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![Page 1: Principles of Mechanical Ventilation RET 2284 Module 1.0 Spontaneous Breathing vs. Negative / Positive Pressure Ventilation.](https://reader036.fdocuments.us/reader036/viewer/2022081415/56649da65503460f94a914f6/html5/thumbnails/1.jpg)
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 patient’s 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 <25 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-compliancefills quickly, but with less air
C: Increased resistancefills 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 <3 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 <3 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
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
Iron Lung
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Types of Mechanical Ventilation
Negative Pressure Ventilation
Iron Lung
Chest Cuirass
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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 patient’s 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 PressurePeak PressurePlateau PressurePressure 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 PressureAt 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