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18.2 Mode Related Questions
18.2.1 What is meant by Targeted Tidal Volume?
The user selects the appropriate volume. This volume is then achieved, limited,
and maintained by pressure. Changes in resistance and compliance are
automatically accommodated.
18.2.2 What are appropriate Tidal Volumes?
Preterm infants 4 - 6ml/kg
Term infants 4 - 8ml/kg
Paediatric patients 10 - 12ml/kg
18.2.3 What is the meaning of Compliance and Resistance?
Compliance is defined as the relationship between a change in volume and a
change in pressure. In the early stages of the disease a lung is often referred to as
non compliant. In other words there is little or no elasticity making the opening up
of the alveoli difficult.
Resistance is the relationship of pressure to flow i.e. the resistance offered by the
airway to the flow. If the resistance is high, it means that the lungs are stiff and
therefore requiring higher pressures to ventilate effectively.
Normal Resistance and Compliance values in term newborn infants
Pulmonary compliance 2 - 2.5ml/cmH
2
O/kg
Chest wall compliance > 4ml/cmH
2
O
Pulmonary resistance 20 - 40 cmH
2
O/L/sec
Resistive work 20 - 30 gm / cm/kg
Newborn RDS Newborn BPD
Pulmonary compliance <0.6ml/cmH
2
O/kg <1.0ml/cmH
2
O/kg
Chest wall compliance ----- -----
Pulmonary Resistance >40 cmH
2
O/L/sec >150cmH
2
O/L/sec
Resistive work 30 - 40gm-cm/kg 40gm-cm/kg
18.2.4 How do I minimise the Apnoea alarm in CPAP?
The trigger sensitivity needs to be appropriately set.It is advisable to remove the
flow sensor. Set the cycle fail alarm at or just above the selected CPAP level. Set
the low pressure alarm below the CPAP level.
18.2.5 In TTV some of the breaths exceed the set TTV. Why?
The spontaneous patient effort may exceed the set volume. The rise time may be
set in such a way that the pressure waveform overshoots and the ventilator
measures the peak pressure.
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