Download RCP 112 Basic Concepts

RCP-112 PATIENT
MANAGEMENT
Basic Terms And Concepts Of
Mechanical Ventilation
Normal Mechanics Of Spontaneous
Ventilation

Respiration – movement of gas molecules
across a membrane.

External respiration – 02 moves from the
lung into the bloodstream, C02 moves from
the blood stream into the alveoli.
Normal Mechanics Of Spontaneous
Ventilation

Internal respiration – cellular level, C02
moves from the cells into the blood and 02
moves from the blood into the cells.
Normal Mechanics Of Spontaneous
Ventilation
Here’s how we breathe:
1.
2.
3.
4.
5.
Muscles of inspiration contract.
The diaphragm descends and enlarges the vertical
size of the thoracic cavity.
The external intercostals contract – and slightly raise
the ribs, increasing the circumference of the thorax.
The activity of these muscles represent the work
needed to inhale.
Exhalation is passive – muscles relax and the volume
of the thoracic cavity decreases and air is forced out.
Normal Mechanics Of Spontaneous
Ventilation

The normal breath
Gas Flow And Pressure Gradients
During Ventilation
You Need To Understand The Basic
Concept Of Airflow!
1. For air to flow through the airways – pressure at one
end must be higher than pressure at the other end.
2. Pressure gradient – air flows from the high-pressure
point to the low pressure point.
Gas Flow And Pressure Gradients
During Ventilation

Lung volumes change as a result of gas
flow caused by changes in pressure.

The conductive airway starts at the mouth
and nose and ends at the small airways
near the alveoli.
Gas Flow And Pressure Gradients
During Ventilation

Gas flows into the lung when the pressure
in the alveoli is < than the pressure at the
mouth and nose.

Gas flows out of the lungs when the
pressure in the alveoli is > than the
pressure at the mouth and nose.
Gas Flow And Pressure Gradients
During Ventilation

A pressure gradient doesn’t exists – when
the pressure in the mouth and alveoli are
the same.

This occurs at the end of inspiration or the
end of expiration – gas flow doesn’t occur
because the pressure across the
conductive airways are =.
Units Of Pressure

Ventilating pressure are measured in cm
H20.

These pressures are referenced to
atmospheric pressure and given a baseline
value of zero.
Definition Of Pressures And Gradients
In The Lungs

Airway opening pressure (Pawo) or mouth pressure
(Pm) is called airway pressure (Paw).

Mouth pressure and airway pressure are the
terms most commonly used.

But this pressure may be called upper airway
pressure, mask pressure or proximal airway
pressure.
Pressures And Gradients In The Lungs

Pbs – pressure at the body surface area:
1. Equal to 0 (atmospheric) unless the person is using a
hyperbaric chamber or a iron lung.
Pressure And Gradients In The Lungs

Intrapleural pressure (Ppl) – measurement of
pressure in the potential space between the
parietal and visceral pleural:
1. Normal is -5 cmH20 at the end of exhalation during a
spontaneous breath.
2. -10 cmH20 at the end of inspiration.
3. Since intrapleural pressure is hard to measure, they
will look at esophageal pressure (Pes) – balloon is
placed in the esophagus and changes in balloon
pressure are used to estimate pressure and pressure
changes in the pleural space.
Pressure And Gradients In The Lungs

Alveolar Pressure (Palv/PA) – also called
intrapulmonary or lung pressure:
1. It normally changes as intrapleural pressure
changes.
2. During inspiration alveolar pressure is about -1
cmH20 and during exhalation it is about +1 cmH20.
Pressures And Gradients In The Lung

4 basic pressure gradients used to describe
normal ventilation:
1.
2.
3.
4.
Transairway pressure
Transthoracic pressure
Transpulmonary pressure
Transrespiratory pressure
Pressures And Gradients In The Lungs

Transairway pressure – (PTA) the pressure
gradient between the airway opening and
the alveolus:
1. PTA = Pawo – PA.
2. Pawo is often called the airway pressure (Paw).
3. It produces airway movement in the conductive
airways.
4. It also represents the pressure caused by resistance
to gas flow in the airways (airflow resistance).
Pressure and Gradients In The Lungs

Transthoracic pressure (Pw) – the pressure
difference between the alveolar space (PA)
or lung and the body’s surface (Pbs):
1. Pw = PA-Pbs.
2. It represents the pressure needed to expand or
contract the lungs and the chest wall at the same
time.
Pressures And Gradients In The Lungs

Transpulmonary pressure – (PL) or transalveolar
pressure, is the pressure difference between the
alveolus PA and the pleural space (Ppl): PL=PA-Ppl:
1. PL maintains alveolar inflation and is sometimes called the
alveolar distending pressure.
2. All modes increase PL, either by decreasing Ppl (- pressure
ventilators) or increasing PA by increasing pressure at the upper
airway ( + pressure ventilators).
3. Transalveolar pressure is used synonymously with transpulmonary
pressure
Pressures And Gradients In The Lung

Transrespiratory pressure (PTR) – the pressure
gradient between the airway opening and the
body surface:
1. PTR = Pawo-Pbs.
2. This pressure is required to inflate the lungs and
airways during positive pressure ventilation.
3. In this situation the body surface pressure is
atmospheric and given the value 0.
4. Thus – Pawo becomes the pressure reading on
the ventilator.
Pressures And Gradients In The Lungs

Transrespiratory pressure continued:
5. It has 2 components – transthoracic pressure
(pressure required to overcome elastance) and
transairway pressure (pressure required to
overcome airway flow resistance)
Pressures And Gradients In The Lungs

Let’s look at the normal spontaneous breath:
1. Remember Boyle’s Law?
2. With spontaneous inspiration – the volume of the
thoracic space increases and the pressure in
the pressure in the pleural space is more negative
in relation to atmospheric pressure.
3. Negative intrapleural pressure goes from -5 cmH20
at end-inspiration to about -10 cmH20 at endinspiration.
Pressure And Gradients In The Lung

Spontaneous breath continued:
4. - pleural pressure is transmitted to the alveolar
space.
5. PA pressure becomes more negative relative to
atmospheric pressure.
6. PL widens – thus the alveoli have a negative
pressure during spontaneous inspiration.
Pressure and Gradients In The Lungs

The spontaneous breath continued:
7. Pressure at the mouth is still atmospheric.
8. There is a pressure gradient between the mouth
and alveolus – thus air flow from the mouth to
the alveolus and they expand.
9. When the volume of gas builds up in the alveoli
and pressure returns to 0, airflow stops.
10. This is the end of inspiration – pressure at the mouth
and in the alveoli is 0.
Pressures And Gradients In The Lungs

Spontaneous breath continued:
11. During the expiratory phase the muscles relax.
12. Thoracic volume decreases to resting and the
intrapleural pressure returns to around -5 cmH20.
13. The pressure inside the alveolus during exhalation
increases and becomes slightly positive.
14. Pressure is lower at the mouth than inside the
alveoli – transairway pressure gradient causes
air to move out of the lungs.
15. Pressure in the alveoli and the mouth are =, exhalation ends.
Changes In Transpulmoanry Pressure
Under Varying Conditions
Passive Spontaneous Ventilation
Pressure
End-Expiration
End-Inspiration
Intraalveolar
0 cmH20
0 cmH20
Intrapleural
-5 cmH20
-10 cmH20
+5 cmH20
+10 cmH20
Transpulmonary
Changes In Transpulmonary Pressure
Under Varying Conditions
Positive Pressure Ventilation
Pressure
End-Expiration
End-Inspiration
Intraalveolar
0 cmH20
9-12 cmH20
Intrapleural
-5 cmH20
2-5 cmH20
Transpulmonary
+5 cmH20
8 cmH20
Lung Characteristics

2 primary characteristics of the lungs:
1. Compliance
2. Resistance
These 2 parameters are evaluated for patients receiving
mechanical ventilation.
Lung Characteristics

2 types of force that oppose the inflation of
the lung:
1. Elastic force – arise from the elastic properties of
the lungs and thorax that oppose inspiration.
2. Frictional forces – which is the result of 2 factors,
the resistance of the tissues and organs as they
move and become displaced during breathing and
resistance to gas flow through the airways.
Lung Characteristics

Compliance:
1. The ease with which the structure distends.
2. A balloon that’s easy to inflate is very compliant.
3. Elastance is the tendency of a structure to return
to its original form after being stretched or acted on
by an outside force
Lung Characteristics

Pulmonary physiology and compliance:
1. Describes the elastic forces that oppose lung
inflation.
2. Defined as change of volume that corresponds
to the change in pressure.
3. C = cV/cP.
Lung Characteristics

Pulmonary physiology and compliance
continued:
4. Volume is measured in liters or milliliters and
pressure in centimeters of water pressure.
5. Compliance of the respiratory system is the sum
of the compliances of both the lung tissue and the
surrounding thoracic structures.
Lung Characteristics

Pulmonary physiology and compliance continued:
6. The value for compliance varies based on a
person’s posture, position and active breathing.
7. It can range from .05 L/cmH20 to 0.17 L/cmH20
(50 to 170 ml/cm H20).
8. For intubated and mechanically ventilated patients
with normal lungs and chest wall – males 40-50
ml/cmH20, up to 100 ml/cmH20 and females 35
- 45 ml/cmH20, up to 100 ml/cmH20.
Lung Characteristics

Pulmonary physiology and compliance
continued:
9. Monitoring changes in compliance is an excellent
way of assessing changes in the patient’s condition.
10. Compliance is usually measured under conditions
of no gas flow – called static compliance.
Lung Characteristics

Pulmonary physiology and compliance:
11. Changes in condition of the lungs and/or chest wall
affect total respiratory system compliance and the
pressure needed to inflate the lungs.
12. Examples ARDS and COPD.
Excercise
Calculate Pressure
If compliance is normal at 0.1 L/cmH20,
calculate the amount of pressure needed to
attain a tidal volume of .5 L (500ml).
Answer
C = cV/cP – so cP = cV/C
.5L/0.1L/cmH20 = 5 cmH20
So a Palv. change of 5 cmH20 would be
needed to achieve a .5 L Vt in a person
with normal compliance.
Lung Characteristics

Pulmonary physiology and compliance:
13. When patients are mechanically ventilated,
compliance calculations use the plateau pressure
measured at static or no flow conditions and the
pressure at the end-exhalation.
14. Tidal volume exhaled from the patient’s lungs
is determined by measuring the volume near the
patient connector.
Equation For Calculating Static
Compliance
CL = Exhaled Tidal Volume/(Plateau
pressure-EEP)
C = VT/Plateau – EEP
Compliance value includes both lung and
thorax
If a patient actively inhales or exhales
during the measurement of a plateau
pressure, the value will be inaccurate.
L
Lung Characteristics

Resistance:
1. Frictional forces seen with ventilation are due to the
anatomical structure of the conductive airways and
the tissue viscous resistance of the lungs and
adjacent tissues and organs.
2. When the lungs and thorax move during ventilation,
the movement and displacement of the lungs, abdominal
organs , rib cage and diaphragm create resistance to
breathing.
Lung Characteristics

Resistance continued:
3. During MV, resistance of the airways is the factor
most often evaluated.
4. Ability of air to flow through the conductive airways
depends on the gas viscosity and density, length
and diameter of the E.T. tube and the flow rate of
the gas through the tube.
Lung Characteristics

Resistance continued:
5. Clinically diameter of the airway lumen and flow
rate of the gas are factors that can change.
6. Tube size, secretions, bronchospasm, edema or
kinks in the ET tube.
7. The flow rate can be controlled in most modes of
ventilation.
Lung Characteristics
The equation
Raw = (PIP-Pplateau)
Flow
Raw = (40-25)
1(L/sec)
= 15 cmH20/(L/sec)
Lung Characteristics
Normal Resistance Values
Unintubated Patient
0.6 to 2.4 cmH20/(L/sec) at 0.5 L/sec flow
Intubated Patient
6 cmH20/(L/sec) or higher (airway resistance increases as ET tube
size decreases.
Lung Characteristics

Factors that can affect Raw:
1.
2.
3.
4.
Smaller internal diameter of the ET tube.
Diseases processes like asthma
Secretions, bronchospasm , kinks in the ET tube
Flow patterns
Lung Characteristics

Disadvantage of higher Raw:
1. More of the pressure for breathing goes to the
airways and not the alveoli (some molecules are
slowed as they collide with the tube and the bronchial
walls – this causes them to exert pressure against the
passages – airways expand and some of the molecules
stay in the airway and don’t reach the alveoli).
2. Less pressure in the alveolus, smaller volume of gas
is available for gas exchange.
Lung Characteristics

Disadvantages continued:
3. With higher resistance, more force needs to be
exerted to try to get the gas flow through the
obstructed airways.
4. For this to happen a spontaneously breathing patient
will use their accessory muscles – this generates
more negative intrapleural pressure and a greater
pressure gradient between the upper airway and the
pleural space to achieve gas flow.
5. The same thing happens on a mechanical ventilator.
Lung Characteristics

More on measuring Raw:
1. Ex. PIP during a mechanical breath is 25 cmH20
and the plateau pressure is 20 cmH20 – the pressure
lost to the airways because of Raw is 5 cmH20.
2. This is normal with a proper size ET tube.
3. Key – the calculation assumes that the flow is
constant and the flow is converted from liters/min.
to liters/sec.
Calculation Time
An intubated, 36-year-old woman is being
ventilated with a volume of .5L (500ml).
The PIP is 24cmH20, Pplateau is 19 cmH20
and baseline pressure is 0. The
inspiratory gas flow is constant at 60
L/min ( 1 L/sec). What are the static
compliance and Raw? Are these values
normal?
Time Constants

Differences in compliance and resistance occur
throughout the lung.

So the characteristics of the lung are
heterogenous – some units may be normal,
others affected by increased resistance,
decreased compliance or both.

This is based on the pathophysiology of the lung.
Time Constants

Differences in C and Raw affect how quickly the lung units fill
and empty.

If a lung unit is stiff (hypocompliant) it fills rapidly.

If pressure is applied to a stiff lung unit for the same time as
to a normal unit, a smaller volume results --- C = V/P.

For the same amount of pressure applied, if compliance is
low, the resulting volume is lower than normal.
Time Constants

If the lung unit is normal, but the airway is very
narrow (increase Raw) the lung unit fills very
slowly.

The gas takes longer to move through the narrow
passage and reach the alveoli.

If the gas flow is applied for the same length of
time as in a normal situation, the resulting volume
is smaller.
Time Constants

Length of time lung units require to fill and
empty can be calculated.

The product of compliance and resistance
is called a time constant.
Calculation Of Time Constant
Time constant = C x R
Time constant = 0.1 L/cmH20 x 1
cmH20/L/sec
Time constant = 0.1 sec.
Time Constant

It expresses the time required for the lung
to fill or empty by a certain amount.

Ex. A patient with a time constant of 0.1
sec. 63% of exhalation (inhalation) occurs
in 0.1 sec – that’s 1 time constant.
Time Constant

2 time constants allow for about 86% of the
volume to be exhaled (inhaled).

3 time constants allow for about 95% to be
exhaled (inhaled).

4 time constants allow 98% of the volume
to be exhaled or inhaled.
Time Constants

5 time constants – the lung is considered to have
exhaled (inhaled) 100% of tidal volume.

In our previous calculation, 5 time constants
would = 5 x 0.1 sec. or .5 sec.

So in half a second a normal lung unit would
empty.
Time Constant

Why is this important:
1. It’s important in setting up a ventilator’s inspiratory
time and expiratory time.
2. An inspiratory time < than 3 time constants can lead
to incomplete delivery of the tidal volume.
3. Increasing the inspiratory time allows even
distribution of ventilation and adequate delivery
of tidal volume.
Time Constants

Why is this important continued:
4. 5 time constants should be used for the IT in
PCV to ensure an adequate volume delivery.
5. ET < than 3 time constants can lead to incomplete
emptying of the lungs – this increases the FRC and
leads to gas trapping in the lungs.
6. The belief is using the 95% to 98% volume emptying
is adequate for exhalation.
7. The key though is careful observation of the patient and
measurement of end-expiratory pressure to decide which time is
better tolerated.
Time Constant

Lung units can be described as fast or
slow.

Fast units have short time constants.

Short time constants are a result of normal
or low Raw and decreased compliance –
ex. interstitial fibrosis.
Time Constant

Fast lung units take < time to fill and empty, but
require more pressure to achieve a normal
volume.

Slow lung units have long time constants, this is a
product of increased resistance or increased
compliance or both – ex. Emphysema.

This units require more time to fill and empty.
Time Constant

Key fact ---- the lung is rarely an even mixture of
ventilating units – some units empty and fill
quickly, others do so more slowly.

The RT determines how most of the lung is
functioning and base treatment decisions on
his/her findings and on patient’s response to
therapy
Types of Mechanical Ventilation

3 basic methods:
1. Negative pressure ventilation – please read in your
book.
2. Positive pressure ventilation
3. High-frequency ventilation – will cover fall.
Positive Pressure Ventilation

This happens when an mechanical ventilator
moves air into the patient’s lungs by way of an
E.T. tube or mask.

At any point during inspiration – the inflating
pressure at the proximal airway = 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

During the inspiratory phase, the pressure
in the alveoli builds and becomes more
positive --- this is transmitted across the
visceral pleura.

Thus the intrapleural space may become
positive at the end of inspiration.
Positive Pressure Ventilation

At end inspiration, the ventilator stops
delivering the positive pressure.

Mouth pressure returns to ambient.

Alveolar pressure is still positive – this
creates a gradient between the alveolus
and the mouth and air flows out.
Definition Of Pressures In Positive
Pressure Ventilation

Baseline pressure:
1. There read from baseline values.
2. Look at figure 2-9 p. 27 – there the baseline pressure
is 0 (atmospheric) – this indicates that no additional
pressure is applied at the airway opening during
expiration and before inspiration.
3. There are times the baseline pressure is > than 0 –
ex. when the RT picks a higher pressure to be
present during exhalation -- this is PEEP.
Definition Of Pressures In Positive
Pressure Ventilation

When PEEP is set – the ventilator prevents
the patient from exhaling to 0
(atmospheric).

It increases the volume of gas left in the
lungs at the end of a normal exhalation.

It increases FRC
Definition Of Pressures In Positive
Pressure Ventilation

PEEP applied by the RT is called extrinsic PEEP.

Auto-PEEP (Intrinsic PEEP) is a complication of
positive pressure ventilation.

Here gas is accidently trapped in the lungs.

This happens when a patient doesn’t have
enough time to exhale before the ventilator gives
another breath
Definition Of Pressures In Positive
Pressure Ventilation



During positive pressure ventilation, you will see
a rise to a peak pressure (Ppeak or PIP or peak
airway pressure.
It is the highest pressure recorded at the end of
inspiration.
This is the sum of 2 pressures – the pressure
needed to force the gas through the resistance of
the airways (PTA) and the pressure of the gas
volume as it fills the alveoli.
Definition Of Pressures In Positive
Pressure Ventilation

Plateau pressure is measured after a breath has
been delivered to the patient and before
exhalation begins.

An inspiratory pause (.5-1.5 sec) to get this
measurement.

With the pause, the pressure inside the mouth
and alveoli are equal (no gas flow).
Definition Of Pressures In Positive
Pressure Ventilation

Relaxation of the respiratory muscles and
the elastic recoil of the lung tissues are
exerting force on the inflated lungs.

This creates a + pressure and reads as a +
pressure on the ventilator.
Definition Of Pressures In Positive
Pressure Ventilation

Plateau pressure is a good estimate of
alveolar pressure (PA).

It reflects the effect of the 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 circuit.