Mechanical Ventilation

POS Seminar SeriesDecember 2010

Dr. J. Wassermann

Department of Anesthesia

St. Michael’s Hospital

University of Toronto

Outline

Definition – what is itIndications – when do you use itVentilator Settings – how do you use itModes of VentilationAdverse EffectsSummary

Mechanical Ventilation – Definition

Mechanical Ventilation =

– Use of a mechanical apparatus to provide (or augment) the requirements of a patient’s breathing (i.e. get O2 into and CO2 out of alveoli)

Mechanical Ventilation – Definition

Use of positive pressure to physically transport gases into and out of lungs

(earlier ventilators used negative pressure)

Usually performed via ETT but not always (noninvasive ventilation)

Mechanical Ventilation

A supportive measure – not a therapy

Must diagnose and treat underlying cause

Use ventilator to support patient until underlying disorder improved – and try not to cause harm in the process

Intubation - Indications

1. Airway patency (obstruction)

2. Airway protection (aspiration)

3. Oxygenation (pO2)*

4. Ventilation (pCO2)*

5. Tracheal Toilet (secretions)

* don’t necessarily need intubation for these

Mechanical Ventilation – Indications

Improve Oxygenation (pO2; SaO2)

Improve Ventilation (pCO2) - or hyperventilate pt

Reduce work of breathing (WOB)(i.e. asthma; COPD)____________________________________________

CHF

Hemodynamic Instability

Inadequate Oxygenation

1. Decreased FIO2/PIO2

2. A/W obstruction

3. Hypoventilation

4. V/Q mismatch*

5. Diffusion

6. Decreased mixed venous O2

7. RL shunt (i.e. intra-cardiac shunt)

Alveolar Gas Equation

PALVO2 = [(PATM – PH2O) × FIO2] – (PCO2 ) RQ

PALVO2 = [(760 – 47) × 0.21] − (40 ÷ 0.8)

PALVO2 = [ 713 × 0.21] − 50 = 100

NOTE: On 100% O2 , PALVO2 should be ~ 650

Inadequate OxygenationV/Q mismatch (low V/Q):

– Pneumonia– Aspiration– Pulmonary edema– Atelectasis/collapse (plug/PTX/HTX/effusions)– ARDS– Pulmonary contusion– Alveolar hemorrhage

Inadequate Ventilation

PaCO2 CO2 production Minute Ventilation (VE )

VE = RR x Vt

Any condition inadequate ventilation

increased pCO2 Altered LOC S. cord injuries/NM disorders weakness

Work of Breathing

WOB ~ ventilatory demands (CO2 prod’n)

~ airway resistance (i.e. severe asthma)

~ compliance (lung, c/w, abdo)

Increased WOB usually O2/CO2 problems;May need mech vent purely for WOB (i.e. asthma)

c/w = chest wall

Summary thus far

Mechanical ventilation indicated in situations with:

1. O2 problems (oxygenation)

2. CO2 problems (ventilation)

3. WOB (often assoc with 1 and/or 2)

Don’t always need an ETT

Mechanical Ventilators

How do you use them……

Ventilator Settings

ModeRateVolume (VT)PressureFIO2PEEPI:E (ratio of insp vs exp time)

Insp Flow rateFlow patternAlarms

Modes of Mechanical Ventilation

Controlled Mechanical Ventilation (CMV)Assist Control (AC)/Volume Control (VC)Intermittent Mandatory Ventilation (SIMV)Pressure Control (PCV)Pressure Support Ventilation (PSV)

Modes of Mechanical Ventilation

Trigger – what initiates a breath

Target – what the vent is trying to achieve

Cycle – what causes the breath to end

Continuous Mandatory Ventilation (CMV)

Trigger –Ventilator initiates all breaths

Patient can not initiate

Target – Volume

Cycle - Time

Continuous Mandatory Ventilation (CMV)

e.g. Settings - Mode: CMV

Rate 10; Vt 700cc

FIO2 0.5; PEEP 5.0vent gives 10 bpm @ 700cc each

pt gets zero extra breaths (even if tries)

very uncomfortable for pt

only used if pt paralyzed (i.e. in O.R.)

Assist Control (Volume Control)

Trigger – machine and patientTarget – volumeSettings-Mode: VC

Rate 10; Vt 700cc

FIO2 0.5; PEEP 5.0

e.g. vent gives 10 bpm @ 700cc each

pt initiates 6 bpm – vent provides 700cc

Synchronized Intermittent Mandatory Ventilation (SIMV)

Trigger – ventilator and patientTarget – ventilator breaths = set volume

patient breaths = patient effortSettings-Mode: SIMV

Rate 10; Vt 700cc

FIO2 0.5; PEEP 5.0e.g. vent gives 10 bpm @ 700cc each

patient takes 6 bpm @ 150 cc each

Pressure Control (PC)

Trigger – ventilator and patientTarget – Pressure (above PEEP)Settings – Mode: PC

Rate 10; Pressure 24 cm H2O

FIO2 0.5; PEEP 5

e.g. vent gives 10 bpm to a peak Paw = 29

pt takes 6 bpm targeted to peak Paw =29

Pressure Support Ventilation (PSV)

Trigger – patient onlyTarget - pressureCycle – patient flow decrease

Settings – Mode: PSV = 14 cm H2O FIO2 0.4; PEEP 5

e.g. pt takes 18 bpm @ Vt = 500ccmachine gives zero breaths

Completely Unassisted Breaths

Trigger – patientCycle – patient effort ceases

Settings: CPAP 5; FIO2 0.4

e.g. patient takes 24 bpm @ 250 cc each

Mechanical Ventilator Settings

Mode (usually VC or PC to start)RateTidal Volume (VC) [or Pressure (PC)]FIO2

PEEP I:E

Choosing a Ventilatory Mode

No proven benefit of any mode of another

Keys are to:

– Support pt to allow time for underlying process to improve

– Avoid/minimize adverse effects of mech ventilation

Adverse Effects of Mechanical Ventilation

Pulmonary:

– Intubation effects– Air leaks (pneumothorax/BPF)

–Ventilator-associated lung injury– Ventilator-associated pneumonia– Dynamic hyperinflation/Auto-PEEP

Adverse Effects of Mechanical Ventilation

Cardiovascular:– Increased CVP (↑intrathoracic pressure)– Decreased venous return– Hypotension– Increased RV afterload

GI:– Stress ulcers/GI bleeding

Adverse Effects of Mechanical Ventilation

Neuro/muscular:

– ↑ ICP– Prolonged sedation– Myopathies/Neuropathies

Ventilator-Associated Lung Injury (VALI)

1. Volutrauma

2. Barotrauma

3. Atelectotrauma

4. Biotrauma

Ventilator-Associated Lung Injury (VALI)

1. Volutrauma – overdistension of alveoli

2. Barotrauma – high alveolar pressures

3. Atelectotrauma – repetitive opening and closing of alveoli

4. Biotrauma – release of infl mediators into systemic circulation organ dysfunction

Ventilation of ARDS (baby lung)

Moderate PEEP (to keep alveoli open)• 10 – 15 cm H20

Small Vt’s (to avoid overdistension)• < 6 cc/kg

Plateau pressure < 30 cm H2OPermissive hypercapnia & hypoxemia

Summary

Mechanical ventilation used to:1. Improve oxygenation

2. Improve ventilation (CO2 removal)

3. Unload respiratory muscles

A support until patients condition improves

Summary

Different modes for ventilation– differ in how breaths are initiated, ended and

assisted– differ in independent and dependant variables

(i.e. what machine controls and what it doesn’t)– no proven advantage of one mode– use ventilator strategies to avoid lung injury

and other adverse effects

Questions?

Weaning from Mechanical Ventilation

Once underlying pathology improves

Need to ensure:– Adequate respiratory muscle strength– WOB not excessive

Ventilatory demands Resistance Compliance

Weaning from Mechanical Ventilation

Volume overload and myocardial ischemia

common causes of failure to wean

RR/Vt = good predictor if <80-100

SIMV inferior to SV trials or CPAP/PSV

Noninvasive Ventilation

Indications for intubation:

1. Airway patency*

2. Airway protection (aspiration)*

3. Oxygenation

4. Ventilation

5. Tracheal suctioning (toilet)*

Noninvasive Ventilation

Avoids intubation and complicationsCan deliver various modes of ventilation

– CPAP/CPAP + PSV most common

Indications:– hypercapneic respiratory failure (COPD exac)– cardiogenic p. edema

Noninvasive Ventilation

Contraindications:– Inability to cooperate (i.e. confusion)– Altered LOC– Inability to clear secretions– Hemodynamic instability

Ventilation Strategies in Specific Situations

ARDSAsthmaIncreased intraabdominal pressure