Mechanical Ventilation Dr Rob Stephens [email protected]

Mechanical Ventilation

Dr Rob [email protected]

www.ucl.ac.uk/anaesthesia/people/stephens

the centre forAnaesthesia UCL

Contents

• Introduction: definition• Introduction: review some basics• Basics: Inspiration + expiration• Details

– inspiration pressure/volume– expiration– Cardiovascular effects– Compliance changes– PEEP

• Some Practicalities

Definition: What is it?

• Mechanical Ventilation=Machine to ventilate lungs = move air in (+ out) – Several ways to..move air in (IPPV vs others)

Intermittent Positive Pressure Ventilation



Intermittent Positive Pressure Ventilation– Several ways to ..connect the ventilator to

the patient

Several ways to ..connect the machine to Pt

• Oro-tracheal Intubation

• Tracheostomy

• Non-Invasive

Ventilation

Several ways to ..connect the machine to Pt is Airway



Intermittent Positive Pressure Ventilation– Several ways to ..connect the machine to Pt– Unnatural- not spontaneous

• consequences

Why do it?- indications

• Hypoxaemia: low blood O2

• Hypercarbia: high blood CO2

• Need to intubate eg patient unconscious so reflexes

• Others eg – need neuro-muscular paralysis to allow surgery– want to reduce work of breathing – cardiovascular reasons

Anaesthesia Drugs

• Hypnosis = Unconsciousness– Gas eg Halothane, Sevoflurane– Intravenous eg Propofol, Thiopentone

• Analgesia = Pain Relief– Different types: ‘ladder’, systemic vs other

• Neuromuscular paralysis– Nicotinic Acetylcholine Receptor Antagonist

Neuromuscular Paralysis

Nicotinic AcetylCholine Channel

Non competitiveSuxamethonium

CompetitiveOthers eg Atracurium

Different propertiesDifferent length of actionParalyse Respiratory musclesApnoea – ie no breathingNeed to ‘Ventilate’

Review some basics

• 1 What’s the point of ventilation?

• 2 Vitalograph, lets breathe

• 3 Normal pressures

Review 1

What’s the point of ventilation?– Deliver O2 to alveoli

• Hb binds O2 (small amount dissolved)

• CVS transports to tissues to make ATP - do work

– Remove CO2 from pulmonary vessels• from tissues – metabolism

Review 2: Vitalograph

TLC

IRV

0RV

FRC

TV

ERV

VC

Normal breath inspiration animation, awake

Diaghram contracts

Chest volume

Pleural pressurePressure difference from lips to alveolusdrives air into lungs

ie air moves down pressure gradientto fill lungs

-2cm H20

-7cm H20

Alveolarpressure falls -2cm H20

Review 3: Normal breath

Lung @ FRC= balance

Normal breath expiration animation, awake

Diaghram relaxes

Pleural / Chest volume

Pleural pressure rises

Review 3: Normal breath

Alveolarpressure rises

Air moves down pressure gradientout of lungs

-7cm H20

+1-2cm H20

The basics: Inspiration

Comparing with spontaneous

• Air blown into lungs– 2 different ways to do this (pressure / volume)– Air flows down pressure gdt– Lungs expand– Compresses

– pleural cavity– abdominal cavity– pulmonary vessels

Ventilator breath inspiration animation

Air blown in

lung pressure Air moves down pressure gradientto fill lungs

Pleuralpressure

-2 cm H20

+5 to+10 cm H20

Ventilator breath expiration animationSimilar to spontaneous…ie passive

Ventilator stops blowing air in

Pressure gradientAlveolus-trachea

Air moves outDown gradient Lung volume

Details: IPPV

• Inspiration– Pressure or Volume?– Machine or Patient initiated?

’control or support’

– Fi02

– Tidal Volume / Respiratory Rate

• Expiration– PEEP? Or no PEEP (‘ZEEP’)

Details: Inspiration Pressure or Volume?

• Do you push in..– A gas at a set pressure? = ‘pressure…..’– A set volume of gas? = ‘volume….’

Time

Pre

ssur

e cm

H20

Time

Pre

ssur

e cm

H20Details: Inspiration

Pressure or Volume?

Time

Pre

ssur

e cm

H20

Time

Pre

ssur

e cm

H20

Details: Expiration

PEEP

PEEP

Positive End Expiratory Pressure

Details: Cardiovascular effects

• Compresses Pulmonary vessels

• Reduced RV inflow

• Reduced RV outflow

• Reduced LV inflow

• Think of R vs L heart pressures– RV 28/5– LV 120/70


IPPV + PEEP can create a shunt !


Normal blood flow


Blood flow: Lung airway pressures


• Compresses Pulmonary capilary vessels• Reduced LV inflow

Cardiac Output: Stroke Volume– Blood Pressure = CO x resistance –

Blood Pressure

– Neurohormonal: Renin-angiotensin activated

• Reduced RV outflow- backtracks to body– Reduced RA inflow– Head- Intracranial Pressure– Others - venous pressure eg liver– Strain: if RV poorly contracting


• Compresses Pulmonary vessels

• Inspiration + Expiration– More pressure, effects on cardiovascular– If low blood volume

• vessels more compressible effects

Details: compliance changes• If you push in..

– A gas at a set pressure? = ‘pressure…..’• Tidal Volume compliance • Compliance = Δ volume / Δ pressure• If compliance: ‘distensibility stretchiness’ changes• Tidal volume will change

– A set volume of gas? = ‘volume….’• Pressure 1/ compliance • If compliance: ‘distensibility stretchiness’ changes• Airway pressure will change

Normal ventilating lungs

Details: compliance changes

Abormal ventilating lungs: Eg Left pneumothorax

Details: compliance changes

Regional ventilation; PEEP

• Normal, awake spontaneous • Ventilation increases as you go down lung

– as ‘top’ ` (non-dependant) alveoli larger already– so their potential to increase size reduced– non-dependant alveoli start higher up

compliance curve

Effects of PEEP: whole lungV

olum

e

Pressure

Compliance=

Volume Pressure

energy needed to open alveoli

?damaged during open/closing

- abnormal forces

‘over-distended’ alveoli

Regional ventilation: PEEPV

olum

e

Pressure

Static Compliance=

Volume Pressure

Spontaneous, standing, healthy

Regional ventilation; PEEP

Lying down, age, general anaesthesia– Lungs smaller, compressed– Pushes everything ‘down’ compliance curve

• PEEP pushes things back up again

• Best PEEP = best average improvement


olum

e

Pressure

Compliance=

Volume Pressure

energy needed to open alveoli

?damaged during open/closing

- abnormal forces

‘over-distended’ alveoli


olum

e

Pressure

Compliance=

Volume Pressure

Raised ‘PEEP’

PEEP: start inspiration from a higher pressure

↓?damage during open/closing

Effects of PEEP

Normal, Awake – in expiration alveoli do not close (closing capacity)– change size

Lying down / GA/ Paralysis / +- pathology– Lungs smaller, compressed– Harder to distend, starting from a smaller volume– In expiration alveoli close (closing capacity)

PEEP– Keeps alveoli open in expiration ie increases FRC– Danger: but applied to all alveoli– Start at higher point on ‘compliance curve’– CVS effects (Exaggerates IPPV effects)

Practicalities

• Ventilation: which route?• Intubation vs others• Correct placement?

• Ventilator settings: • spontaneous vs ‘control’• Pressure vs volume• PEEP?• How much Oxygen to give (Fi02 )• Monitoring adequacy of ventilation (pCO2,pO2)

• Ventilation: drugs to make it possible• Ventilation: drug side effects• Other issues

Practicalities

• Ventilation: which route?• Intubation vs others• Correct placement?

• Ventilator settings: • spontaneous vs ‘control’• Pressure vs volume• PEEP?• How much Oxygen to give (Fi02 )• Monitoring adequacy of ventilation (pCO2,pO2)

• Ventilation: drugs to make it possible• Ventilation: drug side effects

Summary

•IPPV: definition

•Usually needs anaesthesia

•Needs a tube to connect person to ventilator

•Modes of ventilation

•Pressures larger + positive ; IPPV vs spontaneous

•CVS effects

•PEEP opens aveoli, CVS effects

Other reading

• http://www.nda.ox.ac.uk/wfsa/html/u12/u1211_01.htm

Practicalities in the Critically ill

• http://www.nda.ox.ac.uk/wfsa/html/u16/u1609_01.htm

Effects of induction in eg asthma

Effects of position- supine/obese

TLC

IRV

0RV

FRC

TV

ERV

VC

Closing Capacity

TLC

IRV

0RV

FRC

TV

ERV

VC

Closing Capacity

Effects of pathology eg PTx

Documents

Mechanical Ventilation Dr Rob Stephens [email protected]