Review of modes of mechanical ventilation

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

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

3. ANSWER: • In A/C mode, the ventilator has • Time triggered • Patient triggered

4. question • Identify the two most common patient triggers for mechanical ventilators in A/C mode

5. ANSWER: • Pressure trigger • Flow trigger

6. 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?

7. 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.

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

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

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

11. 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.

12. question • 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?

13. 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.

14. 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

15. 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’

16. question • What is the advantage of control mode?

17. 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.

18. question • What is the difference between SIMV and IMV?

19. 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 ANSWER:

20. question • Under what circumstances do we move the patient to PSV?

21. 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.

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

23. 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

24. question • How do we select the correct PSV pressure?

25. 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

26. question Compare PC ventilation to VC ventilation

27. Answer • 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

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

29. 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

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

31. 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 still end immediately on PC just as it does on VC

32. question • Compare CPAP mode to PSV

33. 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.

34. question • In what ways are CPAP and PSV max the same?

35. 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

36. question • When do we select PC ventilation rather than VC?

37. 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

38. question • Identify the indications for SIMV or IMV?

39. 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

40. questions • Identify indications for CPAP

41. 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 ANSWER:

42. question • Describe IRV?

43. 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

44. question • Identify an indication for IRV.

45. 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.

46. question • Explain what happens in ‘Bilevel ventilation’

47. 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

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

49. 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 ANSWER:

50. question • How does bilevel ventilation compare to APRV?