Transcript CPAP an Introduction

CPAP an Introduction
Joe Lewis, M.D.
City and County of Honolulu EMS
What is CPAP ?
Continuous
Positive
Airway
Pressure
Background Information
• In order to understand how CPAP works a little
background information is needed.
• While some of this some anatomy and
physiology of normal lung will seem like review,
bear with it as it will make how CPAP works
crystal clear to you.
• Since the terms used can have different
meanings in different situations they are defined
for this situation now, so their meaning in this
situation is clear, as that is essential to
understanding CPAP.
Background concepts
•
•
All the body cells metabolically consume
oxygen, and discharge carbon dioxide. To
cover this need, respiration takes place in
the lungs and we call it ventilation or
breathing.
Ventilation involves the inhalation of
atmospheric air into the lungs via the nose
and mouth through branching passageways,
and the exhalation of carbon dioxide.
Background concepts
Respiration-the exchange of oxygen
and carbon dioxide between the
atmosphere and the body cells.
This occurs at the level of the cells
and in the alveoli.
Diffusion - the movement of gas from
an area of high concentration to
an area of lower concentration.
Lung Anatomy & Gas Exchange
• The key function of lung is to bring air
and blood into intimate contact in the
alveolar air sacs so that oxygen can
enter the blood, and carbon dioxide
can leave.
Lung Anatomy and Physiology
• Gas exchange occurs in the lungs
between the alveoli and a capillary
network within the alveolar wall.
• Capillaries are microscopic blood vessels
that exchange material between the blood
and body tissues.
Pulmonary Anatomy
– Alveolus-smallest anatomic part of the lung,
where gas exchange takes place between the
oxygen rich atmospheric air and the oxygen
poor blood in the pulmonary capillary.
– Alveolar wall-the physical structure which
separates the alveolus from the interstitium.
– Interstitium the loose tissue separating the
alveoli and the alveolus from the pulmonary
capillary.
Pulmonary Anatomy
• Pulmonary capillary wall-the wall
separating the oxygen poor, carbon
dioxide rich pulmonary capillary blood from
the interstitium.
• Pulmonary capillary-the smallest blood
vessel in the lung where the exchange of
oxygen and carbon dioxide takes place.
The lung is made of millions of alveoli
The Alveolus
The Alveolus and blood supply
Red Blood Cell in capillary in between alveoli
Electron Microscope picture
Oxygen diffusing across the capillary wall and Interstitium to the red blood
cell & carbon dioxide diffusing across same structures to the alveoli
Pulmonary Physiology Terms
• Oxygen concentration- the percentage of
oxygen in the air presented to the
alveolus.
• Partial pressure of oxygen-the
atmospheric pressure on the oxygen
delivered to the alveolus
Pulmonary Physiology Terms
• Ventilation- the mechanical exchange
of air between the lungs and the
atmosphere.
• Alveolar ventilation – the exchange of
air between the alveoli and the
atmosphere.
Pulmonary Physiology Terms
• Respiration-the exchange of oxygen and
carbon dioxide between the atmosphere
and the body cells. This occurs inside cells
and in the alveoli.
• Alveolar respiration-the exchange of
oxygen and carbon dioxide between the
atmosphere and the alveoli.
Pulmonary Physiology
Alveolar Respiration
Alveolar respiration or the exchange of
oxygen and carbon dioxide occurs due two
things:
1. A concentration gradient or difference.
2. A pressure gradient or difference.
Pulmonary Physiology
Concentration gradient
• Oxygen moves or “diffuses” from the
oxygen rich air in the alveoli to the oxygen
poor blood in the alveolar capillary.
• It follows a “concentration gradient” which
means the oxygen moves from a high
oxygen area to a low oxygen area.
Pulmonary Physiology
• When we administer high flow
oxygen, like 15 liters by mask, we
increase the concentration of oxygen
in the alveolus. This increases the
concentration gradient and this
makes more oxygen diffuse or move
into the capillary and therefore the
blood.
Pulmonary Physiology
• Partial pressure of oxygen-the atmospheric
pressure on the oxygen delivered to the
alveolus.
• The atmospheric pressure at sea level is one
atmosphere or 760 mm of mercury.
• The partial pressure of a gas is calculated by
multiplying it’s concentration by the atmospheric
pressure.
• The oxygen concentration in air is 21%, so 760 x
21% gives a partial pressure of oxygen of 159.6.
Pulmonary Physiology
Pressure gradient:
The pressure of the atmosphere on oxygen, also
called the “partial pressure” of oxygen is higher
in the alveolus then the partial pressure of
oxygen in the capillary blood.
This pressure difference creates a “pressure
gradient” similar to the “concentration gradient”
we discussed before, which makes oxygen
“diffuse” or move from the alveolus air into the
blood in the capillary surrounding the alveolus.
Pulmonary Physiology
• When we administer CPAP, or “continuous
positive air pressure” we are adding pressure to
the atmospheric pressure on the oxygen.
• This increases the “partial pressure” of oxygen
in the alveolus.
• This then increases the pressure difference
between the oxygen in the air and the oxygen in
the capillary blood around the alveolus.
• This increase in the pressure difference
increases the “pressure gradient” and makes
more oxygen “diffuse” or move into the capillary
blood around the alveolus.
Pulmonary Physiology
CPAP takes advantage of the two physical
forces which improve alveolar capillary
blood oxygenation;
• Concentration and
• Pressure
This is why CPAP is referred to as oxygen
delivery at it’s highest or most efficient
form.