NEONATAL RESPIRATORY MECHANICS

1. NEONATAL RESPIRATORY
MECHANICS
Dr. Murtaza Kamal
MBBS, MD, DNB
Division of Neonatology
Department of Pediatrics
Safdarjung Hospital &VMMC, New Delhi
DOP-07/11/2015

2. What is expected of us post
talk??
 Compliance
 Resistance
 Time Constant
 Lung Volumes
 Oxygenation
 CO2 removal
 Clinical implications

3. COMPLIANCE

4. 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

5. Clinical Implications
In RDS
 Less compliant lungs
 Requires more pressure to ensure tidal
volume delivery

6. Clinical Implications (cont.)
Role of Surfactant
 Improves compliance
 Results in a rapid decline in pressures
required to deliver the tidal volumes
after surfactant administration

7. 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=??

8. So we got…
 Compliance=
Tidal volume/ Pressure gradient
=14/ (PIP-PEEP)
=14/14
=1ml/cm Hg

9. 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

10. Clinical Implications (cont.)
 These lung
propertiesTypical
sigmoid shape curve
to pressure-volume
loop
 Operate on the
rapid slope (middle
segment) during
ventilation
 Fig.1

11. RESISTANCE

12. 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

13. Resistance
 Resistance is directly proportional to:
 Length
 Viscosity
 1/r4
 Pressure required to drive 1L/min of gas
flow into the lungs

14. 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

15. 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

16. TIME CONSTANT

17. 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

18. Time Constant

19. Time Constant

20. Clinical Implications
RDS:
 Low compliance, normal resistance
 Hence time constant-> less
 Time required to inflate (Ti) or deflate
(Te) the lungs ->Hence short

21. 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

22. Clinical Implications(cont.)

23. LUNG VOLUMES

24. Lung Volumes of our Importance
 Tidal volume
 Minute volume
 Functional residual volume

25. TIDAL VOLUME

26. Tidal Volume
 Volume of the gas going in and out with
each breath
 5-8ml/kg
 Ideally inspiratory and expiratory tidal
volumes are equal

27. 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)

28. DEAD SPACE

29. 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

30. Dead Space
 Some gas in alveoli, due to ill
perfusion, is not a part of gas
exchange
 Physiological dead space- Anatomical+ ill
perfused alveoli

31. So Tidal Volume is…
Alveolar tidal volume+ Anatomical dead
space+ ill perfused alveoli’s vol
Alveolar tidal volume+ Physiological dead
space

32. MINUTE VOLUME

33. 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= ??

34. 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)

35. FUNCTIONAL RESIDUAL
CAPACITY

36. 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

37. 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

38. OXYGENATION

39. Oxygenation
 Parameters determining oxygenation
are:
 Mean Airway pressure (directly
proportional)
 FiO2 (directly proportional)

40. 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

41. 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

42. High MAP will lead to…
 Decreased cardiac output
 Pulmonary hypoperfusion
 Increased risk of barotrauma
(Levels>12cm H2O contributes to
barotrauma)

43. 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)

44. 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

45. So lets scratch our heads now…
 PIP=22
 PEEP=4
 Rate=40/min
 IT=.4 sec
 Calculate MAP-??

46. 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

47. C02 ELIMINATION

48. 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

49. 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)

50. LETS TRY OUT THESE…

51. 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

52. 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

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

54. KEY CONCEPTS

55. 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

56. 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