Pulmonary physiology in ventilated babies

Pulmonary Physiology in Pulmonary Physiology in Ventilated BabiesVentilated Babies

Mohamed T. KhashabaMohamed T. Khashaba Professor of Pediatrics & NeonatologyProfessor of Pediatrics & Neonatology

Head of NICUHead of NICU,,

Mansoura faculty of MedicineMansoura faculty of Medicine

1.1. Pulmonary mechanicsPulmonary mechanics..2.2. Physiology of gas exchange:Physiology of gas exchange:

• COCO22

• OO22

3.3. V/Q mismatching.V/Q mismatching.

4.4. FiOFiO22 and its effects. and its effects.

5.5. Dead space.Dead space.

6.6. Effects of:Effects of:• Elevated intra thoracic pressure.Elevated intra thoracic pressure.• Pulm. Edema.Pulm. Edema.• High PaOHigh PaO22

ABCsABCs

Respiratory system is composed of Respiratory system is composed of millions of air exchange units connected to millions of air exchange units connected to outside by conducting airways.outside by conducting airways.

Lungs inflate by contraction of diaphragm Lungs inflate by contraction of diaphragm rendering intrapleural pressure –ve.rendering intrapleural pressure –ve.

Intercostal muscles contract mainly to Intercostal muscles contract mainly to stabilize the chest wall.stabilize the chest wall.

ABCsABCs

Lungs tend to deflate by 2 forces:Lungs tend to deflate by 2 forces:

A. elastic recoil of stretched tissues.A. elastic recoil of stretched tissues.

B. Surface tension within the air spaces.B. Surface tension within the air spaces.

Structural and Functional Limitations in Structural and Functional Limitations in NeonatesNeonates

1. Neonatal thorax is more cylindrical.1. Neonatal thorax is more cylindrical.

2. More compliant chest wall.2. More compliant chest wall.

3. Low diaphragmatic muscle mass.3. Low diaphragmatic muscle mass.

4. Surfactant deficiency.4. Surfactant deficiency.

5.Small lung volume.5.Small lung volume.

6. Narrower airways.6. Narrower airways.

Lungs and Air Passages have Physical Lungs and Air Passages have Physical (Mechanical) Properties that Resist (Mechanical) Properties that Resist

InflationInflation

1.1. Compliance. Compliance.

2.2. ResistanceResistance. .

ComplianceCompliance = = Volume Volume Pressure Pressure

i.c The higher the compliance, the larger the i.c The higher the compliance, the larger the

delivered volume per unit of pressure: delivered volume per unit of pressure:

• Normal compliance = 0.03-0.06 L/Cm HNormal compliance = 0.03-0.06 L/Cm H22O O

• In RDS compliance = 0.005-0.01 L/Cm HIn RDS compliance = 0.005-0.01 L/Cm H22OO

Static Pressure-Volume loop

Pressure-volume loop in normal Pressure-volume loop in normal and RDS infantsand RDS infants..

Clinical applicationsClinical applications

1.1. Compliance Compliance pressure gradient would pressure gradient would

have to be increased to maintain VT.have to be increased to maintain VT.

2.2. Take care about pressure if compliance is Take care about pressure if compliance is

normal or improving.normal or improving.

3.3. Excessive pressure will make the lung Excessive pressure will make the lung

more stiff “less compliant”.more stiff “less compliant”.

Factors affecting complianceFactors affecting compliance

1.1. Surfactant.Surfactant.

2.2. Pulmonary fluid content.Pulmonary fluid content.

Clinical ApplicationsClinical Applications

1.1. Decrease of lung edema, water content Decrease of lung edema, water content

will improve compliance e.g by diuretics.will improve compliance e.g by diuretics.

2.2. Improvement of RDS is associated with Improvement of RDS is associated with

diuresis.diuresis.

3.3. Rapid infusions increases water flux into Rapid infusions increases water flux into

the interstitium.the interstitium.

Laplace LawLaplace Law

P = 2 ST/ rP = 2 ST/ r

pp : pressure required to counteract : pressure required to counteract tendency of air spaces to collapse.tendency of air spaces to collapse.

STST: surface tension in the air spaces.: surface tension in the air spaces.

RR : radius. : radius.

Clinical ApplicationClinical Application

With improved inflation, pressure has to With improved inflation, pressure has to decrease to avoid volutrauma.decrease to avoid volutrauma.

More pressure is needed in preterms due to More pressure is needed in preterms due to small radius and increased surface tension small radius and increased surface tension

ResistanceResistance (Airway + Tissue Resistance)(Airway + Tissue Resistance)

ResistanceResistance == Pressure Pressure

FlowFlow

ResistanceResistanceAirway or tube DiameterAirway or tube Diameter

Poiseuille’s LawPoiseuille’s Law

R = L / r4R = L / r4

Clinical applicationsClinical applications

• Reduction of radius by ½ results in 16 fold Reduction of radius by ½ results in 16 fold

increase of resistanceincrease of resistance..

• Resistance during inspiration is less than Resistance during inspiration is less than

expiration.expiration.

• Accumulated secretions add to resistance.Accumulated secretions add to resistance.

Viscous resistanceViscous resistanceTissue resistanceTissue resistance

Generated by tissue elements moving past Generated by tissue elements moving past

one another.one another.

High in neonates because of low ratio of High in neonates because of low ratio of

lung volume to lung weight and relative lung volume to lung weight and relative

pulmonary interstitial fluid.pulmonary interstitial fluid.

Time constantTime constant

A measure of how quickly the lungs A measure of how quickly the lungs

can inflate or deflate.can inflate or deflate.

Time Constant = Resistance x ComplianceTime Constant = Resistance x Compliance

• One time constant =One time constant = 63% equilibrium.63% equilibrium.

• 2 time constant =2 time constant = 86% equilibrium.86% equilibrium.

• 3 time constant =3 time constant = 95% equilibrium.95% equilibrium.

• 5 time constant =5 time constant = 100% equilibrium.100% equilibrium.

In healthy infantIn healthy infant

• Resistance 30 cm HResistance 30 cm H22O/ L/sec.O/ L/sec.

• Compliance 0.04 L/cm HCompliance 0.04 L/cm H22O. O.

• One time constant =0.12 sec.One time constant =0.12 sec.

• 3 times constant = 0.36 sec. 3 times constant = 0.36 sec.

• 5 times constant = 0.6 sec. 5 times constant = 0.6 sec.

i.e .i.e .An inspiratory or expiratory phase of 0.6 sec. An inspiratory or expiratory phase of 0.6 sec.

is necessary (Tis necessary (TIIC&TC&TEEC) for full equilibrium.C) for full equilibrium.

Clinical applicationsClinical applications

• Adequate Ti and Te should be provided for Adequate Ti and Te should be provided for

gas exchange,gas exchange,

• In RDS, with improving compliance, Ti is In RDS, with improving compliance, Ti is

to be prolonged .to be prolonged .

Gas Exchange During Assisted Gas Exchange During Assisted Ventilation Ventilation

CoCo22 diffuses readily from blood to alveoli. diffuses readily from blood to alveoli.

It’s elimination largely depends on the total It’s elimination largely depends on the total

amount of air that goes through alveoliamount of air that goes through alveoli

(Alveolar ventilation V(Alveolar ventilation VAA).).

Minute alveolar ventilation =Minute alveolar ventilation =

(Tidal volume- Dead space) x Frequency. (Tidal volume- Dead space) x Frequency.

VVAA= ( V= ( VTT- V- VDD) x F.) x F.

11 . .How to alter PHow to alter PaaCoCo22

?

Answer :Answer :

Increased VA ( alveolar ventilation) Increased VA ( alveolar ventilation) will decrease PaCowill decrease PaCo22

HowHow ? ?

1.1. Increase tidal volume. Increase tidal volume. a)a) What’s it’s definition What’s it’s definition

b)b) What does it mean during ventilationWhat does it mean during ventilation

2.2. Increase Rate (F).Increase Rate (F).

22 . .Can we predict changes in Can we predict changes in PaCoPaCo22 with changes in frequency with changes in frequency

(at constant VT)(at constant VT)

?

ooFF nn P PaaCOCO22== ooPPaaCOCO22 x x nnFF

o = oldo = old

n = newn = new

Effect of metabolism on COEffect of metabolism on CO22

Increased metabolic rate Increased metabolic rate → → more more

COCO22 production production →→ more work of breathing more work of breathing

needed.needed.

Factors increasing CO2 Factors increasing CO2 productionproduction

• ++ VCO++ VCO22 fever , catecholamines.fever , catecholamines.

• + VCO+ VCO22 activity , cold.activity , cold.

• Normal Normal Quiet state.Quiet state.

• Low Low 11stst week of life. week of life.

Clinical applicationsClinical applications

Avoid factors leading to increased Avoid factors leading to increased metabolism e.g infections, temperature metabolism e.g infections, temperature instability, manipulations and instability, manipulations and catecholamines.catecholamines.

Gas Exchange During Assisted Gas Exchange During Assisted VentilationVentilation

OO22

• Depends on MAP “ mean airway pressure” Depends on MAP “ mean airway pressure”

• ie PAW ie PAW

What are the Factors that What are the Factors that Increase Paw ?Increase Paw ?

1.1. Peak inspiratory pressure (PIP).Peak inspiratory pressure (PIP).

2.2. Positive end expiratory pressure (PEEP).Positive end expiratory pressure (PEEP).

3.3. Inspiratory: Expiratory time (I/E ratio ).Inspiratory: Expiratory time (I/E ratio ).

4.4. Inspiratory flow. Inspiratory flow.

Ventilatory Settings that augment Ventilatory Settings that augment PaWPaW

30

20

10

0

2

3

4

1

0 1 2

Air

way

Pre

ssur

e (c

m H

2O) Insp.

flow

PIP

I:E

PEEP

Seconds

How PaW affects OxygenationHow PaW affects Oxygenation??

Through augmenting QThrough augmenting Q//V matchingV matching..

V/Q mismatching V/Q mismatching

1.1. Reduction in perfusion Reduction in perfusion • Collapse of air spaceCollapse of air space. . • Over distension of air space Over distension of air space

2.2. Increase in perfusion Increase in perfusion

ee..g in CHDg in CHD

Resistance

Compliance

The Constant

TidalVolume

PressureGradient

Peak INSP.Pressure

End EXP.Pressure

MinuteVentilation

INSP.Time

EXP.Time

Frequency

I:ERatio

FiOFiO2 2

How changes affect alveolar OHow changes affect alveolar O22 (P (PAAOO22))

• PiOPiO22 = = FiOFiO2 2 (Pb - PH(Pb - PH22OO22))

• PiOPiO22 = = Partial pressure of inspired Oxygen. Partial pressure of inspired Oxygen.

• Pb Pb = = Parametric pressure = 760 mmHg at sea level. Parametric pressure = 760 mmHg at sea level.

• PHPH22OO = = Water vapor pressure = 47 mmHg at sea Water vapor pressure = 47 mmHg at sea

level. level.

• PiOPiO22 = = 0.21(760-47)= 150 mmHg0.21(760-47)= 150 mmHg

PAOPAO22= PiO= PiO22-PACO -PACO 22

• PPAAOO2 2 = = AAlvelveoolar Olar O2 2 pressure pressure ..

• PPAACOCO22 == Alveolar CO Alveolar CO22 partial pressure partial pressure = = 45 45 mmHg. mmHg.

• PPAAOO2 2 = = 150150 - - 4545 = = +100 mmHg at room air+100 mmHg at room air..

Dead Space Volume (VD)Dead Space Volume (VD)

1.1. Automatic VAutomatic VDD • Usually constantUsually constant..• Includes the apparatus dead space. Includes the apparatus dead space. • Of minor consideration, usually VD/VT is < 0.3.Of minor consideration, usually VD/VT is < 0.3.

2.2. Alveolar VD Alveolar VD • In ventilated but poorly perfused lung regions. In ventilated but poorly perfused lung regions. • AD/VT may exceed.0.6AD/VT may exceed.0.6

Positive Intrathoracic Positive Intrathoracic pressure Causespressure Causes

• Compliant lung, rigid chest wall and high Compliant lung, rigid chest wall and high

inspiratory pressureinspiratory pressure. .

• Abdominal distentionAbdominal distention. .

Clinical applicationsClinical applications

1.1. Prone position.Prone position.

2.2. Decompress the bowel as needed.Decompress the bowel as needed.

3.3. Excessive alveolar > capillary pressure Excessive alveolar > capillary pressure temponading pulmonary circulation & temponading pulmonary circulation & V/Q mismatchV/Q mismatch . .

Positive Intrathoracic Pressure Positive Intrathoracic Pressure EffectsEffects

11. . Pulmonary Pulmonary

* * Barotrauma, lung injuryBarotrauma, lung injury..

22. . Cardiovascular Cardiovascular

* * Decreased venous return & right ventricular Decreased venous return & right ventricular

outputoutput

Risks of Elevated PRisks of Elevated PaaOO22

Ventilatory Depression .Ventilatory Depression .

Decreased pulm. Vascular resistance & Decreased pulm. Vascular resistance &

increased systemic vascular resistance. increased systemic vascular resistance.

Depression of erythropeisis. Depression of erythropeisis.

Retinal damage. Retinal damage.

Toxicity to lungs.Toxicity to lungs.

Clinical applicationsClinical applications

Keep Sa OKeep Sa O22 89 -93 % in almost all cases. 89 -93 % in almost all cases.

PaOPaO22 50- 80 mmHg is satisfactory. 50- 80 mmHg is satisfactory.