Review of modes of mechanical ventilation By Elizabeth Kelley Buzbee A.A.S., R.R.T.-N.P.S., R.C.P.

question In A/C mode there are two ways to trigger the breath. What are they?

ANSWER: In A/C mode, the ventilator has Time triggered Patient triggered Flow triggered Pressure triggered Volume triggered NAVA

question 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: 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.

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

ANSWER: A/C or VC modes will rest the patient We can also use these modes with sedation and paralysis to “Control” the patient

question What is the function of a PAV mode?

answer The PAV proportional assist ventilation mode is one in which the ventilator collects data about elasticity and RAW and his flow and volume demands to moderate the PS to maintain a more or less consistent breathing pattern

question Under what conditions would you want to select ATC mode?

answer Automatic tube compensation is a mode in which the PS will be set by the machine based on the RAW of the ET tube.

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

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.

question What has happened to his inspiratory time? Your patient on A/C 10 bpm and he is assisting at a total f of 15 bpm. 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 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 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? A couple of ways: Increase the flow rate to decrease the Ti, 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’

question What is the advantage of control mode?

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.

question What is the difference between SIMV and IMV?

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

question Under what circumstances do we move the patient to pressure support ventilation PSV?

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.

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

ANSWER: In PS, because the patient selects his own VT, inspiratory flow rate and his own VE, his muscle strength and co-ordination are encouraged Because the PS ’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

question How do we select the correct PSV pressure?

ANSWER: There are three methods: Set up the PS pressure to get a VT of 10-15 ml/ kg IBW Titrate the PS to get a spontaneous respiratory rate of less than 25 bpm Give just enough PS to overcome the resistance to the endotracheal or the tracheostomy tube

question Compare pressure control [PC] ventilation to volume Control [VC] ventilation

Answer in PC ventilation, you set the PIP and the VT will vary based on the patient’s time constants In VC ventilation, you set the VT and the PIP will vary based on the patient’s time constants

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

answer 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 alarms go off VT thus VE drops PIP rises, thus PAW rises

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

answer 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 end immediately on PC just as it does on VC

question Compare CPAP mode to PSV

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.

question 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

question When do we select PC ventilation rather than VC?

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

question Identify the indications for SIMV or IMV?

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

questions Identify indications for CPAP

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

question Describe IRV?

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

question Identify an indication for IRV.

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.

question Identify the normal settings for the non-invasive positive pressure ventilation via the BiPap machine

answer IPAP 8 cmH20 EPAP 4 cmH20 Spontaneous mode/ Spontaneous timed Added 02 via 02 line to mask

Question Discuss the indications for NIPPV [BiPap]

answer Indications for NIPPV are the patient who: Acute management of CHF, COPD patient who doesn’t want to get intubated ,recently extubated person who fails & the immunosupressed patient. Long term management of the patient with neuromuscular disorders, with obstructive & central Sleep Apnea and COPD with hx of hypoventilation at night

question Identify the patient who would handle NIPPV best.

answer The patient who would be most successful being placed on NIPPV would be the patient who Can protect his airway; remember no ET tube Is alert Is not claustrophobic nor vomiting Has an intact ventilatory drive Requires only a little extra driving pressure to keep his VE reasonable [PaC02 and pH at base line]

question Explain what happens in ‘Bilevel ventilation’

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

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

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, if the rate is set approprately the patient reverts to PC ventilation

question How does bilevel ventilation compare to airway pressure release ventilation [APRV?]

ANSWER 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?

ANSWER The patient on APRV who goes apnic will now have alternating high and low pressures. If the rate is set appropriately, He will basically revert to PC – IRV.

question You have a blood gas that shows the pH is acidic due to a higher PaC02. What parameters do you adjust to correct this?

ANSWER To control the PaC02 you manipulate the VE. Parameters that manipulate the VE are the respiratory rate [f] and the VT Once the PaC02 returns to norma,l the pH will return to normal [assuming the bicarb is normal]

question 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: 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

question If your patient had the following ABG what would you do to the ventilator? pH 7.47 PaC02 30 Pa02 45 HC03- 26

Answer 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 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 him? What parameters would you monitor? What are the problems associated with the mode you selected? What are the advantages to the mode you selected?

What ventilator mode [modes] might 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 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 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 after 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 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 # 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 would you have to monitor with this mode? What are the problems associated with the mode you selected? What are the advantages to the mode you selected?

What ventilator mode [modes] might 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 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 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 mode you selected? You have complete control over the PaC02 so that there are no alternations in cerebral blood flow As long as the patient has no changes in his RAW and compliance, because he is sedated, you have control over the PAW so that there are minimal changes in the cerebral blood flow

Case 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 her? What would you have to monitor with this mode? What are the problems associated with the mode you selected? What are the advantages to the mode you selected?

What ventilator mode [modes] might 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 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 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 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

question When would we want to select HFV high frequency ventilation?

answer We would select HFV for a patient who needs high mPAW but whose lung compliance is so low that his PIP is excessive and there is a risk of barotrauma and decreased CO. We would raise the mPAW with rate rather than VT [PIP] or PEEP

question Under what conditions do we want to select a pressure regulated, volume control [PRVC] mode?

answer We would select PRVC when we want the advantages of VC [guaranteed VE and VT], but we don’t like the PIP that this mode might create. We can set a lower PIP, so that the inspiration is limited to a PIP that is 5 cmH20 lower than the preset pressure. The flow rate will decrease so that the PIP stays lower

question Under what circumstances might we want to select an auto-mode?

answer Auto-mode is a choice for the patient who is expected to start to breath on his own and we don’t anticipate issues with muscle weakness. Auto mode is a form of automatic weaning from a CMV to a spontaneous mode. It is best used with the basically healthy patient who is s/p surgery for who we expect to get off the ventilator in less than 24-48 hours

question How does auto-mode work?

answer This is a duel mode that only work in when the RCP has selected a CMV mode [PC or VC] . If the patient is resting quietly, the ventilator stays at the CMV mode Once the patient starts to breath on his own [increased assisted breaths] the machine will revert to a spontaneous mode such as PS/CPAP or CPAP. If the patient fails to breath the machine reverts to the previous mode

question How does auto mode differ from apnea parameters?

answer When the patient is on apnea parameters, the ventilator gives a few breaths then allows him to start breathing again, then alarms audibly if he doesn’t. Apnea parameters are clearly an alarm situation while auto-mode is simply the machine going back and forth between CMV and spontaneous modes

question What patient would do well on MMV?

answer Extra PS to increase the spontaneous VT Extra breathes In mandatory minute ventilation, the ventilator is in a partial mode [SIMV at a low rate] and if the patient cannot maintain a minimal VE, there will be help to get this VE back up Extra PS to increase the spontaneous VT Extra breathes

question Discuss the use of the ASV mode.

answer Adaptive support ventilation mode is a mode in which the RCP set the patient’s IBW and a percent of work [VE] that she wants the patient to get from the ventilator If she picks 100%, the ventilator is in full support with all VE needs from the ventilator. If she picks 50%, the patient is in a partial mode in which the half the VE comes from spontaneous efforts [PS] and half from the ventilator