Respiratory Flashcards
What is the Po2 of air? what is the barometric pressure at sea level?
20.93% of the total dry gas pressure.
At sea level the barometric pressure is 760mmHg
What is the water vapour pressure of moist inspired gas at body temperature?
47mmHg
What is the Po2 of inspired air? show the calculation
(20.93/100) x (760-47)= 149mmHg
760 is the barometric pressure at sea level and 47 is the pressure of water vapour
What is the PO2 in the alveoli and why ?
It is 100mmHg. This is because it is the balance between two processes;
1) the removal of O2 by the pulmonary capillary blood
2) continual replenishment by alveolar ventilation on the other (strictly this is not continuous but is breath by breath)
What is the fluctuation of PO2 pressure wise in the alveoli and why?
it is 3mmHg, because the tidal volume is small compared to the volume of gas in the lungs
what determines alveolar PO2?
determined by the balance between the rate of removal of O2 by the blood (which is set by the metabolic demands) and the rate of replenishment of O2 by alveolar ventilation.
What woul cause alveolar PO2 to drop ?
Increased demand that outstrips supply (e.g. fever in a brachycephalic) and decreased supply of O2 (e.g. laryngeal obstruction)
what are causes of hypoventilation
Causes of hypoventilation include;
- drugs as morphine and barbiturates that depress the central drive to the respiratory muscles
- damage to the chest wall or paralysis of the respiratory muscles
- high resistance to breathing (e.g., brachycephalic)
- Some diseases, such as morbid obesity may cause hypoventilation by affecting both central respiratory drive and respiratory mechanics.
What physiological change does hypoventilation always cause
an increased alveolare, and therefore arterial Pco2
what is the alveolar ventilation equation?
Pco2= (Vco2/Va)xK
Vco2 = patient CO2 production
Va= alveolar ventilation
K= constant
Define and what is the alveolar gas equation ?
The relationship between the fall in PO2 and the rise in Pco2 that occurs with hypoventilation
PA02 = PIo2 - (PAco2/R) +F
F= small correction factor (usually 2mmHg for air breathing) which is often ignored.
What happens to alveolar and arterial O2 in hypoventilation
It is ALWAYS reduced EXCEPT when the individual breaths an O2 enriched mixture. In this case the amount of O2 per breath makes up for the reduced flow of inspired gas
If a patient was hypoventilating, and then suddenly starts hyperventilation, why does it take several minuted for the alveolar PO2 and PCO2 to assume their new steady-state values?
Because there are significant higher stores of CO2 in the body in the form of bicarbonate and interstitial fluid.
Is the PO2 of arterial blood the same as alveolar gas?
At the level of the alveolus it should be, however, the arterial blood never quite reaches it.
This is due to;
- incomplete diffusion through the blood gas barrier ( not usually a problem at sea level even when lung disease is present as the RBC spends enough time in the pulmonary capillary to become saturated)
- Shunts;
1) Blood that enters the arterial system without going through ventilated areas of the lung. In the normal lung, some of the bronchial artery blood is collected by the pulmonary veins after it has perfused the bronchi and its O2 has been partly depleted.
2) small amount of coronary venous blood that drains directly into the cavity of the left ventricle through the Thebesian veins.
3) other congenital abnormalities (Pulonary arteriovenous malformation)
The effect of the addition of this poorly oxygenated blood is to depress the arterial PO2.
Draw a schematic of O2 transger from air to tissues showing the changes in arterial PO2 on the Y axis
What is the alveolar shunt
Blood that enters the arterial system without going through ventilated areas of the lung. In the normal lung, some of the bronchial artery blood is collected by the pulmonary veins after it has perfused the bronchi and its O2 has been partly depleted.
Another source is a small amount of coronary venous blood that drains directly into the cavity of the left ventricle through the Thebesian veins.
The effect of the addition of this poorly oxygenated blood is to depress the arterial PO2.
What cardiac pathologies can result in a lower PO2 even when alveolar ventilation is normal ?
Any right to left shunt
How do you calculate shunt flow ?
Qs/Qt = (Cc’o2 -Cao2) / Cc’o2 -CvO2)
Qt= total amount of O2 leaving the system
CaO2= O2 concentration of the arterial blood.
How do you abolish hypoxemia in a shunt?
You cannot. Even giving 100% O2 will not result in this as CO2 is generated by the metabolism and it will shunt this into the arterial system
Is it pointless to give shunt patients 100% O2
No, even though the shunt is still reducing PO2, giving 100% O2 will help relieve some symptoms as the patient will have increased dissolved O2 concentrations (ie non haemoglobin bound)
How ca you diagnose if a shunt is present by giving 100%
When100%O2 is inspired, the arterial PO2 does not rise to the expected level—a useful diagnostic test.
Giving the subject 100% O2 to breathe is a very sensitive measurement of shunt because when the PO2 is high, a small depres- sion of arterial O2 concentration causes a relatively large fall in PO2 due to the almost flat slope of the O2 dissociation curve in this region
A shunt usually does not result in a raised PCO2 in arterial blood, even though the shunted blood is rich in CO2. Why?
The reason is that the chemoreceptors sense any elevation of arterial PCO2 and they respond by increasing the ventilation. This reduces the PCO2 of the un-shunted blood until the arterial PCO2 is normal. Indeed, in some patients with a shunt, the arterial PCO2 is low because the hypoxemia increases respiratory drive
What are the 4 causes of hypoxemia;
1) hypoventilation
2) diffusion
3) shunt
4) ventilation/perfusion missmatch
What is vention- perfusion missmatch
This is when the ventilation and blood flow are missmatched in various regions of the lung resulting in impairment of bth O2 and Co2 to transfer
what determines the ratio f gas in any particular lung unit at any point in time ?
In exactly the same way, the concentration of O2 (or, better, PO2) in any lung unit is determined by the ratio of ventilation to blood flow. This is true not only for O2 but CO2, N2, and any other gas that is present under steady- state conditions.
what happens when you reduce ventilation-perfusion ratio - e.g. by a gradual obstruction to ventilation (blood flow unchanged)?
The O2 in the unit will fall, and the CO2 will rise
In V/Q missmatch, what happens if you gradually obstruct blood flow ?
The O2 rises, and the CO2 falls enventually reaching the composition of the inspired air
What is the ventilation-perfusion ratio equation
Depict the O2-Co2 diagram showing ventilation-perfusion ratio
where is the ventilation-perfusion ratio greatest in the lung and why?
At the top of the lung, as the ventilation slowly increaes from top to bottom of the lung and blood flow increases more rapidly. It is clear that the PO2 of the alveoli (horizon- tal axis) decreases markedly down the lung, whereas the PCO2 (vertical axis) increases much less. Ventilation is less at the top than the bottom, but the differences in blood flow are more marked.
Depict the V/Q based on location within the lung
It is clear that the PO2 of the alveoli (horizon- tal axis) decreases markedly down the lung, whereas the PCO2 (vertical axis) increases much less. Ventilation is less at the top than the bottom, but the differences in blood flow are more marked.
Chart the changes in Po2 when there is no shunt and a shunt
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