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