L08 Alveolar Gas Equation Flashcards
What is Pulmonary Ventilation V̇ ?
= amount of air moved into / out of lungs per minute (minute volume)
V̇ is calculated by?
V̇ = f x TV
f = frequency of breathing (breaths/min) TV = tidal volume (L)
what is normal value for V̇?
~ 6 L/min at rest
Define Alveolar Ventilation V̇A.
= amount of air reaching functioning alveoli (exchange surface) per minute = minute volume for gas exchange
How much of Pulmonary Ventilation is Alveolar Ventilation?
2/3
so Alveolar Ventilation Normal value ~ 4.2 L/min at rest
What is the other third of pulmonary ventilation?
Dead Space Vent.
Equation for dead space ventilation?
TV = VA + VD
Equation for Alveolar Ventilation?
V̇A = f x VA = f x (TV - VD)
VA = volume of air reaching the functioning alveoli (L) VD = volume of dead space (L)
Define dead space volume.
= space in lungs where gas exchange cannot take place
2 types of dead space? Explain each
- Anatomical dead space (born with it)
= parts of respiratory tract acting as passageways (nose, pharynx, trachea, bronchi, bronchioles) = 150 mL - Physiological dead space (physiological) = anatomical dead space PLUS:
a) Space that does not receive blood supply e.g. diseased lung and/or
b) Space in which ventilation is in excess of need to arterialize the blood
Normal total dead space volume = 150 mL
NO PHYSIOLOGICAL DEAD SPACE
is alveolar dead space important?
No, normally negligible
Equation linking PACO2 and V̇A
V̇CO2 = amount of CO2 Exhaled per min.
V̇A = alveolar ventilation
FACO2= fractional conc. of CO2 (partial pressure of CO2)
V̇CO2= V̇A x FACO2
FACO2 depends on what?
The amount of CO2 into capillary and CO2 out of airwayw
How does PACO2 relate to FACO2?
FACO2 = k1 x PACO2
k1 is a constant
therefore PACO2 = partial pressure of CO2 in alveolar air
V̇CO2= V̇A x FACO2 V̇CO2= V̇A x k1 x PACO2 therefore PACO2 = 1/k1 x V̇CO2 / V̇A what does this mean?
At a constant level of CO2 production / metabolic activity (i.e. also constant): increase level of alveolar ventilation V̇A means PACO2 falls
PACO2 varies inversely with V̇A
For body pH to be kept at normal physiological value ~ 7.4, what is the PACO2?
Isocapnic line: PACO2 need to be kept at 40 mmHg for pH = 7.4
What conpensatory measures are taken to counter rise in PACO2? (normal at rest)
Hyperventilation:
ventilation in excess to metabolic demand > ↓ PCO2 in alveolar and systemic blood
Same amount produced in body but less CO2 remains
What measures to counter drop in PACO2? (normal at rest)
Hypoventilation:
inadequate ventilation to meet metabolic demand >
↑ PCO2
> CO2 builds up in arterial systemic blood
Exercise changes metabolic status. What changes to meet metabolic demand?
Hyperpnea:
increased ventilation to meet metabolic demand (e.g. moderate exercise) > normal PCO2 > normal pH
(curve on PACO2/V̇A graph shifted to right. Higher V̇A needed to maintain same PACO2 as at rest)
PAO2- V̇A relationship.
What is the equation for V̇O2?
V̇O2 = k1 x V̇A (PIO2 - PAO2)
PIO2 = inspired PO2 PAO2 = alveolar PO2
Rearranging V̇O2 equation gives?
What does that mean?
How does it compare to PACO2?
V̇O2 = k1 x V̇A (PIO2 - PAO2)
PAO2 = PIO2 - (1/k1) V̇O2/V̇A
This means PAO2 varies directly with V̇A, whilst PACO2 varies inversely with V̇A.
Explain V̇A and PACO2 and PAO2 relationship.
Increase V̇A causes decrease in PACO2, increase in PAO2
At a constant level of O2 consumption / metabolic activity and a fixed inspired O2 concentration: increase level of alveolar ventilation casues PAO2 increases. What is the normal PAO2?
100mmHg at iso-oxic line
Hyperventilation and hypoventilation causes what to PO2?
Hyper = increase PO2 Hypo = decrease PO2
During exercise, metabolic demand for O2 increases. So increased V̇A is needed for the same PAO2
