Review of modes of Review of modes of mechanical ventilationmechanical ventilation

By Elizabeth Kelley Buzbee A.A.S., R.R.T.-N.P.S., R.C.P.

questionquestion

In A/C mode there are two ways to trigger the breath.

What are they?

ANSWER:ANSWER:

In A/C mode, the ventilator has Time triggered Patient triggered

questionquestion

Identify the two most common patient triggers for mechanical ventilators in A/C mode

ANSWER:ANSWER:

Pressure trigger Flow trigger

questionquestion

A/C mode is considered one of the CMV modes.

What is a CMV mode and why is A/C classified as a CMV mode?

ANSWER:ANSWER:

A/C mode is a CMV mode because it handles 100% of the work of breathing. The patient can trigger a breath, but all breaths are controlled by the ventilator .

CMV modes include: A/C in PC or VC One of these modes is used to rest the patient

who is in respiratory failure He does no work at all.

questionquestion

Identify the most common initial ventilator setting used with the patient in respiratory failure who needs to rest?

ANSWER:ANSWER:

A/C or VC modes will rest the patient

We can also use these modes with sedation and paralysis to “Control” the patient

questionquestion

Identify the mode one would select for initial ventilation of the patient with COPD or with asthma who needs to rest?

ANSWER:ANSWER:

We would select SIMV with a rate of 10-12 to rest this patient while minimizing chances of air trapping that can happen during A/C.

If the patient’s exhalation is too long, we may need to decrease the rate even more.

questionquestion Your patient is on A/C 10 and he

is breathing 15bpm. What has happened to his

inspiratory time? What has happened to his expiratory

time? How can you correct this situation?

What has happened to his What has happened to his inspiratory time?inspiratory time?

The inspiratory time is established by the inspiratory flow rate and flow pattern.

If those knobs don’t change, then the inspiratory time doesn’t increase or decrease.

What has happened to his expiratory What has happened to his expiratory time?time?

Because the rate increased from 10 to 15 bpm, the patient’s cycle time decreased.

Cycle time = 60 seconds / rate

60 / 10 = 6 seconds

60 / 15 = 4 seconds

As the cycle time decreases, and the inspiratory time stayed the same, the expiratory time decreased

How can you correct this situation?How can you correct this situation?

A couple of ways: Increase the flow rate to decrease

the inspiratory time, this gives you more time to exhale

Change the patient from A/C to SIMV if you want him to breathe

If you don’t want him to breathe, give him sedation and paralytic agents to return him to ‘Control’

questionquestion

What is the advantage of control mode?

ANSWER:ANSWER:

Controlling the patient will control the VE, thus the PaC02.

When the patient breathes on A/C or SIMV he will alter the VE which will change the PaC02.

questionquestion

What is the difference between SIMV and IMV?

ANSWER:ANSWER: In IMV, the patient will get his time-triggered

breaths right on schedule. If he happens to be exhaling during his spontaneous breath, then he will ‘stack breaths.’ this leads to air trapping & patient discomfort.

In SIMV, the patient’s time-triggered mandatory breath will come in just a fraction of a second early so that the patient and the ventilator are ‘synchronized’ to avoid stacking breaths

questionquestion

Under what circumstances do we move the patient to PSV?

ANSWER:ANSWER:

we add PSV to the SIMV so that the patient can establish a spontaneous VE without increasing his respiratory rate to a dangerous level.

We also select PSV when we want to help the patient breathe, but still allow him to use his own muscles.

questionquestion

What is the advantage of SIMV with PSV over SIMV alone?

ANSWER:ANSWER: In PSV, because the patient selects his

own VT, inspiratory flow rate and his own VE, his muscle strength and co-ordination are encouraged

Because the PSV ’s VT are larger than the patient could get with spontaneous breathing, his WOB is not as excessive as if he was doing all the work, but it is more than if the ventilator was doing all the work

questionquestion

How do we select the correct PSV pressure?

ANSWER:ANSWER:

There are three methods:

– Set up the PS pressure to get a VSet up the PS pressure to get a VT T of 10-15 ml/ kg of 10-15 ml/ kg

IBWIBW– Titrate the PS to get a spontaneous respiratory rate Titrate the PS to get a spontaneous respiratory rate

of of lessless than 25 bpm than 25 bpm– Give just enough PS to overcome the resistance to Give just enough PS to overcome the resistance to

the endotracheal or the tracheostomy tubethe endotracheal or the tracheostomy tube

questionquestion

Compare PC ventilation to VC ventilation

AnswerAnswer

in PC ventilation, you set the PIP and the VT will vary based on the patient’s compliance and RAW

In VC ventilation, you set the VT and the PIP will vary based on the patient’s compliance and his RAW

questionquestion

Describe the effect on the return VT of the patient on VC whose PIP has reached the high pressure limit?

answeranswer

In VC ventilation, when the patient reached the high pressure limit, the breath is immediately cycled off, and exhalation starts.

– Audible and visual High pressure Audible and visual High pressure alarms go offalarms go off

– VVTT thus V thus VEE drops drops

– PIP rises, thus PPIP rises, thus PAWAW rises rises

questionquestion

Describe what happens to the patient on PC ventilation when he reaches the set PIP?

answeranswer A patient on PC ventilation, who reaches

his PIP will continue to get the breath at that pressure until it is time-cycled off.

If however, if something happens so that the patient reaches the high pressure alarm [which is set higher than preset PIP], his breath with still end immediately on PC just as it does on VC

questionquestion

Compare CPAP mode to PSV

ANSWER:ANSWER: In CPAP, the patient is breathing

spontaneously. His VT, inspiratory flow rate and Ti are all determined by the patient. His PAW and the baseline pressure are pretty much the same.

In PSV, the patient triggers a pressurized breath that rises above the baseline. Again, this patient controls his own VT, inspiratory flow and Ti, but in this case the PAW is lower than the PS pressure because there is more difference between baseline and PS pressures.

questionquestion

In what ways are CPAP and PSV max the same?

CPAP and PSV max both require a patient with an intact ventilator drive, & enough muscle strength to create a VE that can get the PaC02 to normal levels

In both of these modes, the clinician must establish [1] VE alarms that will warn of apnea and [2] high respiratory rate alarms to warn of possible fatigue

questionquestion

When do we select PC ventilation rather than VC?

ANSWER:ANSWER:

When VC ventilation has failed due to excessive PIP or Pplateau and there is real danger of barotrauma or decreased CO.

In infants or small children who have gross air leaks around uncuffed endotracheal tubes

questionquestion

Identify the indications for SIMV or IMV?

ANSWER:ANSWER:

To wean the patient by increasing his work load gradually

As an initial ventilatory mode for COPD and asthma patient to minimize airtrapping

To decrease the negative effects of A/C mode on the cardiac output

questionsquestions

Identify indications for CPAP

ANSWER:ANSWER: CPAP or N-CPAP for obstructive sleep

apnea Treating refractory hypoxemia without

respiratory acidosis or hypercapnia Weaning modality just before the patient

is extubated Means of keeping a patient ‘off’ the

ventilator for more than 2 hours without risking atelectasis

questionquestion

Describe IRV?

ANSWER:ANSWER:

IRV is ‘inverse ratio ventilation’ Which is a mode where ventilator is set up so that the inspiratory time exceeds the expiratory time making the ratio 1:1 up to 4:1

questionquestion

Identify an indication for IRV.

ANSWER:ANSWER:

IRV is indicated in patients with poor compliance and normal RAW who have failed conventional ventilation by having PIP so high there is a real risk of barotrauma or decreased CO.

questionquestion

Explain what happens in ‘Bilevel ventilation’

ANSWER:ANSWER:

In bilevel ventilation, the patient breaths at a high level of CPAP that drops down to a lower level of CPAP periodically so that the patient can get rid of excessive C02

questionquestion

What happens to the patient on Bilevel ventilation if he becomes apnic?

ANSWER:ANSWER:

If the patient on bilevel ventilation has been set up properly, as he stops breathing, the changes between high CPAP and low CPAP now are changes between a PIP and a PEEP—in other words, the patient reverts to PC ventilation

questionquestion

How does bilevel ventilation compare to APRV?

ANSWERANSWER

These modes are identical except that in APRV, the patient breaths at the higher CPAP level for a longer time than he breaths at the lower CPAP level.

In Bilevel ventilation, the time spent at higher CPAP is less than at lower CPAP

Describe what happens to the patient on APRV who goes apnic?

ANSWERANSWERThe patient on APRV who goes apnic will now have alternating high and low pressures. He will basically revert to PC – IRV.

questionquestion

You have a blood gas that shows the pH is acidic due to a higher PaC02.

What parameters do you adjust to correct this?

ANSWERANSWER

To control the PaC02 you manipulate the VE. Parameters that manipulate the VE are the respiratory rate and the VT

Once the PaC02 returns to normal the pH will return to normal

questionquestion

You have an arterial blood gas in which the patient’s Pa02 and Sa02 are both lower than normal. How do you adjust the ventilator to treat hypoxemia?

ANSWER:ANSWER:

To treat hypoxemia you increase the Fi02

If the Fi02 changes don’t work—or your Fi02 is at a toxic level, then you increase the PEEP level

questionquestion

If your patient had the following ABG what would you do to the ventilator?

pH 7.47 PaC02 30

Pa02 45

HC03- 26

AnswerAnswer

To correct the low PaC02, you need to decrease the VE

That will fix the pH too To correct the low Pa02, you need

to increase the Fi02 or if it is already at 50% start the patient on a PEEP of 3-5 cmH02

Case studiesCase studies Patient is a 65 year-old WM with respiratory failure

secondary to viral pneumonia. He has a history of COPD. He is alert and anxious with a respiratory rate of 35 bpm.

– What ventilator mode [modes] might work with What ventilator mode [modes] might work with him?him?

– What parameters would you monitor?What parameters would you monitor?– What are the problems associated with the mode What are the problems associated with the mode

you selected?you selected?– What are the advantages to the mode you What are the advantages to the mode you

selected?selected?

What ventilator mode [modes] might What ventilator mode [modes] might work with him?work with him?

He needs to rest, so A/C might be a choice but because he is at risk for airtrapping, one might best select SIMV for his initial mode

What would you have to monitor with What would you have to monitor with this mode?this mode?

Vital signs for increased WOB or compromise of Cardiac output

Sp02 for oxygenation pH and PaC02 for acid/base balance BBS to make sure his breath ends before

the next breath comes in to avoid air trapping

monitor flow/time curve for auto-PEEP and air trapping

What are the problems associated What are the problems associated with the mode you selected?with the mode you selected?

SIMV will result in the patient controlling some of the VE, you will lose fine control over the PaC02—unless you sedate and paralyze him– Then your patient will get muscle atrophy Then your patient will get muscle atrophy

after a few days of this CMVafter a few days of this CMV As the SIMV rate is dropped the patient

must assume more of the VE, , and we don’t want his spontaneous respiratory rate getting too high if his VT is too low

What are the advantages to the What are the advantages to the mode you selected?mode you selected?

SIMV will minimize chances of air trapping,

it will help him keep his muscle strength

maintain his ventilatory drive as long as the Pa02 and PaC02 stay at his baseline

Case study # 2Case study # 2 Patient is a 25 year-old BF suffering from a closed head injury.

The doctor wants to keep the PaC02 at 25-35 mmHg and the Pa02 110-120 mmHg to minimize cerebral edema. Her breath sounds are clear and bilateral when you bag her at a rate of 15 bpm and with 100% Fi02.

– What ventilator mode [modes] might work with her?What ventilator mode [modes] might work with her?– What would you have to monitor with this mode?What would you have to monitor with this mode?– What are the problems associated with the mode you What are the problems associated with the mode you

selected?selected?– What are the advantages to the mode you selected?What are the advantages to the mode you selected?

What ventilator mode [modes] might What ventilator mode [modes] might work with her?work with her?

In situations where the clinician needs complete control over the PaC02 like this one, a control mode of some kind is required. A/C with VC is best

Sedation and paralysis is mandatory

What would you have to monitor with What would you have to monitor with this mode?this mode?

In closed head injuries we worry about sudden changes in the systemic BP because this can change blood flow in the head.

We watch the PAW: PIP and PEEP changes can alter the thoracic pressure thus the blood flow from the head

We watch the Sp02 for hyper-oxygenation We watch the VS for s/s of altered blood

pressure

What are the problems associated What are the problems associated with the mode you selected?with the mode you selected?

If the patient were to wake up and start to breathe, he can drastically alter:

his VE thus his C02

He could air trap as his respiratory rate rises without the flow rate rising to keep the I:E the same

As he fights the ventilator, his PAW can rise which can alter his blood flow from his head

What are the advantages to the What are the advantages to the mode you selected?mode you selected?

You have complete control over the PaC02 so that there are no alternations in cerebral blood flow

You have complete control over the PAW so that there are no changes in the cerebral blood flow

Case study # 3Case study # 3 Patient is a 55 year-old LAF with respiratory failure

following cardiac arrest. She is apnic and unresponsive with a low CO and diffuse crackles in both lungs

– What ventilator mode [modes] might work with What ventilator mode [modes] might work with her?her?

– What would you have to monitor with this mode?What would you have to monitor with this mode?– What are the problems associated with the mode What are the problems associated with the mode

you selected?you selected?– What are the advantages to the mode you What are the advantages to the mode you

selected?selected?

What ventilator mode [modes] might What ventilator mode [modes] might work with her?work with her?

While CPAP, NIPPV or PSV might be indicated for CHF which might well be part of this patient’s problem, she is apnic

She needs to be intubated and ventilated VC or A/C is initial ventilator mode for her. Post-CPR patients are best started with Fi02

100% then get a gas and titrate later

What would you have to monitor with What would you have to monitor with this mode?this mode?

Sp02 for oxygenation and good peripheral perfusion

BBS and P plateau for changes in lung compliance due to CHF—or fluid over load during CPR

VS and heart monitor for cardiac arrhythmias

What are the problems associated What are the problems associated with the mode you selected?with the mode you selected?

If the patient were to wake up and breathe faster, she will increase her VE which will alter her PaC02

If she breathes too fast, she alters her I:E ratio which can decrease venous return to the heart

Each breath on A/C will result in higher intrathoracic pressures- this could confuse her body’s control over urine production and blood pressure

What are the advantages to the What are the advantages to the mode you selected?mode you selected?

We control her PaC02 and her Pa02.

She rests Her WOB is decreased and that will

decrease the work on her heart As long as she is controlled by sedation

and paralysis, her intrathoracic pressures stay the same so that ventilation cannot alter the blood pressure