VENTILACI

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VENTILACIÓN PULMONAR
El arte de medicina consiste en entretener al
paciente mientras la naturaleza cura la
enfermedad. Voltaire (1694-1778)
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Los doctores son hombres que prescriben medicinas
que conocen poco, curan enfermedades que conocen
menos, en seres humanos de los que no saben
nada. Voltaire (1694-1778)
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La función primordial de los pulmones consiste en
garantizar un intercambio de gases adecuado para
las necesidades del organismo, de forma que el
aporte de O2 necesario para las demandas
metabólicas de los tejidos y la eliminación de
CO2 de los mismos se lleven a cabo correctamente
de forma simultánea. Estos dos gases son, junto
con el nitrógeno (N2), los tres elementos
esenciales con los que el pulmón trabaja
constantemente. La presión parcial (P) que ejerce
cualquiera de estos tres gases fisiológicos en
los 300 millones de unidades alveolares
funcionales que constituyen el parénquima
pulmonar viene regulada por tres factores
principales
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1.- La composición del gas inspirado,
dependiente de la fracción inspiratoria de O2
(FIO2) y de la ventilación alveolar (VA) 2.-
Los cocientes o relaciones ventilación perfusión
(VA/Q) pulmonares y 3.- La composición de la
sangre venosa mixta (mezclada) (v), dependiente a
su vez del flujo sanguíneo o gasto cardiaco (QT)
del organismo y del consumo de O2 (VO2) tisular.
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ELEMENTOS DE LA RESPIRACIÓN
Inspiración Espiración - Activa
- Pasiva - Presión Negativa -
Retracción - Expansión de
elástica cavidad torácica y diafragma
RESPIRACIÓN Anatomía - Pared
torácica Ley de Boyle - Mús. Resp. - ?
presión - Diafragma - ? volumen
- Cav. Torácica
(P1 x V1 P2 x V2)
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El flujo de aire se debe a cambios en la presión
alveolar
Los músculos respiratorios se contraen
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Presiones Pulmonares

• PL Presión Transpulmonar P. Alveo. - P.
  Intrap.
• PT Presión Transtorácica P. Intrap. - P.
  Atm.
• PR Presión Respiratoria P. Alveo. - P. Atm.

Presión Atmosférica
PL
Presión Intrapleural
PT
Presión Alveolar
PR
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La presión intrapleural siempre es negativa
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Transpulmonary pressure is the difference between
the alveolar pressure (760mmHg) and the
interpleural pressure (756mmHg). At rest the
forces tending to collapse the lung, surface
tension and tissue elastic recoil, are pulling
the lungs away from the chest wall. Since the
interplural space is filled with fluid, it only
slightly expands. This small increase in size
reduces the interplural pressure below
atmospheric. Thus at rest the lungs are being
held against the chest wall by a small vacuum.
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Presión alveolar
Presión intrapleural
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Pequeñas Presiones!!
1cmH20 0.735mmHg 1mmHg 1.35 cmH2O
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Este módulo explica lo que sucede durante la
respiración en reposo. Examinaremos el ciclo de
la respiración en 5 períodos 1.- Reposo, 2.-
Durante la inspiración, 3.- Final de la
inspiración (equilibrio), 4.- Durante la
espiración, y 5.- Final de la espiración. Mecáni
ca se refiere al estudio de los aspectos
mecánicos de la respiración (en oposición a los
químicos o biológicos), en cuanto a la
interacción de presión, volumen, y flujo aéreo
dentro del sistema respiratorio. El siguiente
gráfico mostrará en que parte del ciclo estamos.
Observe como se mueve la bola roja a través del
ciclo inspiratorio y espiratorio. La escala de
abajo a la derecha correlaciona los cambios de
presión con los colores utilizados en la
ilustración. Observe como se mueven las flechas
que representan las presiones alveolar y pleural.
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En este modelo, el color púrpura indica presiones
subatmosféricas (negativa), y el verde presiones
mayores que la atmosférica (positivas) a color
más intenso más lejos se estará de la presión
atmosférica, a más pálido, más cercano a la
presión atmosférica.
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I- En Reposo (Después que finaliza la
espiración y antes de comenzar la
inspiración.) 1. Con los músculos respiratorios
en reposo, la retracción elástica del pulmón
(PalvgtPpl5) y de la pared torácica (PplgtPbs-5)
son iguales pero opuestas. 2. La presión pleural
es subatmosférica (fíjese en el color púrpura del
espacio pleural). 3. La presión a lo largo del
árbol traqueo- bronquial y en el alveolo es igual
a la presión atmosférica. No existe flujo de
aire. 4. El aire solo se moverá de una zona
de alta presión a una de baja presión. Dado que
la presión alveolar iguala la presión atmosférica
no existe flujo aéreo.
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II- Durante la Inspiración 1. Se contraen el
diafragma y otros músculos respiratorios. 2.
Dado que el diafragma es curvo, su contracción
comprime el contenido abdominal y descomprime el
contenido torácico, produciendo una caída de la
presión pleural. 3. Dado que el volumen inicial
del pulmón no cambia, su presión de retracción
(Palv - Ppl), tampoco se modifica ya que es
volumen depen- diente. De esta manera, la presión
pleural disminuye, y la presión
alveolar disminuye de igual manera, volviéndose
subatmosférica. 4. El flujo aéreo hacia los
pulmones sigue el gradiente de presión desde
la boca hacia los alveolos. 5. Los pulmones y
pared torácica se expanden en volumen,
produciendo un aumento de la presión de
retracción de los pulmones hasta que se alcanza
el equilibrio nuevamente.
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III- Fin Inspiración 1. Existe un equilibrio
después de finalizar la inspiración y antes de
que comience la espiración. 2. El flujo aéreo
desciende según el gradiente de presión hasta que
el pulmón alcanza un nuevo volumen de equilibrio
al cual la presión alveolar se iguala a cero y el
gradiente de flujo deja de existir. 3. Pulmones
y tórax están totalmente expandidos.
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IV- Durante Espiración 1. La relajación de los
músculos respiratorios, producen un aumento
abrupto de la presión pleural a un valor menos
negativo. 2. Dado que el volumen pulmonar aún no
ha cambiado, la presión de retracción del pulmón
sigue siendo la misma, de esta manera el aumento
de la presión pleural produce el mismo aumento de
la presión alveolar. 3. Esto establece un
gradiente de presión desde el alveolo hacia
la boca, a través del cual el aire fluye. 4. Los
volúmenes pulmonar y torácico disminuyen a medida
que el aire fluye hacia afuera, de tal manera que
la presión de retracción pulmonar caiga
también, hasta que se alcanza un nuevo
equilibrio a la CRF, el volumen de equilibrio.
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V- Al Final de la Espiración 1. La cavidad
pleural y el alveolo vuelven a tener la relación
de presión que tenían al comienzo de la
inspiración 2. La presión pleural es -5 y la
presión alveolar es 0.
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Summary
The graph shows temporal changes in each index
within one respiratory cycle. The inflow during
inspiration is shown in negative value, and the
outflow during expiration is shown in positive
value.
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Summary
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When inspiratory muscles, such as the diaphragm,
contract, the thoracic cavity and the lungs
increase in volume. The red line shows the lungs
and the outer grey line shows the chest wall. The
cavity inside the chest wall is the thoracic
cavity. The diaphragm is at the bottom of the
thoracic cavity. Red indicates that the diaphragm
is contracting.When the diaphragm contracts,
the thoracic cavity increases in volume and
inflates the lungs. This process is the
inspiration. The volume of gas inside the lungs
is called 'lung volume'. With diaphragm
contraction, the lung volume increases.
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When inspiratory muscles, such as the diaphragm
relax, the thoracic cavity and lungs decrease in
volume. Blue indicates that the diaphragm is
relaxed. With diaphragm relaxation, the lung
volume decreases. This process is the
expiration.The decrease in volume is due to the
elasticity of the lungs, which will be discussed
later.
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When the lungs are increasing in volume, the
pressure inside the lungs (alveolar pressure)
becomes negative compared to the atmospheric
pressure. Due to the negative pressure, gas
(airs) flows into the lungs. When the lungs
remain increased at a constant volume, the
negative pressure disappears and the flow ceases.
The yellow pressure gauge indicates the
pressure inside the lungs. The pressure inside
the lungs are also called alveolar pressure, to
specify anatomic location. When the lungs are
increasing in volume, the pressure inside the
lungs (alveolar pressure) are negative compared
to the atmospheric pressure. Note that the yellow
pressure gauge is elevated on the lung side,
compared to the outer-side, which is open and
thus indicates the atmospheric pressure.
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When the lungs are decreasing in volume, the
pressure inside the lungs (alveolar pressure)
becomes positive compared to the atmospheric
pressure. Due to the positive pressure, the gas
inside the lungs flows out. When the lungs remain
decreased at a constant volume, the positive
pressure disappears and the flow ceases. When
the lungs are decreasing in volume, the pressure
inside the lungs (alveolar pressure) are positive
compared to the atmospheric pressure. Note that
the yellow pressure gauge is lowered on the lung
side, compared to the outer-side, which is open
and thus indicates the atmospheric pressure.
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The lungs have inward elasticity. This works
towards decreasing the lung volume. The larger
the lung volume, the larger the elastic force.
Let's suppose that there are 'rubber bands'
inside the lungs, and that they are lightly
stretched. When the lung volume is increased, the
'rubber bands' are strongly stretched.
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• The water between the chest wall and lungs,
  keep the lungs from being pulled away from the
  chest wall. Because of inward elastic force of
  the lungs and the outward elastic forces of the
  chest wall, the pressure between the lungs and
  the chest wall is negative.
• The chest wall has outward elasticity, which
  (when the respiratory muscles are relaxed and
  there is no respiratory movement) balances with
  the inward elasticity of the lungs. The pleural
  pressure, which is the pressure between the lungs
  and the chest wall, is negative (note that the
  blue pressure gauge on the lung side is
  elevated).
• There is water between the chest wall and lungs.
• The water keep the two from being apart.

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When the thoracic cavity increases in volume
with inspiration, the negative pleural pressure
becomes larger. With inspiration, the lung
volume increases and the inward elastic force of
the lungs increases. To counteract this force and
increase the lung volume, the chest wall exerts a
larger outward force. This makes the negative
pleural pressure larger.
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When the respiratory muscles are relaxed and
there are no respiratory movement... the
inspiratory muscles such as the diaphragm are
relaxed and the lung volume (within a quiet
breath) is at its minimum. The inward elasticity
of the lungs and the outward elasticity of the
chest wall are balanced. The pleural pressure is
negative. The pressure inside the lungs (alveolar
pressure) is equal to the atmospheric pressure
and there is no gas (air) flow. Since the lung
volume (within a quiet breath) is at its minimum,
the lung elastic force and the pleural pressure
are also at their minimum.
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At the beginning of inspiration... the
diaphragm begins to contract (note that it turned
red!). There still are no changes in pressure,
lung volume, etc.
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During inspiration... inspiratory muscles,
such as the diaphragm, contract, expanding the
chest wall and thoracic cavity. Because of the
water, the lung volume is also increased. Since
this is in the opposite direction of the lung
elasticity (note that the 'rubber bands' are
further stretched), the negative pleural pressure
becomes larger (note the blue pressure gauge).
With lung volume increasing, the pressure
inside the lungs (alveolar pressure) become
negative, compared to the atmospheric pressure,
causing gas (air) inflow into the lungs.
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At the end of inspiration... the lung volume
its reaches maximum (within a quiet breath). The
lung elasticity reaches its maximum (note that
the 'rubber bands' are strongly stretched), thus,
the negative pleural pressure reaches its maximum
(within a quiet breath). The lung volume cease
increasing. Without lung movement, pressure
inside the lungs (alveolar pressure) is equal
to the atmospheric pressure and the air inflow
(inspiration) ceases.
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At the beginning of expiration... the
diaphragm relaxes (note that it has turned blue).
There still are no changes in pressure, lung
volume, etc.
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During expiration... the inspiratory muscles
are relaxed and thus, there are no force
expanding the chest wall nor thoracic cavity.
Thus, due to the elasticity of the lungs, the
lungs and thoracic cavity decrease in volume.
Naturally, the negative pleural pressure is
smaller than at the end of inspiration (beginning
of expiration). Because the lungs decrease in
volume, the pressure inside the lungs (alveolar
pressure) become positive compared to the
atmospheric pressure. The positive pressure
pushes out the gas inside the lungs (because of
O2 and CO2 exchange with the blood, it is no
longer 'air') outward through the trachea
(expiration).
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At the end of expiration... the chest wall has
outward elasticity. When the inward elasticity of
the lungs balances with the outward elasticity of
the chest wall, expiration ends. Naturally, the
negative pleural pressure and lung volume is at
their minimum (within a quiet breath). When the
lung volume cease to decrease, pressure inside
the lungs (alveolar pressure) become equal to the
atmospheric pressure and the gas outflow
(expiration) ceases. This is the same
illustration as in "quiet breath When the
respiratory muscles are relaxed and there are
no respiratory movement". Repeatedly, the
diaphragm contracts and the cycle is repeated.
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Changes in lung volume, air flow, intrapleural
pressure, and alveolar pressure during normal
(tidal) breathing. The dashed intrapleural
pressure line would be followed if there were no
airway resistance). The diagram at the left shows
the lung and a spirometer measuring the changes.
This figure depicts changes in the main
parameters of interest during a normal
ventilatory cycle in which a Tidal Volume of 400
- 500 ml is being inspired and expired with each
breath (quiet breathing). Inspiration is
indicated by a downward deflection in the first
panel. Note that during the entire ventilatory
cycle, the intrapleural pressure (Ppl) remains
negative (2nd panel). The plot normally follows
the solid blue line alternatively it follows the
dashed line when where there is no resistance to
air flow (imaginary situation). Note that
alveolar pressure (bottom panel) has a non-zero
value only when there is air flowing.
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Effect on distribution of ventilation due to
inspiration from FRC (panel A) vs. starting from
RV (panel B).
Because of the way in which the lungs are
suspended in the chest cavity and are subjected
to gravity, a gradient in pleural pressure exists
from the apex (top, blue) to the base, of about
7.5 cm H2O. If a subject starts his inspiration
from Functional Residual Capacity (FRC) (apex
-10, base -2.5 both in cm H2O), apical alveoli
will start off at a larger volume than the basal
ones due to the more negative apical pressure.
Thus apical alveoli will be less able to
accommodate further increase in volume (i.e. they
are already stretched and thus less compliant
than basally). Thus, if inspiration starts from
FRC, most of the incoming air goes preferentially
to basally located alveoli which began less
distended and thus more compliant. If inspiration
starts from RV (following a very energetic
expiration to get below FRC), all the incoming
air goes initially to apical alveoli where
pleural pressure is still negative (-4 cm H2O).
Basal areas are experiencing a positive pleural
pressure of 3.5 cm H2O, which causes the
collapse of small airways. Thus air inflow is
initially prevented in that area until basal
pressures become negative again and allow airway
opening.
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