27. Ventilators Flashcards
How can ventilators be
classified?
Pressure Generator
- Delivers gas to the patient
at a constant inspiratory pressure
set by the operator. - 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.
Flow Generator
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.
A third type of ventilator is called the high-frequency oscillating ventilator
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.
CPAP – Continuous Positive Airway Pressure
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.
PEEP – Positive End Expiratory Pressure
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.
BiPAP – Bi-level Positive Airway Pressure
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.
PC – Pressure Control
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.
PS – Pressure Support
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.
SIMV – Synchronised Intermittent Mandatory Ventilation
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.
PRVC – Pressure Regulated Volume Control
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.
