physiological shunt

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anatomic shunting disease

the flow of blood from the right ventricle to the left side of the heart without going through the pulmonary capillaries

3 disease conditions

-CHD congenital heart disease
-intrapulmonary fistula
-vascular lung tumor

whats Congenital heart disease

any congenital heart defect that results in a right to left flow of blood

whats intrapulmonary fistula

may be congenital or may result from chest trauma or disease

whats vasculat lung tumor

a lung tumor can become very vascular resulting in pulmonary arterial blood passing through the tumot mass and into the pulmonary veins without coming into contact with alveoli

whats acute atelectasis

the most common clinical cause of absoilute shunting(true)
alveolar fuid

cause of acute atelectasis

space occupying lesions of the chest which press on lung tissue and cause compresiion atelectasis
-pneumothorax
-hemothorax
pleural fluid (effusions)
~total obstruction of brochi and or bronchioles (mucus plugging)
causes absortion of the distal alveolar air
absorption atelectasis
~Microatelectasis= the random collapse of alveoli resulting from constant positive pressure ventillatin or increased tension (ARDS)

Alveolar fluid usually due to

alveolar abcess
cardiogenic pulmonary edema
pneumonia
pooled secretions

shunt effect venous admixture
aka

perfusion in excess of ventilation
aka ventilation/ perfusion inequality
most readily correctable by 02 therapy

alveolar hypoventilation common cause of

hypoxemia.
results in decrease alveolar gas delivery
less than normal oxygenation of the blood that is exchanging with alveolar air

uneven distribution of ventilation is common cause of

alveolar hypoventilation
the underventilated alveoli have a decrease in oxygen tension Pa02
this occurs in almost all pulmonary diseases
this is a primary reason for hypoxemia in COPD

diffusion defects

the alveolar capilary membrane is altered so diffusion of all gas is impaired
the blood passing by the alveolus does not have time to equilibrate eith the alveolar oxygen
causes an increas in the alveolar- arterial oxygen difference A-a ration
difficult to differentiate a diffusion defect from a shunt effect because diffusion tests may not be able to be performed on critically ill patients

cardiac output=

HR X SV

total cardiac output can be divided into 2 major cmponents

QC_ capillary portion
QS_ shunted portion

QC is the portion that.....

exchanges perfectly with the alveolar air
contains an end pulmonary capillary oxygen content with a saturation of 100 CC 02

not a measured parameter, it is a mathmatical calculation based on theoretical concepts

QS is the portion of th CO that

does not exchange at all with alveolar air
this is unchanged from the value in the pulmonary artert

END CAPILLARY O2 CONTENT

we assume that the highest level of o2 in the pulmonary capillaries
so we assume that the SO2 is 100%
PAO2 is calculated from the alveolar air equation=
Cc02=(hb x 1.34 x 1.00) + (PAO2 x 0.003)

shunt equation

expressed the relationship between the cardiac output and the shunted cardiac output
Qs/Qt = cco2-ca02/ cc02-cvo2 x 100

normal lung shunt=

equals <10 %

10-19% shunt=

denotes an intrapulmonary abnormality that is seldom of clinical significance in terms of maintaining respiratory homeostasis

20-29% shunt=

reflects significant intrapulmonary disease, in a patient with limite cardiovascular or cns function, this degree of shunt may be life threatening

>30 %

potentially life threatenting and usually requires aggressive cardiopulmonary support therapy
changes in CO cardiac output alone will affect the intrapulmonary shunt calculation

whats alveolar ventilation

the portion of inspired ait that reached the alveoli that is effective in gas exchange

whats deadspace?

the portion of inspired air that does not reach the alveolo which does not participate in gas exchange

what are 3 types of deadspace?

antomic
alveolar
physiologic= anatomic+ alveolar

anatomic deadspace

the volume of gas in the conducting zones
inclufr nose, mouth, larynx,, pharynx and lower airways down to but not including resp broncholes

anatomic deadspace equal to

1ml/ lb of ideal body weight

alveolar deadspace occurs when

the alveolus is ventilated but not perfused with pulmonary blood flow
-the air that enters the alveolus is not effective in terms of gas exchange
-there is no pulmonary capillary blood flow
-the amount of alveolar deadspace is unpredictable and cannot be measured directly

whats physiologic deadspace

the sum of anatomic deadspace and alveolar deadspace
- bc neither of these forms of deadspace is effective for gas exchange the two forms are combined and referred to physiologic deadspace
- we can calculate ohysiologic deadspace as
anatomic deadspace plus alveolar deadspace=physiologic deadspace

calculating physiologic deadspace

VD/VT= PaCO2-PeCO2 / PaCO2

co2 in blood is

always higher than exhaled co2
blood is not getting to alveoli to unload co2 into the lungs

conditions that increase the v/q ratio >0.8

deaspace or wasted ventilation
which are pulmonary emboli
partial or complete obstruction in the pulmonary artert or some of the arterioles
-pressure outside the pulmonary vessels
pneumothoraz hydrothoraz tumors
-destruction of the pulmonary vessels- emphysema
-decreased cardiac output
-PEEP levels too high

conditions that decrease the v/q ratio <.08

=decresed ventilation
=shunted blood
-atelectasis
-obstructive lung disorders like emphysema, chronic bronchitits
-restrictive lung disorders like pneumonia, silicosis, pulmonary fibrosis
-hypoventilation from any cause

shunt vs deadspace

shunt= perfused adequate, ventilation decreased and is calcualted using 02 values

deadspace= ventilation adequate, perfusion decreased and is calculated using c02 values`

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