|
Mechanical Ventilators are Flow Generators
Expiratory support is almost always PEEP/CPAP, which
elevates the baseline airway pressure. The description of the mode of
ventilation refers to the method of inspiratory support, that is, how the
patient is helped up the volume pressure curve.
CPAP is elevated baseline airway pressure
The classification of ventilators refers to the
following elements (which vary from textbook to textbook): this is the
clearest method:
1) Control:
How
the ventilator knows how much flow to deliver
Either
Volume Controlled (volume limited, volume targeted) and Pressure
Variable
or
Pressure Controlled (pressure limited, pressure targeted) and
Volume Variable
or
Dual Controlled (volume targeted (guaranteed) pressure limited)
2) Cycling:
how the ventilator switches from inspiration to
expiration: the flow has been delivered to the volume or pressure target -
how long does it stay there?
Time cycled - such in in pressure
controlled ventilation
Flow cycled - such as in pressure support
Volume cycled - the ventilator cycles to
expiration once a set tidal volume has been delivered: this occurs in
volume controlled ventilation. If an inspiratory pause is added, then
the breath is both volume and time cycled
3)
Triggering: what causes the ventilator to cycle to
inspiration. Ventilators may be time triggered, pressure triggered or
flow triggered.
Time: the ventilator cycles
at a set frequency as determined by the controlled rate.
Pressure: the ventilator
senses the patient's inspiratory effort by way of a decrease in the
baseline pressure.
Flow: modern ventilators
deliver a constant flow around the circuit throughout the respiratory
cycle (flow-by). A deflection in this flow by patient inspiration, is
monitored by the ventilator and it delivers a breath. This mechanism
requires less work by the patient than pressure triggering.
4) Breaths are either:
what causes the ventilator to cycle from inspiration
Mandatory (controlled) - which is
determined by the respiratory rate.
Assisted (as in assist control,
synchronized intermittent mandatory ventilation, pressure support)
Spontaneous (no additional assistance in
inspiration, as in CPAP)
5) Flow pattern:
constant, accelerating, decelerating or sinusoidal
Sinusoidal = this is the
flow pattern seen in spontaneous breathing and CPAP
Decelerating = the flow
pattern seen in pressure targeted ventilation: inspiration slows down as
alveolar pressure increases (there is ahigh initial flow). Most
intensivists and respiratory therapists use this pattern in volume
targeted ventilation also, as it results in a lower peak airway pressure
than constant and accelerating flow, and better distribution
characteristics
Constant = flow continues
at a constant rate until the set tidal volume is delivered
Accelerating = flow
increases progressively as the breath is delivered. This should not be
used in clinical practice.
6) Mode or Breath
Pattern:
there are only a few different modes of ventilation:
CMV = Conventional controlled ventilation,
without allowances for spontaneous breathing. Many anesthesia
ventilators operate in this way.
Assist-Control = Where assisted breaths are
facsimiles of controlled breaths.
Intermittent Mandatory Ventilation = Which
mixes controlled breaths and spontaneous breaths. Breaths may also be
synchronized to prevent "stacking".
Pressure Support = Where the patient has
control over all aspects of his/her breath except the pressure limit.
High Frequency Ventilation = where mean
airway pressure is maintain constant and hundreds of tiny breaths are
delivered per minute.
Knowing the mechanisms of the
above modes is more than enough to be familiar with the practices in the
majority of intensive care units. However, more modes exist, which are
worth mentioning. Airway pressure release ventilation (BiPAP/BILEVEL),
proportional assist ventilation and automatic tube compensation, are
modern pressure targeted modes of ventilation which feature enhanced
patient interactivity.
|
