NEONATAL RESPIRATORY MECHANICS

Dr. Murtaza KamalMBBS, MD, DNB

Division of NeonatologyDepartment of Pediatrics

Safdarjung Hospital & VMMC, New DelhiDOP-07/11/2015

What is expected of us post talk??

Compliance Resistance Time Constant Lung Volumes Oxygenation CO2 removal Clinical implications

COMPLIANCE

Compliance

Measurement of distensibility

C= Volume change (V) / Pressure change(P)

Volume change per unit pressure

Lung which is more compliant is more distensible and vice versa

Clinical Implications

In RDS

Less compliant lungs Requires more pressure to ensure

tidal volume delivery

Clinical Implications (cont.)

Role of Surfactant

Improves compliance Results in a rapid decline in

pressures required to deliver the tidal volumes after surfactant administration

Lets try this

3kg neonate, tidal volume delivered-14ml, pressure required to drive this volume is PIP of 18 and PEEP of 4

Calculate compliance=??

So we got… Compliance=

Tidal volume/ Pressure gradient=14/ (PIP-PEEP)=14/14=1ml/cm Hg

Clinical Implications (cont.)

Collapsed/ overstretched lungs poor compliance

PEEP Pressure required to open the lungs and keep it inflated

High PIP (excess pressure) which over distends the lungs, does not result in better volume delivery

Clinical Implications (cont.) These lung

propertiesTypical sigmoid shape curve to pressure-volume loop

Operate on the rapid slope (middle segment) during ventilation

Fig.1

RESISTANCE

Resistance

The oppositional force for air flow into the lungs

Higher the resistance, greater is the pressure required to drive the gases into the lungs

Depends on: Airway diameter Airway length Viscosity of gas

Resistance Resistance is directly proportional to:

Length Viscosity 1/r4

Pressure required to drive 1L/min of gas flow into the lungs

Clinical Implications

As length increases- resistance increases

Eg: long ET tube flow sensor or capnograph

Trim the ET tube to 2.5cm outside the upper lip

Clinical Implications (cont.) A small decrease in airway diameter

causes a large change in resistance

Removal of airway secretions and largest diameter ET tube that fits the glottis to be chosen

Higher air flows increases the resistance by causing turbulence to gas flow

TIME CONSTANT

Time Constant

Time taken to empty the gases from lungs

It nearly takes 3-5 time constants to completely empty the lungs

Time constant(sec)= Compliance x Resistance

Time Constant

Time Constant

Clinical Implications

RDS: Low compliance, normal resistance Hence time constant-> less Time required to inflate (Ti) or

deflate (Te) the lungs ->Hence short

Clinical Implications(cont.)

MAS Resistance is high, Compliance

decreases little bit Hence, time constant increases Time required to inflate (Ti) or deflate

(Te) the lungs hence is long A short set expiratory time on

ventilator can lead to inadequate lung emptying and hence gas trapping

Clinical Implications(cont.)

LUNG VOLUMES

Lung Volumes of our Importance

Tidal volume

Minute volume

Functional residual volume

TIDAL VOLUME

Tidal Volume

Volume of the gas going in and out with each breath

5-8ml/kg

Ideally inspiratory and expiratory tidal volumes are equal

Clinical Implications

Whenever peritubal leak during MV, air leaks more during inspiration (higher PIP and wider airways)

Hence, in modes of ventilation where tidal volume is being targeted, it is better to measure the expiratory volume (volume of gas that actually goes to the lungs)

DEAD SPACE

Dead Space Respiratory component-

Terminal bronchiole, Alveolar sacs Alveoli

Anatomical dead space- Part of tidal volume which is not a part of gas exchange (airways) 2ml/kg

Dead Space

Some gas in alveoli, due to ill perfusion, is not a part of gas exchange

Physiological dead space- Anatomical+ ill perfused alveoli

So Tidal Volume is…

Alveolar tidal volume+ Anatomical dead space+ ill perfused alveoli’s vol

Alveolar tidal volume+ Physiological dead space

MINUTE VOLUME

Minute Volume

Total volume of the gas moving in and out of the lungs per minute

MV=TV X RR

200-480 ml/kg/min

Alveolar MV= ??

Clinical Implications

MV, esp the alveolar MV, is determinant of CO2 removal

Co2 removal hastened either by increasing the TV or rate

Better to increase the TV more energy efficient (Increasing TV--dead space is constant but increasing rate– increases dead space too)

FUNCTIONAL RESIDUAL CAPACITY

FRC

Volume of the gas present in the lungs at the end of expiration

Allows gas exchange to be a continuous process

25-30ml/kg

RDS-FRC low MAS-FRC high

Clinical Implications PVR- Least at normal FRC PVR increases as FRC increases of decreases

Indicators of normal FRC: 6-8 intercostal spaces on CXR Fall in FiO2 when increasing the PEEP

when a neonate is on CPAP or MV

CPAP and surfactant—Interventions done to allow normal FRC in neonatal lungs

OXYGENATION

Oxygenation

Parameters determining oxygenation are:

Mean Airway pressure (directly proportional)

FiO2 (directly proportional)

Mean Airway Pressure

The average pressure exerted on the airway and the lungs from the beginning of inspiration until the beginning of next inspiration

Most powerful influence on oxygenation

MAP=K(PIP*Ti)+(PEEP*Te) / Ti+Te

Pressure-Time Graph

MAP is the area under the curve for one respiratory cycle

Slope of pressure rise is dependent on flow

K depends on the slope

High MAP will lead to…

Decreased cardiac output Pulmonary hypoperfusion Increased risk of barotrauma

(Levels>12cm H2O contributes to barotrauma)

Clinical Implications MAP can be increased by:

Increasing PIP Increasing PEEP Increasing I/E ratio Increase the flow rate( converts the

sign wave pressure time graph to a square wave)

So for increasing oxygenation…

Prefer increasing MAP when: Lung disease is severe (Pneumonia) Lung volume is small (RDS)

Prefer increasing FiO2 when: Lung volume is increased(MAS) When there is air leak

So lets scratch our heads now…

PIP=22 PEEP=4 Rate=40/min IT=.4 sec

Calculate MAP-??

Lets get it… Rate =40/min So, one cycle of breath=60/40=1.5 sec IT=0.4sec So, ET=1.5-0.4=1.1sec So, MAP=K(PIP*Ti)+(PEEP*Te) / Ti+Te

=1(22*0.4)+(4*1.1)/1.5 =1(8.8)+(4.4)/1.5 =13.2/1.5 =8.8cm H20

C02 ELIMINATION

CO2 Elimination

Depends on MV (alveolar minute volume more specifically) and RR

CO2 elimination directly proportional to alveolar tidal volume and respiratory rate

=(PIP-PEEP)*RR

Achieve CO2 elimination by:

Decreasing dead space Excess ET tube Secretions Partial block

Increasing PIP Increasing rate Decreasing PEEP (only if there is lung

hyperinflation)

LETS TRY OUT THESE…

Question 1

1]True about RDS is? A] Low compliance, High resistance B]High compliance, Low resistance C]Low compliance, Normal

Resistance D]High compliance, High resistance

Question 2

2]Which of the following will affect PaCO2 maximum?

A]Secretions in the ET tube B]Respiratory Rate C]Tidal volume D]Dead space

Question 3

3] Which is a wrong match? A] FRC—PVR B] PaO2—MAP C] PaCO2—TV D] MAP—FiO2

KEY CONCEPTS

Key Concepts

Compliance is distensibility, resistance is the oppositional force and time constant is the time required to empty the lungs

Lung tissue determine the compliance, airways determine the resistance and both compliance and resistance determine the time constant

Key Concepts(cont…)

Ventilation is intermittent but gas exchange is continuous. Alveolar Minute Volume is a measure of ventilation and FRC influences gas exchange

MAP and FiO2 regulate oxygenation while alveolar tidal volume and respiratory rate regulate PaCO2 when a neonate is on MV