MECHANICAL VENTILATION

1
MECHANICAL VENTILATION

• Gregory A. Schmidt, MD
• Professor, University of Chicago

2
A 23 yo woman is admitted to the ICU with status
asthmaticus. She is ventilated with Vt 600cc
RR20 FiO2 0.5 and has PO2 135 PCO2 73 pH 7.08.
What do you do next (assume therapy is maximal)?
3
The following changes are made Vt 600 to
800 RR 20 to 32PO2 105 PCO2 66 pH 7.09 HR
136.Meanwhile, the airway pressure begins to
alarm flow is changed from square to
decelerating (the alarm stops).The heart rate
rises to 160, blood pressure falls to 88/68, the
ventilator begins alarming again and, while the
patient is being moved for a CXR, she arrests.
4
Two Aspects of MV

• 1. To control PO2 (oxygenation)
• When mask therapy is insufficient
• Adjust FiO2, PEEP
• 2. To control PCO2 (ventilation)
• When the patient cant
• When wed rather the patient not
• Adjust Mode, rate, tidal volume

Intubation differs from MV. ETT may be needed
when MV isnt.
5
What is PEEP?
6
Surgery by T.Whitehead Video by
G.Volgyesi Producer A. S. Slutsky
ZEEP/PEEP
7
(No Transcript)
8
Setting the PEEP

• PEEP recruits alveoli, raising PO2
• Some lesions are not PEEP-responsive
• PEEP may protect the lung
• PEEP makes the lung bigger
• Pneumothorax, but only if Vt is excessive
• Hypotension, but only if critically dry
• Range 5-30, average 10 cmH2O
• Use enough to get the SpO2 0.88 and FiO2
• Major error PEEP too low

9
AutoPEEP

• Lungs dont empty at end-expiration
• Usually associated with airflow obstruction
• Problems
• Increased WOB and difficulty triggering
• Barotrauma, hypotension
• You have to look for it
• Also called intrinsic PEEP

10
AUTOPEEP Ventilator Changes
VOLUME
TIME
11
Measuring AutoPEEP
12
Reducing AutoPEEP

• Reduce VE
• Lower Vt
• Lower RR
• Raise inspiratory flow (square)
• Treat airflow obstruction
• Use HeO2
• Accept hypercapnia

13
Ventilation Control of PCO2
1. Ventilator produces a constant pressure of
gas PCV,PSV 2. Ventilator produces a constant
flow of gas ACV, IMV
14
PCV Flow, Pressure, Vol
15
Volume Assist-Control
16
Rrs vs Crs
1. Low shoulder 2. Elevated Pplat-PEEP 3. Normal
expiratory flow
17
Comparing/Contrasting Modes

• All can do most anything, used properly
• Patient comfort is generally greatest with
  pressure modes (PCV, PSV)
• Doctor comfort is greatest with volume modes
  (ACV, SIMV) ARDSNet used ACV
• Useful information about the patient is most
  readily available from volume modes
• SIMV impairs weaning
• Mixed modes and fancy new modes are unnecessary

18
Initial Ventilator Settings

• Normal mechanics and gas exchange
• ACV with large Vt (8-15cc/kg)
• f 8-12 Flow 60L/min
• FiO2 0.5 PEEP 5-8
• ACRF
• Consider NIV
• Other (SA, ACRF, ARDS, CW)
• ACV with small Vt 5-7cc/kg
• f 16-36 Flow 60L/min, square wave
• FiO2 1.0 PEEP 5-30

19
After Initial Stabilization

• 1. Use the airway pressure and waveform displays
  to determine the physiology and to follow changes
  with time and treatment
• 2. Ascertain your goals, eg. rest, lung
  protection, exercise
• 3. Adjust the ventilator in accord with the
  patients physiology and comfort and with your
  goals

20
Case Mrs. SP
A 34 yo woman, 3 years s/p kidney transplant, is
admitted with fever, hypotension, and acute renal
failure. She is intubated for obtundation and
respiratory distress. BP 85/30 HR 131 RR 33
accessory muscle use responds to deep pain. CXR
clear lungs, ETT in position. ABG before ETT
(room air) 105/20/7.19. Waveforms show...
21
Waveforms Mrs. SP
22
Waveforms Mrs. SP
23
A/C Pre- and Post-NMB
24
Subsequent Adjustments

• Normal
• Rapidly reduce FiO2 using oximetry
• Use PaCO2 or ET-CO2 to set VE
• On ACV, adjust flow rate for comfort
• If drive is high, raise flow, VT, or Pinsp
• If goal is full rest, ACV or PCV
• May switch to PSV if drive and strength can be
  relied upon
• Use sighs and PEEP to prevent atelectasis

25
NIV in ACRF
Brochard etal. NEJM 333817, 1995
26
Improving NIV Success

• Commitment
• Select patients carefully
• Array of masks
• Use PSV and escalate gradually

• Verbal and pharmacologic sedation
• Use and adjust PEEP
• Detect and correct leaks
• Patient-ventilator synchrony

27
Subsequent Adjustments

• ACRF
• Reduce FiO2 using oximetry
• Adjust PEEP to ease triggering (e.g. 5-10)
• Follow autoPEEP and Pplat (
• Titrate VE to goal PaCO2 or ET-CO2
• Avoid excessive VE
• If full rest is not desired, may change to PSV

28
ARDSNet VT and Mortality
ARDSNetwork. NEJM 3421301, 2000
29
Subsequent Adjustments

• ARDS
• Adjust PEEP to
• least PEEP c/w SpO2 0.87 with FiO2
• Lower FiO2 to 0.6
• Reduce VT below 6mL/kg IBW, if needed, to keep
  Pplat

30
MV in Status Asthmaticus

• Effect of increasing VE or VT on Ppk, Pplat, VEI,
  and VEE
• VEI
• Normocapnic ventilation caused hypotension in 7/9

Tuxen and Lane, ARRD 1987136872
31
PCO2
Lung Injury, AutoPEEP
32
Permissive Hypercapnia

• Sacrifice PCO2/pH for lung protection
• Well-tolerated and standard therapy for status
  asthmaticus, ARDS

33
Flow Rate and Flow Profile
34
The following changes are made Vt 600 to
800 RR 20 to 32PO2 105 PCO2 66 pH 7.09 HR
136.Meanwhile, the airway pressure begins to
alarm flow is changed from square to
decelerating (the alarm stops).The patient
arrests.
35
Subsequent Adjustments

• Status Asthmaticus
• Reduce FiO2 using oximetry
• Flow 60-80L/min, square wave
• Reduce VE until Pplat significantly
• May use PCV with Pinsp
• Follow autoPEEP and Pplat (or VEI)
• Raise PEEP to limit the work of triggering

36
Subsequent Adjustments

• Chest Wall/Lung Restriction
• Minimize VT and VE, especially if cardiac output
  is threatened
• Raise FiO2 in preference to PEEP

37
Case Ms. ACSCVP Tracing
38
Weaning Liberation

• The spontaneous breathing trial greatly assists
  judgment regarding extubation
• Most patients dont need to be weaned
• Respiratory failure is in the patient, not the
  ventilator
• IMV delays weaning

39
Spontaneous Breathing Trial

• 100 ICU patients considered ready for weaning
  after 8.2 1.1 days of MV
• Compared VE, MIP, f/VT, CROP
• Best predictor of weaning f/VT ? 105
• T-piece CPAP or PSV ? 5
• Example rate 28, VT .300, f/VT 93
• This RSBI has become the standard

Yang KL, Tobin MJ N Engl J Med 3241445, 1991
40
Effect of SBT

• 300 ICU patients screened daily
• PaO2/FiO2 200 PEEP ? 5
• Adequate cough no sedatives or vasoactive drugs
• f/VT ? 105 on CPAP for 1 minute
• Randomized to 2 hour SBT or not
• Result communicated verbally and with pre-printed
  message
• Your patient has successfully completed a 2-hour
  trial of spontaneous breathing and has an 85
  chance of successfully staying off mechanical
  ventilation for 48 hours.

Ely EW, et.al N Engl J Med 3351864, 1996
41
Effect of SBT

• Duration of MV
• SBT group 4.5 d
• No SBT 6 d
• Complications (self-extubation tracheostomy
  reintubation 21 days MV)
• SBT group 20
• No SBT 41
• Costs Reduced in SBT group

Ely EW, et.al N Engl J Med 3351864, 1996
42
Weaning Modes Tested

• IMV Initial rate 10 reduced, if possible, at
  least 2x/d, by 2-4 breaths/min
• PSV Initial PSV 18cmH2O reduced, if possible,
  at least 2x/d, by 2-4 cmH2O
• SBT1 T-piece up to 2h/d
• SBT2 T-piece (or CPAP ? 5) gradually up to 2h/d,
  at least 2x/d

Esteban A et.al NEJM 332 345, 1995
43
Extubation Criteria

• IMV Tolerate 5 breaths/min for 2h
• PSV Tolerate PSV 5 for 2h
• SBT1 Tolerate 2h unassisted breathing
• SBT2 Tolerate 2h unassisted breathing

Esteban A et.al NEJM 332 345, 1995
44
Modes and Weaning
Esteban A et.al NEJM 332 345, 1995
45
Conclusions

• PO2 is controlled by FiO2 and PEEP
• Dont be afraid of PEEP
• AutoPEEP must be sought and reduced
• Ventilator settings should take into account
  whats wrong with the lung
• Waveforms can provide useful information
• Facility with ACV and PSV will meet all your
  needs
• Liberate, dont wean use the SBT