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Ventilation: Breathing Quantified


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VENTILATION

P rocess by which fresh gas moves in and outof the lungs.

Alveolar ventilation: process by which gasmoves between the alveoli and externalenvironment.Minute ventilation: volume of air that entersor leaves the lung per minute.MV=f x Vt


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VENTILATION(cont.)

Each minute at rest on an average 300ml of O2 is taken up and 250ml of CO2 is removedby alveolar ventilation.Thus the partial pressure of O2 and CO2 in thealveolar air are determined by alveolarventilation, by O2 consumption and by CO2production.


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VENTILATION(cont.)

RESP IRATORY QUOTIENT :Ratio of CO2 excreted(Vco2) to the O2

taken up(Vo2) by the lungs.It varies between 0.7 and 1.0 and consideredto be 0.8 under usual circumstances.


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D ISTRIBUTION OF VENTILATION

Ventilation is not evenly distributed in thelung.

Regional differences in ventilation due toeffects of gravity.In upright position,ventilation per unit of volume is greatest in lower regions of the lungcompared with the upper lung regions.


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D ISTRIBUTION OF VENTILATION(cont.)

In upright position the intrapleural pressure isless negative in the lower gravity dependentregions.P leural pressure increases by +0.2cm of waterfor every cm of vertical displacement from topto the most dependent part of the lung.This pressure gradient is due to effect of gravity and weight of lung.


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In upright position alveoli near the apex aremore expanded than alveoli at the base.

This is because transpulmonary pressure ismore in the apical regions than in the base.As inspiration begins, alveoli at the apex andat the base of lung have different volumes andare therefore at different locations on thepressure volume curve.


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Intrapleural P ressure Gradient


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Intrapleural P ressure Gradient


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Regional D istribution of AlveolarVentilation from FRC


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Regional D istribution of AlveolarVentilation from RV


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Single breath nitrogen test: used to examine thedistribution of ventilation in patients.Exhales to RESI D UAL VOLUME and then inspires100% O2 maximally.D uring the subsequent exhalation N2 conc. Of exhaled air is measured.Initially there is no nitrogen in the exhaled air.As alveolar emptying occurs the N2 conc. In theexpired air begins to rise reaching a plateau.


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SINGLE BREATH NITROGEN TEST


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P ulmonary blood flow

Lung is the only organ in the body thatreceives blood from two separate sources.P ulmonary blood flow and bronchial bloodflow.P ulmonary circulation arises from rightventricle and receives 100% of blood flow.

Bronchial circulation arises from aorta andreceives only 2% of left ventricular output.


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PulmonaryPulmonary CirculationCirculation


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Thin walled vessels at all levels.

P ulmonary arteries have far less smoothmuscle in the wall than systemic arteries.

Consequences of this anatomy- the vesselsare: D istensible. Compressible.

Thin walled vessels at all levels.P ulmonary arteries have far less smooth

muscle in the wall than systemic arteries.Consequences of this anatomy- the

vessels are:

D istensible.Compressible.

Functional Anatomy of theFunctional Anatomy of the

P ulmonary CirculationP ulmonary Circulation


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P ULMONARY BLOOD FLOW

P ulmonary capillary bed has the largestsurface area of any vascular bed in the body.Many of these capillaries are closed at restand open periodically during periods of increased flow.At rest it takes 4 to 5 seconds for a red cell to

travel through the pulmonary circulation with0.75 secs of this time spent in pulmonarycapillaries.


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P ulmonary capillaries have diameter of about 6micrometer slightly smaller than the diameter of a redcell.D

uring exercise CO and thus pulmonary blood flow canincrease to as much as 25lts/min/m2 and thepulmonary capillary volume increases with little or noincrease in the vascular resistance.This increase in volume occurs through two processes:

1)recruitment2)distension


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RECRUITMENT AND D ISTENSION


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P ulmonary veins are situated within looseinterlobular connective tissue septae andreceive blood from many lung units.They return the blood to left atrium and dueto large numbers and thin walls provide alarge reservoir of blood.

Larger pulm. vessels contain smooth muscleand have a motor supply through thesympathetic nervous system.


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BRONCHIAL BLOOD FLOW

P erfuses the respiratory tract to the level of terminal bronchioles.D istal to it lung is directly oxygenated by diffusionfrom alveolar air and receive nutrients from theblood in pulmonary circulation.Usually 3 in number arise either from aorta or

from intercostal arteries.P ressure is equal to that in the systemiccirculation.


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BRONCHIAL BLOOD FLOW(cont.)

The return of bronchial blood to the heart is througheither of the following:

1) true bronchial veins2) bronchopulmonary veins

True bronchial veins drain into azygous,hemiazygous orintercostal veins and finally into right atrium.Bronchopulmonary veins are formed by anastomosis of both pulmonary and bronchial vessels which with anadmixture of blood from both vessels returns to leftatrium through pulm. veins.


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BRONCHIAL BLOOD FLOW(CONT.)

Lung transplant studies have shown that adultlungs can function normally in absence of bronchial circulation .However they are important in neonates andyoung children in bringing nutrients to thedeveloping lungs .

Another importance is in diseased states in whichreciprocal change in flow in the two circulationsprovides a steady supply of nutrients.


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EFFECTS OF GRAVITY ONP ULM. FLOW

Because the pulm. Circulation is low pressure,low resistance system it is effected by gravitymuch more dramatically than the systemiccirculation.Leads to an uneven distribution of blood flowin the lung.

In upright position there is a linear increase inblood flow from the apex to the base of thelung.


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EFFECTS OF GRAVITY ONP ULM.FLOW(cont.)

Under conditions of stress the difference in bloodflow between apical and basal regions becomesless due mainly to the increase in blood flow.Upon leaving the pulmonary artery blood musttravel up to the apex against gravity in uprightposition.

For every increase of 1cm in height above theheart there is a corresponding decrease inhydrostatic pressure in vessels.


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ZONES OF LUNG

When referring to blood flow lung has beenclassically divided into three zones:

a) zone 1 represents the apex where it ispossible to have no blood flow. Under normalconditions this zone does not exist.

b) zone 2 represents upper 1/3 rd of the

lung.C) zone 3 comprises of basal areas of lung.


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ZONES IN THE LUNG


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EFFECT OFP RESSURE CHANGES ONEXTRA ALVEOLAR AND ALVEOLAR

VESSELS Changes in alveolar and intrapleural pressure can

also influence pulmonary blood flow.

These effects are different depending on thelocation of the vessels.The EA vessels are larger vessels and aregenerally not affected by alveolar pressure

changes.They are affected by pleural and interstitialpressure changes.


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EFFECT OFP RESSURE CHANGES ONEXTRA ALVEOLAR AND ALVEOLAR

VESSELS As the transpulmonary pressure increases thecaliber of extra alveolar vessels increases andvice-versa.As the alveolar pressure increases the alveolarvessels get constricted and caliber decreases.Thus high lung volumes have opposite effecton alveolar and extra alveolar vessels andtheir resistance.


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P ULMONARY VASCULAR RESISTANCE

The resistance of alveolar and extra alveolarvessels are additive to each other at any lungvolume because the alveolar and extraalveolar vessels are in series to each other.This results in U shaped curve of totalpulmonary resistance.P VR is lowest near FRC and increases at bothlow and high volumes


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P ULMONARY VASCULAR RESISTANCE


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P ulmonary Vascular Resistance

With positive pressure mechanical ventilationthere is both an increase in alveolar pressureand a decrease in transmural pressuregradient.this results in compression of both alveolarand extra alveolar capillaries and can blockblood flow to the area.Thus it can create zone 1 blood flow.


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HYP OXIC VASOCONSTRICTION INP ULM. VESSELS

This occurs in small arterial vessels inresponse to decreased arterial po2.

Response is local and is believed to be aprotective response as it shifts the blood flowfrom the hypoxic areas to normal areas in aneffort to enhance gas exchange.


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VVentilation perfusion relationships

V/Q ratio is the ratio of ventilation to bloodflow (perfusion).

It can be defined for a single alveolus, a groupof alveoli, or for the entire lung.The relationship between ventilationperfusion that is V/Q ratio is the majordeterminant of normal gas exchange.


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V/Q relationships (cont.)

At rest in normal individuals alveolarventilation is 4 lts per minute.P

ulmonary blood flow is 5 lts per minute.Overall V/Q ratio is 0.8.If ventilation and perfusion are mismatchedimpairment of both O and CO transferoccurs .


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V/Q relationships (contd )

Ventilation-perfusion mismatching occurs withincreasing age and with various lung diseases.

When ventilation exceeds perfusion, the V/Q ratio > 1.When perfusion exceeds ventilation, the V/Q

ratio < 1


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REGIONALD IFFERENCES IN VENT. AND P ERFUSION

Because of the regional differences inventilation and perfusion, even in the normallung the V/Q in different areas of the lung is

greater or less than the normal value of 0.8.In upright position in a lung from top tobottom ventilation increases more slowlythan perfusion does.As a result V/Q at the top is high while as atthe bottom it is abnormally low.


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VENTILATION,P ERFUSION AND V/P RATIO


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ALVEOLAR O2 AND CO2 LEVELS INRELATION TO V/P RATIO


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O 2 O 2 DI S OW ING EN TIL TION -PE F SION TIO LIN E


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Effects of ventilation-perfusioninequality on overall gas exchange

Lung with ventilation perfusioninequality is not able to transfer as

much O2 and CO2 as a lung that isuniformly ventilated and perfused,other things being equal .


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Thank you


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O2 = 150

CO2 = 0.3

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