57: Ventilation Perfusion Flashcards
Alveolar Ventilation (VA)
4L/min at rest
Perfusion (Q)
cardiac output 5L/min
Alveolar- arterial oxygen (A-a O2) gradient
5 to 15 mm
Hg is due to venous admixture
Hypoxemia
PaO2 below 85 mm Hg
Increased venous admixture results in low O2 tension in
the blood
Dead Space
V(D)
Volume of inhaled air that does not
participate in gas exchange
Anatomic dead
space
Conducting airways
150 mL
Alveolar dead
space
Alveoli with no perfusion
Physiologic Shunt
total venous
admixture
Cause 1 of Physiologic Shunt?
Anatomic shunt
right –to –left shunt
ex: bronchial circulation
Cause 2 of Physiologic Shunt?
50% from low VA/Q
ex: partially obstructed airway
“wasted air”
all inspired air
does not participate
in gas exchange
anatomic + alveolar = physiologic dead
space
“wasted blood”
all blood entering the lung is not fully oxygenated
anatomic + low (VA/Q) ratio = physiologic shunt
Lung is slinky
top: coils are pulled apart, weight pulling down on it
middle: coils tighter than the top, half of weight pulling down on it
bottom: coils are tighter, little weight pulling down on it
* lungs rest on abdomen
Pressure Volume at FRC
apex less compliant than base
at rest: apex 70% and base 15% distended
What is the change in volume at apex during inspiration?
Significantly less than base
ventilation gradient aligned with gravity
Pulmonary circulation
high-flow,
low-pressure, low-resistance system
Is the base or apex of the lung more compliant?
The base of the lung
There is more resistance to blood flow toward the apex of the lung b/c upward flow runs against…….
Hydrostatic pressure
High alveolar volume in the apex reduces ,,,,,,,,
Blood flow (this restriction is not present when alveolar pressure falls below pulmonary arterial pressure)
Apex of Lung: Ventilation
- Intrapleural pressure more………..
- Greater transmural pressure gradient
- Alveoli ………….
- Less ventilation
Negative
Larger, Less compliant
Apex of Lung; Perfusion
- Low Intravascular pressures
- …….. Recruitment and Distention
- …….. Resistance
- Less blood Flow
Less Recruitment
Higher Resistance
Base of Lung: Ventilation
- Intrapleural pressure less negative
- ……….. Transmural Pressure Gradient
- Alveoli…………
- More Ventilation
Smaller Transmural Pressure Gradient
Alveoli smaller, more compliant
Base of Lung: Perfusion:
- ………. Vascular Pressures
- ……….. Recruitment and Distention
- Lower resistance
- Greater Blood Flow
Greater vascular pressures
More Recruitment
………….. and ……… are both gravity dependent
Ventilation and Blood Flow
Ventilation-perfusion ratio ……..down the lung
Decreases
……….. is more efficient at the apex of the lung as compared to the base.
Gas Exchange
For low V/Q ratio (lots of blood or too little ventilation); causes the blood coming into the area to be directed to other parts of the lung.
Hypoxic vasoconstriction
for high V/Q ratio, the bronchi will constrict slightly to increase the resistance and decrease the amount of ventilation coming into an area that is not well perfused thus limiting the amount of alveolar dead space
Bronchoconstriction
The patient with a V/Q = 0 may most likely not benefit from supplemental oxygen because there is ………
No ventilation
Regional low VA/Q ratio causes A-a O2 gradient to …..
Increase
An Anatomic shunt causes A-a O2 gradient to …..
Increase
Diffusion block causes A-a O2 gradient to …..
Increase
How does hypoventilation effect A-a O2 gradient?
No Change
If the patient has hypoxemia and PaCO2 is less than 40 and A-a is outside of the 5-20 mmHg points to …….
Hypoventilation
…….. has high alveolar pressure –which increases resistance in pulmonary circulation
Apex of lung
V/Q = 0 means
Perfusion of an area with no ventilation i.e. a shunt
V/Q = infinity means
Dead space –ventilation with no perfusion
The healthy lung has ……. the majority of respiratory segments blood flow (by controlling pulmonary capillary flow) to ventilation –and therefore have a V/Q of ~1
Matched
V/Q mismatch always ……the oxygen content of the venous admixture and increases the A-a gradient.
Lowers
not necessarily hypercapnia though
↑V/Q = >1 =
↑PO2 ↓PCO2
V/Q = 1 =
PO2 100, PCO2 40
↓V/Q = <1 =
↓PO2 ↑PCO2
With high altitude, alveolar PO2 ……… which results in decreased arterial PO2 (hypoxemia)
Decreases
………. is stimulated by the hypoxemia experienced by carotid/aortic bodies
Ventilation
Minute alveolar ventilation ……….in an attempt to restore blood oxygen levels
Increases
During a rapid Altitude Increase, leads to:
- Hypoxemia –stimulated ventilation
- PCO2 declines producing resp. alkalosis
- These tend to counter the increase in ventilation due to hypoxia
- Over 2-3 days –blood pH is corrected (acidified) by renal excretion of HCO3-. CSF also returns to normal with HCO3excretion.
- Full hypoxic ventilatory drive is then restored
How can polycythemia be caused by hypoxemia?
Low PO2 of kidney stimulates the release of erythropoietin –which in turn stimulates bone marrow to increase RBC production
During mountain sickness, the pulmonary hypoxic vasoconstriction ……. pulmonary vascular resistance
–……… hydrostatic pressure in the pulmonary circulation
Increases
Increasing
During mountain sickness, ………sympathetic stimulation and cardiac output due to arterial chemoreceptor stimulation
Increase
…….. is sensitive to hypocapnea –it is a strong cerebral vasoconstrictor
Cerebral Circulation
Common symptom of altitude sickness
Headaches, mental confusion
Low PO2 leads to cerebral vessel ……….. and hyperperfusionof the vessels. Increases the likelihood of cerebral edema
Vasodilation
