Highlights of Unit 3: Highlights of Unit 3: Classification of Classification of

mechanical ventilation mechanical ventilation

By Elizabeth Kelley Buzbee AAS, RRT-NPS, RCP

The parts of a mechanical The parts of a mechanical ventilator ventilator

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

Mechanical ventilators and the Mechanical ventilators and the WOBWOB

is dependent on the patient’s RAW, his compliancethe volume required and the elastic recoil of the lung

We classify ventilators by these We classify ventilators by these questionsquestions

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?

Input Power Input Power How does this machine get power to How does this machine get power to operate?operate?

electrical power: A/C D/C 110-115 volt current

pneumatically-powered. 50-60 psigBattery powered [emergency only or

transport]◦Internal batteries◦External batteries◦Run about one hour then require 8-12 hours to

recharge

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

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

Output control valves: Output control valves: How does it control the flow of gas into the How does it control the flow of gas into the patient?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

Fluidic and pneumatic controlFluidic 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

Means of Communication:Means of Communication:How does it communicate information to the How does it communicate information to the operator in such a manner that the RCP can operator in such a manner that the RCP can monitor the patient’s responses and modify the monitor the patient’s responses and modify the ventilator’s actionventilator’s action??

Monometers/◦bourdon gauges- measure pressure◦digital monometer may be displayed as a bargraph, or as numbers

Spirometers: Volume measurementsWaveformsalarms

Where do we set the alarm limits?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 delayLow or high humidifier temperature- keep

at 32-340 C [high 37 max]

Where do we set the alarm limits?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?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

Control variables:Control variables:

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

Open vs. closed loop control of Open vs. closed loop control of ventilator outputventilator outputopen; 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 A control variablecontrol variable is the primary is the primary variable that the ventilator variable that the ventilator manipulates to cause an inspiration:manipulates to cause an inspiration:

Pressure controlled [PC]Volume controlled [VC]Flow controlledTime [usually based on the other

parameters]

Only one control, the other two will be variables

Pressure control:Pressure control:

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

Volume controlled:Volume controlled:

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

Flow controlledFlow 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

Phase Variables: Phase Variables:

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

Phase variables: 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

What event triggers inspiration?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

Pressure triggered: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 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

Flow triggeredFlow 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

Problems with flow triggersProblems 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’

Volume triggeredVolume triggered

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

NAVANAVA

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.

What happens when triggering is not What happens when triggering is not accurate or responsive?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◦

Limit: what limits the actual Limit: what limits the actual inspiratory cycle? 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

IMPORTANT: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.

Cycle: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.

◦

Flow cyclingFlow 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

Baseline: What changes the base Baseline: What changes the base line pressure? line pressure?

We can raise the pressure during exhalation phase from zero to a positive number;

we have raised the baseline

PEEP or CPAP?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

Effects of increased baseline Effects of increased baseline pressurepressure

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

With PEEP 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, ◦I:E ratio [if the exhalation time is too short more gas can trap

◦time constant of the lung units.

basic types of PEEP/CPAP devicesbasic 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.

Types of threshold PEEP valvesTypes of threshold PEEP valves

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

When can a threshold resistor When can a threshold resistor become a flow restrictor?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