Mechanical Ventilation Dr Rob Stephens

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

Picture of a ventilator

Video of a ventilator

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 –Several ways to..connect the ventilator to the patient

Several ways to..connect the machine to Pt Oro-tracheal Intubation Tracheostomy Non-Invasive Ventilation Picture of a tubes connected to patients

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

Picture of a ett in patient

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 –Several ways to..connect the machine to Pt –Unnatural- not spontaneous Consequences –of drugs needed to tolerate it –of IPPV itself –route for infection

Why do it?- indications Hypoxaemia: low blood O 2 Hypercarbia: high blood CO 2 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 competitive Suxamethonium Competitive Others eg Atracurium Different properties Different length of action Paralyse Respiratory muscles Apnoea – ie no breathing Need to ‘Ventilate’ Picture of a NM drugs

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 O 2 to alveoli Hb binds O 2 (small amount dissolved) CVS transports to tissues to make ATP - do work –Remove CO 2 from pulmonary vessels from tissues – metabolism

Review 2: Vitalograph

TLC IRV 0 RV FRC TV ERV VC

Normal breath inspiration animation, awake Diaghram contracts  Chest volume  Pleural pressure Pressure difference from lips to alveolus drives air into lungs ie air moves down pressure gradient to fill lungs -2cm H cm H 2 0 Alveolar pressure falls -2cm H 2 0 Review 3: Normal breath FRC= balance

Normal breath expiration animation, awake Diaghram relaxes Pleural / Chest volume  Pleural pressure rises Review 3: Normal breath Alveolar pressure rises Air moves down pressure gradient out of lungs -7cm H cm H 2 0

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 (inside chest) –abdominal cavity –pulmonary vessels

Ventilator breath inspiration animation Air blown in  lung pressure Air moves down pressure gradient to fill lungs  Pleural pressure -2 cm H to+10 cm H 2 0

Ventilator breath expiration animation Similar to spontaneous…ie passive Ventilator stops blowing air in Pressure gradient Alveolus-trachea Air moves out Down gradient  Lung volume

Details: IPPV Inspiration –Pressure or Volume? –Machine or Patient initiated? ’control or support’ –Fi0 2 –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….’

TimePressure cm H 2 0 TimePressure cm H 2 0 Details: Inspiration Pressure or Volume?

TimePressure cm H 2 0 TimePressure cm H 2 0 Details: Expiration 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 mmHg –LV 120/70 mmHg ~10 cmH 2 0 =~ 7 mmHg =~1KPa

Details: Cardiovascular effects IPPV + PEEP can create a shunt !

Details: Cardiovascular effects Normal blood flow

Details: Cardiovascular effects Blood flow:  Lung airway pressures

Details: Cardiovascular effects 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

Details: Cardiovascular effects Compresses Pulmonary vessels Inspiration + Expiration –More pressure,  effects on cardiovascular –If low blood volume eg bleeding 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 lung Volume Pressure Compliance=  Volume  Pressure energy needed to open alveoli ?damaged during open/closing - abnormal forces ‘over-distended’ alveoli

Regional ventilation: PEEP Volume 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

Effects of PEEP: whole lung Volume Pressure Compliance=  Volume  Pressure energy needed to open alveoli ?damaged during open/closing - abnormal forces ‘over-distended’ alveoli

Effects of PEEP: whole lung Volume 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 (Fi0 2 ) Monitoring adequacy of ventilation (pCO 2, pO 2 ) 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 (Fi0 2 ) Monitoring adequacy of ventilation (pCO 2, pO 2 ) Ventilation: drugs to make it possible Ventilation: drug side effects

Summary IPPV: definition Usually needs anaesthesia- triad of drugs Needs a tube to connect person to ventilator Modes of ventilation Pressures larger + positive ; IPPV vs spontaneous CVS effects PEEP opens aveoli, CVS effects

Stuff to know Why use IPPV/definition Airway- connecting to the machine Modes: inspiration Pressure/volume; control/support Expiration: PEEP Pressures in the cycle vs spontaneous CVS effects IPPV vs spontaneous Side effects – tubes and pressure General Anaesthesia – see previous lecture Drugs- triad

Other reading 211_01.htm Practicalities in the Critically ill 609_01.htm

Thank you Any questions

Effects of induction in eg asthma

Effects of position- supine/obese

TLC IRV 0 RV FRC TV ERV VC Closing Capacity

TLC IRV 0 RV FRC TV ERV VC Closing Capacity

Effects of pathology eg PTx