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.