Principles of Mechanical Ventilation

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Principles of Mechanical Ventilation

• RET 2284
• Module 1.0
• Spontaneous Breathing vs. Negative / Positive
  Pressure Ventilation

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Spontaneous Breathing

• Ventilation and Respiration
• Spontaneous Breathing or Spontaneous Ventilation
• The movement of air into and out of the lungs
• Main Purpose
• Bring in fresh air for gas exchange into the
  lungs and to allow the exhalation of air that
  contains CO2

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Spontaneous Breathing

• Ventilation and Respiration
• Respiration
• The movement of gas molecules across a membrane
• External Respiration
• Oxygen moves from the lung into the blood stream,
  and CO2 moves from bloodstream into the alveoli
• Internal Respiration
• Carbon dioxide moves from the cells into the
  blood, and oxygen moves from the blood into the
  cells

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Spontaneous Breathing

• Ventilation and Respiration
• Normal Inspiration
• Accomplished by the expansion of the thorax. It
  occurs when the muscles of inspiration contract.
  
• Diaphragm descends and enlarges the vertical size
  of the thoracic cavity
• External intercostal muscles contract and raise
  the ribs slightly, increasing the circumference
  of the thorax
• The activities of these muscles represent the
  work required to inspire, or inhale

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Spontaneous Breathing

• Ventilation and Respiration
• Normal Exhalation
• Does not require any work, it is passive
• The muscles relax
• The diaphragm moves upward to its resting
  position
• The ribs return to their normal position
• The volume of the thoracic cavity decreases and
  air is forced out of the alveoli

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Spontaneous Breathing

• Gas Flow and Pressure Gradients During
  Ventilation
• Pressure Gradient
• For air to flow through a tube or airway,
  pressure at one end must be higher than pressure
  at the other end
• Air always flows from the high-pressure point to
  the low pressure point

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Spontaneous Breathing

• Gas Flow and Pressure Gradients During
  Ventilation
• Pressure Gradient
• Gas flows into the lungs when the pressure in the
  alveoli is lower than the pressure at the mouth
  and nose
• Conversely, gas flow out to the lungs when the
  pressure in the alveoli is greater than the
  pressure at the mouth and nose
• When the pressure in the mouth and alveoli are
  the same, as occurs at the end of inspiration or
  the end of expiration, no gas flow occurs

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Spontaneous Breathing

• Mechanics of Spontaneous Respiration

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Spontaneous Breathing

• Lung Characteristic
• Two Primary Characteristic of the Lung
• Compliance and Resistance
• Two types of force oppose inflation of the lungs
• Elastic force
• Arise from elastic properties of lung and thorax
  that oppose inspiration
• Frictional force
• Resistance of tissues and organs as they move and
  become displaced during breathing and resistance
  to gas flow through the airways

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Spontaneous Breathing

• Lung Characteristic
• Compliance
• The relative ease with which a structure distends
• Pulmonary physiology uses the term compliance to
  describe the elastic forces that oppose lung
  inflation (lung tissue and surrounding thoracic
  structures)
• Described as the change in volume that
  corresponds to a change in pressure
• Compliance ?V / ?P

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Spontaneous Breathing

• Lung Characteristic
• Compliance
• For spontaneous breathing patients, total
  compliance is about 100 mL/cm H2O
• Range 50 170 mL/cm H2O
• For intubated and mechanically ventilated
  patients, compliance varies
• Males 40 50 mL/cm H2O
• Females 35 45 mL/cm H2O
• When compliance is measured under conditions of
  no gas flow, it is referred to as STATIC
  Compliance

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Spontaneous Breathing

• Lung Characteristic
• Compliance
• Monitoring changes in compliance is a valuable
  means of assessing changes in the patients
  condition during mechanical ventilatory support
• Calculate Pressure
• If compliance is normal at 100 mL/cm H2O,
  calculate the amount of pressure needed to attain
  a tidal volume of 500 mL

500 ml 5 cm H20 100 ml/cm H2O
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Spontaneous Breathing

• Lung Characteristic
• Compliance
• Static Compliance (CS ) When compliance is
  measured under conditions of no gas flow
• Normal value is 70 100 mL/cm H2O
• When CS is lt25 cm H2O, the WOB is very difficult
• Exhaled tidal volume/Plateau pressure PEEP
• VT / PPlat PEEP
• 500 mL / 25 cm H2O 5 cm H2O
• CS 25 mL/cm H2O

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Spontaneous Breathing

• Lung Characteristic
• Compliance
• Changes in the condition of the lungs or chest
  wall (or both) affect total respiratory system
  compliance and the pressure required to inflate
  the lungs
• Diseases that reduce the compliance of the lung
  increase the pressure required to inflate the
  lung, e.g., ARDS
• Diseases that increase the compliance of the lung
  decrease the pressure required to inflate the
  lung, e.g. emphysema

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Spontaneous Breathing

• Lung Characteristic
• Resistance
• Frictional forces associated with ventilation are
  the result of the anatomical structure of the
  conductive airways and the tissue viscous
  resistance of the lungs and adjacent tissue and
  organs
• During mechanical ventilation, resistance of the
  airways (Raw) is the factor most often evaluated

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Spontaneous Breathing

• Lung Characteristics
• Airway Resistance (Raw)
• Expressed in (cm H2O/L/sec)
• Unintubated patients
• Normal 0.6 2.4 cm H2O/L/sec
• Intubated patients
• Approximately 6 cm H2O/L/sec or higher
• Increase is caused by artificial airway smaller
  the tube the greater the resistance
• Diseases of the airway can also cause increases
  in Raw

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Spontaneous Breathing

• Lung Characteristics
• Airway Resistance (Raw)
• With higher airway resistance, more of the
  pressure for breathing goes to the airways and
  not the alveoli consequently, a smaller volume
  of gas is available for gas exchange

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Spontaneous Breathing

• Time Constants
• Compliance x Resistance
• A measure of how long the respiratory system
  takes to passively exhale (deflate) or inhale
  (inflate)
• The differences in C and Raw affect how rapidly
  the lung units fill and empty

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Spontaneous Breathing

• Time Constants
• Normal lung
• Lung units fill within a normal length of time
  and with a normal volume
• Low-compliance
• Lung units fill rapidly
• Increased resistance
• Lung units fill slowly

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Spontaneous Breathing

• Time Constants
• A Normal lung unit
• B Low-compliance
• fills quickly, but with less air
• C Increased resistance
• fills slowly. If inspiration were to end at the
  same time a unit A, the volume in unit C would be
  lower

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Spontaneous Breathing

• Time Constants
• Calculation of Time Constants
• Time constant C x R
• Time constant 0.1 L/cm H2O x 1 cm H2O/(L/sec)
• Time constant 0.1 sec
• In a patient with a time constant of 0.1 sec.,
  63 of passive exhalation or inhalation occurs in
  0.1 sec., 37 of the volume remains to be
  exchanged

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Spontaneous Breathing

• Time Constants
• TC of lt3 may result in incomplete delivery of
  tidal volume
• Prolonging the inspiratory time allows even
  distribution of ventilation and adequate delivery
  of tidal volume

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Spontaneous Breathing

• Time Constants
• TC of lt3 may result in incomplete emptying of the
  lungs, which can increase the FRC and cause air
  trapping
• Using TC of 3 4 may be more adequate for
  exhalation (95 98 volume emptying level)

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Types of Mechanical Ventilation

• Negative Pressure Ventilation
• Attempts to mimic the function of the respiratory
  muscles to allow breathing through normal
  physiological mechanisms
• Applies subatmospheric pressure outside of the
  chest to inflate the lungs
• Removing the negative pressure allows passive
  exhalation

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Types of Mechanical Ventilation

• Negative Pressure Ventilation
• A negative pressure device designed for
  resuscitation by Woillez in 1876

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Types of Mechanical Ventilation

• Negative Pressure Ventilation

Iron Lung
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Types of Mechanical Ventilation
Iron Lung

• Negative Pressure Ventilation
• This is the Iron Lung ward at Rancho Los Amigos
  Hospital, Downey, California, in the early 1950s,
  filled to overflowing with polio patients being
  treated for respiratory muscle paralysis

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Types of Mechanical Ventilation

• Negative Pressure Ventilation

Chest Cuirass
Iron Lung
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Types of Mechanical Ventilation

• Negative Pressure Ventilation

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Types of Mechanical Ventilation

• Negative Pressure Ventilation
• Advantages
• Upper airway can be maintained without the use if
  an endotracheal tube or tracheotomy
• Patients can talk and eat
• Fewer physiological disadvantages than positive
  pressure ventilation

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Types of Mechanical Ventilation

• Negative Pressure Ventilation
• Disadvantages
• Decreased accessibility to the patient
• Abdominal venous blood pooling
• Decreased venous return, cardiac output, systemic
  blood pressure (hypotension) tank shock
• Negative pressure ventilators have primarily been
  replaced by positive pressure ventilators that
  use a mask, nasal device or tracheostomy tube

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Types of Mechanical Ventilation

• Positive Pressure Ventilation
• Occurs when a mechanical ventilator moves air
  into the patients lungs by way of an
  endotracheal tube or mask (NPPV).

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Types of Mechanical Ventilation

• Positive Pressure Ventilation
• At any point during inspiration, the inflating
  pressure at the upper (proximal airway) equals
  the sum of the pressure required to overcome the
  compliance of the lung and chest wall and the
  resistance of the airways

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Types of Mechanical Ventilation

• Positive Pressure Ventilation

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Pressures in Positive Pressure Ventilation

• Baseline Pressure
• Peak Pressure
• Plateau Pressure
• Pressure at End of Exhalation

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Pressures in Positive Pressure Ventilation

• Baseline Pressure
• Pressures are read from a zero baseline value

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Pressures in Positive Pressure Ventilation

• Baseline Pressure
• Continuous Positive Airway Pressure (CPAP)

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Pressures in Positive Pressure Ventilation

• Baseline Pressure
• Positive End-Expiratory Pressure (PEEP)
• Prevents patients from exhaling to zero
  (atmospheric pressure)
• Increases volume of gas left in the lungs at end
  of normal exhalation increases FRC

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Pressures in Positive Pressure Ventilation

• Peak Pressure (PIP)

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Pressures in Positive Pressure Ventilation

• Peak Pressure
• The highest pressure recorded at the end of
  inspiration (PPeak, PIP)
• It is the sum of two pressures
• Pressure required to force the gas through the
  resistance of the airways and to fill alveoli

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Pressures in Positive Pressure Ventilation

• Plateau Pressure

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Pressures in Positive Pressure Ventilation

• Plateau Pressure
• At baseline pressure (end of exhalation), the
  volume of air remaining in the lungs is the FRC.
  
• At the end of inspiration, before exhalation
  starts, the volume of air in the lungs is the VT
  plus the FRC. The pressure measured at this
  point with no flow of air is plateau pressure

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Pressures in Positive Pressure Ventilation

• Plateau Pressure
• Measured after a breath has been delivered and
  before exhalation
• Ventilator operator has to perform an inflation
  hold
• Like breath holding at the end of inspiration

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Pressures in Positive Pressure Ventilation

• Plateau Pressure
• Reflects the effect of elastic recoil on the gas
  volume inside the alveoli and any pressure
  exerted by the volume in the ventilator circuit
  that is acted upon by the recoil of the plastic
  circuit

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Pressures in Positive Pressure Ventilation

• Pressure at End of Expiration
• Pressure falls back to baseline during expriration

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Pressures in Positive Pressure Ventilation

• Pressure at End of Expiration
• Auto-PEEP
• Air trapped in the lungs during mechanical
  ventilation when not enough time is allowed for
  exhalation
• Need to monitor pressure at end of exhalation

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Pressures in Positive Pressure Ventilation

• Pressure at End of Expiration
• Auto-PEEP