Mechanical Ventilation Dr Rob Stephens robcmstephens@googlemail.com www.ucl.ac.uk/anaesthesia/people/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

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

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

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

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 Analgesia = Pain Relief 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’

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

IRV VC TLC TV FRC ERV RV

Review 3: Normal breath Normal breath inspiration animation, awake Lung @ FRC= balance Diaghram contracts -2cm H20 Chest volume Pressure difference from lips to alveolus drives air into lungs ie air moves down pressure gradient to fill lungs Pleural pressure -7cm H20 Alveolar pressure falls -2cm H20

Review 3: Normal breath Normal breath expiration animation, awake -7cm H20 Diaghram relaxes Pleural / Chest volume  Pleural pressure rises +1-2cm H20 Alveolar pressure rises Air moves down pressure gradient out of lungs

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 -2 cm H20  lung pressure Air moves down pressure gradient to fill lungs +5 to+10 cm H20  Pleural pressure

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 Expiration 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….’

Details: Inspiration Pressure or Volume? Pressure cm H20 Time Pressure cm H20 Time

Details: Expiration Pressure cm H20 PEEP Time Positive End Expiratory Pressure Pressure cm H20 PEEP Time

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

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 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 Airway pressure will change

Details: compliance changes Normal ventilating lungs

Details: compliance changes Abormal ventilating lungs: Eg Left pneumothorax

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 ‘over-distended’ alveoli Compliance= Volume  Pressure Volume energy needed to open alveoli ?damaged during open/closing - abnormal forces Pressure

Regional ventilation: PEEP Spontaneous, standing, healthy Static Compliance= Volume  Pressure Volume Pressure

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 ‘over-distended’ alveoli Compliance= Volume  Pressure Volume energy needed to open alveoli ?damaged during open/closing - abnormal forces Pressure

Effects of PEEP: whole lung Compliance= Volume  Pressure Volume PEEP: start inspiration from a higher pressure ↓?damage during open/closing Pressure Raised ‘PEEP’

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? Ventilator settings: 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? Ventilator settings: 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

IRV VC TLC TV FRC Closing Capacity ERV RV

IRV VC TLC TV FRC Closing Capacity ERV RV

Effects of pathology eg PTx