DAVID AYMOND, PGY-II

Ventilator Principles and Management

Goals and Objectives

Understand major physiological/pathophysiological consequences of mechanical ventilation and how they are used clinically

Define objectively when a patient needs to be intubatedUnderstand the basic principles of ventilator mechanicsUnderstand basic modes of mechanical ventilationUnderstand and discuss initial settings on the ventilatorDescribe how to tailor ventilator settings to correct an

ABGDefine 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, Diarrhea5. Renal: RF for AKI6. CNS: increase in ICP7. 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 BPM2. Rise in PCO2>10mmHg3. PaO2 on supplemental O2 <55mmHg4. 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=MVMV= RR x Tidal Volume (Vt)Vt: the volume of air moved into the lungs

during quiet breathing.MV=RR x VtNormal MV~6L/minCMV= Controlled Mechanical VentilationAC= Assist ControlSIMV= Synchronized intermittent mandatory

ventilation

Ventilator Principles

1. Type of Breaths: Volume controlled, Volume Assist, pressure controlled, Pressure Assist and pressure support. Our ventilators can be set to any of these.

1. 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.

2. 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.

3. PS breaths can be given on SIMV and is a separate mode in its-self. PS ventilation provides driving pressures for each breath.

2. 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, 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:1. 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 option3. 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.

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 causes1. 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 Process1. 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 T-piece 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 reversedObjective 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 < -30If 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)

What is an Air Leak

An air leak is a good thingWe 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