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Mechanical Ventilation

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

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Mechanical Ventilation

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

  2. Contents • Introduction: definition • Introduction: review some basics • Basics: Inspiration + expiration • Details • inspiration pressure/volume • expiration • Cardiovascular effects • Compliance changes • PEEP • Some Practicalities

  3. 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

  4. Picture of a ventilator

  5. Picture of a ventilator

  6. Video of a ventilator

  7. 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

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

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

  10. Picture of a ett in patient

  11. 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

  12. 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

  13. 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

  14. 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

  15. Review some basics • 1 What’s the point of ventilation? • 2 Vitalograph, lets breathe • 3 Normal pressures

  16. 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

  17. Review 2:Vitalograph

  18. IRV VC TLC TV FRC ERV RV 0

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. Details: IPPV • Inspiration • Pressure or Volume? • Machine or Patient initiated? ’control or support’ • Fi02 • Tidal Volume / Respiratory Rate • Expiration • PEEP? Or no PEEP (‘ZEEP’)

  25. Details: Inspiration Pressure or Volume? • Do you push in.. • A gas at a set pressure? = ‘pressure…..’ • A set volume of gas? = ‘volume….’

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

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

  28. Details: Cardiovascular effects • Compresses Pulmonary vessels • Reduced RV inflow • Reduced RV outflow • Reduced LV inflow • Think of R vs L heart pressures • RV 28/5mmHg • LV 120/70mmHg ~10 cmH20 =~ 7 mmHg =~1KPa

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

  30. Details: Cardiovascular effects Normal blood flow

  31. Details: Cardiovascular effects Blood flow:  Lung airway pressures

  32. 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

  33. Details: Cardiovascular effects • Compresses Pulmonary vessels • Inspiration + Expiration • More pressure,  effects on cardiovascular • If low blood volume eg bleeding • vessels more compressible •  effects

  34. Details: compliance changes • If you push in.. • A gas at a set pressure? = ‘pressure…..’ • Tidal Volume  compliance • Compliance = Δvolume / Δpressure • 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

  35. Details: compliance changes Normal ventilating lungs

  36. Details: compliance changes Abormal ventilating lungs: Eg Left pneumothorax

  37. 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

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

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

  40. 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

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

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

  43. 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)

  44. 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

  45. 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

  46. 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

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