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ASSESSMENT OF LUNG AS AN OXYGENATOR CONCEPT OF VENTILATION – PERFUSION RATIO, DEAD SPACE, SHUNT Speaker : Dr. S Sai Janani University College of Medical.

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Presentation on theme: "ASSESSMENT OF LUNG AS AN OXYGENATOR CONCEPT OF VENTILATION – PERFUSION RATIO, DEAD SPACE, SHUNT Speaker : Dr. S Sai Janani University College of Medical."— Presentation transcript:

1 ASSESSMENT OF LUNG AS AN OXYGENATOR CONCEPT OF VENTILATION – PERFUSION RATIO, DEAD SPACE, SHUNT Speaker : Dr. S Sai Janani University College of Medical Sciences & GTB Hospital, Delhi

2 CONTENTS Ventilation – perfusion ratio Regional differences in ventilation Regional differences in perfusion Ventilation – perfusion ratio and anesthetic implications Dead space Shunt Anesthesia and dead space, shunt and V/Q mismatch Assessment of lung as an oxygenator

3 VENTILATION- PERFUSION RATIO Defined as the ratio between ventilation and perfusion in a lung unit. Ventilation(V) = 4 L/ min Perfusion(Q) = 5 L / min V/Q = 0.8 (young adult) Differs in different lung regions due to regional differences in ventilation and perfusion.

4 Regional differences in ventilation: Compliance of the alveolus PRESSURE SUPPORT NEEDED AT BASE > APEX MORE LUNG VOLUME AT THE BASE

5 Regional differences in ventilation Lung is suspended from the hilum The weight of the lung needs more pressure support from below than above Pressure at the base > apex Intra pleural pressure is less negative at the base and more negative at the apex

6 Regional differences in Ventilation AT APEX: INTRAPLEURAL PRESSURE IS MORE NEGATIVE ALVEOLI NEAR APEX MAXIMALLY INFLATED DURING INSPIRATION, WHEN-VE THORACIC PRESSURE IS CREATED ∆PRESSURE BASE > ∆PRESSURE APEX VENTILATION BASE>APEX IN AWAKE ERECT POSITION

7 Compliance…….

8 Regional differences in ventilation SUPINE POSITION: Dorsal region is dependent. Hence, ventilation is better dorsally than ventrally. Thus, the diaphragm is DESIGNED TO HAVE GREATER EXCURSIONS POSTERIORLY than anteriorly.

9 Ventilation in anesthetised pt. 1.FRC < CV. THE ALVEOLI IN THE DEPENDENT REGION COLLAPSE 2. THE PREFENTIAL DORSAL EXCURSION OF DIAPHRAGM IS LOST IN PARALYSIS

10 Atelectasis after anesthesia in the dependent parts Augmented when higher FiO2 is used or during pre – oxygenation ABSORPTION ATELECTASIS

11 Ventilation in anesthetised pt THE VENTILATION OF NON DEPENDENT PART > DEPENDENT PART IN SUPINE : ANTERIOR > POSTERIOR IN LATERAL DECUBITUS: NONDEPENDENT LUNG > DEPENDENT LUNG

12 REGIONAL DIFFERENCES IN PERFUSION Primary determinant – GRAVITY Healthy adult – erect position: Distance from apex to base = 30 cm Assuming that pulm. Artery enters the lung midway b/w apex and base - 15 cm H20 ( overcome) + 15 cm H20(gain) PERFUSION AT BASE > APEX PASCAL’S LAW P = h ῥ g

13 Regional differences in perfusion

14 Based on the differences in perfusion, the lung is divided onto West’s zones: 1.Alveolar pressure is constant from apex to base 2.If the pulm. Artery system is considered a continuous column of blood, the pressure exerted by the column above is greater at the base than at the apex (by 30cmH2O)

15 Regional differences in perfusion: The West’s Zones ZONE I : MINIMAL BLOOD FLOW DEPENDS ON a-v DIFFERENCE

16 Regional differences in perfusion: The West’s Zones ZONE I : MINIMAL BLOOD FLOW DEPENDS ON a-v DIFFERENCE INDUCTION OF ANESTHESIA. ↓Q T, “WASTED VENTILATION”,ALVEOLAR DEAD SPACE

17 Regional differences in perfusion: The West’s Zones ZONE II: WATERFALL EFFECT DEPENDS ON a-A DIFFERENCE

18 Regional differences in perfusion: The West’s Zones ZONE III: HIGH PERFUSION DEPENDS ON a-v DIFFERENCE

19 Regional differences in perfusion Supine position: West’s zones

20 VENTILATION – PERFUSION RATIO ↑ PERFUSION > ↑ VENTILATION FROM APEX TO BASE V/ Q AT APEX > V/Q AT BASE ↑ PERFUSION > ↑ VENTILATION FROM APEX TO BASE V/ Q AT APEX > V/Q AT BASE

21 Types of alveoli based on V/Q SHUNTDEAD SPACE

22 Ventilation –Perfusion ratio SHUNTDEAD SPACE PHYSIOLOGICALLY THESE UNITS ARE NOT ABSOLUTE AND OCCUR AS A SPECTRUM PHYSIOLOGICALLY THESE UNITS ARE NOT ABSOLUTE AND OCCUR AS A SPECTRUM

23 Arterial blood gases and V/Q O2=40 Co2 = =100 Co2=40 02 = 103 Co2 = 0 02 = 103 Co2 = 0 SHUNT DEAD SPACE

24 Regional differences in gas exchange

25 MEASUREMENT OF V/Q Regional differences- Measured by radioactive gases: Xe Single and multiple N 2 breath tests By measuring (A-a)PO 2 PAO 2 measured by alveolar gas equation PAO 2 = PIO 2 – PACO 2 + F R PACO 2 = PaCO 2

26 MULTIPLE BREATH TEST

27 Hypoxic Pulmonary Vasoconstriction Adaptation of pulmonary blood flow to ventilation. Constriction of pulm. Blood vessels in areas Of reduced ventilation GA attenuates HPV > inhalational agents Less attenuation when Thiopentone is used for induction

28 DEAD SPACE That portion of the respiratory tree in which no gas exchange takes place V / Q = ∞ Volume occupied by the conducting system. Represents “wasted ventilation”

29 DEAD SPACE Types: Anatomical dead space (Vd anat) Physiological dead space (Vd phys) Apparatus dead space

30 Dead space Anatomical dead space:  The volume of the respiratory passages, extending from nostrils and mouth down to (but not including) the respiratory bronchioles.  Varies with age and sex  Normal = 150 mL  Young women = 100 mL  Old men = 200 mL  Normally = 1 mL / lb body weight or 2.2 mL / kg body weight

31 Dead space….. Anesthesia and Anatomical dead space: Head tilt- chin lift (+40 mL) Depression of jaw with flexion of head (Airway obstruction) = - 30 mL Tracheostomy / Pneumonectomy = ↓

32 Measurement of anatomical dead space Modification of FOWLER’S Single breath N2 wash out method

33 Dead space Physiological dead space (Vd phys):  Fraction of tidal volume not available for gas exchange.  Anatomical + Alveolar dead space  Alveolar dead space = ↑ V/Q regions = wasted ventilation

34

35 Physiological dead space Normally, Anatomical dead space = Physiological dead space = 1/3 tidal volume Expressed as that fraction of tidal volume that contributes to “wasted ventilation” (Vd / Vt) Vd / Vt = 0.25 – 0.4

36 Vd phys Factors affecting Vd phys: Old age Upright position ↑ Tv ↑ RR Atropine T inspi < 0.5 secs during CMV ↑V/Q states Hypotension ( esp. with head up position)

37 Measurement of Physiological Dead space PHYSIOLOGICAL DEAD SPACE AND ANESTHESIA: Roughly estimated by Cooper’s formula: Measurement of Vd phys: (Vt – Vd)PACO2 = PeCO2 X Vt OR (subtract apparatus dead space) Vd phys = PaCO2 – PeCO2 PaCO2 Vd phys = PaCO2 – PeCO2 PaCO2 Vd/ Vt = 33 + age /3 % ENGHOFF’S MODIFICATION OF BOHR EQUATION

38 Bohr Equation

39 Dead space… Vd phys and the capnogram: (Non invasive measurement) (PaCO 2 – ETCO 2 ) gradient increases when there is significant Vd alveolar thus increasing vd phys Hence at that time, ETCO2 is an unreliable monitor for PaCO2 Vd phys and the capnogram: (Non invasive measurement) (PaCO 2 – ETCO 2 ) gradient increases when there is significant Vd alveolar thus increasing vd phys Hence at that time, ETCO2 is an unreliable monitor for PaCO2

40 PaCO 2 PeCO 2 PaCO 2 PeCO 2 PaCO 2

41

42 Dead space APPARATUS DEAD SPACE: Volume of gas contained in any anesthetic apparatus between the patient and that point in the system where rebreathing of exhaled carbon dioxide ceases to occur

43 Dead space Anatomical face mask = ↑ Vd / Vt= 0.68 COPA or LMA = Vd/Vt = 0.3 – 0.4 ETT or Tracheostomy = ↓Vd/ Vt Breathing circuits – Length Diameter Compressible volume ( 2 -3 mL gas for every 1 cmH2O inspiratory pressure) Anatomical face mask = ↑ Vd / Vt= 0.68 COPA or LMA = Vd/Vt = 0.3 – 0.4 ETT or Tracheostomy = ↓Vd/ Vt Breathing circuits – Length Diameter Compressible volume ( 2 -3 mL gas for every 1 cmH2O inspiratory pressure)

44 Dead space Pathologies producing dead space : Anatomic: Rapid shallow breathing Alveolar dead space: Acute pulmonary embolus Redistribution of pulmonary perfusion: ↓cardiac output Acute pulm. Hypertension Ventilation > Perfusion PPV Alveolar septal destruction

45 SHUNT Defined as blood that enters the arterial system without going through ventilated areas of the lung. Defined as blood that enters the arterial system without going through ventilated areas of the lung.

46 Shunt….. Types: 1.Anatomical shunt: “True shunt” due to area in which there is absolutely no ventilation, but perfusion exists. 2.Physiological shunt: Normal degree of venous admixture due to true shunt + ↓V/Q ratio 3.Pathological shunt: Those forms of anatomical shunt which are not present in a normal subject. 4.Atelectatic shunt: Blood which has passed through collapsed regions of the lung. Types: 1.Anatomical shunt: “True shunt” due to area in which there is absolutely no ventilation, but perfusion exists. 2.Physiological shunt: Normal degree of venous admixture due to true shunt + ↓V/Q ratio 3.Pathological shunt: Those forms of anatomical shunt which are not present in a normal subject. 4.Atelectatic shunt: Blood which has passed through collapsed regions of the lung.

47 Anatomical shunt

48 Physiological shunt PAO2 = 101 PaO2 = 97 (A- a) O2 = mmHg

49 SHUNT (A – a) PO2 = 5 – 25 mmHg PAO2 = 101 PaO2 = 97 AMOUNT OF MIXED VENOUS BLOOD ADDED INORDER TO PRODUCED AN OBSERVED (A-a) – VENOUS ADMIXTURE AMOUNT OF MIXED VENOUS BLOOD ADDED INORDER TO PRODUCED AN OBSERVED (A-a) – VENOUS ADMIXTURE

50 Shunt……….. Sources of venous admixture (5% CO) ↓V/Q ratio Shunt

51 Shunt………. ↓V/Q RATIO SHUNT OXYGENATION IN ↓V/Q AREAS – IMPROVES WITH ↑FiO2 SHUNT IS REFRACTORY TO ↑FiO2

52 Calculating the shunt fraction….. The shunt equation is derived as: Pulmonary capillary blood flow (Qc) + blood flow through shunt(Qs) = cardiac output(Qt) In terms of O2 content: ( CcO2 X Qc)+(CvO2 X Qs) = (CaO2 XQt) Qc + Qs = Qt

53 The shunt equation Since, Qc = Qt – Qs, (CcO 2 X Qt) – (CcO 2 X Qs)+ (CvO 2 X Qs) = (CaO 2 X Qt) Dividing both sides by Qt, Qs = CcO 2 – CaO 2 Qt CcO 2 - Cvo 2 Qs = CcO 2 – CaO 2 Qt CcO 2 - Cvo 2

54 Shunt equation Inorder to remove CvO2 from the equation, We add and subtract CaO2 in the denominator. Qs = (CcO2 – CaO2) Qt (CcO2 – CvO2 + CaO2 –Cao2) Since CaO2 – CvO2 = 5 in non critical patients, And 3.5 in critical patients, we substitute in the equation to get the Estimated shunt Equation

55 Estimated physiologic shunt equation…… Does not need a pulm. Artery sample. Non critical patients: (Spontaneously breathing,mod. FiO2, mod. CPAP) Estimated Qs= (CcO 2 – CaO 2 ) Qt [5+(CcO 2 -CaO 2 )]

56 Estimated shunt equation….. For critical patient: (Mechanical ventilation, High FiO2, high level of positive end-expiratory pressure) Estimated Qsp = (CcO2 –CaO2) Qt [3.5 + (CcO2 – caO2)] Estimated Qsp = (CcO2 –CaO2) Qt [3.5 + (CcO2 – caO2)]

57 Shunt Equation….

58 Shunt…..

59 Shunt producing pathology… Anatomic: 1.Congenital heart disease 2.Pulmonary fistula 3.Vascular lung tumors Capillary shunting: 1.Acute atelectasis 2.Alveolar fluid 3.Consolidation Perfusion > Ventilation 1.Hypoventilation 2.Uneven distribution of ventilation 3.Diffusion defect

60 Anesthesia and V/Q mismatch, shunt and dead space BEFORE INDUCTION: AWAKE SUPINE SUBJECT HPV Dead space = Anatomical dead Space Minimal shunt Ventilation perfusion relationships during anesthesia. Hedenstierna G, Thorax 1995; 50:

61 After induction, with anatomical face mask and inhalational agent ANESTHESIA INDUCED ANATOMICAL FACE MASK INHALATIONAL AGENT: HPV attenuated Absorption atelectasis- ↑ shunt Hypotension by induction agent- ↑ alv Dead space Face mask – ↑ apparatus dead space ↑ dead space, shunt and V/Q mismatch areas Arterial oxygenation compromised

62 After endotracheal intubation and mechanical ventilation TRACHEA INTUBATED ON MECHANICAL VENTILATION Dead space reduced to awake levels Slight widening of V/Q distribution Shunt almost the same

63 Shunt and V/Q mismatch in anesthetised and awake individuals

64 ASSESSMENT OF LUNG AS AN OXYGENATOR Clinical assessment: Level of consciousness ~ adequate cerebral oxygenation Vital signs: RR, HR, Blood pressure, temp. General physical examination: Head and neck: Inspection of face – any nasal flaring cyanosis pursed lip breathing Neck: position of trachea Shifted in upper lobe collapse pneumothorax, pleural effusion lung tumor

65 Inspection: Shape of the chest Kyphosis Scoliosis Flattening Over inflation Measurement of chest: Rate of respiration Rhythm Chest expansion Symmetry Percussion: Resonance/ dullness Auscultation: Breath sounds, adventitious sounds

66 Lung as an oxygenator…….. ABG: PaO2 = 80 – 100 mmHg paCO2 = mmHg P(A-a)O2 = mmHg (↑ V/Q mismatch, shunt) PaO2/ FiO2 = 100 /0.2 = 500 O2 content = 19.8mL/dl blood SpO2 ETCO2 V/Q mismatch shunt

67 Lung as oxygenator: Oxygenation indices: PaO2 (A – a) PO2 PaO2 / FiO2 PaO2 / FiO2 x Paw Pao2 / PAO2 PaO2 / PvO2

68 Bibliography 1.Respiratory Physiology- John.B.West 7 th ed 2.Egan’s Respiratory Care- 4 th ed 3.Miller’s Anesthesia- 5 th ed 4. Textbook of Anesthesiology- Morgan 5 th ed 5. Applied Respiratory Physiology- J.F.Nunn 6. Textbook of Anesthesia- gray and Nunn 7.Mechanical ventilation- Chang- 4 th ed 8.Clinical Application of Blood Gases- Shapiro- 5thed. 9.understanding Anesthesia equipment- dorsch & dorsch- 7 th ed. 10.A practice of Anesthesia-Wiley 5 th ed. 11.review of Medical Physiology – W.F. Ganong 12.V/Q distribution and correlation of atelectasis in anesthetised paralysed humans- JAP 13. ventilation – perfusion ratio inAnesthesia –hedenstierna et al – Thorax ; 50:

69 THANK YOU


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