MECHANICAL VENTILATIONMECHANICAL VENTILATION

Things “I” wish I knew when I was an Intern

Amit Gupta, MD

Internal Medicine

North Mississippi Medical Center

Mechanical VentilationMechanical Ventilation

1. Indications for Intubation and Ventilation

2. Principles of Mechanical Ventilation

3. Patterns of Assisted Ventilation

4. Ventilator Dependence: Complications

5. Liberation from Mechanical Ventilation: Weaning

6. Troubleshooting

7. Arterial Blood Gases

Indications for Mechanical Indications for Mechanical VentilationVentilation

“….An opening must be attempted in the trunk of the trachea, into which a tube or cane should be put; You will then blow into this so that lung may rise again….And the heart becomes strong….”

-Andreas Vesalius (1555)

Indications for Mechanical Indications for Mechanical VentilationVentilation

1. “Thinking” of Intubation: elective v/s emergent

2. “Act of weakness?”3. Endotracheal tubes are not a disease and

ventilators are not an addiction4. And the usual elective and emergent

indications that you all know!

Objectives of Mechanical Objectives of Mechanical VentilationVentilation

Improve pulmonary gas exchangeReverse hypoxemia and Relieve acute respiratory acidosis

Relieve respiratory DistressDecrease oxygen cost of breathing and reverse respiratory muscle fatigue

Alter pressure-volume relationsPrevent and reverse atelectasisImprove CompliancePrevent further injury

Permit lung and airway healingAvoid complications

Strategies for Mechanical Strategies for Mechanical VentilationVentilation

Ventilatory Parameter

Traditional Lung-Protective

Inflation Volume 10-15 ml/kg 5-10 ml/kg

End-insp. pressure

Peak Pr<50cm water

Plateau Pr<35

PEEP PRN to keep FiO2<0.6

5-15 cm of water

ABG Normal, pH 7.36-7.44

Hypercapnia allowed, pH 7.2-7.4

Monitoring Lung MechanicsMonitoring Lung Mechanics

Proximal Airway Pressures (end-inspiratory)

1. Peak Pressure Pk

Function of: Inflation volume, recoil force of

lungs and chest wall, airway resistance

2. Plateau Pressure Pl

Occlude expiratory tubing at end-inspiration

Function of elastance alone

Use of Airway PressuresUse of Airway Pressures

Pk increased Pl unchanged:

Tracheal tube obstruction

Airway obstruction from secretions

Acute bronchospasm

Rx: Suctioning and Bronchodilators

Use of Airway PressuresUse of Airway Pressures

Pk and Pl are both increased:Pneumothorax

Lobar atelectasis

Acute pulmonary edema

Worsening pneumonia

ARDS

COPD with tachypnea and Auto-PEEP

Increased abdominal pressure

Asynchronous breathing

Use of Airway PressuresUse of Airway Pressures

Decreased Pk:

System air leak: Tubing disconnection, cuff leak

Rx: Manual inflation, listen for leak

Hyperventilation: Enough negative intrathoracic

pressure to pull air into lungs may drop Pk.

ComplianceCompliance

Static Compliance (Cstat):Distensibility of Lungs and Chest wallCstat = Vt/PlNormal C stat: 50-80 ml/cm of waterProvides objective measure of severity of illness in a pulmonary disorderDynamic Compliance:Cdyn: Vt/Pk*Subtract PEEP from Pl or Pk for compliance measurementUse Exhaled tidal volume for calculations

Patterns of Assisted VentilationPatterns of Assisted Ventilation

Assist Control Intermittent Mandatory Ventilation Pressure Controlled Ventilation Pressure Support Ventilation Positive end-expiratory ventilation Continuous Positive Airway Pressure

Assist Control VentilationAssist Control Ventilation

Volume-cycled lung inflation

Patient can initiate each mechanical breath or Ventilator

provides machine breaths at a preselected rate

Maintain I:E ratio to 1:2 to 1:4. An increase in Peak flow

decreases the time for lung inflation and increases the I:E

Ratio

I:E ratio of <1:2 can cause hyperinflation by air trapping

Diaphragmatic contraction continues during ACV and

increases the work of breathing.

Assist Control VentilationAssist Control Ventilation

Adverse effects:In a tachypneic patient>>Lead to overventilation and

severe respiratory alkalosis>> Hyperinflation and

Auto-PEEP>> Lead to Electromechanical

dissociation

Intermittent Mandatory VentilationIntermittent Mandatory Ventilation

Delivers volume cycled breaths at a preselected rate with spontaneous breathing between machine breaths

Less Alkalosis and Hyperinflation Synchronized IMV

Intermittent Mandatory VentilationIntermittent Mandatory Ventilation

Disadvantages:Increased work of Breathing:

Spontaneous breathing through a high resistance circuit

Solution: Add Pressure support

Cardiac Output Changes:

C O decreased by decreasing ventricular filling

C O increased by reducing ventricular afterload

More significant decrease in patients with LV dysfunction

IMV vs. ACVIMV vs. ACV

Switch to IMV for:

Rapid breathers with alkalosis and over-

Inflation

Switch to ACV for:

Patients with respiratory muscle weakness and

LV dysfunction

Pressure Controlled VentilationPressure Controlled Ventilation

Pressure cycled breathing, fully ventilator controlled Inspiratory flow rate decreases exponentially during lung

inflation (+)Reduces peak airway pressure and improves gas

exchange (-)Inflation volume varies with changes in mechanical

properties of the lungs. Suited for patients with neuromuscular diseases and

normal lung mechanics

Inverse ratio VentilationInverse ratio Ventilation

PCV combined with prolonged inflation time Inspiratory flow rate is decreased I:E ratio reversed to 2:1 Helps prevent alveolar collapse (-) Hyperinflation, Auto-PEEP and decreased

cardiac output Use: ARDS with refractory hypoxemia or

hypercapnia ?mortality benefit

Pressure Support VentilationPressure Support Ventilation

Pressure augmented breathing Allows patient to determine the inflation volume

and respiratory cycle duration Uses: augment inflation during spontaneous

breathing or overcome resistance of breathing through ventilator circuits (during weaning)

Popular an a non-invasive mode of ventilation via nasal or face masks

Positive end-expiratory pressurePositive end-expiratory pressure

Alveolar pressure at end-expiration is above atmospheric pressure : PEEP

Extrinsic PEEP

Auto PEEP

Positive end-expiratory pressurePositive end-expiratory pressure

EXTRINSIC PEEP Applied by placing pressure limiting valve in the

expiratory limb of ventilator circuit Prevents end-expiratory alveolar collapse and

recruits collapsed alveoli This decreases intrapulmonary shunting, improves

gas exchange and improves lung compliance, allowing the FiO2 to be reduced to less toxic levels

Positive end-expiratory pressurePositive end-expiratory pressure

Cardiac Performance:

Greater reduction in cardiac filling and cardiac output (Q),

irrespective of level of PEEP!

It is a function of PEEP induced increase in mean

intrathoracic pressure

Oxygen transport Do2:

Do2 = Q X 1.3 X Hb X SaO2

Systemic O2 delivery may vary with the effect of PEEP on

the Cardiac Output.

Positive end-expiratory pressurePositive end-expiratory pressure

Best PEEP: Monitor Cardiac Output Another measure: Venous Oxygen Saturation If VOS decreases after PEEP applied= Drop CO Swan-Ganz catheter may be indicated in most

patients on PEEP

Positive end-expiratory pressurePositive end-expiratory pressure

CLINICAL USES: Reduce toxic levels of FiO2 (ARDS not

pneumonia) Low-volume ventilation Obstructive lung disease (Extrinsic=Occult PEEP)

Positive end-expiratory pressurePositive end-expiratory pressure

CLINICAL MISUSES: Reducing Lung Edema Routine PEEP Mediastinal Bleeding after CABG

Continuous positive Airway PressureContinuous positive Airway Pressure

Spontaneous breathing Patient does not need to generate negative

pressure to receive inhaled gas CPAP replaced spontaneous PEEP Use: Non-intubated patients (OSA, COPD)

Occult PEEPOccult PEEP

Intrinsic or Auto-PEEP or Hyperinflation Incomplete alveolar emptying during expiration Ventilator Factors: High inflation volumes, rapid rate,

low exhalation time Disease factors: Asthma, COPD Consequences: Decreased CO/EMD, Alveolar

rupture, Underestimation of thoracic compliance, increased work of breathing.

If extrinsic PEEP does not increase Pk, then occult PEEP is present

Complications of Mechanical VentilationComplications of Mechanical Ventilation

Toxic effects of Oxygen Decreased cardiac output Pneumonia and sepsis Psychological problems Ventilator dependence

Complications of Mechanical VentilationComplications of Mechanical Ventilation

Purulent sinusitis Laryngeal Damage Aspiration :Value of routine tracheal suctioning Tracheal Necrosis (pressure below 20mm water) Alveolar rupture: Pneumothorax,

pneumomediastinum, subQ emphysema, pneumoperitoneum

Basilar and sub-pulmonic air collections in the supine position, as seen on X-ray

Liberation from Mechanical Ventilation: Liberation from Mechanical Ventilation: WeaningWeaning

Weaning: Gradual withdrawal of mechanical ventilation Misconceptions:

Duration- longer duration, harder to wean

Method of weaning determines ability to wean

Diaphragm weakness is a common cause of failed weaning

Aggressive nutrition support improves ability to wean

Removal of ET tube reduces work of breathing

Bedside Weaning ParametersBedside Weaning Parameters

Parameter Normal Adult range

Threshold for weaning

PaO2/FiO2 >400 200

Tidal Volume 5-7ml/kg 5ml/kg

Resp. Rate 14-18/min <40/min

Minute Ventl. 5-7L/min <10L/min

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

Bedside Weaning ParametersBedside Weaning Parameters

Maximal Inspiratory Pressure

>-90 cm Water (F)

>-120 cm water (M)

-25cm of water

Rate/Tidal Volume <50/min/L <100/min/L

Maximal Inspiratory PressureMaximal Inspiratory Pressure

Pmax: Excellent negative predictive value if less than –20 (in one study 100% failure to wean at this value)

An acceptable Pmax however has a poor positive predictive value (40% failure to wean in this study with a Pmax more than –20)

Frequency/Volume ratioFrequency/Volume ratio

Index of rapid and shallow breathing RR/Vt Single study results:

RR/Vt>105 95% wean attempts unsuccessful

RR/Vt<105 80% successful• One of the most predictive bedside parameters.

T-Piece WeaningT-Piece Weaning

On-off toggle switch that circulates between on and off the ventilator

Inhaled gas is delivered at a high flow rate Varied protocols: like 30min-2hr on and off, or keep as

long as possible and if tolerated for >2-4hr…. Deemed successsful (RR, TV, HR, diaphoresis, sat)

Failed T piece: Resume Vent support till comfortable, 24h

vent Airflow with CPAP

patient

T-Piece with VentilatorT-Piece with Ventilator

Drawback: increased resistance due to vent tubing and actuator valve in circuit

Provide minimum pressure support (PSV) :Pmin Pmin= PIFR X R PIFR is during spontaneous breathing R is airflow resistance during mech ventilation R= Pk-Pl/Vinsp (Vinsp:inspiratory flow rate delivered by the vent)

IMV WeaningIMV Weaning

Gradual decrease in no of machine breaths in between the spontaneous breaths

False security: It does not adjust to patient’s ventilatory demands to maintain constant MV

End point in IMV weaning is the T-piece trial Most important to recognize when a patient is

capable of spontaneous unassisted breathing T-piece more rapid than IMV

Complicating FactorsComplicating Factors

DYSPNEA Anxiety and dyspnea are detrimental (low dose

haloperidol or morphine) CARDIAC OUTPUT Increased LV afterload can reduce CO, impair

diaphragm function, promote pulmonary edema (Use Swan to monitor CO, may use dobutamine) ELECTROLYTE DEPLETION OVERFEEDING

The Problem WeanThe Problem Wean

RAPID BREATHING: Check TVLow TV>> Resume vent supportTV not low…….. Check arterial pCO2Arterial pCO2 decreased>sedate (anxiety)Arterial pCO2 not decreased> Resume vent

The Problem WeanThe Problem Wean

ABDOMINAL PARADOX Inward displacement of the diaphragm during inspiration

is a sign of diaphragmatic muscle fatigue HYPOXEMIA May be due to low CO and MVO2 HYPERCAPNIA Increase in PaCO2-PetCO2: increase dead space

ventilation Unchanged gradient: Respiratory muscle fatigue or

enhanced CO2 production

Tracheal DecannulationTracheal Decannulation

Successful weaning is not synonymous with tracheal decannulation

If weaned and not fully awake or unable to clear secretions, leave ETT in place

Contrary to popular belief, tracheal decannulation increases the work of breathing due to laryngeal edema and secretions

Do not perform tracheal decannulation to reduce work of breathing

Inspiratory StridorInspiratory Stridor

Post extubation inspiratory stridor is a sign of severe obstruction and should prompt reintubation

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

Steroids have no roleMost need reintubation followed by

tracheostomy

ARDS and Low Volume VentilationARDS and Low Volume Ventilation

ARDS Network trial : NEJM May 4, 2000 p1301-08 Traditional: TV 10-15ml/kg, keep plateau<50cm water Low TV ventilation: TV 6ml/kg, keep plateau<30cm water Need high RR in Low TV group to prevent acidosis Permissive hypercapnia tolerated well, if needed, use IV bicarb

to maintain pH May add PEEP in addition to the low TV group to prevent

atelectrauma (open-close alveoli>> alveolar fracture) Results: Lower mortality in the Low TV group (31% v/s 39.8%

p<0.007); Higher days without vent use and lower average plateau pressures in low TV group.