Transcript Ventilator Principles and Management

Ventilator Principles and
Management
DAVID AYMOND, PGY-II
Goals and Objectives
 Understand major physiological/pathophysiological
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consequences of mechanical ventilation and how they are
used clinically
Define objectively when a patient needs to be intubated
Understand the basic principles of ventilator mechanics
Understand basic modes of mechanical ventilation
Understand and discuss initial settings on the ventilator
Describe how to tailor ventilator settings to correct an
ABG
Define when to wean from the ventilator and discuss the
best evidence based approach
Mechanical Ventilation
 Following an inspiratory trigger, a pre-determined
mixture of air is forced into the central airways down
to the alveoli increasing intra-alveolar pressure and
causing the lungs to inflate. A termination signal
eventually causes the ventilator to stop forcing air
into the central airways and CAP decreases.
Expiration then follows passively from an area of
high pressure (Alveoli) to an area of lower pressure
(central airways)
Physiology/Pathophysiology of Ventilation
1.
Respiratory System
-Barotrauma/Volutrauma: trauma to the alveoli from
pressure causing overdistention and damage
-Hyperoxia: O2 radicals cause inflammation
-Atelectotrauma: repetitive alveoli recruitment and
decruitment
-VALI: inflammation of the lung from the vent.
-Physiologic Dead Space: alveolar area that is not
involved in gas exchange due to decreased perfusion.
Mechanical ventilation increases this space by
increasing ventilation in areas without perfusion.
Physiology/Pathophysiology (cont)
Respiratory System
-Physiologic shunt: this exist when there is blood flow
through the pulmonary parenchyma that is not involved
in gas exchange due to poor ventilation. Pt’s with
respiratory failure have increased physiological shunting
(Pathologic shunting), why?
*On 100% FiO2, a pts PaO2 with normal lung
physiology=750mmHg; for every decrease of 100mmHg
in PaO2, this corresponds to 5% shunting.
-Diaphragm: very rapid disuse atrophy in 18hrs
-Resp Muscles: Atrophy
- Muco-cilliary clearance is largely decreased.
Physiology/Pathophysiology (cont)
2. Hemodynamics
-decrease in CO from decrease venous return from PPV
-decrease in RV output from PPV compressing
pulmonary vasculature
-decrease in LV output from bulging interventricular
septum from the increase in RV pressure, which
decreases diastolic return
-The extent of these hemodynamic effects will vary
according to lung compliance. Airway pressures are
transmitted the greatest with high lung compliance
(COPD) and the least with low lung compliance
(ARDS,CHF).
Physiology/Pathophysiology (cont)
Hemodynamics
-This process of transmural pressures will cause false
elevations in CVP. To correct for this:
-If NL lung compliance, subtract ½ of the intrinsic
PEEP from the CVP=real CVP
-If Compliance is decreased, subtract ¼ of the
intrinsic PEEP from CVP=real CVP.
4. GI: increase in GI Bleeds, increase in ALT/AST and
LDH, Diarrhea
5. Renal: RF for AKI
6. CNS: increase in ICP
7. Weakness
Indications
Acute or Chronic Respiratory Failure: defined as
insufficient oxygenation, insufficient ventilation, or both.
The parameters used to assess the need for mechanical
ventilation are widely accepted.
1. RR >35 BPM
2. Rise in PCO2>10mmHg
3. PaO2 on supplemental O2 <55mmHg
4. Most important is your clinical interpretation based on
ABG, setting, and Vitals.
5. Inability to protect airway (large CVA, AMS, etc)
Must choose to intubate before its an emergency if at all
possible. If decision is made to intubate, see RSI
guidelines for sedation and tube choice
Definitions
Minute Ventilation=MV
MV= RR x Tidal Volume (Vt)
Vt: the volume of air moved into the lungs during quiet
breathing.
MV=RR x Vt
Normal MV~6L/min
CMV= Controlled Mechanical Ventilation
AC= Assist Control
SIMV= Synchronized intermittent mandatory
ventilation
Ventilator Principles
Type of Breaths: Volume controlled, Volume Assist, pressure controlled,
Pressure Assist and pressure support. Our ventilators can be set to any of
these.
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VC breaths are ventilator initiated breaths with a set inspiratory flow rate. Inspiration is
terminated once a set tidal volume has been given. Only in CMV, assist control and
synchronized IMV.
VA breaths are patient initiated breaths with a set inspiratory flow rate. Inspiration is
terminated once a set tidal volume has been given. Only in AC and SIMV.
PS breaths can be given on SIMV and is a separate mode in its-self. PS ventilation
provides driving pressures for each breath.
Each Breath has the following:
-Trigger: breaths can be triggered by a timer (set respiratory rate) and/or
by patient effort. When the pt’s effort causes a significant change in the
pressure or flow of the circuit, they get a breath with sp. settings
-Target: the flow of air into the lung can target a predetermined flow
rate=peak inspiratory flow rate
-Termination: can be volume, time or flow related; volume is easiest to
understand, so once a set Vt has been delivered, inspiration ceases
Types of Breaths
 Our ventilators are set to Volume Control and/or Volume Assist by default,
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but we do have the option to change to pressure control or pressure assist.
The optimal type of breath given is strictly operator dependant and there
has never been one breath type shown to be superior to the other.
In Volume control (VC) the only trigger is the time (Rate) we set on the
ventilator. The target is the inspiratory flow rate. The termination is the
tidal volume. The modes that can be VC are CMV, AC and SIMV.
In Volume assist (VA) there are 2 triggers: patient initiated breaths (either
by changes in pressure or flow) and time=ventilator initiated breaths (the
set rate). The target is the inspiratory flow rate. The termination is the Tidal
volume.
AC and SIMV give volume assisted breaths and volume controlled breaths.
Meaning the trigger is set to time (rate). Also, the trigger is either by
changes in pressure or flow initiated by the patient.
CMV can only give volume controlled breaths.
Flow and Pressure Trigger
 Pressure Triggering: there is a release valve on the ventilator,
once the pt tries to initiate a breath, only if he/she can
generate the set pressure (usually -1cm H2O- -3cm H2O) to
release the valve will they get a breath. Once the pressure is
generated the patient will get a breath with our settings (AC)
or what the individual can do (SIMV)
 Flow Triggering: the ventilator has a censor that detects flow
from the patient. Once the flow from the patient is below the
set thresh-hold (i.e. the pt is trying to inspire) the ventilator
will release and the patient will get a breath with our settings
(AC) or what the individual can do (SIMV)
 These 2 triggers only apply in Volume Assisted breaths which
are possible on AC and SIMV modes.
 Our ventilators are pressure triggered by default, but can be
changed to flow triggered.
Ventilator Principles (cont)
3. Modes of Volume Controlled Ventilation
-CMV: the MV is determined entirely by the set RR and Vt; the pt does
not initiate any MV above that set on the ventilator
-AC: the MV is determined by the set RR and Vt; the pt can increase
the MV by triggering additional breaths, if they trigger they receive
our set Vt from the ventilator.
Ex: If we set the vent to RR 20 and Vt of 500, MV is 5 L/min. If the pt
triggers 5 more breaths, they will get 5 more breaths of 500mL of
Vt.
-IMV: similar to AC except each additional breath will have a Vt of
whatever the patient can generate
-SIMV: is IMV but with pressure support, why is this important?
-CPAP: Pressure supported breaths, very little ventilatory support if
settings low
Ventilator Principles (cont)-Settings
4. Initial Settings:
-Trigger: -1cm H2O to -3cm H2O for pressure, and
1-3 L/min for flow
-Vt: 5-8 mL/kg of IBW, unless ARDS 4-6 mL/kg of
IBW
-RR: 12-16 bpm
-PEEP: 5cm H2O, may need up to 20cm H2O, can
go as high as needed as long as plateau pressures are
<30
Initial Settings (cont)
-Inspiratory Flow Rate: 50-60 L/min. Our ventilators have an
auto-flow option so we do not have to set the flow rate.
-FiO2: titrate to keep PaO2 >60, in ARDS PaO2 of 55mmHg ok
-Inspiratory time and I:E ratio: determined by the Peak
Inspiratory flow rate. The Inspiratory flow time (IT)=
Vt/Inspiratory flow rate (Flow). 1:2 in normal patients, 1:3 or higher
in COPD pts or ARDS pts retaining CO2. Typically, we will set a RR
and Vt, and this will adjust the I:E ratio automatically.
-Pressure Support: Only in SIMV or CPAP, 10cm H2O initially
-Airway Pressures: Peak and Plateau, depend on the ventilator
settings and patient related variables (compliance, airway
resistance). Airway pressures are increased by large Vt, PEEP, high
peak flow, poor compliance, or increased airway resistance. Goal is
to keep <30 cmH2O
Plateau Pressure
 How to access plateau pressure on the vent:
On the upper right side of the ventilator there is a
button called specials. Push this. Its beside the
peak and plateau pressure reading on the vent.
2. Next screen shows inspiratory hold as an option
3. At the end of inspiration, just before expiration,
push and hold the inspiratory hold button for 3
seconds. The alarm will sound, this is fine
4. Wait a few seconds and a plateau pressure will
appear on the home screen right beside the peak
pressure tab located in the upper right hand corner.
1.
Tailoring the Ventilator
 MV is the sole contributor to CO2, so if we want to
change the CO2 we only need to change the Vt or the
RR. But do not change the Vt unless change
inspiratory flow rate b/c of asynchrony.
 PEEP and FiO2 are the sole contributors to PaO2
and O2%, so if need to change oxygenation change
these 2 variables
 Monitor the peak and plateau pressures, be sure they
stay below 30 mm Hg
 Monitor patient Asynchrony (see following slides)
Sedation
See RSI guidelines for initial sedation and paralytics. Just remember: etomidate for
sedation, succinylcholine for paralysis, and lidocaine if evidence of head trauma or
stroke.
-For continuous sedation there are several variables to consider. The most important
being:
1. Etiology of the distress: For distress due to anxiety, benzodiazepines
(Midazolam=“Versed”); distress due to dyspnea or pain, opioids (Fentanyl)
2. Duration o f Therapy: Drugs (propofol, midazolam) with brief duration of sedation
(<24 hours) should be used if brief sedation is anticipated or the patient needs to
be frequently awakened. If a longer duration of sedation is needed, use Ativan
(Lorazepam)
Currently, the Society of Critical Care Medicine practice guidelines suggest the
sustained use of sedatives/analgesics be given via intermittent infusion, with the
initiation of continuous infusions with daily interruption in patients who require
intermittent infusions more often than every 2 hours. They also rec not using
benzos.
The ideal sedation goal is for the patient to be awake and comfortable with minimal to
no distress. Ramsay Sedation Scale of 3-4. Currently the accepted scale for sedation
is the Richmond Agitation Sedation Scale (RASS).
Asynchrony
 Defined as phases of breath delivered by the
ventilator do not match that of the patient. This
leads to dyspnea, increased work of breathing, and
prolonged mechanical ventilation
 3 major causes
1. Ineffective Triggering of a Ventilator-Delivered
Breath (not sensitive enough)
2. Double Triggering of Ventilator-Delivered Breaths
(too sensitive)
3. Prolonged Inspiratory Time
Weaning
This is a 2 Step Process
1. Readiness Testing- The purpose of readiness testing is
to identify patients who are ready to wean from
mechanical ventilation and to identify patients who are
not ready for weaning thereby protecting them against
the risks of premature weaning. 2 randomized
controlled trials found 85% of patients tolerated d/c of
ventilation on the same day that their readiness to
wean was first assessed and passed.
2. Weaning- the process of decreasing the amount of
support that the patient receives from the mechanical
ventilator, so the patient assumes a greater portion of
the ventilatory effort. Currently, this is termed
liberation from mechanical ventilation.
Readiness Testing
Due to studies finding an inability of doc’s to adequately
assess when weaning should begin, a consensus
conference composed of experts in mechanical
ventilation proposed criteria for patients who are ready
to wean. They developed 5 criteria:
-The cause of the respiratory failure has improved
-PaO2/FiO2 > 150 mmHg or O2% > 90 while receiving
FiO2 <40% and a PEEP < 5
-pH >7.25
-SBP >90 mmHg but <180 mm Hg
- Pt is able to initiate an inspiratory effort
*This study found that up to 30% of pts who never satisfy
these criteria can be successfully weaned.
Readiness Testing-Weaning Parameters
This consensus performed a systematic review and
found that there are only 3 weaning predictors
(parameters) that were most accurate:
1. RSBI: frequency divided by tidal volume (f/Vt);
should be <105 br/m/L on no ventilatory support
2. NIF: should be <-30 cm H2O (more negative)
3. Minute Ventilation-not as accurate
Weaning
3 Traditional Methods:
1. Spontaneous breathing trials refers to a pt breathing
through the ET tube without any ventilator support
(T-piece) or with minimal ventilator support (low
level of pressure support or CPAP)
2. Progressive decreases in the number of ventilator
assisted breaths during SIMV
3. CPAP with decreasing PEEP and PS
Weaning
There is no clear consensus, but current evidence suggests SBT as the
best method because they are simple, efficient, safe, and effective.
-In a trial of 546 patients receiving mechanical ventilation, 416
underwent successful SBT using a T-piece. Of these, 340 patients
were successfully extubated in 24 hrs. The 130 who failed SBT were
assigned to daily weaning on all 3 methods. The SBT arm were more
likely to wean than the other 2 methods
- In another trial, 300 pts were randomly assigned to receive daily SBT
(via T piece or 5cm H2O) or usual care (weaning at the discretion of
the attending physician). The daily SBT group had a shorter
duration of mechanical ventilation (4.5 versus 6 days) and fewer
complications of resp failure (20 vs 41%). Complications of
respiratory failure included re-intubation, trach, or prolonged
mechanical ventilation.
Spontaneous Breathing Trials
 The type of SBT (T piece vs CPAP) is dictated by the availability and
clinician preference, with the caveat that a small, high resistance
ETT (<7 mm) requires a PS of 10 cm H2O to overcome the
resistance of the tubing and therefore overcome the added work of
breathing. This was shown in a study of patients failing a 30 minute
T-piece trial. Once they failed, immediate conversion of PS to just 7
cm H2O for an additional 30 mns resulted in weaning success in
68% of patients.
 If using CPAP, the pt is hooked to the ventilator and the monitoring
system can alert the clinician about changes in f or MV, whereas Tpiece does not.
 There is no evidence that one SBT method is superior to others. A
randomized trial comparing SBTs using a T-piece to SBTs using
CPAP (5 cm H2O) found no differences in re-intubation rates.
Spontaneous Breathing Trials
Duration of a weaning trial
-A multicenter trial randomly assigned 526 pt’s
receiving mechanical ventilation to undergo SBTs
using a T-piece for 30 mns and for 120 mns. The
rates of weaning failure and re-intubation were
virtually identical in both groups, suggesting that a
30 minute SBT is sufficient to determine whether
mechanical ventilation can be d/c.
How to Define Failure of A Weaning Trial
 The patients sedation must be reversed
 Objective criteria include tachypnea, respiratory
distress (accessory muscle, paradoxical breathing
and diaphoresis), tachycardia (Increase in HR by 20
beats per minute), HTN (significant change),
oxyhemoglobin desaturation (<88%), and AMS
(somnolence or agitation), PaO2 <50 mmHg, and pH
<7.3
Weaning Trial
 If the weaning trial is passed, the patient should then
subsequently undergo weaning parameters (RSBI,NIF).
 There is no clear consensus, but studies commonly use a
cut off of 105 for the RSBI on a t-piece.
 The NIF should be < -30
 If these weaning parameters are met, the pt is a
candidate for extubation if the following are appropriate:
Pt has air leak (next slide)
 Pt can protect their airway (sufficient cough and adequate level of
consciousness)
 Not requiring frequent suctioning (volume of resp secretions minimal)
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What is an Air Leak
 An air leak is a good thing
 We take 5cc of air, after intubation, in a syringe and inflate a
cuff on the endotracheal (ET) tube. This cuff keeps the ET
tube in place. To test for an air leak, we deflate that cuff and
tell the patient to take a breath. When they take a breath, you
should hear air move around the ET tube IF there is no
laryngeal edema. This is called an AIR LEAK. If there is
laryngeal edema, you will hear no air move. Meaning there is
NO AIR LEAK.
 To prevent laryngeal edema, and therefore allow an air leak,
we can give dexamethasone breathing treatments prior to
extubation. If you extubate and hear stridor, racemic
epinephrine should be given. I could find no evidence
supporting this, but it is accepted clinical practice by RRMC
respiratory department.
How to Put it All Together
Step 1: Daily sedation vacation, if patient tolerates go
to step 2
Step 2: Readiness testing=Assessment criteria (See
slide 21), if pass go on
Step 3: SBT for 30 minutes using CPAP with PS 8,
PEEP 5; unless pt has CHF, then do T-piece trial
Step 4: Conclusion of SBT: If pt passes SBT (see slide
27), get ABG and Weaning parameters
Step 5: If pH, PaO2, PCO2 NL, RSBI <80, and NIF < 30, extubate to face tent or NIPPV.
Summary
 Basic definitions (Slide 9)
 When to intubate (Slide 8)
 Choose a mode (slide 11)
 Set the initial vent settings (slides 12-13)
 Tailor the vent settings (slide 14)
 Assess readiness with testing and begin liberation
ASAP