principles of mechanical ventilation ret 2284 module 1.0 spontaneous breathing vs. negative /...

Principles of Mechanical Ventilation

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

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


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


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


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


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


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


Mechanics of Spontaneous Respiration


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


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



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



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



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



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


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


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


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


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


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


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


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


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


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)

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


Negative Pressure Ventilation A negative pressure device designed for

resuscitation by Woillez in 1876



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

Iron Lung



Iron Lung

Chest Cuirass


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


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


Positive Pressure Ventilation Occurs when a mechanical ventilator

moves air into the patient’s lungs by way of an endotracheal tube or mask (NPPV).


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


Positive Pressure Ventilation

Pressures in Positive Pressure Ventilation

Baseline PressurePeak PressurePlateau PressurePressure at End of Exhalation


Baseline Pressure Pressures are read from a zero baseline

value


Baseline Pressure Continuous Positive Airway Pressure (CPAP)


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


Peak Pressure (PIP)


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


Plateau Pressure


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


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


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


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


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


Pressure at End of Expiration Auto-PEEP

principles of mechanical ventilation ret 2284 module 1.0 spontaneous breathing vs. negative /...

Documents

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