how ventilators work

• On/off switch


  Controls the main electrical power source
• Ventilator operation


  1. Electricity provides energy to operate motors, electromagnets, potentiometers, rheostats, and microprocessors

  2. Microprocessors control timing mechanisms for inspiration and expiration, gas flow, and alarm systems

  3. Electricity operates devices like fans, bellows, solenoids, and transducers

  4. Ensures controlled pressure and gas flow to the patient

  
• Electrically powered and controlled ventilators

  • Lifecare PLV-102
  • Pulmonetics LTV 800, 900, and 1000
  • Newport HT50

  
• Pneumatically powered ventilators


  Use one or two 50-psi gas sources with built-in internal reducing valves
• Pneumatic ventilators


  • Use needle valves, Venturi entrainers, flexible diaphragms, and spring-loaded valves to control flow, volume delivery, and inspiratory and expiratory function

  
• Pneumatic ventilator


  • Bird Mark 7

  
• Fluidic ventilators


  Rely on principles of wall attachment and beam deflection to control gas flow
• Fluidic ventilator


  • Bio-Med MVP-10

  
• Most pneumatically powered ICU ventilators also have an electrical power source to energize a computer that controls the ventilator functions

  
• The gas sources, mixtures of air and oxygen, supply the power for ventilator function and allow for a variable fractional inspired oxygen concentration (FIO2)
• The electrical power is required for operation of the computer microprocessor, which controls capacitors, solenoids, and electrical switches that regulate the phasing of inspiration and expiration, and the monitoring of gas flow
• The ventilator's preprogrammed ventilator modes are stored in the microprocessor's read-only memory (ROM), which can be updated rapidly by installing new software programs
• Random access memory (RAM) is used for temporary storage of data, such as pressure and flow measurements and airway resistance and compliance
• Positive pressure ventilators


  Gas flows into the lung because the ventilator establishes a pressure gradient by generating a positive pressure at the airway opening
• Negative pressure ventilators


  Generate a negative pressure at the body surface that is transmitted to the pleural space and then to the alveoli
• Open-loop systems


  The operator sets a control (e.g. tidal volume) and the ventilator delivers that volume to the patient circuit, without being able to respond to changing conditions
• Closed-loop systems


  Compare the set control variable to the measured control variable, allowing the ventilator to respond to changes in the patient's condition

  
• Mandatory minute ventilation is an example of a closed-loop system where the ventilator monitors the patient's spontaneous minute ventilation and increases its output if it falls below the operator's set minimum
• Control panel (user interface)


  Located on the surface of the ventilator and monitored/set by the operator, it has various controls to regulate flow, volume, pressure, and time

  
• The operator can also set alarms to respond to changes in monitored variables like high/low pressure and low volume
• Closed-loop system


  A system where the measured parameter is compared to the set parameter and the output is adjusted to match the set value
• Open-loop system


  A system where the desired parameter is set but the output is not measured or adjusted
• Potential advantages and disadvantages of closed-loop vs open-loop systems

  • Advantages of closed-loop: More accurate, self-adjusting, compensates for changes
  • Disadvantages of closed-loop: More complex, potential for instability, more expensive
  • Advantages of open-loop: Simpler, less expensive, more reliable
  • Disadvantages of open-loop: Less accurate, no self-adjustment, can't compensate for changes
• Control panel (user interface)

  Located on the surface of the ventilator, monitored and set by the ventilator operator. The internal control system reads and uses the operator's settings to control the function of the drive mechanism.

  
• Components of the control panel

  • Knobs or touch pads for setting tidal volume, rate, inspiratory time, alarms, FIO2
• Ventilatory variables


  Flow, volume, pressure, time
• The manufacturer provides a list of the potential ranges for each ventilatory variable

  
• The operator can set alarms to respond to changes in a variety of monitored variables, particularly high and low pressure and low volume
• Pneumatic circuit


  A series of tubes that allow gas to flow inside the ventilator and between the ventilator and the patient
• Single-circuit ventilator


  The ventilator's internal circuit allows the gas to flow directly from its power source to the patient
• Double-circuit ventilator


  The primary power source generates a gas flow that compresses a mechanism such as a bellows or "bag-in-a-chamber", and the gas in the bellows or bag then flows to the patient
• Most ICU ventilators manufactured today are classified as single-circuit ventilators
• Basic elements of a patient circuit

  • Main inspiratory line
  • Adapter
  • Expiratory line
  • Expiratory valve
• Exhalation valve


  Allows the release of exhaled gas from the expiratory line into the room air
• In most current ICU ventilators, the exhalation valve is located inside the ventilator and is not visible

  
• Typical components included in a patient circuit

  • Pressure manometer
  • Upper airway pressure monitor line
  • Expiratory valve line
  • Expiratory valve
  • Expiratory line
  • Expired volume measuring device
  • Temperature measuring or sensing device
  • Main inspiratory line
  • Humidifier
  • Heater and thermostat
  • Main flow bacterial filter
  • Oxygen analyzer

  
• Adjuncts used with a patient circuit

  • Device to warm and humidify inspired air
  • Thermometer or sensing device to measure temperature of inspired air
  • Apnea or low-pressure alarm
  • Nebulizer line
  • Volume-measuring device
  • Bacterial filters
  • Pressure gauge
  • In-line suction catheter

  
• Power transmission and conversion system


  The internal hardware that converts electrical or pneumatic energy into the mechanical energy required to deliver a breath to the patient
• Drive mechanism


  A mechanical device that produces gas flow to the patient, e.g. a piston powered by an electrical motor
• Output control


  One or more valves that regulate gas flow to the patient