Ventilators

1. Ventilators Tuesday, 20 April 2004 Bill McCulloch

2. Types of Ventilator • Positive Pressure Ventilators • Gas blown into lungs • All Current Itu and Theatre Ventilators • Unphysiological but practical • Negative Pressure Ventilators • “Iron Lung” • Cuirass (breastplate) ventilators • Physiological but impractical

3. History • Need arose from polio epidemics in 1950s and changes in anaesthetic techniques (muscle relaxants) • Originally engineering challenge • Inflexible

4. Classification • Most classifications obsolete but need to be known • Based on cycling • Pressure cycling – cycles when pressure attained in system • Compensates for leaks • Vt changes with changes in compliance • Volume cycling – cycles when preset volume delivered • Doesn’t compensate for leaks • Will generally deliver preset volume (unless limit reached) • Time cycling – cycles after given time • Unresponsive to leaks or compliance changes • or Inspiratory flow patterns • Flow generation • High powered ventilator can deliver constant flow through inspiration – flow rate unaffected by patient characteristics • Pressure generation Low powered ventilator delivering decreasing flow through inspiration -

5. Anaesthetic Ventilators • Need to be capable of being attached to anaesthetic machine and scavenging • Less sophisticated / flexible than itu ventilators • Nowadays , generally must be usable with circle

6. Manley Ventilator • Minute Volume divider • Vt set by operator. Rate=FGF/Vt • Driving Force = Fresh Gas Pressure

7. Penlon Nuffield • Tubing from ventilator plugs into bag port on bain or circle • Uses “Fluid Logic” (coanda effect) • Used in paediatrics (with Newton Valve)

8. Ohmeda • Bag in bottle • Driving gas blown into bottle , compressing bellows (“bag”) • Bellows contain anaesthetic gas • “Pneumatic bag squeezer” • Controlled by electronic management of driving gas.

9. IMV • Originally , entailed attaching a t-piece onto the inspiratory limb of a ventilator • Allowed patient access to spontaneous breaths • PEEP had to be adjusted to be equal in spont & controlled circuits

10. sIMV • Allows imv within the normal breathing circuit • Breathing cycle ( which will contain 1 mandatory breath) broken into 3 parts • 1. Spontaneous breathing allowed • 2. Spontaneous breath will trigger the mandatory breath • 3. If spontaneous breath not taken in 2 , mandatory breath delivered • Reduction in sIMV rate not considered useful weaning method

11. Pressure vs volume control • Generally volume control used to initiate ventilation • Changed to pressure control where lungs susceptible to damage by high pressures (ards) • Volume delivered under pressure control variable

12. Pressure Support • System for reducing work of breathing • Patient inspiration spontaneous but breathes from pressurised reservoir • Applied to any breathing mode • Gradual reduction of level of pressure support is valid means of weaning

13. BiLevel (BiPap) • 2 levels of peep set • Patient can breathe spontaneously at any phase of respiration • Change in peep level-> change in volume within lungs