Physiology of respiration

respiratory physiology relevent to anesthesiologist

1. PHYSIOLOGY OF RESPIRATION BY:- DR. DEEPTI SHARMA MODERATOR: DR. VIRENDRA

2. Introduction Respiratory system can be broken down into: 1. upper airway 2. conducting airway 3. alveolar airway (lung parenchyma/ acinar tissue)

3. Upper Airway Consists of nose/ nasal cavity, mouth, pharynx and larynx. Nose – primary point of entry. Two important functions:- 1 . filtering out large particulates to prevent them from reaching the conducting and alveolar airways. Particulates till size of 5-10 μ m can be filtered in nasopharynx . 2 . to warm and humidify air as it enters body.

4. Conducting Airway Begins at trachea B ranches dichotomously. First 16 generation Transports gas Epithelium – pseudostratified C iliated , secretory and basal cells. provide innate immunity serve as progenitor cells during injury.

5. Cont.. Smaller particles (2-5 μ m) can enter but generally falls on the walls of bronchi or can initiate reflex bronchial constriction or coughing. Epithelium is ciliated. It can move particles away from lungs @16mm/min. Defective motility can lead to chonic sinusitis, recurrent lung infection and bronchiectesis . Innervated by autonomic nervous system. Receptors show rapid adaptation.

6. Alveolar Airway Last 7 gernerations form transitional and respiratory zones. Includes transitional and respiratory bronchiloes , alveolar ducts and alveoli. 2.5cm ² in trachea to 11,800 cm² in alveoli. 300 million alveoli, total alveolar surface in contact with capillaries is 70m². Alveoli are lined by 2 types of epithelial cells. Type I pneumocytes Type II pneumocytes :- produce surfactant.

7. Cont.. Alveoli surrounded with pulmonary capillaries. Air and blood separated only by alveolar epithelium and capillary endothelium, about 0.5 μ m apart. Contains alveolar macrophages, lymphocytes, plasma cells, neuroendocrine cells and mast cells. PAMs are important component of pulmonary defense mechanism.

8. Respiratory Muscles Lungs are positioned within the thoracic cavity, composed of ribs, sternum and spinal column. INSPIRATION Movement of diaphragm accounts for 75% of the change in intrathoracic volume during quiet inspiration. Diaphragm moves downward like a piston. Distance it moves ranges from 1.5 -7 cm with deep inspiration. Diaphragm has 3 parts:- costal portion, crural portion and central tendon. Costal and crural portions are innervated by different parts of phrenic nerve. As the dome of diaphragm descends, it displaces the abdominal contents caudally.

9. Cont.. The fall in pleural pressure and accompanying lung expansion cause ↑ abdominal pressure and outward movement of abdominal wall . Supine and trendelenberg position or surgical retractors significantly interfere with this abdominal motion . External oblique :- obliquely downward and forward from rib to rib. Principal accessory mucsles :- sternocleidoid and scalene muscles. D iaphragm / external intercostal alone can maintain adequate ventilation at rest. Transection of spinal cord above 3 rd cervical segment is fatal.

10. Cont.. EXPIRATION Passive process:- elastic recoil of lungs and chest wall. Decrease in intrathoracic volume and forced expiration requires expiratory muscles. Internal intercostals :- obliquely downward and posteriorly from rib to rib.

11. Radiographic view of lungs during expiration and inspiration

12. Lung Pleura Pleural cavity and its infoldings serve as a lubricating fluid that allows for lung movement within thoracic cavity. 2 layers:- parietal pleura visceral pleura Pleural fluid:- 15-20ml, forms a thin layer between the pleural membranes and prevent friction.

13. Mechanics of Respiration I nteraction b/w 2 opposing forces:-chest wall and the lungs. Lung and chest wall move together as a unit. By air tight thoracic cavity and intimate contact b/w the layers of pleura which is maintained by a negative intrapleural pressure. Lungs slide on chest wall easily, but resist separation . I ntrapleural pressure :- subatmospheric (-5cm H ₂O). If chest wall is opened, lungs collapse and if lungs lose elasticity , chest expands and becomes barrel shaped .

14. Pressure in alveoli & pleural space

15. Intrapleural pressure at the base of lungs is about -2.5mmHg at the start of inspiration, decreases to -6mmHg. Lungs are pulled in more expanded position, pressure in the airway becomes slightly negative and air flows into the lungs. At the end of inspiration, the lung recoil begins to pull the chest back to expiratory position, pressure in airway becomes slightly positive and air flows out of the lungs. Strong inspiratory efforts can reduce intrapleural pressures as low as -30mmHg.

16. Lung Volumes and Capacities

17. Tidal Volume(TV):- The amount of air that moves into the lungs with each inspiration (or the amount that moves out with each expiration) during quiet breathing. ( 500–750 mL ) Inspiratory reserve volume (IRV ):- The air inspired with a maximal inspiratory effort in excess of the TV (typically 2 L). Expiratory reserve volume (ERV):- expelled by an active expiratory effort after passive expiration is ( 1 L) Residual volume (RV):- air left in the lungs after a maximal expiratory effort is the (RV; ~1.3 L).

18. Total lung capacity(TLC):- all four of the above components together (~5 L). Vital lung capacity (VC):- the maximum amount of air expired from the fully inflated lung, or maximum inspiratory level (TV + IRV + ERV). (~3.5 L) Inspiratory capacity (IC):- maximum amount of air inspired from the end-expiratory level (IRV + TV). (~2.5 L) Functional residual capacity (FRC):- the volume of the air remaining in the lungs after expiration of a normal breath (RV + ERV). (~2.5 L)

19. Dynamic lung volumes Forced vital capacity (FVC):- amount of air that can be expired after a maximal inspiratory effort . measured clinically as an index of pulmonary function. FEV1 (forced expiratory volume in the first second):- The fraction of the vital capacity expired during the first second of a forced expiration .

20. Respiratory minute volume (RMV):- ~ 6 L (500 mL / breath × 12 breaths/min). Maximal voluntary ventilation (MVV):- maximum volume of gas that can be moved into and out of the lungs in 1 min by voluntary effort. M easured over a 15 sec → a minute N ormal values :- 140-180 L/min Changes in RMV and MVV - lung dysfunction.

21. Compliance C = ∆V/∆P M easured in the pressure range where the relaxation pressure curve is steepest. normal values :- 0.2 L/cm H ₂ O. Dynamic compliance:- ∆ V / ∆ P of the respiratory system measured at the instant gas flow. In a ventilated patient, this ∆ P = peak airway pressure - PEEP.

22. Static compliance:- ∆ V/ ∆ P of the respiratory system measured at a point of no gas flow and when the pressure gradient has been allowed to equilibrate in the entire airway. ∆ P = plateau airway pressure - PEEP.

23. Airway Resistance Change of pressure (ΔP) from the alveoli to the mouth divided by the change in flow rate (V). Airflow resistance :- 1 cm H ₂ O/L/s . (spontaneously breathing) I n COPD and asthma- ↑ upto 5 to 10 cm H ₂ O/L/s. At a flow of 1 L/sec, 8mm ID ETT - 5 cm H ₂ O/L/sec 7mm ID ETT - 8 cm H ₂ O/L/sec.

24. Airflow resistance – mainly by larger airways: mouth and pharynx - 40 % larynx and large airways- 40 % small airways (3-mm)- 20 % Airway resistance - inversely proportional to lung volume ( ↑ exponentially as lung deflates below FRC).

25. CLOSING CAPACITY & VOLUME CC = CV + RV -Normal value : 15-20% of VC - ↑ with age-loss of parenchymal support -When alveolar unit ↓below its CC-PAO₂↓ & eventually leads to atelectasis CV = Lung volume below which smaller airways begin to close during expiration

26. Surfactant Surface acting material- responsible for ↓surface tension on alveolar membrane Secreted by – 2 types of cells:-(LAMELLAR BODIES) 1)Type 2 pneumocytes 2)Clara cells Components: 1) Dipalmitoyl-phosphatidylcholine (DPPC) 2)Lipids(triglycerides & phosphatidyl glycerol) 3)Proteins-(SP-A,B,C,D) 4)Ions-Calcium

27. Law of Laplace: P = 2T / r Where, P = Distending Pressure T = Tension r = Radius P = 2T / r

28. FUNCTIONS 1) ↓ surface tension- prevents lung collapse 2) Inflates lung after birth (Deficiency-Infant/Adult Respiratory Distress Syndrome / Hyaline Membrane Disease) 3) Defence mechanism ( opsonise microbes) 4) Prevents pulmonary edema (unopposed surface tension-20mm Hg force-Transudation of fluid : Blood-Alveoli)

29. WORK OF BREATHING Work = force x distance or pressure x volume (Denote ongoing energy expenditure) Oxygen requirement- less than 2% of normal basal oxygen consumption(3-4 ml/kg/min) In COPD, it ↑ to 200ml/kg/min Normally- corresponds to - RR :15 -16/min Obstructive lung disease- ↓ Restrictive lung diseases- ↑

30. Distribution Of Ventilation Smaller alveoli - middle & lower portion of lungs - ↑Ventilated (than Larger alveoli- superior lung regions) L ower alveoli - steeper portion ( ↑ complaint) D uring a spontaneous breath → inspired gas → enter open alveoli near Lung base (most compliant)

31. Pulmonary Hemodynamics N ormal mean pulmonary artery pressure 9 to 16 mm Hg P ulmonary circulation - ↑ flow , ↓ pressure Factors responsible: 1)Extremely thin-walled vasculature- ↑compliant reservoir- ̴3.2L/min/m² 2)↓PVR (<250 dynes.sec.cm¯⁵) PVR = [( P PA - PAOP) / CO] x 79.9 PAOP - pulmonary artery catheter occlusion pressure (reflect left atrial pressure) CO - cardiac output (L/min ) Factor- 79.9( mm Hg/L /min- units of absolute resistance)

32. Ventilation & Perfusion

33. DEAD SPACE Volume of air that does not take part in gaseous exchange 2 types: 1)Anatomical /Airway dead space –Conducting Airways (= B.Wt in pounds) 2)Physiological/A lveolar dead space- ventilated but not perfused ( West’s zone 1 )

34. Measured by: BOHR’S EQUATION P ECO ₂ × V T = Pa C0 ₂ × ( V T – V D ) + P ICO ₂ × V D P ECO ₂ - Pco ₂ of expired gas V T - tidal volume Paco ₂ – Pco ₂ of arterial blood V D -dead space volume P I co ₂ - Pco ₂ of inspired air

35. SHUNT / VENOUS ADMIXTURE T he portion of venous blood returned to the heart that passes to arterial circulation without being exposed to normally ventilated lung units 1.Pulmonary 2.Extra-pulmonary venous blood passing through lung regions with decreased or no alveolar ventilation. venous blood that does not pass through the lungs.Eg : 1) Thebesian veins 2)Bronchial veins

36. QS/Q T=Cc’O2 - CaO2/Cc’O2 - CvO2 QS/QT= fraction of total cardiac output (QT) that is shunt (QS) CaO2= arterial O2 content Cc’O2 =Pulmonary capillary O2 content CvO2 =mixed venous ( pul.arterial )O2 content

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