Highlights of Unit 3: Classification of mechanical ventilation

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Highlights of Unit 3 Classification of
mechanical ventilation

• By Elizabeth Kelley Buzbee AAS, RRT-NPS, RCP

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The parts of a mechanical ventilator

• obtains power and converts this power into a
  force that can move gas into a patients lung.
• Sends gas down a circuit to the patient interface
  and back to ventilator for analysis of data

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Mechanical ventilators and the WOB

• is dependent on the patients RAW,
• his compliance
• the volume required
• and the elastic recoil of the lung

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We classify ventilators by these questions

• 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 patients responses and modify the
  ventilators action?

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Input Power How does this machine get power to
operate?

• 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

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Power transmission and conversion How does it
use this power to drive gas into the patient?

• Drive mechanisms
• Compressors
• piston-driven-
• rotarydriven- 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

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Output control valves How does it control the
flow of 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

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Fluidic and pneumatic control

• 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

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Means of CommunicationHow does it communicate
information to the operator in such a manner that
the RCP can monitor the patients responses and
modify the ventilators action?

• Monometers/
• bourdon gauges- measure pressure
• digital monometer may be displayed as a bargraph,
  or as numbers
• Spirometers Volume measurements
• Waveforms
• alarms

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Where do we set the alarm limits?

• 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 temperature- keep at
  32-340 C high 37 max

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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

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Where do we set the alarm limits?

• Fi02 alarms -5 /-.
• High/low rate alarms more than 10-20 from
  baseline
• Loss of PEEP/CPAP alarms are generally set about
  3-5 below the PEEP

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Control variables

• How does the mechanical ventilator control the
  various parameters of ventilation such as
  starting and stopping a breath?

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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.

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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
  

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Pressure control

• a PC pressure controlled breath is one in which
  the pressure stays the same, but changes in the
  patients condition will alter the delivered
  volume and the flow rate.
• The doctor orders a PIP which will deliver a VT

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Volume controlled

• During VC ventilation, the PIP varies with
  changes in the patients conditions, while the
  volume and the flow stay constant.
• The doctor orders a VT

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Flow 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

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Phase Variables

• What event triggers inspiration, what stops the
  breath, what changes the breath?

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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

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What event triggers inspiration?

• 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

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Pressure triggered

• 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
  patients 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

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Flow 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

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Problems with flow triggers

• 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

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Volume triggered

• only the Drager Baby Log actually uses the volume
  inspired by the patient to trigger a breath

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NAVA

• Neutrally adjusted ventilatory assist
• A probe is sent down the esophagus and as the
  phrenic nerve fires, the probes sensor notes the
  breath effort and triggers the ventilator.

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What happens when triggering is not accurate or
responsive?

• 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

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Limit what limits the actual inspiratory cycle?

• 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

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IMPORTANT

• 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.

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Cycle

• 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 time-cycled
  off
• Pressure cycled- in VC ventilation, if the high
  pressure alarms goes off and the breaths stops we
  can say that the breath was pressure-cycled.

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Flow cycling

• 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

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Baseline What changes the base line pressure?

• We can raise the pressure during exhalation phase
  from zero to a positive number
• we have raised the baseline

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PEEP or CPAP?

• 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

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Effects of increased baseline pressure

• 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

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With PEEP

• 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,
• IE ratio if the exhalation time is too short
  more gas can trap
• time constant of the lung units.

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basic types of PEEP/CPAP devices

• 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.

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Types of threshold PEEP valves

• Water Column 
• Weighted ball
• Flexed springs
• Venturi diaphragm
• Electromagnetic valve

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When can a threshold resistor become a flow
restrictor?

• 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