases in various anesthetic system: 

Relative humidity of gases in various anesthetic system Anaesthetic system Percentage Humidity of Gases Non-breathing Valve 0 T-Piece 0 To and fro 40-100 Closed circle 40-60 HUMAN NOSE 100 25-04-2012 8

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Lower RS  larynx, trachea ,principal bronchi bronchial tree ,pleura mediastinum lungs & diaphragm. The term “Airway” refers to the upper airway consisting of: Nasal cavity Oral cavity Pharynx Larynx Trachea Principal bronchi. 25-04-2012 9

Human Airway : 

Human Airway 25-04-2012 10

Anatomy of larynx.: 

Anatomy of larynx. 25-04-2012 11

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SITUATION AND EXTENCT Lies opposite the 4 th 5 th and 6 th cervical vertebrae. Anterior midline of neck Extends from the root of the tongue to beginning of trachea Adult females and children lies at a higher level. 44 mm in males and 36 mm in females 25-04-2012 13

Cartilages of Larynx.: 

Cartilages of Larynx. 3 Paired and 3 Unpaired UNPAIRED CARTILAGES:- Thyroid Cricoids Epiglottis PAIRED CARTILAGES:- Arytenoid Corniculate Cuneiform 25-04-2012 14

Muscles Of The Larynx: 

Muscles Of The Larynx EXTRINSIC MUSCLES Inferior constrictor Sternothyroid Thyrohyoid Stylopharyngeus & palatopharyngeus – few fibers Indirect Elevators Geniohyoid Stylohyoid Mylohyoid Indirect depressors Sternohyoid Omohyoid 25-04-2012 15

Intrinsic Muscles: 


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ACTIONS OF THE INTRINSIC LARYNGEAL MUSCLES 3.ABDUCTORS OF THE CORD Posterior cricoarytenoid 25-04-2012 18 4.ADDUCTORS OF THE CORD Transverse arytenoid Lateral cricoarytenoid Cricothyroid Thyroarytenoid 5.TENSORS OF THE CORD Cricothyroid 6.RELAXERS OF THE CORD Thyroarytenoid Vocalis 1.SPHINCTERS OF THE VESTIBULE  Aryepiglotticus 2.OPENER OF THE LARYNGEAL INLET  Thyroepiglotticus


NERVE SUPPLY OF LARYNX MOTOR SUPPLY All intrinsic laryngeal muscles – Recurrent laryngeal N. Except Cricothyroid – External laryngeal nerve. SENSORY SUPPLY (MUCOSA ) Above vocal cords – Internal laryngeal Nerve. Below vocal cords – Recurrent laryngeal nerve 25-04-2012 19

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Damage to the Int. Laryngeal N. abolishes the cough reflex – foreign bodies can easily enter the larynx Larynx ( glottis ) is the narrowest part of the adult respiratory tract – foreign body impaction Subglottic area is the narrowest in children Cricoid cartilage forms a complete ring unlike thyroid ,so used in sellick’s maneuver to prevent aspiration. Vocal cords are wide open during deep inspiration, to avoid injury to cords intubation and extubation should be carried out during inspiration 25-04-2012 21



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Laryngeal nerve injury.: 

Laryngeal nerve injury. EXTERNAL LARYNGEAL NERVE Lies in close proximity to the superior thyroid vessels close to the superior pole of the thyroid Liable to injury during ligation of superior thyroid A during thyroid surgery. Artery should be ligated close to the gland to prevent it’s injury It’s injury – Cricothyroid muscle palsy – Hoarseness Temporary – compensated by the over action of the opposite side muscle RECURRENT LARYNGEAL NERVE Close proximity to inferior thyroid A next to the lateral lobe of thyroid Right side – mostly posterior. Left side – may be in any of the above 3 positions Wise to ligate the artery as far from the thyroid gland as possible Causes :Thyroid malignancies Lung and esophageal tumours Malignant or inflamed nodes Aortic arch aneurysms Mitral stenosis – Left pulmonary A pushed upwards Nerve gets compressed between the L Pul. A and the arch of aorta. Left Recurrent laryngeal N paralyzed twice as often as the right because of it’s close proximity to the thorax The abductor fibers are more susceptible to injury  Moderate trauma can cause pure abductor palsy  They are the first to be paralyzed and last to recover . Pure adductor palsy does not occur as a clinical entity 25-04-2012 24

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25-04-2012 25 Normal vocal cord movements

Vocal cord palsy.: 

25-04-2012 26 PURE ABDUCTOR PALSY – LEFT On phonation both cords meet in the midline Right false cord tends to lie slightly anterior to that on left On inspiration the left cord is unaffected The right cord shows full abduction Vocal cord palsy.

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COMPLETE PALSY – LEFT On phonation the left cord maintains paramedian position The right cord crosses the midline for apposition, the right false cord tends to remain anterior to the lest During inspiration the left cord maintains in the paramedian position, the left cord shows complete abduction 25-04-2012 27

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BILATERAL RECURRENT LARYNGEAL PALSY Might occur during removal of thyroid gland Position of the cords will depend on the severity of injury Mild injury – abductor fibers affected – cords tend to remain adducted – airway reduced to a mere chink Patient shows signs of severe respiratory obstruction esp. in cases of tachypnoea When both nerves are severely damaged the cords maintain a b/l paramedian position No major resp. obstruction – during severe resp. efforts the cords tend to be sucked in 25-04-2012 28

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25-04-2012 29 BILATERAL REC. LARYNGEAL NERVE PALSY WITH EXT. LARYNGEAL NERVE PARALYSIS Both cords are in the paramedian position The cords are no longer tensed True cadaveric position


Formed of rings of cartilage that are def post. Extend:C6 –T5,Lower border of cricoid to bi-furcation into right and left bronchi. Length:10 –11 cm Moves with respiration and position of head. Deep inspiration, carina descends 2.5cm. Lining: Ciliated columnar epithelium. Trachea. 25-04-2012 30

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RIGHT MAIN BRONCHUS 2.5 cm long, shorter, wider and more vertical than the left. The rt main br. gives off branches to upper and middle lobes before becoming continuous with the lower lobe bronchus M/c sites for the dev of lung abcess,the post seg of upper lobe and apical segment of lower lobe. The apical segments are more vulnerable to inhaled materials in the supine position. LEFT MAIN BRONCHUS  Narrower than the right,5cm long.  Terminates at the origin of the upper lobe bronchus becoming the main stem to the lower lobe.  Presence of 5cm of un-interrupted lumen by branching makes it suitable for intubation and blocking in thoracic surgery.  Lt upper lobe bronchus: Upper division -apical, posterior & anterior branches. Lower division –lingular bronchus.  Lt lower lobe bronchus :apical, basal bronchi –anterior, posterior and lateral basal branches. 25-04-2012 31

Bronchopulmonary segments.: 

Bronchopulmonary segments. 25-04-2012 32 Rt upper lobe bronchus: Apical,posterior and anterior. Rt middle lobe bronchus: Medial and Lateral. Rt lower lobe br : Apical branch, medial basal branch.anterior,lateral & posterior branches Lt upper lobe bronchus: Upper division -apical, posterior & anterior branches. Lower division –lingular bronchus.  Lt lower lobe bronchus :apical, basal bronchi –anterior, posterior and lateral basal branches.

Division of tracheo-bronchial tree: 

300 million alveoli, 1/3mm in diameter => 50-100 m2 surface area Z0 - Trachea Z1 - Bronchi Z4 - Bronchioles Z16 - Terminal bronchioles Z17-19 - Respiratory bronchioles Z20-22 - Alveolar ducts Z23 - Alveolar sac  Z0 to Z16 - Conducting airway : No gas exchange i.e. anatomical dead space ,About 150 mL.  Z17 to Z23 - Transitional and respiratory zone : About 2.5L to 3 L Alveolar are polyhedral, not spherical, and not all surface areas are available for gas exchange Division of tracheo-bronchial tree 25-04-2012 33

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Markings on chest X-Ray.: 

Markings on chest X-Ray. 25-04-2012 35

Overview …!: 

1.Respiratory anatomy Upper respiratory system ; Nose, pharynx. Lower respiratory system ; Larynx, tracheobronchial tree. 2.Respiratory physiology. Ventilation –lung volumes. Dead space Blood flow & metabolism-pulmonary circulation. Ventilation-Perfusion Relationships:shunt,V/Q ratio ,water's zone of lung. Mechanics of Breathing ; muscle’s of respiration , compliance, airway-resistance ,work of breathing , Airway closure & Closing volume . Overview …! 25-04-2012 36

Lung volumes : 

Lung volumes 25-04-2012 37

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Expiratory reserve volume (ERV) = 15 mL/kg Tidal volume (VT) = 7-10 mL/kg Inspiratory reserve volume (IRV) = 45 mL/kg Capacity Total lung capacity (TLC) = RV + ERV + VT + IRV = about 75-80 mL/kg Vital capacity (VC) = ERV + VT + IRV = about 60-70 mL/kg Functional residual capacity (FRC) = RV + ERV = 30 mL/kg Inspiratory capacity = VT + IRV 25-04-2012 39

Functional Residual Capacity (FRC) : 

FRC is essentially the balance point between the tendency of the chest wall to spring outwards and the tendency of the lung to recoil --> Intrapleural pressure is negative Factors affecting FRC FRC increases with Increasing height Erect position (30% higher) Reduced lung recoil FRC decreases with Decreased height Supine position Increased lung recoil Obesity Muscle paralysis Pregnancy Anaesthesia FRC does not change with age Functional R esidual Capacity (FRC) 25-04-2012 40

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Functions of FRC Oxygen store Buffer to maintain a steady PaO2 Prevent atelectasis Minimize work of breathing Minimize pulmonary vascular resistance Minimize V/Q mismatch * By keeping lung volume above closing capacity Keep airway resistance low. 25-04-2012 41

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Vital capacity : It is amount of air maximally an individual can exale following maximal deep inspiration. VC(4000 ml)=TV(500 ml)+ IRV (2500 ml)+ERV(1000 ml) conditions associated with reduced vc 1.alteration in muscle power 2.pulmonary diseases occupying lesions of in the chest 4.abdominal tumors which impede the descent of diaphragm 5.abdomial pain 6.abdominal splinting 7.alteration in posture. Significance during anesthesia Spontaneous breathing= no issues Artificial ventilation= lungs feel stiff ,slight reduction in VC SIGNIFICANT REDCUTION OF VC in tension pneumothorax,large haemothorax,diaphragmatic hernia,exompholas,NM-Diseases also post operatively following thoracic and upper abdominal surgeries associated with difficulty to cough & clear the secretion. 25-04-2012 42

Concept of Dead Space: 

Definition of dead space - the volume occupied by gas which does not participate in gas exchange in lung. A few different types, including: 1.anatomical dead space 2.physiological dead space 3.alveolar dead space 4.apparatus dead space Concept of Dead Space 25-04-2012 43

Anatomical dead space : 

Anatomical dead space is the volume of the conducting airways i.e. from nostrils and mouth down to respiratory bronchioles. => about 150mL in an average adult => or 2.2mLs/kg ,it is constant regardless of circulation. Anatomical dead space 25-04-2012 44

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25-04-2012 46 Dead space and alveolar ventilation in normal and diseased lungs.

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Factors influencing anatomical dead space Size of subject => increases with body size Age , sex => at infancy, anatomical dead space is higher for body weight (3.3mL/kg) ,it may be 100 ml in young woman and 200 ml in old man. Posture => sitting 147mL, supine 101mL Position of neck and jaw ; Depression of jaw with flexion of head ( as occurs in respiratory obstruction in anesthetized person) can reduce dead space by 30 ml.on contrary protrusion of jaw with neck extension may increase by 40 ml. Lung volume at the end of inspiration => anatomical dead space increases by 20mL for each L of lung volume Drugs e.g. bronchodilator will increase dead space 25-04-2012 47

Physiological dead space : 

Physiological dead space is that part of the tidal volume which does not participate in gas exchange. Includes: anatomical dead space alveoli with no perfusion (i.e. infinite V/Q) (e.g. West's zone 1) In normal man, anatomical & Physiological dead space numerically remains equal and is about 1/3 of tidal volume. Physiological dead space 25-04-2012 48

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Increase in physiological dead space old age, upright position, large tidal volume, high RR, after atropine administration, during controlled ventilation with inspiratory time reduced to 0.5 or less, in presence of lung diseases, pulmonary embolism, lung hemorrhage, hypotensive anesthesia. Chronic bronchitis & asthma physiological dead space may rise to 50-80% of tidal volume. 25-04-2012 49

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Cooper suggested estimation of physiological DS by a formula Phy DS = 33+ [ Age/3 ] % . The most quantitative technique used to measure physiologic dead space uses a modification of the Bohr equation: 25-04-2012 50

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Alveolar dead space Alveolar dead space is the part of the inspired gas which passes through the anatomical dead space to mix with gas at the alveolar level, but does not participate in gas exchange. (i.e. infinite V/Q) Factors influencing alveolar dead space Low cardiac output can increase alveolar dead space (increasing West's zone 1) Pulmonary embolism 25-04-2012 51

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Apparatus dead space Volume of gas contained in any anesthetic apparatus between the patient and that point in the system where rebreathing of exaled CO2 ceases to occur ( e.g. ; expiratory valve in Magill's system or side arm in Ayer’s T-piece). It is very important factor to be considered in anesthetizing newborns and small children. 25-04-2012 52

Overview …!: 

1.Respiratory anatomy Upper respiratory system ; Nose, pharynx. Lower respiratory system ; Larynx, tracheobronchial tree. 2.Respiratory physiology. Ventilation –lung volumes. Dead space Blood flow & metabolism-pulmonary circulation. Ventilation-Perfusion Relationships:shunt,V/Q ratio ,water's zone of lung. Mechanics of Breathing ; muscle’s of respiration , compliance, airway-resistance ,work of breathing , Airway closure & Closing volume . Overview …! 25-04-2012 53

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1. The pulmonary artery is thin, arterial branches are very short. 2.pulmonary arteries and arterioles have larger diameters, Vessels are more distensible, Vessels have larger capacity. 3.The pulmonary capillary pressure is low about 7 mm Hg , in comparison with a considerably higher functional capillary pressure in the peripheral tissues of about 17 mm Hg . 4.The pulmonary capillaries are relatively leaky to protein molecules so that the colloid osmotic pressure of the pulmonary interstitial fluid is about 14 mm Hg, in comparison with less than half this value in the peripheral tissues. 5. In the systemic circulation, arteries carry oxygenated blood to the tissues from the left ventricle of the heart. In the pulmonary circulation, the pulmonary artery carries deoxygenated blood to the lungs via the right ventricle. PECULIARITIES OF PULMONARY CIRCULATION 25-04-2012 55

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7. In the systemic circulation, veins carry deoxygenated blood to the heart ,emptying into the right atrium of the heart. In the pulmonary circulation, the pulmonary vein carries oxygenated blood to the heart emptying into the left atrium. 8. The systemic circulation not only delivers oxygen to the tissues (exchanging it for carbon dioxide), but it picks up nutrients from other tissues, such as the intestines, and delivers cellular wastes to the kidneys. The pulmonary circulation is concerned primarily with gas exchange, making it more specialized. 9. flow is more pulsatile than in the systemic circuit 25-04-2012 56

Pressures at pulmonary & systemic circulation.: 

Pressures at pulmonary & systemic circulation. 5 12 0-5 6-12 25-04-2012 57

Hypoxic pulmonary vasoconstriction: 

Hypoxic pulmonary vasoconstriction is a paradoxical, physiological phenomenon in which pulmonary arteries constrict in the presence of hypoxia (low oxygen levels) without hypercapnia (high carbon dioxide levels), redirecting blood flow to alveoli with a higher oxygen content. However, it is explained by the fact that constriction leads to redistribution of blood flow to better-ventilated areas of the lung, which increases the total area involved in gaseous exchange. This improves ventilation/perfusion ratio and arterial oxygenation, but it less helpful in the case in long-term whole-body hypoxia. This is seen in COPD at altitude and in heart failure. Several factors inhibit this process including increased cardiac output, hypocapnia hypothermia , acidosis /alkalosis , increased pulmonary vascular resistance, inhaled anesthetics , calcium channel blockers, PEEP, HFV, isoproterenol , nitrous oxide , and vasodilators . Hypoxic pulmonary vasoconstriction 25-04-2012 58

Overview …!: 

1.Respiratory anatomy Upper respiratory system ; Nose, pharynx. Lower respiratory system ; Larynx, tracheobronchial tree. 2.Respiratory physiology. Ventilation –lung volumes. Dead space Blood flow & metabolism-pulmonary circulation. Ventilation-Perfusion Relationships:shunt,V/Q ratio ,water's zone of lung. Mechanics of Breathing ; muscle’s of respiration , compliance, airway-resistance ,work of breathing , Airway closure & Closing volume . Overview …! 25-04-2012 59

Ventilation-Perfusion Relationships ; Concept of shunt: 

A pulmonary shunt is a physiological condition which results when the alveoli of the lung are perfused with blood as normal, but ventilation (the supply of air) fails to supply the perfused region. Perfusion in excess of ventilation ,opposite of dead space. The physiologic shunt : is that portion of the total cardiac output that returns to the left heart and systemic circulation without receiving oxygen in the lung. Normally its less than 5 % Ventilation-Perfusion Relationships ; Concept of shunt 25-04-2012 60

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Dead space v shunt…: 

Dead space v shunt… 25-04-2012 62

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Interpretation of shunt percent (%): 

Interpretation of shunt percent (%) 25-04-2012 64 Physiological shunt Interpretation < 10% Normal 10% - 20% Mild Shunt 20%-30% Significant Shunt 30% Critical and severe Shunt

Classification of causes of ‘’TRUE-SHUNT’’ (after Nunn).: 

Physiological Shunt (NORMAL SHUNT ) Pathological Shunt (ABNORMAL SHUNT ) Extra-Pulmonary Thebesian veins Congenital diseas e of heart or great vessels with RIGHT TO LEFT SHUNT. Intra-Pulmonary Bronchial veins Possibly some slight degree of atelectasis Atelectasis Pulmonary edema, Pulmonary contusions, Pulmonary hemorrhage Pulmonary infections (pneumonia, consolidation) Pulmonary arteriovenous shunts, Pulmonary neoplasms including haemangioma. Classification of causes of ‘’TRUE-SHUNT’’ (after Nunn). 25-04-2012 65

Ventilation Perfusion ratio: 

Efficacy with which O2 & CO2 exchange at the alveo-capillary level depends on matching of capillary perfusion & alveolar ventilation. Blood flow in lungs is gravity dependent Relationship between pulmonary artery pressure-Ppa, alveolar pressure PA, and pulmonary venous pressure Ppv determines the lung perfusion. Ventilation Perfusion ratio 25-04-2012 66


WEST - ZONES OF LUNG 25-04-2012 67

West-zones of Lung contd…..: 

Zone 1  pulmonary A lveolar pressure(PA) exceeds pulmonary artery pressure(Ppa),which is negative at this height, so vessels are collapsed-no blood flow, no gas exchange, hence wasted ventilation-alveolar dead space. Under normal conditions little or no zone 1 exists. But in conditions where Ppa is greatly reduced-hypovolemic shock or where PA is greatly increased-large tidal volume ventilation or high PEEP ventilation during IPPV. West-zones of Lung contd….. 25-04-2012 68

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In zone 2  T he Ppa exceeds PA pressure & blood flow begins. But, PA still exceeds Ppv so it’s the Ppa_PA which determines the flow. WATERFALL EFFECT- The height of the upstream river before reaching the dam is the Ppa & the height of the dam is the PA.So the rate of water flow over the dam is equivalent to the diff between the height of the upstream river & the height of the dam(Ppa-PA). It does not matter how far below the dam the height of the downstream river bed is- Pv. Also known as STARLING resistor, WEIR / SLUICE effect. 25-04-2012 69

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In zone 3  Ppv becomes positive and also exceeds PA and the capillary systems are thus permanently open and blood flow is continuous down zone 3. In this region, blood flow is governed by the pulmonary arteriovenous pressure difference (Ppa - Ppv) 25-04-2012 70

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In zone 4  Ppa > PISF > Ppv > PA A region of the lung from which a large amount of fluid has transuded into the pulmonary interstitial compartment or is possibly at a very low lung volume. blood flow is governed by the arteriointerstitial pressure difference (Ppa - PISF), which is less than Ppa - Ppv difference, and therefore zone 4 blood flow is less than zone 3. This produces positive interstitial pressure, which causes compression of extra-alveolar vessels, increased extra alveolar vascular resistance, and decreased regional blood flow. 25-04-2012 71

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RECRUITMENT (opening of previously unperfused vessels)- as Ppa & Ppv increase from-low to moderate DISTENSION - ( widening of previously perfused vessels)- moderate to high TRANSUDATION- (of fluid from very distended vessels )-high to very high levels. 25-04-2012 73

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The normal alveolar ventilation (V)is in an adult is 4 l/min,and total perfusion is 5 l/min (Q). So proportion of ventilation to perfusion is 4/5= 0.8 ,this ratio is known as VENTILATION-PERFUSION RATIO. In an erect person, Ventilation increase from apex to base * 0.24 L/min ----> 0.82L/min Perfusion increase from apex to base * 0.07 L/min --> 1.29 L/min Because the increase in perfusion is greater than that of ventilation --> V/Q decreases from apex to base --> 3.3 to 0.63 25-04-2012 74

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25-04-2012 75 Regional differences in gas exchange down the normal lung.

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1.Respiratory anatomy Upper respiratory system ; Nose, pharynx. Lower respiratory system ; Larynx, tracheobronchial tree. 2.Respiratory physiology. Ventilation –lung volumes. Dead space Blood flow & metabolism-pulmonary circulation. Ventilation-Perfusion Relationships:shunt,V/Q ratio, water's zone of lung Mechanics of Breathing ; muscle’s of respiration , compliance, airway-resistance ,work of breathing , Airway closure & Closing volume . 25-04-2012 76

Mechanics of Breathing: 

The movement of gases in and out of the lungs is a mechanical process, which is dependent on the following factors The respiratory muscles and their actions Compliance of chest wall & the lungs Gas flows in the airways Mechanics of Breathing 25-04-2012 77

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Muscle’s of respiration Inspiratory muscles Diaphragm The diaphragm descends about 1.5 cm during tidal inspiration. 1 cm downward movement causes 350 ml of air to enter lungs. During a vital capacity breath, total movement is about 6-10cm. Diaphragmatic contraction is responsible for 70% of the tidal volume Motor innervations is by phrenic nerve-C 3,4,5. Moves in vertical plane, increases intra abdominal pressure, elevates & expands the lower rib cage increasing dimensions of chest cavity. High oxidative capacity,50% of fibers have slow twitch response to electrical activity, hence fatigue resistant. Total paralysis of diaphragm by b/l phrenic nerve injury greatly reduces ability to ventilate lungs although adequate tidal exchange for rest and light activity can be still maintained, Unilateral phrenic nerve crush can produce 15-20 redu ction in max-breathing capacity, with normal tidal exchange . 25-04-2012 78

Video demo of diaphragmatic movement: 

Video demo of diaphragmatic movement 25-04-2012 79

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Expiratory muscles ; most imp muscles are abdominal wall muscles including external, internal oblique ,rectus abdominis & transverse abdominis. These also contract forcefully during coughing, vomiting & defecation. Internal intercostals assist active expiration. 25-04-2012 80

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Accessory muscles of inspiration-scalene muscles, elevate 1 st two ribs, SCM raises the sternum-increasing AP diameter of chest wall. cervical strap muscles most imp inspiratory accessory muscles even during ventilation at rest. 25-04-2012 81

Process of Breathing: 

Inhaling (active process) – Air moves in. Why?? Gases move from an area of high pressure to low pressure During inspiration – diaphragm pulls down and lungs expand When lungs expand, it INCREASES the VOLUME, which DECREASES the PRESSURE inside lungs Lung pressure is lower than outside pressure, so air moves in Process of Breathing 25-04-2012 82

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Exhaling (passive process) – breathing out Diaphragm and muscles relax Volume in lungs and chest cavity decreases, so now pressure inside increases Air moves out because pressure inside is HIGHER than OUTSIDE atmosphere 25-04-2012 83

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Increased volume causes a drop in pressure if the system is closed. Where is the system closed to the outside? The pleural cavity! The decrease in intrapleural cavity pressure is translated to the lungs via the inner visceral pleural membrane because it is attached to the outer surface of the lung Thus, an increase in volume causes a decrease in intrapleuralpressure because it is a closed system. 25-04-2012 84

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The lungs are an open system via the trachea and as such as intrapleural pressure drops lung volume increases and lung pressure also decreases. However, because the lungs are open to the outside, air rushes into the lungs to equalize the pressure. Thus, the drop in pulmonary pressure is transient. Pulmonary pressure returns to zero as air moves into the lungs to take up the volume change (drop in pressure). 25-04-2012 85

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Changes in intrapleural (intrathoracic) and intrapulmonary pressure relative to atmospheric pressure during inspiration and expiration: 

25-04-2012 90 Changes in intrapleural (intrathoracic) and intrapulmonary pressure relative to atmospheric pressure during inspiration and expiration


Compliance 25-04-2012 91

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Lung compliance is defined as change in volume (lung expansion )per unit of pressure change(work of breathing) .Its a measure of distensibility of the lung. The slope of the pressure-volume curve, or the volume change per unit pressure change, is known as the compliance. Total compliance=C(chest wall) +C(lung) Normal Chest compliance is 200 ml/cmH2o Normal total compliance is 100 ml/cmH2o 25-04-2012 92

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Compliance is made up of lung compliance and chest compliance --> 1/Ct = 1/Cl + 1/ Ccw C = Δ V/ Δ P * i.e. Summation of elastance =1/compliance Thus, 1/Ct = 1/200 + 1/200 --> Ct = 100 mLs/cmH2O 25-04-2012 93

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Factors affecting compliance 1.Lung elastic recoil  Due to: a) Surface tension in the alveoli b) Stretched elastic fibers in the lung parenchyma Surface tension accounts for 70% of the elastic recoil 2.Lung volume  The slope of the P-V curve is not constant across different lung volumes. At high lung volume --> Elastic fibers already stretched --> Greater pressure is required to inflate lung --> Reduced compliance At very low volumes --> Alveoli radius reduced --> (according to Laplace's Law), pressure required to inflate alveoli is increased --> Reduced compliance Other factors affecting compliance via effect on lung volume  Posture ,Restriction of chest expansion 3.Disease state. 25-04-2012 95

Types of compliance : 

1.Static compliance  compliance measured when there is NO gas flow into or out of the lung. Static C = Plateau pressure-PEEP ,It reflects elastic resistance of the lung and chest wall 2.Dynamic compliance  compliance measured when there is gas flow into or out of the lung Dynamic C =peak airway pressure-PEEP , It reflects condition of airway ( non-elastic ) and elastic properties of chest wall and lung. Airway resistance is a critical factor in its measurement . Types of compliance 25-04-2012 96

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Low compliance : means less distensibilty,stiff lungs seen in  pulmonary fibrosis,pul-edema, atelectasis, prolonged under ventilated lung with low volume, pulmoary venous hypertension. Cause of refractory hypoxaemia. Features : restrictive lung defect, low FRC,low LUNG VOLUMES,low minute ventilation.  Low static C  Atelectasis,ARDS,Tension pnumothorax,obesity,retained secretions. Low dynamic C bronchospasm,kinking of ET tube,airway obstruction. 25-04-2012 97

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High compliance : over distensibility seen in emphysema ,aging lung ,during asthmatic attack. Features : obstructive lung defect, air flow obstruction, incomplete exalation,poor gas exchange. 25-04-2012 98

In a tube it is the pressure difference between the two ends at a given flow rate. Flow= pressure gradient/ airway resistance Under normal conditions resistance comprises of force required to drive air to & fro along bronchial tree from mouth to alveoli. Every piece of anaesthetic apparatus-gas flows, shape & diameter of breathing tubes, number,stylet & setting of valves, size & packing of soda lime canister increases resistance Airway Resistance 25-04-2012 99

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Normal total airway resistance is about 0.5–2 cm H 2 O/L/s, with the largest contribution coming from medium-sized bronchi (before the seventh generation). Factors affecting airway resistance Raw = (P alv – P atm )/ Flow Lung volume Density and viscosity of inspired gas Contraction of bronchial smooth muscles Small airway caliber 25-04-2012 100

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1.lung volume ; => At residual capacity, airway resistance is at its greatest => At total lung capacity, airway resistance is at its lowest => i.e. airway resistance is inversely proportional to the lung volume Alternatively, Conductance (1/resistance) is directly proportional to the lung volume. 25-04-2012 101

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2. Density and viscosity of inspired gas The higher the density => Resistance increases => turbulent flow more likely The higher the viscosity => Resistance decreases => laminar flow more likely In practice, density is a lot more significant. 25-04-2012 102

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3. Contraction of bronchial smooth muscles Innervations of bronchial smooth muscles: autonomic nervous system vagus nerve (motor) Relaxation (i.e. decreasing AWR) adrenergic receptor stimulation (mainly beta2) epinephrine isoproterenol Contraction (i.e. increasing AWR) irritation e.g. smoke parasympathetic stimulation via: acetylcholine, M3-receptor acetylcholine decreasing PACO2 histamine (act on smooth muscle in alveolar ducts) 4. Small airway caliber 25-04-2012 103


LAMINAR FLOW Air molecules move smoothly in same direction. Poiseuille’s law- airway resistance is proportional to viscosity of gas, length of the tube but inversely proportional to the fourth power of radius. occurs in small airways distal to terminal bronchioles . TURBULENT FLOW A s the rate of flows reach a critical velocity, flow is turbulent particularly through branched & irregularly shaped,no fresh gas reaches the end of tube until gas in the tube equals volume in the tube. Only in trachea where the radius is large & linear air velocity is high-during exercise & coughing TRANSITIONAL FLOW Occurs throughout the tracheo-bronchial tree. TYPES OF FLOWS 25-04-2012 104

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25-04-2012 105

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Flow Changes from Laminar to Turbulent when Reynolds Number exceeds 2000 Turbulent flow tends to take place when gas density, linear velocity & tube radius are large. Gas flows, sharp angles in the tube, branching in tube & change in tube diameter-changes laminar to turbulent flows. 25-04-2012 106

Work of Breathing: 

The effort required to inspire air into the lungs. WOB accounts for 5% of total body oxygen consumption in a normal resting state but can increase dramatically during acute illness. To inflate lungs certain forces has to be overcome, which prevents its inflation .These are  1.elastic resistance of lung. 2.non-elastic resistance (structural resistance) 3.airflow resistance at tracheo-bronchial tree. where P = pressure, W = work of breathing, V = tidal volume Work of Breathing 25-04-2012 107

Components of Work of breathing. : 

Elastic work - work to overcome: lung elastic recoil thoracic cage displacement abdominal organ displacement Frictional work - work to overcome: air-flow resistance (major) viscous resistance (lobe friction, minor) Inertial work - work to overcome: acceleration and deceleration of air (negligible due to low mass of air) acceleration and deceleration of chest wall and lungs (negligible due to over damping) 25-04-2012 108 Components of Work of breathing.

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1. Elastic work Work is stored as potential energy Eventually work is dissipated as heat Elastic recoil is due to * Surface tension * Intrinsic elasticity of tissue fibers. 2. Resistance work Work is required to overcome: Airway resistance Tissue resistance 25-04-2012 109

Factors influencing elastic work The higher the elastic recoil --> the more work required to overcome elasticity : 

1. Intrinsic elasticity of fibers Age --> Elastic recoil reduces as age increases Pathology * e.g. emphysema reduces elastic recoil 2. Surface tension Surfactant is responsible for 70% of elastic recoil --> Increased in surface tension increases recoil Size of alveoli --> Larger alveoli reduce pressure required for inflation * Laplace law 3. Other factors Lung volume Large lung volume --> More stretched fibers --> Higher recoil At low lung volume, compliance is reduced, but it has nothing to do with recoil. Respiratory rate Give the same minute volume, Increased RR --> Decreased work due to recoil Factors influencing elastic work The higher the elastic recoil --> the more work required to overcome elasticity 25-04-2012 110

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Factors influencing resistance work 1. Airway resistance AWR is increased by: Reduced lung volume * Opposite to elastic recoil Increased bronchial smooth muscle tone Increased density and/or viscosity of gas Increased turbulent flow Increased respiratory rate * Given the same minute volume --> increased RR INCREASES work due to airflow resistance 2. Viscous tissue resistance Probably inherent. May be affected by ?pulmonary hypertension 25-04-2012 111

Minimizing work of breathing : 

Among the factors influencing work of breathing, two factors are important in minimizing work: Lung volume (at FRC) Respiratory rate  Lung volume Work of breathing is minimized at FRC , because.. high pulmonary compliance (on steep part of the pressure-volume curve) --> Elastic work is low Low airway resistance --> Resistance work is low (but not lowest) Partial inflation and being at a volume above the closing capacity --> No work required to open collapsed parts of the lung or closed airways At low lung volume, resistance work is increased (due to increased airway resistance) At high lung volume, elastic work is increased (due to already stretched fiber). Minimizing work of breathing 25-04-2012 112

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 Respiratory rate Given the same minute volume, Increasing RR increases work due to air flow resistance. Decreasing RR increases work due to elastic recoil. There is a optimal RR which minimizes the total work required. RR > Optimal rate --> Decreased tidal volume --> Increased work due to AWR RR < Optimal rate --> Increase work due to elastic recoil 25-04-2012 113

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Effects of disease In Restrictive lung disease --> Increase work due to elastic recoil --> Optimal RR increases In Obstructive lung disease --> Increase work due to increase airway resistance --> Optimal RR decreases 25-04-2012 114

Airway closure & Closing volume : 

Closing capacity “The lung volume at which the small airways in (usually the dependent part of) the lung first start to close ‘’. --> Impairs gas exchange and increase venous admixture --> Decrease PaO2 Closing capacity = RV + closing volume Airway closure & Closing volume 25-04-2012 115

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25-04-2012 116 The relationship between functional residual capacity, closing volume, and closing capacity

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Variation of closing capacity Closing capacity increases with age FRC does not change with age Young subject, closing capacity = 10% of VC In erect position, closing capacity = FRC = 40% of VC at 66 yrs of age. In supine position, closing capacity = FRC at 44 yrs of age. In neonates, lung elastic recoil is reduced --> More airway closure --> Closing capacity > FRC --> Reduced PaO2 25-04-2012 117

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Relation of CC to FRC In subjects with normal lungs CC becomes equal to FRC at 66 yr ,and at 44 yr in supine, after this CC continues to rise as age advances, and arterial Po2 begins to fall. Rise in CC is seen in  smokers,obesity,rapid i.v transfusions, early chronic bronchitis, LVF and following MI. It is always increased following Sx,and an important cause of post-op hypoxaemia. Use of PEEP raises Po2 by increasing FRC above CC. 25-04-2012 118


Effect of anaesthesia on gas exchange Increased dead space, Hypoventilation, Increased intrapulmonary relative shunt. There is increased scatter of v /q ratios. Increases in alveolar dead space. General anesthesia commonly increases venous admixture to 5–10%, as a result of atelectasis and airway collapse in dependent areas of the lung. 25-04-2012 119 summary

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Inhalation agents, including nitrous oxide, also can inhibit hypoxic pulmonary vasoconstriction in high doses. PEEP is effective in reducing venous admixture and preventing hypoxemia during general anesthesia Prolonged administration of high inspired O 2 concentrations (> 50%) is associated with increases in absolute shunt. complete collapse of alveoli with previously low V /Q ratios is thought to occur once all the O 2 within is absorbed (absorption atelectasis). 25-04-2012 120

Take home message : 

There is no substitute for hard work. There is no substitute for for sound basics and fundamental principles. Knowing the basic sciences (anatomy,physiology,pharmocology ) is a paramount for safer anesthesia practice and to become a safe anaesthetist. 25-04-2012 121 Take home message


1. Wylie's 5 th Edition, a practice of anaesthesia. 2.Barash, Paul G.; Cullen, Bruce F.; Stoelting, Robert K. Clinical Anesthesia, 6th Edition. 3. Miller's Anesthesia 7 th Edition 4. West, John B. The Essentials of Respiratory Physiology: 8th Edition 5. Morgan’s Clinical Anesthesiology, Fourth Edition 6.David W ,Chang’s ; Mechanical ventilation. 7. Concise Anatomy for Anaesthesia : Andreas G Erdmann 8. Ganong ‘s ‘REVIEW OF MEDICAL PHYSIOLOGY’ 9.Various Net sources. References 25-04-2012 122

Thank you ! : 

Thank you ! 25-04-2012 123