Continuous, reliable measurement of static compliance of the lung and thorax is essential in mechanical ventilation. Dynamic compliance is dependent on airway resistance and can be misleading. An inflation hold maneuver measures static compliance by holding air in the lungs at the end of inspiration to measure plateau pressure. Poor compliance can indicate collapsed or overdistended lungs, while surfactant improves compliance and reduces pressure needs.

1. ‫حححثي‬
Tageldin Aly
Pediatric and neonatal mechanical ventilation
Pulmonary mechanics
Part 3

2. Basics of respiratory mechanics
Compliance
Resistance
Time Constant

3. Volume
Pressure
D V
D P
C =
D V
D P
compliance
elastic properties of lung & chest wall

4. Ventilators deliver gas to the
lungs using positive pressure
at a certain rate.
The amount of gas delivered
can be limited by time,
pressure or volume.
The duration can be cycled by
time, pressure or flow.
Volume Change
Time
Gas Flow
Pressure Difference

5. Volume
PressurePressure
Volume
Pressure VentilationVolume Ventilation
Decreased Tidal Volume Increased Pressure
Compliance 
Compliance

Decreased PressureIncreased Tidal Volume
and do not forget, the time constant (T = Crs x Rrs) will change too!

6. TYPES OF COMPLIANCE
Volume change per unit pressure

7. Continuous, reliable measurement of
static compliance

8. Cdyn vs. Cstat
Measurements of respiratory mechanics during mechanical
ventilation
Continuous, reliable measurement of static compliance of
the lung and thorax is of the upmost importance in
state-of-the-art mechanical ventilation.
Dynamic compliance cannot be considered a satisfactory
substitute, as it is dependent on the airway resistance
and can be misleading in various clinical conditions.
However, Cdyn is used in mode to estimate the maximum
tidal volume allowed, which is calculated as
: Cdyn x Pmax ( Pmax = set alarm limit (Phigh)-10-PEEP).

9. . PIP Cdyn (over come airway resistance ) always
higher than plat pressure (Cstat)
. Cdyn ( resistance and alveolar diseases )
Alveolar disease only ))lower than Cstat

10. Cdyn diseases vs. Cstat diseases
.Cdyn diseases like airway obstructions
,alveolar disease like atelectasis ,
pneumonia, pneumothorax
.Cstat diseases like Alveolar ( atelectasis ,
pneumonia )

12. Inflation hold or inspiratory pause
hold air in the lung at end of inspiration (P plateau)
to measure Cstat

13. Pplat measurement in volume mode
ventilator

14. clinical Implications
In RDS
Less compliant lungs
Requires more pressure to ensure tidal
volume delivery

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

16. 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=??

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

18. 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 deliver

21. Who’s Watching the Patient?
Pierson, IN: Tobin, Principles and Practice of Critical Care Monitoring

22. Key points
. Elastic properties of lung & chest wall
. Continuous, reliable measurement of static compliance .
. Inhalation hold or inspiratory pause test give you clue
about what is maximum pressure (Plateau ) needed
..Collapsed/ overstretched lungs --- poor compliance