Highlights of Unit 3:
Classification of
mechanical ventilation
By Elizabeth Kelley Buzbee AAS, RRT-NPS, RCP
obtains power and converts this power
into a force that can move gas into a
patient’s lung.
 Sends gas down a circuit to the patient
interface and back to ventilator for
analysis of data

The parts of a mechanical
ventilator
is dependent on the patient’s RAW,
 his compliance
 the volume required
 and the elastic recoil of the lung

Mechanical ventilators and the
WOB




How does it get power to operate?
How does it use this power to drive gas
into the patient and how does it control
the flow of gas into the patient?
How does it control the various
parameters of ventilation such as
starting and stopping a breath?
How does it communicate information to
the operator in such a manner that the
RCP can monitor the patient’s responses
and modify the ventilator’s action?
We classify ventilators by these
questions
electrical power: A/C D/C 110-115
volt current
 pneumatically-powered. 50-60 psig
 Battery powered [emergency only or
transport]

◦ Internal batteries
◦ External batteries
◦ Run about one hour then require 8-12 hours to
recharge
Input Power
How does this machine get power to
operate?

Drive mechanisms
◦ Compressors
 piston-driven◦ rotary–driven- delivers a Sine wave
◦ linearly driven-- constant flow pattern .
 Bellows: start with constant flow but as
pressures rise and RAW increases results in
descending
◦ spring,
◦ a weight
◦ gas pressure
Power transmission and conversion
How does it use this power to drive gas into the
patient?





microprocessors are tiny computers
that do only one or two tasks
solenoid valves control flow to the
patient by electronic switching
Electromagnetic
Pneumatic poppet valves
Proportional solenoid valves
Output control valves:
How does it control the flow of gas into the
patient?

Fluidics use gas power, but differs from
pneumatic in that there are no moving
parts.
◦ Coanda effect- gas moves along the side of the
wall and we can direct gas to go down another
tube by application of gas into that flow to
move it

pneumatic control: uses gas but there
are moving parts- mushroom valves ect
Fluidic and pneumatic control
Means of Communication:
How does it communicate information to the
operator in such a manner that the RCP can
monitor the patient’s responses and modify the
ventilator’s action?

Monometers/
◦ bourdon gauges- measure pressure
◦ digital monometer may be displayed as a
bargraph, or as numbers
Spirometers: Volume measurements
 Waveforms
 alarms

Apnea alarm: adults 20 seconds; when
these alarms go off the apnea parameters
on many ventilators will start breathing
for the patient
 Loss of electrical power alarm: if

battery operational will come on with indicator
light
 Loss of gas power alarm: may ventilate
patient will remaining gas
 Disconnect alarms: may have a time delay

Low or high humidifier temperaturekeep at 32-340 C [high 37 max]
Where do we set the alarm limits?

High/Low VE : VE needs to stay within 10-
20% +/- [or 2-5 LPM above and below]
 VT alarms: 100 ml lower than set VT

Low airway pressure alarm- about 5-10
cmH20 below average PIP.

High airway pressure – 10-20 above
average PIP; when this goes off, the breath will
stop [pressure-cycled]
Where do we set the alarm limits?

Fi02 alarms -5% +/-.

High/low rate alarms: more than 1020% from baseline—

Loss of PEEP/CPAP alarms: are
generally set about 3-5 below the PEEP
Where do we set the alarm limits?

How does the mechanical ventilator
control the various parameters of
ventilation such as starting and
stopping a breath?
Control variables:
Open vs. closed loop control of
ventilator output

open; we dial in a VT or a f and the
machine delivers the VT to the circuit. the
open loop machine will not adjust.

In an closed loop system the ventilator
is smart enough to monitor and interpret
changes in such a way that the machine
will alter the next breath to maintain the
VT.
A control variable is the primary
variable that the ventilator
manipulates to cause an inspiration:
Pressure controlled [PC]
 Volume controlled [VC]
 Flow controlled
 Time [usually based on the other
parameters]


Only one control, the other two will be
variables
a PC [pressure controlled] breath is one in
which the pressure stays the same, but
changes in the patient’s condition will
alter the delivered volume and the flow
rate.
 The doctor orders a PIP which will deliver
a VT

Pressure control:
During VC ventilation, the PIP varies with
changes in the patient’s conditions, while
the volume and the flow stay constant.
 The doctor orders a VT

Volume controlled:
mechanical ventilators had consistent flow
rate and volumes, but the airway pressure
changed with patient parameter changes.
 The doctor will order a VT but we will set
up the flow rate and the Ti to deliver this
VT

Flow controlled

What event triggers inspiration, what
stops the breath, what changes the
breath?
Phase Variables:
Trigger: what starts the inspiratory
phase?
 Limit: what limits the actual
inspiratory cycle without stopping it?
 Cycle: what cycles the inspiratory
phase off- starts exhalation?
 Baseline: what changes the base line
pressures? PEEP or CPAP

Phase variables:
 Time triggering. At 10 BPM, there is a
breath initiated by the ventilator every 6
seconds [cycle time]
 Patient triggered: the inspiration is started
by the patient demand. is called the
“Sensitivity.”
◦
◦
◦
◦

pressure trigger
flow trigger
volume trigger
NAVA
Manual trigger: push a button on the
ventilator to trigger a breath– used during
suctioning
What event triggers inspiration?



set the Sensitivity knob to -.5 to -1.5
cmH20.
if the Sensitivity is adjusted from -1 to -3, we
say that the sensitivity is decreased; the
patient’s WOB is increased.
The patient creates a pressure gradient
◦ If there is a leak in the system the pressure may
not drop.
◦ Complicated by having to drop the pressure all the
way back to the ventilator
◦ If baseline pressure rises, may not be able to
pressure trigger
◦ If there is auto-PEEP from air trapping, the
pressure cannot drop enough to trigger a breath
Pressure triggered:
There is always a small constant flow
moving through the circuit
 2 Pneumotachymeters measure and
compare the flow coming to patient and
going away from patient.
 As the patient pulls in the gas, there is
now less expiratory flow than inspiratory
flow, and it is this flow gradient that will
trigger a breath
 Usual set 1-3 LPM in adults

Flow triggered
Water in the circuit can mimic a breath
and trigger more breathes than patient
needs
 Leaks can also alter the constant flow so
that the machine may ‘auto-cycle’ or
‘chatter’

Problems with flow triggers

only the Drager Baby Log actually uses
the volume inspired by the patient to
trigger a breath
Volume triggered
Neutrally adjusted ventilatory assist
 A probe is sent down the esophagus and
as the phrenic nerve fires, the probe’s
sensor notes the breath effort and
triggers the ventilator.

NAVA

If not sensitive enough
◦ Increased WOB
◦ ‘asynchrony with the ventilator’
◦ ‘Fighting the ventilator’

If too sensitive
◦ Triggers too many breaths– called ‘chattering
‘or ‘auto-cycling’
◦ Could lead to air trapping and baratrauma
◦
What happens when triggering is not
accurate or responsive?

A limit on a breath is some parameter
that affects the breath without stopping it.
◦ A pressure limit may mean that the patient
continues to deliver the VT but the flow slows
down in an attempt to keep the airway
pressures down
◦ The actual VT delivered is usually decreased,
but still higher than it would be if the breath
was pressure cycled off
Limit: what limits the actual
inspiratory cycle?
many manufacturers use the term
“limit” when discussing alarms.
 If a high pressure alarm is set and the
breath stops being delivered once that PIP
limit is exceeded, it is not pressure
limiting; it is pressure cycling off.

IMPORTANT:

what parameter cycles the inspiratory
phase off- starts exhalation?
◦ Volume cycled-when preset VT is reached.
Most VC breaths are also volume-cycled
◦ Time cycled- the breathes initiated by the
ventilator can be time triggered and maybe
time cycled off. Most PC breaths will be timecycled off
◦ Pressure cycled- in VC ventilation, if the high
pressure alarms goes off and the breaths stops
we can say that the breath was pressurecycled.
◦
Cycle:

flow cycle
◦ Some ventilators will cycle off once a preset
low flow rate is noted.
◦ You can see this on the graphic when we watch
the descending flow wave suddenly drops to
zero
◦ This occurs always with PS breaths and you
might be able to chose flow cycling with the VC
mandatory breaths
Flow cycling
We can raise the pressure during
exhalation phase from zero to a positive
number;
 we have raised the baseline

Baseline: What changes the base
line pressure?
◦ Both raise the baseline pressure
◦ Both used to treat refractory hypoxemia
◦ Both will increase the FRC and can increase
the lung compliance
◦ PEEP positive end-expiratory pressure ‘ with a
ventilator rate set [full or partial support] the
lower pressure
◦ CPAP- ‘continues positive airway pressure’
without a ventilator rate set [a spontaneous
mode] the only pressure
PEEP or CPAP?

Keep more air inside alveoli and airways
◦ Raises RV [residual volume] which raises the FRC


Return the FRC to normal will generally increase
the lung compliance and decrease the WOB
Excessive PEEP
◦ hampers CO, increases VD and causes air trapping and
can damage the lung tissue
◦ If patient is on PC ventilation, raising PEEP might
decrease the VT because the driving pressure drops.

Excessive CPAP can decrease VT
which will raise the PaC02
Effects of increased baseline
pressure
small amount of gas are trapped
because the exhalation valves closes
before the circuit pressure drops back
to zero.
 How much gas is left in the lung is a
function of :

◦ level of PEEP selected,
◦ I:E ratio [if the exhalation time is too
short more gas can trap
◦ time constant of the lung units.
With PEEP
flow restrictor: the exhalation port is
too small for the gas to all escape. The
faster the flow through the tiny hole, the
more back pressure the flow
restrictor creates
 threshold resistor: creates back
pressure that is independent of flow rate.
In these types of PEEP valves, the gases
passes freely until some balances of
forces on the other side equalize and the
pressure is held in the circuit.

basic types of PEEP/CPAP devices

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Water Column:
Weighted ball:
Flexed springs:
Venturi diaphragm:
Electromagnetic valve:
Types of threshold PEEP valves


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
When the flow is too fast for the exhalation
valve to get all the gas out
Ventilator circuits are rated for their
resistance to flow and a max flow rate will
be suggested.
Failure to keep the flow rate below this max
will result in PEEP rising as the flow rate
rises.
This is a real issue with neonatal circuit that
are so tiny that airway resistance rises
quickly
When can a threshold resistor
become a flow restrictor?
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Highlights of Unit 3: Classification of mechanical ventilation

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