TYPES OF VENTILATORS
Introduction
The
mechanical; ventilator is a device that
functions as a substitute for the action of thoracic cage and diaphragm
Types
- Positive Pressure Ventilator
- Negative Pressure Ventilator
Positive Pressure Ventilator
These
ventilators use positive pressure to inflate the lungs of the patient .During
inspiration a positive intra thoracic and alveolar pressure is caused by the
pressure created by the ventilator. Expiration is largely passive. Different
types of positive pressure ventilators are described below.
1.Pressure
Cycled Ventilators .They produce an
inspiratory flow of gas that inflate the lungs until a preset or predetermined
pressure is reached. Volume delivered depends upon changes in the airway
resistance, lung compliance and integrity of ventilator circuit .Newer machine combine
pressure cum volume cycle.
2.Time
Cycled Ventilators. Time cycled
ventilators terminate the inspiratory phase after the preset time They are
mainly used for newborn and premature neonates because they provide precise control over time and inspiration-to-expiration
ratio.
3.Volume
Cycled Ventilators. These ventilators
terminate the inspiratory phase when the predetermined volume of gas is delivered (12ml/kg body weight. Usually
starting volume).These ventilators can also be used for newborn and premature
neonates.
Negative Pressure Ventilators
This
type of ventilators apply negative pressure around the chest wall which causes
intra airway pressure to become negative,thus drawing air into the lungs .They
are old generation mechanical ventilators that are not in use now.
Modes of Mechanical Ventilation
1.Controlled mode Ventilation(CMV)
a.Volume
Controlled Ventilation :the ventilator initiates the breaths at a preselected
rate and volume. All breaths are compulsory .The patient cannot trigger a
breath .Only patients who have no voluntary respiratory efforts can use this
mode of ventilator.(eg.anaesthetised
patient s)or those who are unable to respire spontaneously .(eg paralysed
patient)
b.Pressure
Controlled Ventilation (PCV):It is used when all breaths are time or patient
triggered ,pressure limited and the time
cycled .The ventilator provides a constant pressure of air to the patient
during respiration .which is set by the therapist .The tidal volume is based on
the compliance and resistance of the patient lungs and the patient pressure.
2.Assisted Control Volume
This is a mode of ventilation in which a fixed
minimum rate and volume are delivered by the machine ,but each of patient’s
spontaneous breaths trigger the ventilator to deliver a selected volume .with
assisted control mode every breath is a
mandatory breath even though the patient-can trigger breath at a more rapid rate
than the set machine rate.This mode allows the patient to adjust respiration to
changing metabolic status.
3.Intermittent Mandatory Ventilation(IMV):IMV
is a mode of ventilation in which the ventilator delivers preset tidal volume
and respiratory rate . The client can take unassisted spontaneous breaths
between preset breaths .
4.Synchronised Intermittent mandatory
Ventilation(SIMV);This mode is similar to
IMV except that it has a predetermined time interval. The ventilator
senses the patient effort and assist the patient by synchronously delivering a
mandatory breath.SIMV eliminates the problem of ‘breath stacking’ where machine
breaths occur at the same time as the patient spontaneous breath. This mode is
very useful as a weaning method to allow the patient to gradually assume more
of the work of breathing .The SIMV/IMV mode has fewer cardiovascular side
effects than most other mode of ventilatior.
Spontaneous Modes
1.Pressure Support Ventilation(PSV):Pressure
Support Ventilation is a special form of assisted ventilation that is used in
spontaneously breathing patients .The ventilator provides a constant pressure
of gas once the patient has made an inspiratory effort .It is always assisted
or patient triggered .In pure PSV there is no preset tidal volume or frequency
The therapist sets the inspiratory and expiratory pressure and sensitivity. The
patient determines the timing of the breaths ,tidal volume receive and rate of breathing
.
2.Continous Positive Airway Pressure (CPAP): CPAP
refers to positive end expiratory pressure (PEEP)applied in conjunction with
spontaneous ventilation.With CPAP all patient breaths are humidified and
oxygenated air from the ventilator system. During the expiratory phase ,the
patient exhales to the preset positive and expiratory pressure level. While
applying positive pressure through out the respiratory cycle ,it helps to keep
the alveoli open and promotes oxygenation. Providing CPAP through the ventiolator circuit during spontaneous
breathing also offers the advantage of an alarm system and may reduce the
patient anxiety during early trials if the patient has been taught that the
machine is keeping tack of his breathing
.However CPAP may increase resistance
for spontaneous breath, so it is often
used in conjunction with pressure support ventilation.
