4 - How Mechanical Ventilators Work

major components of MV

• power source/input power

  • electric or pneumatic

• positive/negative pressure generator

• control systems and circuits

  • open-/closed-loop systems to control ventilator function

  • control panel

  • pneumatic circuit (internal/external)

• power transmission and conversion system

  • volume displacement, pneumatic designs

  • flow-control valves

• output

  • pressure, volume, and flow waveforms

pneumatic circuit

tubes that allow gas to flow inside ventilator and between ventilator and patient

internal pneumatic circuit

pneumatic circuits that can be single (common in ICU vents) or double

external pneumatic circuit

pneumatic circuit that connects vent to patient’s airway

power transmission and conversion system

system that enables ventilator to perform mechanical/pneumatic operation

hardware:

• drive mechanism for gas flow

• output control mechanism that regulates gas flow

compressors

MV components that reduce internal volumes to make positive pressure

types:

• piston

• blades

• diaphragms

• bellows

volume displacement designs

• bellows (out of production)

• pistons (linear/rotary drive)

• concertina bags

• “bag-in-a-chamber” systems

output control

MV components that control pressure, volume, flow, and time

types:

• mechanical

• pressure reducing valves

• solenoid valves

• needle valves

volume flow-controlled valves

valves that regulate flow

• open and close completely or in increments

• types

  • solenoid valves

  • digital valves

classification of mechanical breaths

• control

  • pressure, volume

• trigger

  • patient and/or time

• limit

  • pressure, volume, flow

• cycling

  • time, volume, flow

volume control

• pressure waveform

volume control

• volume waveform

volume control

• flow waveform

pressure control

• pressure waveform

pressure control

• volume waveform

pressure control

• flow waveform

trigger variables

time or patient

• patient can trigger breaths between timed breaths

trigger sensitivity

• pressure

inspiration starts due to loss of circuit pressure (NIF)

• usually -2 cmH₂O below set PEEP

trigger sensitivity

• flow

inspiration starts due to change in flow during exhalation

• most patients find this comfortable

trigger sensitivity

• volume

inspiration starts due to volume drop during exhalation

limit variables

what limits inspiration before cycle variable is reached

• can be pressure, volume, or flow

cycling variables

• pressure

inspiration ends once set pressure is reached

cycling variables

• volume

inspiration ends once set volume is reached

cycling variables

• flow

inspiration ends once flow has decreased to a set percentage

cycling variables

• time

inspiration ends once set time elapses (i-time)