ENTC 4350

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ENTC 4350

• Modern Ventilators

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Modern Ventillators

• Ventilation assistance is provided under either
  of two conditions
• (1) breathing initiated by a timing mechanism or
  
• (2) patient-initiated breathing.

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• Automatically timed breathing is usually provided
  for patients who cannot breathe on their own.
• It provides inspiration and expiration at fixed
  rates and durations except for periodic sigh a
  sigh is a rest period for the patient.

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• Patient-initiated breathing may be given to one
  who has difficulty breathing due to high airway
  resistance.
• The patients effort to inhale triggers the
  respirator unit to deliver air at the positive
  pressure prescribed.

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Ventilator Modes of Operation

• The following definitions are commonly used to
  describe respirator/ventilator operation
• CMV Continuous mandatory ventilation Once
  initiated by either the ventilator operator or
  the patient, the breath is driven to the
  patient.
• CPAP Continuous positive airway pressure Breaths
  are spontaneous, unless the operator
  intervenes. The spontaneous breaths are
  determined entirely by patient effort. However,
  the air/oxygen mixture is set by the ventilator.

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• SIMV Synchronized intermittent mandatory
  ventilation These breaths are initiated by
  either the machine, the operator, or the
  patient. The breaths may be either spontaneous
  or mandatory. That is, if the patient does not
  breathe within a preset time period, the
  ventilator will deliver a breath.
• PEEP Positive end-expiratory pressure The
  pressure maintained by the ventilator that the
  patient must exhale against.

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• Apena The patient has stopped breathing.
• Sigh A breath delivered by the ventilator that
  differs in duration and pressure from a nominal
  breath.
• Nebulizer A device for producing a fine spray of
  liquid or medication into the patients air.

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• The block diagram shows the external flexible
  tubing and the ventilator unit.

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• Air from the ventilator during patient
  inspiration passes through a bacterial filter and
  humidifier.
• A nebulizer may spray medication into the air.
  This air then forces valve 1 up to close off the
  spirometer and deliver air to the patient.

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• After the inspiration air is turned off by the
  ventilator, valve 1 drops and the patient exhales
  into the bellows, which has its outlet valve held
  closed pneumatically by the ventilator unit.
• During the subsequent patient inspiration cycle,
  that valve will open, causing the bellows to fall
  and empty.

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• During patient expiration, the direction of the
  air in the pneumatic system is determined by the
  main solenoid, which is switched appropriately by
  the system electronics.
• Room air is drawn from the air inlet filter by
  the main compressor and is directed through the
  main solenoid to hold closed the upper outlet
  valve of the bellows located inside the unit.

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• Next, the weight of the bellows causes the bottom
  bellows chamber outlet valve to open, as the main
  solenoid directs air to close the inlet bellows
  chamber valve.
• The weight of the falling bellows draws
  oxygen-enriched air into it in preparation for
  the patient-inspiration part of the cycle.

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• The oxygen content of the air flowing into the
  bellows is controlled by a percentage control
  valve, which regulates the resistance to room air
  and oxygen appropriately.

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• At the end of patient expiration, the system
  electronics trip the main solenoid, thereby
  initiating the patient-inspiration part of he
  cycle.

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• During patient inspiration, the compressor draws
  room air through an air filter and then through
  the main solenoid.
• It forces the bottom inlet valve of the internal
  bellows chamber open and forces the bottom
  bellows chamber outlet valve closed.

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• The high pressure in the bellows chamber
  compresses the bellows, forcing open the upper
  outlet valve set free by the main solenoid.
• This allows the oxygen-enriched air to pass
  through the main bacteria filter into the
  external tubes and then to the patient lungs.

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• A sensitivity control monitors the negative
  pressure necessary to initiate inspiration when
  the respirator is used in the patient-initiated
  breathing mode called the assist mode.
• A nebulizer compressor may draw air from the
  bellows and force it through an aspirator to mix
  medication into the patient-inspired air.

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• When inspiration is complete, the main solenoid
  switches the direction of the pneumatic air to
  repeat the expiration cycle, and so on.

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• The black bag at the end of the tubing simulates
  a compliant lung.
• This respirator may be operated using compressed
  air from the hospital air supply.

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• In that mode, the ventilator can be removed from
  its internal compressor, thereby decreasing it in
  size turning off the compressor also reduces
  problems in instrument noise control.

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• To aid patient respiration, hospital air and
  oxygen enter the pneumatic compartment, where it
  is filtered.
• A check valve reduces the pressure to a nominal
  10 psi.

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• A proportional solenoid valve assembly allows the
  air/oxygen mix to be controlled by the system
  electronics.
• A check valve directs air to the patient during
  the inspiration cycle.

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• During the subsequent expiration phase, the
  system electronics opens the check valve, CV5, to
  provide a vent for the patient exhalation air.
• In this case, the pneumatically operated valves
  of older ventilators have been replaced by valves
  controlled with microprocessor-based electronics.

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• A small positive-pressure ventilator is
  illustrated with both front and back views.

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PNEUMOTACHOGRAPH AIRFLOW MEASUREMENT

• Patient airflow may be measured by changes in
  resistance of a thermistor in the airstream due
  to the cooling effect of flowing air.
• But it must be calibrated to compensate for
  changing ambient temperature.

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• To eliminate this disadvantage, a strain-gauge
  wire mesh is often used.

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• The airflow in either direction puts a strain on
  the screen and changes the resistance of its
  strain gauge.

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• The strain gauge is a component of a Wheatstone
  bridge.

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• Here the change in resistance, DR, is
  proportional to the airflow, F, past the wire
  mesh.

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Voltage divider rule
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Getting a common denominator
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If R DR
For the diff amplifier with a gain of AD ,
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Integrator Circuit
For the integrator circuit,
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Note that the term inside the integral is a
constant,
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Pneumotachograph Volume Measurement

• A volume exhaled by a patient is measured with
  the pneumotachograph by first closing and then
  opening the reset switch.
• This sets the initial charge on the capacitor to
  zero and fixes Vout at zero.

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• The patient is then asked to exhale through the
  pneumotach mouthpiece.
• The resulting change in DR creates a voltage VF
  as a function of time in proportion to flow.

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• The air volume expired by the patient, beginning
  at t 0, when the reset is activated, equals the
  area under the flow vs. time curve.
• Mathematically, this area is computed by
  integration.

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• The output voltage, Vout ,is proportional to the
  volume of air expired from time t 0 to the
  time, t, desired.
• The flow, F, is a function of time that may
  increase, decrease, or stay constant, so long as
  it goes in one direction.

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This implies that Vout is proportional to the
total volume of air that is passed through the
pneumotach for the time of observation.
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THE PLETHYSMOGRAPH

• The pneumotachograph can be used to measure the
  rate of airflow during respiration and the vital
  air capacity of the lung VC.
• It cannot, however, measure the total lung
  capacity, TLC.

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• The reason for this is that the pneumotachograph
  can only measure the amount of air a person can
  exchange in respiration, and cannot detect the
  residual volume of air, RV, left in the lung
  after a forced exhaling.
• To measure the TLC, a body plethysmograph may be
  used.

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• The plethysmograph consists of an airtight
  chamber the patient can enter and sit in.

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• The principle of operation of the plethysmograph
  depends directly on the gas law for an ideal gas
  of volume V and pressure P, namely Boyles Law
• where k1 is a constant and T is the absolute
  temperature (K). In the chamber, the temperature
  remains constant.

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• To measure TLC, the patient enters the chamber.
• The door is sealed, and the valve on the
  mouthpiece is closed.

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• Since the patient cannot breathe with the valve
  closed, the air pressure in the mouthpiece equals
  that in the lung, PT.
• That is, when the flow of air is zero, the
  pressure drop from mouthpiece to lung is also
  zero.

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• With the valve closed, a formula for the thoracic
  volume is derived as follows.
• The gas equation for constant temperature holds
  inside the lung

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• Using the previous equation, the TLC can be
  written as

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• Boyles Law also holds in the chamber, so that
• Here VOLC is the chamber volume and PC is the
  chamber pressure.

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• Because the chamber is closed, any increase in
  the thoracic volume introduced by breathing
  motions causes a decrease in the chamber volume
  of air.
• That is

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• Combining the equations
• with the previous fact

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• This yields

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• During the test PT PC approximately, since the
  changes in pressure induced by breathing motions
  are small when the patient is resting.
• Thus,

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• This equation gives the means for measuring the
  lung volume, TLC, by the following steps
• Close the mouthpiece valve on the patient sealed
  in the chamber.
• Ask the patient to make breathing motions.
• Read the change in pressure dPT on meter 1.
• Read the change in pressure dPC in the chamber on
  meter 2.
• Since the chamber volume VOLC is a known
  specification of the plethysmograph, use the
  result in steps 3 and 4 to compute TLC.