Ventilation and Perfusion

dr. Sri Lestari Sulistyo Rini, MSc

1

TERM

• VENTILATION :– the rate at which blood is supplied with O2

• PERFUSION :– the rate at which O2 is removed (blood flow)

• Ventilation/perfusion ratio :– Ratio of ventilation to blood flow for a single alveolus

(VA/Q), or entire lung (VA/Qt)

2

Anatomical DEAD SPACE

Tidal volume is distributed into dead space (VD) and alveolar volume (VA)

Conducting airways

Gas exchange airways

The oropharynx, trachea and upper airways in the lung, which do not participate in gas exchange, comprise VD

Alveoli comprise the gas exchange compartment or respiratory zone, VA

3

Anatomical Dead Space• Normally represents 20-30% of the minute

ventilation• Influenced by

– Size– Age : neonates 3.3 ml/kg, adults 2 ml/kg– Posture : supine < standing - tidal volume– Head and neck position - respiratory rate– Tracheal intubation/ tracheostomy– Bronchodilating/constricting drugs

4

V V fT T

Total minute ventilation

V V fD D

V V - V fA T D

( )

Dead space ventilation

Alveolar ventilation

How much of total minute ventilation is "wasted"?

Dead space can be measured by two methods:

Fowler's single breath N2 washout Bohr's method

5

Alveolar Dead Space

• Volume of alveoli that is ventilated but not perfused

• Usually negligible, unless– Low cardiac output– Pulmonary embolism– Posture

6

Physiology Dead Space

• Anatomic + Alveolar Dead Space• In healthy individu represents 25-35% of the

minute ventilation• Factors influencing

– Age : increases with age– Sex : slightly higher in men– Body size : app 2 mL/kg– Posture : due to anat dead space– Pathology : pulmonary embolism, smoking

7

• Anatomic and physiologic dead space are essentially equal in the normal lung

• Normal values for VD/VT = 0.20-0.35 (or 20-35%)

8

Alveolar ventilation

9

Minute volume = VExp = VT x f

Alveolar ventilation rate = VA = (VT -VD) x f

10

Ventilation-perfusion ratio (V/Q)It is the ratio of alveolar ventilation to pulmonary blood flow per minute. The alveolar ventilation at rest (4.2L/min) and is calculated as:

Alveolar ventilation = respiratory rate x (tidal volume – dead space air).The pulmonary blood flow is equal to right ventricular output per minute (5L/min).

11

12

VA

Q<< 0.8 VA

Q~ 0.8

Let’s assume that there is a blockage of one alveolar region

13

• Normally the V/Q ratio is closer to 0.8.

• If V/Q ratio is 1

– capillary PO2 will reach equilibrium with alveolar PO2

– there will be no alveolar arterial PO2 difference.

• If V/Q ratio is Zero :

– alveolar pO2 and pCO2 = mixed venous blood (shunt)

• If V/Q ratio is Infinity :

– alveolar pO2 and pCO2 = inspired gasses (alveolar dead space)

• where ventilation and perfusion have normal matching, the PO2 will be about 100 and the PCO2 will be about 40 torr.

14

This value is an average value across the lung.At the apex, V/Q ratio = 3.At the base, V/Q ratio = 0.6.So the apex is more ventilated than perfused, and the base is more perfused than ventilated.

During exercise, the V/Q ratio becomes more homogenous among different parts of the lung.

15

Regional Gas Exchange in the Lung

• Even in normal healthy individuals there is a V/Q heterogeneity.

• produced by an uneven distribution of ventilation and perfusion among regions of the lung.

• Ventilation is greater in the lower (caudal) region of the lung than in the upper (cranial) region

• Blood flow is also greater in the caudal compared with the cranial region of the lung

16

• Single-breath 133Xe test 

• Xe has very low water solubility, so remains within the airspace; imaged using external detectors

• Regional VA is indeed greater at the base of the lung in an upright individual

Regional heterogeneity in ventilation

•

17

PO2 = 40PCO2 = 45

Low VA/Q.

Normal VA/Q.

PO2 = 100PCO2 = 40

High VA/Q.

PO2 = 150PCO2 = 0

PO2 (mm Hg)

PC

O2 (

mm

Hg)

50 100 150

50

Base

Apex

18

19

PAPa Pv

Zone 1PA>Pa>Pv

Low Flow

PA

Pa Pv

PA

Pa Pv

Zone 2Pa>PA>Pv

Waterfall

Zone 3Pa>Pv>PA

Hi Flow

perfusion

Zones of the lung

20

21

• The lung may be considered to comprise 3 compartments:– Ventilated but unperfused

alveoli• Alveolar dead space

– Perfused but unventilated alveoli

• Intrapulmonary shunt

– Ideally perfused & ventilated alveoli

22

Characteristics of the Pulmonary Circulation

23

“Special” Characteristics of the Pulmonary Circulation

Systemic Circ. Pulmonary Circ.

C.O. (L/min) 6.0 ≈ 5.9

Arterial B.P. (mm Hg) 100 >> 15

Venous B.P. (mm Hg) 2 “≈” 5

Vascular resistance (∆P/flow) 100-2/6=16.3 > 15-5/5.9=1.7

Vascular compliance (∆V/∆P) Csystemic << Cpulm

24

Ability to promote a decrease in resistance as blood pressure rises

Special Characteristics of the Pulmonary Circulation: high compliance

R =8lr4

Remember that resistance to Flow =

viscosity length

radius25

Special characteristic of blood vessels surrounding alveoli: hypoxic vasoconstriction

When PO2 within the alveoli decreases there is a decrease in blood flow to that alveolus

This is called hypoxic vasoconstriction

Thought to be the result of O2-sensitive K+ channels in the smooth muscle membrane. At low O2 the K+ channels close, the Em rises,

and the cell reaches threshold and depolarizes and contracts.

smooth muscle cell This phenomenon is just the opposite of the response to hypoxia you get with arteriole smooth muscle in the systemic circulation, but it is an important feature of the pulmonary circulation that helps to match perfusion with ventilation

26

Pulmonary blood vessels are much more compliant than systemic blood vessels. Also the system has a remarkable ability to promote a decrease in resistance as the blood pressure rises.

Two reasons are responsible:

Recruitment: opening up of previously closed vessels

Distension: increase in caliber of vessels

27

Inspired air:

PO2 = 158 mm Hg

PCO2 = 0.3 mm Hg

Expired air:

PO2 = 116 mm Hg

PCO2 = 32 mm Hg

Pulmonary vein

Aorta

Arterial blood

PO2 = 95 mm Hg

PCO2 = 41 mm Hg(physiological shunt)

Gas exchange at alveolar and systemic capillaries

Left HeartRight Heart

28

Gas exchange is optimal when ventilation and perfusion are going to the same places and to the extent that they are not, gas exchange suffers.

In an extreme case, if all the blood flow went to the right lung and all the ventilation went to the left, the person might have normal cardiac output and normal alveolar ventilation and no gas exchange.

29

Ventilation/perfusion mismatch

• Main cause of hypoxemia in lung diseases

• O2 transport/perfusion inhomogeneity probably also in other organs

30

Matching respiration & blood flow: the Ventilation-Perfusion Ratio

Alveolar ventilation, VA

VA = (VT - VD) x resp. rate

= (0.5 - 0.15) x 12 = 4.2 L/min

Cardiac output = C.O. = Q

Q = stroke vol. x heart rate

= (0.086) x 70 = 6.0 L/min

= ventilation/perfusion ~ 0.8VA

Q

Ventilation

Perfusion

31

32

33

Normal Emphysema AsthmaPulm. Circ.

Exercise Capillary enlargement (e.g., Mitral Stenosis)

Longer paths for diffusion

Pathological Examples of Altered Respiratory Mechanics

34

SHUNTS• the mixing of deoxygenated blood from systemic veins

with oxygenated blood coming from pulmonary capillaries. – Shunt occurs when blood flows from the venous

system to the arterial system without being oxygenated.

• Shunts are classified as anatomic or physiologic; (a small anatomic shunt [2-3%] is normal).– Physiologic shunts are caused by alveolar collapse

or alveoli filled with a substance other than air.35

• Shunt (the maximal ventilation perfusion mismatch [V/Q ratio of zero]) significantly reduces PaO2.

• Causes ;– the Thebesian circulation which perfuses the left

ventricle and empties directly into the left ventricle without passing through the lung.

– Lung tissue itself must be perfused (bronchial circulation), and this blood empties into pulmonary veins, mixing with pulmonary capillary blood.

– Congenital heart defects (Tetralogy of Fallot)– Pulmonary pathology (Athelectasis/PNEUMONIA)

36

37

38

39

40

41

Forced capacity (FVC & FEV1)Normal

Obstructive

Restrictive

VCNor

FEVN

)(

1)(

VCN

FEVN

)(

1)(

VCNor

FEVN

)(

1)(

42

FORCEDFORCEDEXPIRATIONEXPIRATION

NORMALNORMAL

FEVFEV11 = 3.0L = 3.0L

FVC = 4.2LFVC = 4.2LFEVFEV11/FVC = 72%/FVC = 72%

OBSTRUCTIVEOBSTRUCTIVE

FEVFEV11 = 0.9L = 0.9L

FVC = 2.3LFVC = 2.3LFEVFEV11/FVC = 40%/FVC = 40%

RESTRICTIVERESTRICTIVE

FEVFEV11 =1.8L =1.8L

FVC = 2.3LFVC = 2.3LFEVFEV11/FVC = 78%/FVC = 78%

1 SECOND1 SECOND

FEVFEV11

FEVFEV11

FEVFEV11

TIDALTIDALBREATHINGBREATHING

43

44

Terima kasih