05 - Gas Transport Pt. 1 Flashcards
What is the portion of alveoli that does not participate in gas exchange?
Residual volume
In the normal state, we are breathing a ____ fraction of lung capacity. What is this volume called?
small
tidal volume; Vt = (1/10) of total capacity
What is the equation for functional residual capacity?
FRC = RV + ERV
What do PiO2, PaO2, and PAO2 stand for?
PiO2 = partial pressure of inspired O2 PaO2 = arterial O2 PAO2 = alveolar O2
What pressure changes do the gases go through as it travels from the environment to the arteries?
atmosphere - 150 mmHg pO2
alveolar - 100 mmHg pO2; 40 mmHg pCO2
arterial - 40 mmHg pCO2; 100 mmHg pO2
venous - 45 mmHg pCO2; 40 mmHg pO2
Which partial pressure of oxygen is greater? PAO2 or PiO2?
PAO2 < PiO2
Is the pressure air in the residual volume alveoli greater or less than the air in the capillary beds?
neither; they are equal
What is the PAO2 and PACO2 in RV and FRC?
they will diffuse. both are 40mmHg
Once PAO2 and PACO2 begin to diffuse it will cause a gradient from ____ mmHg from the atmosphere to ____ mmHg inside the lungs.
150 mmHg atm; 100 mmHg inside lungs
What is the alveolar gas equation?
PAO2 = PiO2 - PaCO2/Rq
How does CO2 diffuse between the alveoli and the capillary bed?
it goes from high (45 mmHg in the capillary bed) to low (40 mmHg in the alveoli) very quickly
How does O2 diffuse between the alveoli and the capillary bed?
alveoli has high concentration of O2
40 mmHg venous to 100 mmHg arterial.
What is the equation for PiO2 in the alveolar gas equation?
PiO2 = (Pb - PH2O) x FiO2
Pb = barometric pressure = 760 mmHg PH2O = water vapor = 47 mmHg
What is PiO2 at sea level? (Pb = 760 mmHg; PH2O = 47 mmHg, FiO2 = 21%)
PiO2 = (Pb - PH2O) x FiO2
PiO2 = (760 - 47) x (0.21) = 150 mmHg
Is PAO2 a reliable measure of the patient’s O2 level?
No. It can vary depending on what is being exchanged.
During expiration, the mix of alveolar gas with deadspace gas will dilute PAO2
How do we measure PACO2?
There is no direct way to measure PACO2.
PaCO2 and PACO2 equilibrate quickly, so it is safe to assume the ABG measured PaCO2 is equal to PACO2
What is Rq?
respiratory quotient
of CO2 molecules produced depending on your diet (assume = 0.8)
If PiO2 is 150 mmHg and PaCO is 40 mmHg, what is PAO2?
PAO2 = PiO2 - PaCO2/Rq
PAO2 = 150 mmHg - 40 mmHg/0.8 PAO2 = 150 - 50 = 100 mmHg
Normal room air PAO2 is 100-110 mmHg
As we get higher does the barometric pressure go up or do? PiO2?
Both barometric pressure and PiO2 go down
What is the normal Alveolar-Arterial O2 gradient?
< 15 mmHg
How does age affect the A:a gradient?
it increases with age up to 20-30 mmHg
What is the normal PaO2 range?
60-100 mmHg (without age factor or significant dz)
PaO2 = 102 - Age/3
Do all alveoli ventilate equally?
No. It changes depending on the part of the lung that is being ventilated (V/Q mismatch)
Under mechanical ventilation (FiO2 = 1.0), what do you expect the PaO2 to be?
550 mmHg (actual) high 600's mmHg (calculated)
use 550 mmHg = 100% for quick estimate
i.e. 50% = (0.5)(550) = 275 mmHg
Why do the alveolus have different amount of blood flowing near them?
The body will shunt blood to alveolus with better ventilation via hypoxic pulmonary vasoconstriction (HPV).
A patient is considered clinically hypoxic when they fall below ___ mmHg or ___ % of O2.
PaO2 < 60 mmHg, O2 sat < 90%
Once you go below 60 mmHg, the pt will begin to desaturate slowly. T/F
F; they will desaturate quickly
What shape is the Hgb Dissociation Curve?
Sigmoid or S-shaped. Know how to recreate!
What percentage of oxygen do we extract from the hemoglobin?
~25%
What is the mixed venous O2 saturation?
65-75% or PaO2 of 45-55mmHg
What are 4 physiological reasons for a widened A:a gradient?
R to L Heart Shunting
V/Q Mismatch
Diffusion
Mixed Venous Tension
What is an example of R to L shunting?
Shunting blood around the heart. Not allowing the blood to participate in gas exchange.
Hole in the heart or septal defect. Pressure gradient from right to left. Issues of lung parenchyma.. collapse of lung (atelectasis). The O2 will dilute down in the left heart and can cause widened A:a gradient, eventually leading to hypoxia.
What is an example of a diffusion problem?
Thickened or fibrotic tissue deposited between the alveoli and capillary tissue would impair diffusion of gases from occurring.
What is an example of mixed venous tension?
When you extract too much O2 from the tissue too quickly, you may not be able to pick up enough O2 to compensate.
Examples: decreased CO, increase O2 consumption (septic/febrile), lose Hgb (bleeding)
What is V/Q Mismatch?
When the blood is not picking up as much oxygen as it could. Results in blood returning to left ventricle that is not containing as much O2 and eventually hypoxemia.
What are non physiological causes of hypoxia?
Low inspired O2
Hypoventilation
What is the term when N2 diffuses into alveoli, resulting in diluted O2 and eventual hypoxemia?
This occurs when ventilating with N2O.
diffusional hypoxemia
What are possible causes of low inspired O2?
high altitudes, low FiO2, rebreathing, diffusional hypoxemia
What are possible causes of hypoventilation?
COPD, NMD, narcotics, volatiles
How do you treat hypoxia in a patient with a normal A:a gradient?
increase O2 through increasing RR and increasing FiO2 1.0
How do you treat hypoxia in a patient with an increased A:a gradient?
PEEP, FiO2 1.0
What occurs with V/Q of infinity, >1, <1, 0?
infinity - deadspace
>1 - normal
<1 - low
0 - shunt
What is a pulmonary embolus?
blood clot blocking blood going to part of the lung
If a patient has a pneumothorax, what would the V/Q ratio be in the collapsed area of the lung?
would cause shunting. V/Q = infinity
Which abnormalities are unmasked at high levels of exercise, not much of an issue at rest, and is not a big problem in the perioperative period?
Diffusion abnormalities
What is hypoxic pulmonary vasoconstriction (HPV)?
The body’s protective mechaniism to balance out V/Q mismatch. The arteries increase vascular tone and shunts blood away from alveoli getting low O2. Problem: chronically hypoxic - all vascular resistance will incr.
What is the normal PEEP during spontaneous ventilation?
5 cm H2O held by the vocal cords. Upon intubation, the tube removes normal PEEP because the vocal cords can’t close.
What is the equation for Minute Ventilation?
MV = RR x Vt
What is the waste product from metabolism?
CO2
