Ventilators

Tuesday, 20 April 2004

Bill McCulloch

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

History

Need arose from polio epidemics in 1950s and changes in anaesthetic techniques (muscle relaxants)Originally engineering challengeInflexible

ClassificationMost classifications obsolete but need to be knownBased 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 generationLow powered ventilator delivering decreasing flow through inspiration

-

Anaesthetic Ventilators

Need to be capable of being attached to anaesthetic machine and scavengingLess sophisticated / flexible than itu ventilatorsNowadays , generally must be usable with circle

Manley Ventilator

Minute Volume dividerVt set by operator. Rate=FGF/VtDriving Force = Fresh Gas Pressure

Penlon Nuffield

Tubing from ventilator plugs into bag port on bain or circleUses “Fluid Logic” (coanda effect) Used in paediatrics (with Newton Valve)

Ohmeda

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

IMV

Originally , entailed attaching a t-piece onto the inspiratory limb of a ventilatorAllowed patient access to spontaneous breathsPEEP had to be adjusted to be equal in spont & controlled circuits

sIMVAllows imv within the normal breathing circuitBreathing 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

Pressure vs volume control

Generally volume control used to initiate ventilationChanged to pressure control where lungs susceptible to damage by high pressures (ards)Volume delivered under pressure control variable

Pressure Support

System for reducing work of breathingPatient inspiration spontaneous but breathes from pressurised reservoirApplied to any breathing modeGradual reduction of level of pressure support is valid means of weaning

BiLevel (BiPap)

2 levels of peep setPatient can breathe spontaneously at any phase of respirationChange in peep level-> change in volume within lungs