27. Ventilators

Question 1

How can ventilators be
classified?

Pressure Generator

Answer 1

1. Delivers gas to the patient
   at a constant inspiratory pressure
   set by the operator.
2. Usually time cycled
   (i.e. the operator sets the pressure,
   the number of breaths per minute
   and the inspiratory:expiratory time
   ratio and the machine will deliver
   breaths accordingly).

3.
Inspiratory flow rates and tidal volume
achieved will depend on the patient's
lung compliance 
(i.e. the stiffer the lungs 
the lower the resulting
tidal volume delivered).

4.
Risk of barotrauma is low.

5.
Risk of volutrauma is higher
(so set volume limits).

6.
The system has some ability
to compensate for leaks, as it always
acts to deliver a preset pressure
for a set amount of time.

Question 2

Flow Generator

Answer 2

1.
Delivers gas to patient at a 
constant inspiratory flow
rate until it has delivered
a pre-set tidal volume

2.
Cycles when the set tidal volume 
has been delivered 
(i.e. the operator sets
the volume to be delivered, 
number of breaths and the I:E ratio).

3.
Inspiratory pressures reached depend
on the patient’s lung compliance (i.e. the
lower the compliance, the higher the peak
inspiration pressure).

4.
Higher risk of barotrauma
so set pressure
limits).

5.
Lower risk of volutrauma.

6.
Any leak in the circuit is not compensatedfor, as the ventilator will perceive that
the lost volume has
been delivered to the patient.

Question 3

A third type of ventilator is called the high-frequency oscillating ventilator

Answer 3

A third type of ventilator is called
the high-frequency oscillating ventilator
(HFOV).

HFOV employs an ‘open lung’ strategy, 
using high PEEP and
very small tidal volumes (1–3 mL/kg) 
at respiratory rates of up to 15 Hz
(i.e. 900 breaths per minute!).

It aims to reduce distending pressures
in poorly compliant lungs (e.g. in ARDS)

and is recommended for
those patients requiring a

high FiO2 > 0.60

with high mean airways pressures > 24 cm H2O.

The mechanism of oxygenation with
this type of ventilation is not fully
understood,

but diffusion, convection and

Pendelluft 
(i.e. movement of gas
between different alveolar
 units with 
different time constants) 

thought to play a part.

Question 4

CPAP – Continuous Positive Airway Pressure

Answer 4

CPAP – Continuous Positive Airway Pressure

• A positive pressure (cm H2O) is applied
to the airway of a
spontaneously breathing
patient via a facial or nasal mask.

The pressure is constant through
all phases of the ventilatory cycle.

• The positive pressure helps to prevent 
alveolar and airway closure
during expiration and 
improves lung compliance 
by moving the lungs
up the compliance curve.

• This mode of ventilation is used in the
treatment of obstructive sleep apnoea.

Question 5

PEEP – Positive End Expiratory Pressure

Answer 5

PEEP – Positive End Expiratory Pressure

• This is very similar to CPAP,
except that it applies to mechanically
ventilated patients.

• PEEP is a set level of pressure (cm H2O) 
below which the circuit is
not allowed to fall at the 
end of expiration. 
It is usually set between
5 and 10 cm H2O.

• It helps to prevent alveolar and 
airway closure during expiration and
improves lung compliance by
 moving the lungs up the 
compliance curve.

Question 6

BiPAP – Bi-level Positive Airway Pressure

Answer 6

BiPAP – Bi-level Positive Airway Pressure

• This is a trade name of a particular
make of non-invasive ventilators.

• BiPAP is given via a face mask,
usually to a conscious and
spontaneously ventilating patient.

• The operator sets two levels of pressure;
the first is effectively the PEEP,
i.e. the level below which the circuit
is not allowed to fall.

The second is the positive inspiratory pressure,
which the ventilator delivers to the patient.

• The machine senses when the
patient is taking a breath
(via a pressure transducer
which senses negative pressure in the system)

and then augments their breath
to the set pressure.

In most cases, cycling is controlled
by the machine sensing
the patient’s respiratory effort.

Some models can deliver 
time-cycled positive pressure if
the patient fails to make 
any respiratory effort after a 
set time has elapsed.

Question 7

PC – Pressure Control

Answer 7

PC – Pressure Control

• Pressure control is used in a
mechanically ventilated patient.

It is the most basic mode of ventilation.

• The operator sets either the desired
tidal volume or desired pressure
and the number of breaths per minute,
and the ventilator will deliver these.

• Any respiratory effort that the patient
makes is ‘ignored’ by the ventilator.

Question 8

PS – Pressure Support

Answer 8

PS – Pressure Support

• Pressure support is used for
weaning patients from the ventilator.

• As in BiPAP, the ventilator senses
the patient’s inspiratory effort and
augments it with a pre-set inspiratory pressure.

• It is usual to set two levels of pressure
the principle being the same
as that described above for BiPAP.

Question 9

SIMV – Synchronised Intermittent Mandatory Ventilation

Answer 9

SIMV – Synchronised Intermittent Mandatory Ventilation

• This mode is a ‘half-way house’
between PC and PS.

• The operator sets the 
desired tidal volume or
inspiratory pressure
and the 
number of breaths per minute. 

The ventilator will then
deliver these breaths.

• However, if the patient makes an
inspiratory effort the machine will
sense this and augment their breath.

Question 10

PRVC – Pressure Regulated Volume Control

Answer 10

PRVC – Pressure Regulated Volume Control

• In this mode, 
the operator sets the tidal volume 
and the machine will deliver this volume
 in such a way as to give the 
lowest resultant inspiratory pressure.

• This mode has been developed 
to try to reduce the risk of
barotrauma and to address the issue
 of different areas of the lung
having different compliance in 
lung diseases such as ARDS.