1. Speaker : Dr. S Sai Janani
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
MORE LUNG
VOLUME
AT THE BASE
PRESSURE SUPPORT
NEEDED AT
BASE > APEX

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
IN AWAKE ERECT POSITION
AT APEX:
INTRAPLEURAL
PRESSURE
IS MORE NEGATIVE
DURING INSPIRATION,
WHEN-VE THORACIC
PRESSURE IS CREATED
ALVEOLI NEAR APEX MAXIMALLY
INFLATED
∆PRESSURE BASE >
∆PRESSURE APEX
Capacity to inflate> base than the apex. Thus due to the larger pressure gradient at the base than at the apex, the ventilation is greater at the base.
VENTILATION
BASE>APEX

7. Compliance…….
Thus ventilation is always better in the dependent portion than in the non dependent portion in the awake individual.
In the erect position, the base is the dependent portion. Hence in the erect position, ventilation at the base is > apex

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.
However, the bulk of the lung volume lies posteriorly, and this has to bear the weight of the entire anterior ans middle mediastinum too.

9. Ventilation in anesthetised pt.
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)
PASCAL’S LAW
P = hῥg
+ 15 cm H20(gain)
PERFUSION AT BASE > APEX

13. Regional differences in perfusion

14. Regional differences in perfusion
Based on the differences in perfusion, the lung is divided onto West’s zones:
Alveolar pressure is constant from apex to base
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. ↓QT , “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

21. Types of alveoli based on V/Q
SHUNT
DEAD SPACE

22. Ventilation –Perfusion ratio
SHUNT
DEAD SPACE
Due to the presence of a spectrum, the v/q of the lung is non uniform. This has a profound influence on the arterial blood gases.
PHYSIOLOGICALLY THESE UNITS ARE NOT ABSOLUTE
AND OCCUR AS A SPECTRUM

23. Arterial blood gases and V/Q
SHUNT
02 = 103
Co2 = 0
O2=40
Co2 = 45
DEAD SPACE
02 =100
Co2=40
02 = 103
Co2 = 0
Thus the arterial oxygenation depends on the number of normal alveoli, v/q mismatch , amount of shunt and dead space.thus inc. in shunt or deadspace will reduce arterial oxygenation.this is important for us because anesthesia changes the balnce between the

24. Regional differences in gas exchange

25. MEASUREMENT OF V/Q
Regional differences- Measured by radioactive gases: Xe
Single and multiple N2 breath tests
By measuring (A-a)PO2
PAO2measured by alveolar gas equation
PAO2 = PIO2 – PACO F R
PACO2 = PaCO2

26. MULTIPLE BREATH TEST
After this we now go to 1 end of the spectrum – that is dead space

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

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/ Vt = 33 + age /3 %
Vd phys = PaCO2 – PeCO2
PaCO2
ENGHOFF’S MODIFICATIONOF BOHR EQUATION

38. Bohr Equation

39. Dead space…
Vd phys and the capnogram: (Non invasive measurement) (PaCO2 – ETCO2) gradient increases when there is significant Vd alveolar thus increasing vd phys Hence at that time, ETCO2 is an unreliable monitor for PaCO2
This is seen in conditions like COPD esp. emphysema where there is inc. in alveolar dead space due to air trapping.

40. PaCO2
PeCO2
PaCO2
PeCO2
PaCO2
PeCO2

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)

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.

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

47. Anatomical shunt NORMAL ABNORMAL Extra pulmonary Thebesian veins
CnHD with R – L shunt
Pulmonary
Bronchial veins
Atelectasis
Pulm infection
Pulm AV shunts
Pulm neoplasm
Circn in edematous lung

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

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

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

51. Shunt………. OXYGENATION IN ↓V/Q AREAS – IMPROVES WITH ↑FiO2
↓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,
(CcO2 X Qt) – (CcO2 X Qs)+ (CvO2 X Qs)
= (CaO2 X Qt)
Dividing both sides by Qt,
Qs = CcO2 – CaO2
Qt CcO2 - Cvo2

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= (CcO2 – CaO2)
Qt [5+(CcO2-CaO2)]

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)]

57. Shunt Equation…. <10% 10 -20% 20 -30% >30% PHYSIOLOGIC SHUNT
INTERPRETATION
<10%
NORMAL
10 -20%
MILD
20 -30%
MODERATE
>30%
SEVERE

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 7th ed
2.Egan’s Respiratory Care- 4 th ed
3.Miller’s Anesthesia- 5th 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- 4th ed
8.Clinical Application of Blood Gases- Shapiro- 5thed.
9.understanding Anesthesia equipment- dorsch & dorsch- 7th 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