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

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

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

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 V E, thus the PaC0 2.  When the patient breathes on A/C or SIMV he will alter the V E which will change the PaC0 2.

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

19. 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

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 V E 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 V T, inspiratory flow rate and his own V E, his muscle strength and co- ordination are encouraged  Because the PSV ’s V T 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 V T 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 V T will vary based on the patient’s compliance and R AW  In VC ventilation, you set the V T and the PIP will vary based on the patient’s compliance and his R AW

28. question  Describe the effect on the return V T 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 –V T thus V E drops –PIP rises, thus P AW 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 V T, inspiratory flow rate and Ti are all determined by the patient. His P AW 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 V T, inspiratory flow and Ti, but in this case the P AW 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 V E that can get the PaC0 2 to normal levels  In both of these modes, the clinician must establish [1] V E 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 P plateau 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. 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

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 R AW 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 C0 2

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

49. 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

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

51. 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

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

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

54. question  You have a blood gas that shows the pH is acidic due to a higher PaC0 2.  What parameters do you adjust to correct this?

55. ANSWER  To control the PaC0 2 you manipulate the V E. Parameters that manipulate the V E are the respiratory rate and the V T  Once the PaC0 2 returns to normal the pH will return to normal

56. question  You have an arterial blood gas in which the patient’s Pa0 2 and Sa0 2 are both lower than normal. How do you adjust the ventilator to treat hypoxemia?

57. 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

58. question  If your patient had the following ABG what would you do to the ventilator?  pH 7.47  PaC0 2 30  Pa0 2 45  HC0 3 - 26

59. Answer  To correct the low PaC0 2, you need to decrease the V E  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

60. 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?

61. 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

62. What would you have to monitor with this mode?  Vital signs for increased WOB or compromise of Cardiac output  Sp0 2 for oxygenation  pH and PaC0 2 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

63. What are the problems associated with the mode you selected?  SIMV will result in the patient controlling some of the V E, you will lose fine control over the PaC0 2 —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 V E,, and we don’t want his spontaneous respiratory rate getting too high if his V T is too low

64. 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 Pa0 2 and PaC0 2 stay at his baseline

65. Case study # 2  Patient is a 25 year-old BF suffering from a closed head injury. The doctor wants to keep the PaC0 2 at 25-35 mmHg and the Pa0 2 110-120 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?

66. What ventilator mode [modes] might work with her?  In situations where the clinician needs complete control over the PaC0 2 like this one, a control mode of some kind is required. A/C with VC is best  Sedation and paralysis is mandatory

67. 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 P AW : PIP and PEEP changes can alter the thoracic pressure thus the blood flow from the head  We watch the Sp0 2 for hyper-oxygenation  We watch the VS for s/s of altered blood pressure

68. 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 V E thus his C0 2  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 P AW can rise which can alter his blood flow from his head

69. What are the advantages to the mode you selected?  You have complete control over the PaC0 2 so that there are no alternations in cerebral blood flow  You have complete control over the P AW so that there are no changes in the cerebral blood flow

70. 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?

71. 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 Fi0 2 100% then get a gas and titrate later

72. What would you have to monitor with this mode?  Sp0 2 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

73. What are the problems associated with the mode you selected?  If the patient were to wake up and breathe faster, she will increase her V E which will alter her PaC0 2  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

74. What are the advantages to the mode you selected?  We control her PaC0 2 and her Pa0 2.  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