Gas exchange; ventilation and perfusion relationship: Johnson Flashcards
There has to be a relationship between ventilation and perfusion for what to occur?
gas exchange
so gas exchange CANNOT if either ventilation or perfusion is missing
What is the goal of the respiratory system?
to ensure that ventilation and perfusion are matched as much as possible for efficient gas exchange
What is the major determinant of gas exchange in the pulmonary system?
the ratio of ventilation to perfusion
What is the most common cause of arterial hypoxemia?
mismatching of ventilation and perfusion
the PAO2 is 100 but PaO2 is less than 100
When the ratio of ventilation to perfusion is greater than 1, what does that mean?
ventilation exceed perfusion
The best gas exchange occurs where in the lungs?
at the base
- the best blood flow and ventilation occurs at the base of the lung
- the alveoli at the base has better compliance and started with a lower air volume; gravity causes blood flow to be highest at the base
How does the ratio of V/Q change from base to apex of lung?
base: lowest ratio
apex: highest ratio
Why can account for O2= 150 torr and CO2= 0 torr?
airway is completely open but there is a blood clot and the perfusion is blocked
- CO2 cannot equilibrate with the alveolar air and be expelled
- O2 cannot mix with the blood as the O2 cannot be extracted from the air if the blood cannot come in contact with the air
If partial pressures of CO2 and O2 mimic their respective values in alveolar air pressure, what does this means?
there is no perfusion
If partial pressures of CO2 and O2 mimic their respective values in mixed venous blood, what does this means?
absent ventilation
What are physiological shunts? What is the V/Q ratio for shunts?
- complete blockage of airway, all ventilation is diverted to adjacent alveolus
- perfusion is equally distributed between both units
- this results in mixed venous blood with no gas exchange
V/Q=0
What occurs during emphysema?
- lung parenchyma loses its ability to recoil: a huge increase in compliance (reflective through the large alveolar air spaces)
- destruction of blood capillaries
- very poor gas exchange
have regions of lung that have normal V/Q ratio and other regions where there are high V/Q around 10
What occurs during chronic bronchitis?
Disease of the airway
Excessive inflammation and obstruction to airflow
Associated with smoking
- some region in the lung that have ability to perform normal lung function (normal v/Q ratio)
- some other regions have limited gas exchange due to low amounts of ventilation even though there is high amount of perfusion causing problems with gas exchange
Regions where you cannot have gas exchange is physiologically similar to what other space in the lung?
dead air space
What is hypoxia vasoconstriction?
- causes the vasculature in pulmonary side to constrict leading to pulmonary HTN and can also serve as a an autoregulatory mechanism
- it has the ability to divert blood to areas where ventilation is available (where alveoli have O2) so in this way the pt has compensatory mechanism that kicks in to help him or her to produce the best possible gas exchange and bringing V/Q to normal ratio
Does a normal V/Q value always indicate normal lung?
NO because there are compensatory mechanisms
High V/Q ratio are typically found where in the lung? Low V/Q ratio?
- High V/Q: apex
- Low V/Q: base
High PO2 environment is found where in the lung?
apex
-this can affect the distribution of pulmonary tuberculosis
High blood perfusion occurs where in the lungs?
base
Describe the blood perfusion in Zone I, II, and III due to gravitational forces.
Zone I: very little blood perfusion; right underneath the heart
Zone II: has blood that is most reflective of PaO2
Zone III: best blood flow; follows a pressure gradient; no collapse capillaries
What is the alveolar-arterial difference (AaDO2)? What difference would a perfect lung have? What about a normal individual?
refers to the difference between the partial pressure in the alveolar air and in the arterial blood
perfect lung would have a different of 0
normal lung should be <15 mmHg; <25 is the upper limit of normal
An increase in AaDO2 is the hallmark of what?
inefficient or abnormal gas exchange
What does PaO2 mean in an arterial blood gas?
PaO2 is the amount of O2 dissolved in the plasma not the O2 bound to hemoglobin
What are some factors that contribute to AaDO2????
venous admixture due to anatomic shunt ??
Some bronchial and mediastinal veins drain into pulmonary veins (bronchopulmonary anatomoses) that result in venous admixture.
Oxygenated and deoxygenated blood mixing causes a reduction in the PaO2
PAO2= 77 mmHg PACO2= 45 mmHg
Would you expect hypoventilation or hyperventilation?
hypoventilation
CO2 too high and O2 is too low
What is atelectasis?
when you partial collapse of the alveoli; there are patches of alveoli with no O2 and as a result that impacts gas exchnage losing the ability for efficient gas exchange
these patches could be due to mucus plug, airway edema, foreign body, tumor in the airway)
How can congenital heart defects, anatomic shunts, tetralogy of Fallot, and Guillain-Barre syndrome induce hypoxia?
congenital heart defects cause cyanosis (bluish discoloration of lips and fingers) which is characteristic of hypoxia
Anatomic shunts (hypoxemia)
Tetralogy of Fallot: pulmonary valve stenosis, ventricular septal defect (hole between right and left ventricles) and overriding aorta
Guillain-Barre
syndrome: diaphragm weakness cannot allow efficient pressure gradient for air to flow into the lung in the first place for gas exchange to even occur
Hallmark of hypoventilation is a normal AaDO2. Gas exchange and perfusion to the alveolus are normal; that is, the lung is functioning normally.????
?????
Anesthesia and sleeping pills can cause what?
hypoventilation and thus increased PaCO2/ PACO2 and decreased PaO2/PAO2
For the different causes of hypoventilation, what should be done?
fix the underlying cause to restore gas exchange because the lung itself is normal
