Ch. 5 West Flashcards
T/F: The alveolar PO2 is largely determined by the level of alveolar ventilation
True
The PO2 of inspired air is, at sea level, with a body temperature of 37*C and a water vapor pressure of about 50 mmHg is _______ mmHg
150 mmHg
Five causes of hypoxemia:
Low inspired FiO2
Hypoventilation
Diffusion impairment
R-to-L shunt
V/Q mismatch
Alveolar PO2 is determined by a balance between the rate of _____ of O2 by the blood, and the rate of ____ of the O2 by alveolar ventilation.
Rate of removal of O2 by the blood
Rate of replenishment of O2 by alveolar ventilation
Based on the relationship between ventilation and CO2 level seen in the alveolar ventilation equation, decreasing the ventilation by 50% will result in the PCO2 _____ under steady state conditions.
Doubling
In the alveolar gas equation, “R” represents the respiratory quotient. Explain what that is.
The respiratory exchange ratio, i.e. the ratio between CO2 production and O2 consumption, which is determined by the metabolism of the tissues in a steady state and varies based on the balance of fuels consumed (carbs, fats, proteins etc.).
Alveolar gas equation
P(AO2) = P(IO2) - [P(ACO2)/R]
Normal value of R is about 0.8
Shows that the normal fall in alveolar PO2 is slightly greater than the rise in PCO2 during hypoventilation
Why does alveolar CO2 take longer to reach equilibrium if a patient goes from hypoventilation to hyperventilation?
Because the body stores of CO2 are pretty high in the form of bicarb and therefore it takes longer to come to equilibrium
T/F: Hypoventilation always increases the alveolar and arterial PCO2
True
T/F: Hypoxemia associated with hypoventilation is rectified by adding additional oxygen to the inspired gas.
True
Why does diffusion limitation rarely cause hypoxemia?
Because the red blood cells spend enough time in the pulmonary capillary to allow nearly complete equilibrium with the alveolar O2.
What does the term “shunt” indicate as pertains to pulmonary circulation?
Shunt indicates that blood is entering the arterial system without having gone through ventilated areas of the lung.
What are two sources of partially oxygenated/unoxygenated blood that is normally re-introduced to the systemic circulation?
Some of the bronchial artery blood is collected by the pulmonary veins after it has perfused the bronchi and its O2 has been partially depleted.
Another source is a small amount of coronary venous blood that drains directly into the LV through the thebesian veins.
In healthy individuals, the normal % shunt that occurs from drainage of the bronchial and thebesian circulation is:
a. 1%
b. 5%
c. 10%
d. 15%
b. 5%
Which of the following is an important feature of a shunt?
a. Provision of 100% oxygen can completely abolish hypoxemia
b. With provision of oxygen, the shunt can be overridden (i.e. the PaO2 experiences a typical linear response to oxygen therapy)
c. Hypoxemia cannot be completely abolished by provision of 100% oxygen, and in general the response to oxygen therapy is poor.
d. Most patients are not oxygen responsive at all.
c. Hypoxemia cannot be completely abolished by provision of 100% oxygen, and in general the response to oxygen therapy is poor.
Hypoxemia cannot be completely abolished by provision of 100% oxygen. The shunt will always continue to depress the PaO2, however the patient may still benefit from oxygen therapy since it will hyper-oxygenate the non-shunted blood and can tip the scale in the patient’s favor. The smaller the shunt, the more oxygen-responsive a patient will be.
Why do some patients with a shunt have a low arterial CO2? And why doesn’t a shunt cause increased PCO2?
Chemoreceptors sense any elevation of arterial PCO2 and respond by increasing ventilation. This reduces the PCO2 of un-shunted blood until the arterial PCO2 is normal.
Some patients with shunt will have a low PCO2 because their respiratory rates are higher (increased drive due to hypoxemia)
In a setting of V/Q of zero (i.e. no ventilation, but still has perfusion), the alveolar gas and end-capillary blood will start to look similar to _____
Mixed venous blood
In the setting of V/Q of infinity (lots of ventilation but no perfusion), the alveolar gas and end-capillary blood will start to look similar to ____
Inspired gas
T/F: Ventilation increases slowly from top to bottom of the lung, and blood flow increases more rapidly
True
T/F: The PO2 of the alveoli decreases markedly down the lung, whereas the PCO2 INCREASES proportionally less.
True
If ventilation is less at the top of the lung, why is the V/Q ratio high at the top of the lung?
Ventilation is less at the top than the bottom, but the differences in blood flow are more marked and there is less perfusion at the top of the lung.
The changes in PO2 from top to bottom of the lung vary over ___ mmHg whereas the PCO2 varies much less.
over 40 mHg
The difference between the CO2 level at the top and bottom varies less because the CO2 level relies on ____ more than circulation.
Ventilation
T/F: Lung units with high V/Q (i.e. high ventilation with low perfusion) do not increase the oxygen concentration of the patient’s blood as much, despite their relatively high PO2.
True
The net result is a depression of the arterial PO2 below that of the mixed alveolar PO2 i.e. the alveolar-arterial gradient
What is a normal V/Q ratio?
About 1.0
T/F: Lung units with abnormally high V/Q are inefficient at decreasing the CO2
True, because while they can ventilate, they cannot uptake CO2 from the blood since there is no blood flow
Why is the CO2 often normal despite V/Q mismatch clinically in our patients?
Chemoreceptors trigger increase RR to clear the CO2
Why is increased RR less useful in raising the PaO2 (as compared to its ability to clear the CO2)?
Because the oxygen dissociation curve is almost flat at the top meaning that units with moderately low V/Q ratios will benefit from appreciably from the increased ventilation. However, some hypoxemia will remain.
T/F: The different behavior of the two gases results from the different shapes of their dissociation curves.
True
T/F: Hypoventilation results in no A-a difference, and responds well to oxygen.
True
No A-a gradient/no abnormal gradient
Responds well to O2
T/F: Diffusion impairment results in an increased A-a gradient and responds well to oxygen.
True
A-a gradient elevated
Responds well to O2
T/F: Shunt results in an increased A-a gradient and responds poorly to oxygen but clinically can be useful in determining the degree of shunt (subjectively).
True
T/F: V/Q mismatch results in increased A-a gradient and typically does respond well to oxygen.
True
Normal A-a gradient value
10-15 mmHg
What are the two causes of hypercapnia?
Hypoventilation
VQ mismatch
