Modes of Mechanical Ventilation

Created by charlami 

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What does the mode of ventilation describe?

The breath type and the pattern of breath delivery to a patient.

What 3 factors determine the mode?

1. Type of breath
2. Targeted control variable
3. Timing of breath delivered

What are the different types of breaths? (3 types)

1. Mandatory
2. Assisted
3. Spontaneous

What are the 2 target control variables?

1. Pressure
2. Volume

What are the 3 different ways a breath is time delivered?

1. Continuous Mandatory Ventilation (CMV)
2. SIMV
3. Spontaneous

What is a mandatory breath?

A breath where the ventilator controls the timing (how many breaths each minute), tidal volume (how much volume in each breath), or both

What is a spontaneous breath?

It is a breath where the patient controls the timing and tidal volume.

The volume or pressure of a breath is based on the patient's what?

Demand and lung characteristics, not a set value or control.

What is an assisted breath?

They have characteristics of both mandatory and spontaneous breaths. All or part of the breath is generated by the ventilator which does all or part of the work of breathing.

When does the breath become assisted?

When the airway pressure rises above baseline during inspiration.

What is the control variable in volume targeted breaths?

The control variable is volume.

Volume targeted breaths are _______ of lung characteristics or when patient effort _______.

Independent, changes.

What remains constant in volume targeted breaths?

The volume delivered remains constant.

When are volume targeted breaths usually used? (AKA goal)

When maintaining a certain PaCO2 level

What is the ONE advantage to volume targeted breaths?

1. Guaranteed volume regardless of changes in lung.

What are the FOUR disadvantages of volume targeted breaths?

1. Increased peak & alveolar pressures when lung conditions worsen leading to alveolar overdistention.
2. Flowrate is fixed and may not meet the patient demand
3. Sensitivity level many not be set appropriately making it harder for patient to trigger inspiration
4. Disadvantages can lead to patient-ventilator dyssynchrony

How do lung characteristics affect pressures during volume ventilation? (2 ways)

1. Lower compliance leads to increased PIP & Plateau pressures
2. Increased Raw leads to increased peak pressures

How does the inspiratory flow pattern affect pressures during volume ventilation? (2 ways)

1. Peak pressure is higher with constant flow (square flow) and lower with descending flow
2. High gas flow leading to increased peak pressure

How does the volume setting have an affect on pressures during volume ventilation?

Higher volumes produce higher peak & plateau pressures

How does PEEP affect pressures during volume ventilation?

Increasing PEEP increases the peak pressure

How does auto-PEEP have an affect on pressures during volume ventilation?

Increases in auto-PEEP increase the peak inspiratory pressure

What is the control variable in pressure targeted breaths?

The control variable is pressure.

Pressure is _________ of the patient's lung characteristics and if the patient's effort ________.

Independent, changes.

What varies in pressure targeted breaths? Why is that?

The volume a patient gets will vary. This happens due to the changes in lung characteristics.

When is pressure targeted ventilation used? (AKA the goal)

When the goal is to maintain a certain PaCO2 level

What are the TWO advantages of pressure targeted breaths?

1. Allows maximum pressure to be set which reduces risk of overdistention.
2. Delivers a descending flow pattern (machine will match patient's flow)

What are the TWO disadvantages of pressure targeted breaths?

1. Volume delivery varies
2. MD's and RT's are less familiar with P.V.

How does the pressure setting affect the volume delivery during pressure ventilation?

A higher set PIP produces a larger volume and vice-versa

How does the pressure gradient affect volume delivery during pressure ventilation?

Increasing EEP, which is PEEP + auto-PEEP, while maintaining constant PIP decreases volume delivery and vice-versa.

How does the patient's lung characteristics affect the volume delivery during pressure ventilation? (2 ways)

1. Lower compliance leads to lower volume
2. Increased Raw leads to lower volume (if active flow present). If flow = 0, Raw doesn't affect volume.

How does the inspiratory time affect the volume delivery during pressure ventilation?

Increased I-time leads to increased volume (as long as flow continues)

How does the patient's effort affect the volume delivery during pressure ventilation?

Active inspiration can increase volume.

What is a mode?

It is a set of operating characteristics that control functions of vent

How many modes are used together to get a desired effect?

Normally, 2 or more modes are used together.

What types of modes are offered on a majority of ventilators?

The basic and oldest modes - A/C, SIMV, PEEP

What types of breaths are in CMV?

All the breaths are mandatory.

Is CMV pressure or volume targeted?

It can be either or.

How are the breaths triggered in CMV?

They can be patient triggered or time triggered.

What is the CMV mode called when the breath is time triggered?

The mode is called Controlled Ventilation or the "Control Mode"

What is the CMV mode called when the breath is patient triggered?

The mode is often called "Assist Control"

In control mode, all breaths are triggered by what?

Time.

The control mode is only appropriate for what kind of patients?

For patients who cannot make an inspiratory effort (paralysis due to drugs, spinal cord injury, etc.)

Why is the control mode rarely used today?

Because newer modes allow for all the elements of the control mode but allow for patient initiated breaths.

In A/C mode, the breaths are pressure or volume targeted?

They can be either or.

What type of breath triggers are in the A/C mode?

The breaths can be triggered by time or patient.

What is set in the A/C mode?

A minimum respiratory rate.

What is special about the A/C mode?

It incorporates the ability of the ventilator to sense a patient's inspiratory effort and trigger additional breaths with the same preset volume or pressure as the time triggered breaths.

How is a mandatory breath given in A/C mode?

Through the monitoring of circuit pressure. The vent would monitor the circuit pressure and would deliver a breath when a negative pressure deflection was noted.

What will happen on the pressure/time waveform if a breath is patient triggered?

The waveform will show a negative deflection when the breaths are patient triggered.

What are the common problems of A/C?

Autotriggering and hyperventilation.

What is VC-CMV?

Volume Targeted Continuous Mandatory Ventilation

How much WOB does the patient do in VC-CMV?

33-50%, it was believed that the vent did all of the WOB.

When does most of the WOB occur in VC-CMV?

It occurs when active inspiration occurs.

Why does the most WOB occur during active inspiration?

The set gas flow does not match the patient's inspiratory flow demand.

What is PC-CMV?

Pressure Targeted Continuous Mandatory Ventilation AKA Pressure Control Ventilation

What kind of ramp flow does PC-CMV utilize?

A decelerating ramp flow which improves gas distribution and allows the patient to vary inspiratory gas flow during spontaneous breathing efforts.

What is the trigger, limit and cycle for PC-CMV?

It is pressure trigger, pressure limited, and time cycled.

The max pressure limit is set how much above the PIP?

10 cmH2O

Why is it set that much higher than the PIP?

Because the PIP is not the max possible pressure (pressures increase with cough).

What is IMV?

Intermittent Mandatory Ventilation

IMV is the earliest mode that offered what?

Partial ventilatory support.

What kind of breaths are delivered during IMV and what triggers them?

The breaths can be either pressure or volume targeted and they are time triggered.

What does the IMV mode allow the patient to do?

It allows them to breathe spontaneous breaths at any tidal volume the patient is capable of between mandatory breaths.

What is the spontaneous breath tidal volume determined by?

The patient's inspiratory effort.

How can the WOB be lessened on spontaneous breaths?

The newer vents allow for pressure support and PEEP/CPAP to be used

What is the major drawback of IMV?

It is the lack of timing between spontaneous and mandatory breaths.

Who sets the IMV rate?

The therapist

Why is the lack of timing a major drawback?

The machine is time triggered based on the rate that is set regardless of any spontaneous ventilation. So if a patient takes a spontaneous breath and the ventilator time triggers a mandatory breath before the patient exhales, breath stacking occurs.

What is SIMV?

Synchronized Intermittent Mandatory Ventilation

What is SIMV similar to?

It is similar to IMV.

What kind of breaths does SIMV deliver and what triggers them?

They are volume or pressure targeted breaths with a time trigger.

What can the patient do between mandatory breaths in SIMV?

The patient can take spontaneous breaths, the volume of the breath is determined by the patient's inspiratory effort.

In SIMV, what can also be used to reduce the WOB of spontaneous breaths?

Pressure support and PEEP/CPAP

What is the main difference between IMV and SIMV?

The delivery of the mechanical breaths is synchronized with the patient effort. When the time arrives for a mandatory breath, the ventilator will wait briefly for a patient effort (flow or pressure trigger).

What happens when a trigger is detected in SIMV?

The machine immediately delivers the mandatory breath. If no trigger is detected, the mandatory breath will be delivered via the time trigger.

What is the time interval called prior to time triggering in which the ventilator is responsive to the patient's spontaneous inspiratory effort?

Synchronization window.

How long is the synchronization window?

Typically it is 0.5 seconds.

What is the main benefit of SIMV?

It allows the patient to perform part of the work of breathing.

When is SIMV frequently used?

It is used to wean patients from the mechanical ventilator.

How do you wean using the SIMV?

You wean by slowly lowering the mandatory breath rate. This forces the patient to increase the number of spontaneous breaths.

Can you deliver full support via SIMV?

Yes, but only if a high enough respiratory rate is set high enough to meet the patient's demands.

What are the 3 types of spontaneous modes?

1. Spontaneous breathing
2. Continuous Positive Airway Pressure
3. Pressure Support Ventilation

What is spontaneous breathing sometimes referred to?

The T-piece method

Why is spontaneous breathing called the t-piece method?

It mimics having the patient's endotracheal tube connected to a Briggs adapter.

What is the advantage of using the Spontaneous mode rather than a T-piece?

The ventilator can act as a monitor.

What is the disadvantage of using spontaneous breathing on a vent?

Some vents require a lot of patient effort to open the inspiratory valves to receive gas flow.

What are the benefits of using CPAP via a ventilator?

Same as noninvasive CPAP
1. Improved oxygenation by increasing FRC
2. Alveolar recruitment

What is the advantage of using CPAP via a vent?

The vent can act as a monitor.

What is Pressure support ventilation?

It applies a preset pressure plateau to the patient's airway for the duration of a spontaneous breath and augments spontaneous tidal volume.

PSV has what trigger, what limit and what cycle?

Flow or pressure trigger, pressure limited, and flow cycled.

Using PSV helps lower the patient's....

Work of breathing.

When can PSV be used?

Any time a spontaneous breath is available (IMV, SIMV, CPAP)

What are the 3 basic functions of PSV?

1. Overcome WOB for spontaneously breathing patients
2. Reduce WOB further in CPAP or SIMV by setting the pressure level higher than that required to overcome system resistance.
3. Can be used to provide full ventilatory support in SPECIFIC conditions. The patient must be breathing and have a stable lung condition.

What are the indications for PSV? (3)

1. It can be used anytime spontaneous breaths are allowed by the ventilator
2. PSV can be used during SIMV, especially in difficult to wean patients, to augment the patient's spontaneous tidal volumes
3. Can be used in conjuction with CPAP but a backup apnea alarm must be used in case the patient stops breathing.

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