Principles of Mechanical ventilation

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Principles ofMECHANICAL VENTILATION

In the name of GOD

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Terminology

– f: rate of breathing

– Vt: Tidal volume

– VA: Alveolar ventilation

– VD: dead space = 2.2 ml/ kg

– FiO2: fraction of inspired O2

– PEEP: Positive End Expiratory Pressure

– PASB : Pressure Above Spontaneous Breathing

– ASB : Assisted Spontaneous Breathing

Basic pulmonary physiology Air that moves in and out of a patient's lungs per

minute that is 7-10 L/min Minute volume (MV)

MV =Vt x f Alveolar ventilation (VA) in contact with the

alveolar-capillary gas exchange interface VA = (Vt - VD ) x fVolume-pressure relation: P = V/C

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Plateau pressure (static pressure)

• … is the pressure at the end of inspiration with a short breath hold

• It should not be exceed 30 cmH2O

• P plateau ~ 1/compliance (P = V/C)

• volume = P plateau

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Pressure-time diagram forvolume controlled constant

flow ventilation

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Peak Airway Pressure (dynamic pressure)

• …. is pressure during inspiration

• So related to both airway resistance and compliance

• compliance and/or resistance P peak

• P peak – P plateau < 4 cm H2O (normal gradient)• P peak is a vital sign for mechanically ventilated patients.• P peak < 35 cmH2O

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Changes in inspiratory airway resistance

Paw-peak increased but Plateau pressure unchanged:

1. Tracheal tube obstruction and kinking

2. Airway obstruction from secretions

3. Acute bronchospasm

Rx: Suctioning and Bronchodilators

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Changes in complianceIn this situation P-P gradient is fixid.increasing compliance → plateau and peak pressures falldecreasing compliance → plateau and peak pressures rise

Paw-peak and Plateau pressure are both increased:

1. Pneumothorax2. Lobar atelectasis3. Acute pulmonary edema4. Worsening pneumonia5. ARDS6. COPD with tachypnea and Auto-PEEP7. Increased abdominal pressure8. Asynchronous breathing

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Decreased Paw-peak:

1.Inadequate gas supply, inadvertent change in setting, system air leak, Tubing disconnection, cuff leak, unintended extubation and failure of the ventilator

Rx: Manual inflation, listen for leak

2. Hyperventilation: Enough negative intrathoracic pressure to pull air into lungs may drop Paw-Peak

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Measurements

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Measurements

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P –T

curve

At the start of inflation, the airway pressure immediately rises because of the resistance to gas flow (A), and at the end of inspiratory gas flow the airway pressure immediately falls by the same pressure (A) to an inflexion point.

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P/T F/T

V/T curves in

VCV

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Increase in Ppeak–Pplat gradient

• Increased airway resistance caused by heat and moisture exchanger (HME)• Patient biting endotracheal tube• Kinked or twisted endotracheal tube• Obstruction of endotracheal tube by

secretions, mucus, blood• Bronchospasm• Obstruction of lower airways

Schematic of two superimposed pressure-time curves showing a small increase in peak inspiratory pressures (Ppeak) with a greater increase in plateau pressures (Pplat). This is characteristic of decreased lung compliance

Unchanged or decreased Ppeak–Pplat gradient

• Pneumonia• Atelectasis• Mucus plugging of one lung• Unilateral intubation• Pneumothorax• Pulmonary edema (noncardiogenic and

cardiogenic)• Abdominal distention/pressure

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Spontaneous Breathing

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Positive End Expiratory Pressure

• … is the pressure at the end of expiration

• Serial Elevated PEEP P plateau and FRC

1- Extrinsic PEEP (applied PEEP by MV)

• 3 - 20 cm H2O and be started on 5 cm H2O• It improves the oxygenation not CO2 removal• It may be increased 3-5 cmH2O Q 10-15 min• It has some side effects: biotraumas and

hemodynamic compromise

• What is the optimal PEEP? 1- increasing PEEP until a complication occurs2- assessing P plateau

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Optimal PEEP

1. P-V curve monitoring

2. Cardiac Output monitoring and Venous Oxygen Saturation

If SmvO2 decreases after PEEP application Drop C.O.

In this situation PEEP and/or tidal volume23

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P-V curve

Adequate PEEP Inadequate PEEP

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PEEP

PEEP Disadvantages: • 1. Decrease BP & CPP

2. Increase PCO2

3. because alveolar injury is often heterogeneous, appropriate PEEP in one region may be suboptimal in another and excessive in another

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2- Intrinsic PEEP

• …is incomplete alveolar emptying during expiration due to air trapping

• Ventilator Factors: High inflation volumes, rapid rate, low exhalation time

• Disease factors: Asthma, COPD (collapsed airway)• And has some effects: – Decreased C.O.– Alveolar rupture– increased work of breathing

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Int. PEEP

• Int. PEEP may be detected in two ways:

(1) evaluating the flow-time trace (exp. Flow not returned to zero before next breath)

(2) disconnecting the patient from the ventilator and listening for additional exhaled gas after an exhalation has occurred.

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• Volume cycled

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Pressure cycled

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Indications for mechanical ventilation

No absolute contraindications Loss of airway anatomy Loss airway protection Respiratory and cardiac Failure

Apnea / Respiratory Arrest Inadequate ventilation (acute vs. chronic) Inadequate oxygenation Eliminate work of breathing Reduce oxygen consumption

Neurologic dysfunction Central hypoventilation/ frequent apnea Comatose patient, GCS < 8 Inability to protect airway

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Control Mechanisms

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Control mechanisms

1. Spontaneous breathing (PSV)

2. Pressure targeted ventilation

3. Volume targeted ventilation

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1. Spontaneous breathing:

• Setting: FiO2 and PEEP

• Flow and f is dictated by patient

2. Pressure-Cycled (targeted):

Alters gas flow and volume fixed preset airway pressure (Paw) for the duration of a preset inspiratory time (Ti )

Advantage: fixed pressure limit or eliminate alveolar over-distention and barotraumas

One problem: changes in compliance or resistance

with fixed Paw variable received volume

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3 . Volume/ Flow-Cycled (targeted)

Deliver a preset volume of gas ( VT) Paw is variable

Advantage: delivery a constant VT changes in compliance or resistance with fixed

volume changes airway pressure

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• There are no clinical outcome studies showing benefit of one breath-targeting strategy over the other.

• Pressure-targeting provide a variable flow tends to synchronize better with patient effort.

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Modes

1. Controlled Mechanical Ventilation /Assist Control (CMV/AC)

2. Intermittent Mechanical Ventilation (IMV)/Synchronized IMV (SIMV).

3. Continuous Positive Airway Pressure (CPAP) /pressure support ventilation (PSV)

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1. Controlled Mechanical Ventilation (CMV)

Assist Control (AC)

Control Mode VentilationContinuous mandatory ventilationContinuous mechanical ventilationControlled mandatory ventilation Intermittent Positive Pressure Ventilation (Dräger)

…is a full support mode (machine breaths) All breaths are supported regardless of initiation of

breathing and can be set to VCV and PCV Used for: apneic patients, respiratory muscle

weakness and LV dysfunction

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(CMV/AC )Sensitivity pressure 0.5 to 2 cm H2O (1-3 cm

H2O Tintinalli)

The higher sensitivity the greater work of breathing

After spontaneous breath the ventilator`s timer resets from this time

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(CMV /AC)

CMV preferred and most commonly used initial mode for acute phase of respiratory failure in ED

but CMV : 1. Poor toleration in awake patients 2. Worsening of volume retention in COPD /

asthma43

A/C mode

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2. Intermittent Mechanical Ventilation (IMV)

Synchronized IMV (SIMV)

SIMV (Dräger, Hamilton) SIMV (VC) + PS (Maquet) VCV-SIMV (Puritan-Bennett, Respironics)Volume SIMV (Viasys)Intermittent demand ventilation

IMV combination of spontaneous vent. and AC IMV is a partial support mode This ventilator mode provides breaths at a preset rate

(machine breath) similar to the AC mode Can be set to PCV and VCV

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(IMV / SIMV ) But in spontaneous breath only receive a spontaneous VT with

no support from the ventilator and has a high work of breathing

The synchronized version of IMV coordination spontaneous and machine breaths

SIMV:

1. To prevent excess VT delivered (stacking) decreased hyperinflation, barotrauma

2. During exhalation from a spontaneous breath exhalation compromised

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3. Continuous Positive Airway Pressure (CPAP)

pressure support ventilation (PSV)

Assisted Spontaneous Breathing(Dräger)Spontaneous mode(Hamilton, Puritan-Bennett)Pressure support (Maquet)CPAP (Respironic)Pressure Support Ventilation (Viasys)continuous positive-pressure breathing (CPPB ) EPAP

Is a partial support mode Breathing control by the patient (spontaneous mode ) ,

and peak Paw control by machine (pressure targeted)

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PSV

PSV / ASB / CPAP

Help the patient overcome the resistance of the circuit decreased work of breathing

This is not useful in apnea

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CPAP / PSV / ASB

CPAP the least amount of support and used with IPPV or NIPPV

Most commonly used in (COPD) , CHF and obstructive sleep apnea with NIPPV ( via a tight-fitting nasal or full face-mask)

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CPAP / PSV / ASB With IPPV CPAP is typically used as a weaning

mode

The CPAP level when transitioning from IMV/PSV or AC mode should be the PEEP level that was being used

Reducing the CPAP below the previous PEEP :1. Loss of alveolar recruitment2. Atelectasis3. Hypoxia4. Increased work of breathing

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CPAP / PSV

In partial support: CPAP range is from 0 up to 35 cm H20 pressure Some ventilators may deliver PSV that achieves a

greater range. The average starting point is:

10 cm H20 Use PSV for approximation of spontaneous Vt and

the set Vt for mandatory breaths

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(CPAP) benefits

Eliminate the work of breathing

To aid in weaning from IMV-base ventilation and is frequently part of a transition strategy from IMV to CPAP

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Breath type & examples

DRÄGER Evita 2 CMV

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DRÄGER Evita 4 CMV

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DRÄGER Evita 2 SIMV

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DRÄGER Evita 4 SIMV

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DRÄGER Evita 2 CPAP

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DRÄGER Evita 4 CPAP

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DRÄGER Evita 2 PCV

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DRÄGER Evita 4 PCV

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DRÄGER Evita XL SIMV

Adjunct Ventilator Settings and

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1. Oxygen

The fraction of inspired O2 (FiO2) is set in a range from 21% (room air; not generally indicated) to 100%

In the ED it is common to start at 100% FiO2 to ensure adequate oxygenation and titrate the FiO2 down to nontoxic levels (FiO2< 60%) following the SaO2 via the pulse oximeter (SaO2> 90%) during first 72 h.

some practitioners recommend using 95% O2 as the upper limit of FiO2

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2. Inspiration : Expiration (I:E) Ratio

• The normal I:E ratio in a spontaneously breathing, non intubated patient is 1:4

• Intubated patients commonly achieve I:E ratios of 1:2

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3. Flow rate (Q )• This is the rate of gas delivery (L/min). • The range of flows that can be achieved by current

ventilation is from 10 to 160 L/min. • Common flow settings are from 40 to 75 L/min. • The higher the flow rate, the faster the ventilator

will reach its set volume or pressure. • Start at Q=60 L/ min

• A faster flow rate decreased ins. time• A slower flow rate decreased exp. time

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Flow-time diagram

VCV PCV

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Initial ventilator setting

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Pressure triggering

• The sensitivity of the trigger can be adjusted by changing the pressure drop required for inspiratory cycling to be triggered, and this can be set to a value between −1 cm H2O (very sensitive) and −20 cm H2O (very insensitive)

• If the setting is too sensitive, minor fluctuations in breathing circuit pressure (e.g. from cardiac pulsations) can trigger the ventilator inappropriately.

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Flow triggering

• The reduction in return flow that has to be detected for triggering to occur can be adjusted between 1 (very sensitive) and 10 (insensitive) L/min

• Flow triggering is considered to require less work from the patient to initiate a breath and therefore may enhance patient comfort and reduce the work of breathing.

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Advanced modes

• BREATH TO BREATH– Pressure-regulated volume control (PRVC)– Auto flow– Volume control plus (VC+)– Adaptive pressure ventilation (APV)– Variable-pressure control (VPC)

• WITHIN A BREATH– Volume-assured pressure support ventilation (VAPSV)– Pressure augmentation

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Other modes• High Frequency Ventilation• Proportional assist

ventilation• Airway Pressure Release

Ventilation (Bi-level ventilation) PCV

• T high : 4 – 6 s T low : 0.2 – 1.5 s

• P high : up to 40 cmH2O P low : 10 cmH2O

NON INVASIVE VENTILATION

TUMS 79

TUMS 80

Noninvasive Ventilation• For prevention of invasive MV in selected patients

• No need to definitive airway control

• Candidates :

COPD, CHF, Asthma, hypoxia, DNR patients and Immunocompromised patients

1. the airway must be patent,2. the respiratory drive must be intact, 3. the patient must be cooperative (i.e., awake and alert). TUMS 81

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• Serial assessment and close monitoring Q 30 min• ABG Q 1-2 h• Contraindications:– Near arrest– Severe GIB– Anatomic defect of face– Up airway obstruction– AMS– Risk of aspiration and defect in secretion clearance

Nasal mask

TUMS 84

Advantages of NIV

The potential benefits of NIV over MV are:

1. a decrease in potential airway injury2. decrease in VAP3. probably a shorter length of stay4. Eating and speech reserved

TUMS 85

Disadvantages of NIV

1. Pulmonary baro-trauma (volu-trauma)2. pressure necrosis of the facial skin, subcutaneous tissue

and musculature and patient discomfort3. aerophgia gastric dilation vomiting and aspiration4. hemodynamic compromise

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TUMS 87

Bi-level ventilation.A: Baseline pressure cycles between Plow and Phigh with spontaneous, unsupported, patient breaths during high and low phases.Transition from low to high phase is synchronized to the patient’s inspiration, and transition from high to low phase is synchronized to patient’s expiration.B: As in A, but now inspiratory efforts during the Plow phase trigger support (Psupp).

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Bi-level ventilation.C: As in A, but now inspiratory effort during both Plow and Phigh phase trigger support which is targeted to the same absolute support pressure (Psupp). If Phigh is greater than Psupp, patient effort during Phigh becomes unsupported.D: As in A, but now inspiratory effort during both Plow and Phigh phase trigger support which is set specifically for each phase,relative to the baseline pressure of the phase.

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Approach to NIPPV

• In hypoxemia: EPAP + 2 cmH₂O and fix interval IPAP

• In hypercapnia: IPAP + 2 cmH₂O and EPAP= 40% IPAP

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High flow nasal cannula• … can deliver warm

and humidified air up to 40 L/min

TUMS 92

5 Special Circumstances and applying ventilatory support

1. Severe Acute Lung Injury and ARDS• Preferred PCV• consider permissive hypercapnia. If able to achieve P02> 60 mm Hg on FiO2<60%, the PCO2

may be allowed to be >40 mm Hg if pH> 7.25. if needed, use NaHCO3

• Tv 6 – 8 ml / kg• F 20 – 25 / min• PEEP 8• Monitor P plateau: if > 30 cm H2O Tv : 4 ml/kg

TUMS 93

2. Severe Asthma and COPD

• The defect is decreased gas flow.• In conventional ventilation use higher flow rate and lower

respiratory rate to allow more time for exhalation.• Tv 5 -8 ml / kg• F 8 – 10 / min• Q 80 l/min• PEEP 5 (50 - 80% intrinsic PEEP)• Detection of intrinsic PEEP with wean and chest

compression• Optimal P plateau < 30 cmH2O

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Monitoring of treatment in asthma

3. Pulmonary edema

• NIPPV is preferred• If the patient is intubated PEEP is useful• But in hypotensive patients min. PEEP with

continuous evaluation

TUMS 96

5. Traumatic brain injury

• Do not lower PC02 < 35 mm Hg, as it may induce severe cerebral vasoconstriction and lead to cerebral ischemia.

• The goal is PC02: 35-40 mm Hg. • Acceptable to hyperventilate for a patient with an

acute herniation syndrome as a bridging maneuver for definitive therapy.

TUMS 97

General Guidelines for Initial Invasive Ventilator Settings in Various Clinical Settings / Rosen`s EM

Mode FIO2 (%) VT (mL/kg) F / min I/E PEEP (cm H2O)

Overdose in healthy patient

CMV, A/C, IMV, SIMV 95 8–10 10–12 1:2 0–5

Status asthmaticus

CMV, A/C, IMV, SIMV 95 5–10 8–12 1:4 2.5–10

COPD exacerbation

CMV, A/C, IMV, SIMV 95 5–10 10–12 1:3–1:4 2.5–10

pulmonary edema CMV, A/C, IMV,

SIMV 95 8-10 10–12 1:2 2.5–15

ARDS CMV, A/C, IMV, SIMV 95 6-8 20-25 1:2 2.5-10

Hypovolemic shock

CMV, A/C, IMV, SIMV 95 8-10 - 1:2 0-5

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complications

• Pneumothorax• Ventilator induced lung injury• Hemodynamic instability• Difficult trigger ventilation• Auto-cycling (seizure, shivering)• Outstripping and double cycling (demand over Vt)• Straining over the ventilator (demand over Q)• Coughing• Failure to MV

100Approach to res. distress

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Weaning

Indications for extubation

• Clinical parameters• Resolution/Stabilization of disease

process• Hemodynamically stable• Intact cough/gag reflex• Spontaneous respirations• Acceptable vent settings

• FiO2< 40%, PEEP < 8, PaO2 > 75, pH > 7.25

No weaning parameter completely accurate when used alone

Numerical Parameters Normal Range

Weaning Threshold

P/F > 400 > 200

Tidal volume 5 - 7 ml/kg 5 ml/kg

Respiratory rate 14 - 18 breaths/min < 40 breaths/min

Vital capacity 65 - 75 ml/kg 10 ml/kg

Minute volume 5 - 7 L/min < 10 L/min

Greater Predictive Value

Normal Range

Weaning Threshold

NIF (Negative Inspiratory Force)

> - 90 cm H2O > - 25 cm H2O

RSBI (Rapid Shallow Breathing Index) (RR/TV)

< 50 < 100

Marino P, The ICU Book (2/e). 1998.

Criteria : • The cause of respiratory failure is

improving or has been eliminated,• FiO2 < 0.40 • PEEP < 8cm H2O • No pressors other than dopamine at <5 μg/kg / min or

Epinephrine or NorEpi. at < 0.05 µg /kg / min.

TUMS 103

weaning

Parameter Normal Adult range

Threshold for weaning

PaO2/FiO2 >400 200Tidal Volume 5-7 ml/kg 5 ml/kgRR 14-18 /min <40 /minMinute Vent. 5-7 L/min <10 L/minVital capacity 65-75 ml/kg 10 ml/kg

Peak Inspiratory Pressure

>90 cmH2O (F)>120 cmH2O (M) 25 cmH2O

RSBI (f/Vt) <50 /L <105 /LTUMS

The Problem Wean

• RAPID BREATHING 1• 1. Check Vt Low Vt Resume vent. support Vt not low 2• 2. Check PaCO2 PaCO2 decreased sedate (anxiety) PaCO2 not decreased Resume vent.

TUMS 106

• ABDOMINAL PARADOX : Inward displacement of the diaphragm during

inspiration is a sign of diaphragmatic muscle fatigue

• HYPOXEMIA : May be due to low C.O. • HYPERCAPNEA: – Increase in PaCO2-PetCO2 = increase dead space ventilation–Unchanged gradient: Respiratory muscle fatigue or

enhanced CO2 production

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Weaning

Tracheal Decannulation• Successful weaning is not synonymous with tracheal

decannulation• If weaned and not fully awake or unable to clear

secretions, leave ETT in place• Tracheal decannulation increases the work of

breathing due to laryngeal edema and secretions• Do not perform tracheal decannulation to reduce

work of breathing

TUMS 109

Inspiratory Stridor

• Post extubation inspiratory stridor is a sign of severe obstruction and should prompt re-intubation

• Laryngeal edema (post-ext) may respond to aerosolized epinephrine in children

• Steroids have no role• Most need reintubation followed by tracheostomy

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TUMS 111

Casemanagement

TUMS 112

Case 1

• An l8-year-old otherwise healthy 60-kg female presents with an overdose of benzodiazepines.

• She requires intubation for airway protection and ventilatory support.

• There is no evidence of aspiration or an intrinsic lung problem.

TUMS 113

With VCV setting

Target Vm = 7.2 L / minIt is reasonable to assume a normal need for Vm

since she has no evidence of hypoperfusion or infection, and she has not ingested an medications known to cause a metabolic acidosis that would require a higher Vm to buffer by induced hypocarbia.

TUMS 114

Dräger evAita 2

Patient data Ventilator setting

Alarm setting

TUMS 115

Vt and f

• F= 12 /min • Vt = 600 mL (7-10 mL/kg)

• This setting will guarantee the desired Vm even if the patient continues to develop respiratory depression from the benzodiazepine ingestion.

TUMS 116

Pressure Support Ventilation.

• Initiate PSV at 10 cm H20 pressure, and then titrate up or down to achieve a spontaneous breath Vt approximately equal to that of the set Vt.

TUMS 117

• Gas Delivery Waveform: Begin with a decelerating waveform.

• Maximal Inspiratory Flow : Set the initial Q at 60 L/min.

lower Q(Q = 50 L/min) in hypoxemia higher flow (Q = 70 L/min) in exhalation obstruction

(e.g. COPD), then evaluate the resultant Paw-peak.

TUMS 118

• If the Vt is appropriate, reduce the flow rate by 5 L/min and re-evaluate; repeat if necessary.

• If the Q is reduced to 40 L/min and the Paw-peak remains high, :

1) the Vt is, in fact, too large for the available lung mass,

2) there is a tube obstruction (partial) 3) the patient has a pleural space occupying

disorder (pneumo-, hemo-, hemopneumo-, or hydro-thorax),

4) the patient requires a different mode 5) there is a ventilator dysfunction

TUMS 119

Order writing

1SIMVf : 12/ minFiO2 : 95%PEEP : 5 cm H2OASB / CPAP / PSV : 10 cm

H2OFlow : 60 L/min Ramp

wave

2 Then titrate PSV to achieve Vt

spont. 600 ml

• ABG 20 min later

TUMS 120

The same patient with PCV setting

Vm = 7.2 L/min Mode = ( in E4 , 2 )

PCV+ SIMV, .PCV. PSV. ASB

Q = 160 L/min ( devise adjust the Q in

response to the patient`s breath.)

Wave = ramp F = 12/ min

FiO2= 95% PSV =10 cm H2O PEEP=5 cm H2O

(Paw Peak ) =PC+ PEEP =25

TUMS 121

PC=[(Paw peak) – PEEP] ×2/3 = (35 – 5)× 2/3 =20 cm H2O

Order writing

1SIMVf : 12/ min

PC : 20 cm H2OTi : 1 sFiO2 : 95 %

2PEEP :5 cm H2OPSV : 10 cm H2OQ : max (160) L/minWave : ramp

ABG 20 min later

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Case 2

• A 45-year-old, 70-kg male presents with high fevers, cough, and shortness of breath.

• The ED portable chest film demonstrates bilateral infiltrates as well as lower lobe air bronchograms.

• While awaiting admission to the hospital for presumed community-acquired pneumonia, the patient develops hypoxia, suffers respiratory failure, and requires urgent intubation.

TUMS 123

• Mode : SIMV-As before, allow the patient to breathe spontaneously when the short acting neuromuscular blockers or heavy sedative (e.g., etomidate, fentanyl, midazolam) used to facilitate intubation wears off.

• Target Vm :Given the likely acidosis, this patient's Vm needs to exceed the lower limit of normal value.

• Thus, a target of 9.8 L/minute is a 40% increase above the baseline .

TUMS 124

• initial f= 14 / min • Vt = 700 ml

• One must evaluate the resultant Paw-peak after starting at this setting.

• One may also need to reduce the tidal volume and increase the rate if the Paw-peak is high and no other mode of ventilation is available.

TUMS 125

• Fi02 : Begin with FiO2 :95% then titrate.• PEEP : ≥5 (to maintain FRC and alveolar

recruitment with V/S consideration )• Flow:

• A longer Ti is ideal for alveolar recruitment and a slow flow rate will complement the decelerating waveform and further prolong the Ti. • Then start with a Q of 50 L/min

TUMS 126

• Start with a higher PSV because the pulmonary compliance is less than the normal lungs.

• Initiate PSV at 15 cm H20 and titrate to achieve similar Vt

with the machine and spontaneous breaths.

• In PCV : the same MODE & Vm …

• Only set the Ti & PC (longer Ti)

• Then reevaluate the patient`s condition

TUMS 127

TUMS 128

The end