Ventilation Flashcards
Define pulmonary minute ventilation
inspired air/minute
How do you calculate minute ventilation?
Vt x RR
Define anatomical dead space
upper/conducting airways + terminal bronchioles
For a given tidal volume, what does increasing anatomical dead space do?
decreases amount of fresh air reaching alveoli
What is normal anatomical dead space in a 70 Kg subject at rest?
150 mL
What are some protective roles of dead space ventilation?
warming and humidifying inspired gas filtering particles
How do you calculate alveolar ventilation?
RR x (Vt - Vd)
If tidal volume where to equal anatomical dead space volume, what would happen?
O2 alveolar content would continuously decrease and CO2 would increase because during inspiration no fresh air would enter the alveolar space, and during expiration none of the alveolar air would leave the airways
What is the normal ratio of tidal volume to anatomical dead space?
Vt = 3x Vd
How do you calculate dead space ventilation?
Vd x RR
If you want to maintain minute ventilation but decrease dead space ventilation, what should you do?
breathe slowly and deeply with the same overall tidal volume Vd will decrease with RR, Va will increase with decreasing RR
What is a consideration that impedes low RR to optimize alveolar ventilation?
mechanical considerations - they don’t work very well at low frequencies, which limits how low RR can go
Define alveolar dead space
Air that is anatomically alveolar and is ventilated, but does not participate in gas exchange
What causes alveolar dead space?
a lack of perfusion
What is physiologic dead space
the sum of anatomical and alveolar dead space
What determines rate of O2 consumption and CO2 production?
metabolism
In steady state, what must be true of CO2 production and expiration?
They must be equal
What is true of the net difference between O2 (inspired and expired) and O2 consumption at rest?
they must be equal
Define the respiratory quotient - what information is this giving you about ventilation?
RQ i.e. respiration x blood flow, where R = CO2 ventilation over O2 ventilation
What fixes the respiratory quotient? (RQ)
metabolism and diet
What dietary source has an R of 1?
carbohydrates
What dietary source has the lowest R?
lipids
What is a normal physiologic R value?
0.8
What are two differences between alveolar air and ambient air?
- addition of water vapor 2. addition of circulatory CO2
What must be true of the pressure of gas in the alveoli v. ambient atmospheric pressure (assuming no flow and an open glottis)
sum of partial pressures in alveoli must equal Pb- Ph20
When there are equal amounts of O2 consumed as CO2 produced, what is the RQ?
1
Why does it make sense that inspired pressure of O2 (PiO2) is equal to the sum of PAO2 and PAc02?
because CO2 is essentially zero in room air, so PiCO2 = 0
What is a normal PACo2 pressure at a normal Pb?
40 mmHg
What is the normal range of PAO2 values at normal atmospheric pressure?
90 to 105 mm Hg
What is the consequence of an RQ of 0.8 on the calculation of alveolar O2 pressure?
expired volume is less than the inspired volume, so PACO2 must be divided by the RQ to correct for this
What is the alveolar gas equation? What does it tell you?
Alveolar pressure of O2
What is the alveolar ventilation equation? What 3 relationships does it illustrate?
Ventilation v. metabolism v. blood gas composition
Under conditions of hypoventilation, what is true of the relationship betwen PACo2 and alveolar ventilation?
CO2 is higher at any given alveolar ventilation rate
For relatively modest changes in metabolism, why must alveolar ventilation rates change?
Because small changes in metabolism can generate very big changes in PACo2 that have a narcotic effect without changes in ventilation rate
What is the hallmark of hyperventilation rate: CO2 levels or alveolar ventilation rate?
CO2 LEVELS
you can ONLY be hyper or hypoventilated if you have hypocapnia or hypercapnia because PACo2 depends on BOTH metabolic rate and ventilation rate to change, not just ventilation rate
What is the ventilatory response to increases in metabolic rate with constant levels of arterial PCO2?
hyperpnea
What is the equation for alveolar ventilation rates using O2? what is an adjustment that must be made for O2 v. CO2?
O2 is both inspired and expired, so the denominator must express the change in inspired v. alveolar O2 to give an accurate ventilation rate.
Can you assume that alveolar PO2 and arterial PO2 are the same?
NO. the Alveolar ventilation equation using O2 ONLY refers to the alveolar gas, not arterial gas
What relationship to alveolar PO2 amounts have to inspired PO2 amounts/
Hyperbolic
if PaCO2 = 40 mmHg at f = 10/min, what is the PaCO2 at 32/min? What would happen to arterial pH if this occurred to an adult on a respirator?
The first gas expired is that which was in the dead space and CO2 is essentially zero. As alveolar air is expired the PCO2 will rapidly rise toward the PACO2. As CO2 is constantly entering the alveoli from the pulmonary blood, the measured PCO2 should continue to slowly rise during expiration and will approach the mixed venous PCO2 (PvCO2).
In conditions of unchanged meatbolism and constant VCo2, what happens to PACO2 if VA is halved? Doubled?
If VA is halved, PACO2 doubles
if VA is doubled, PACO2 is halved
Which value(s) cannot be calculated with spirometry?
FRC and RV
What role does nitrogen play in atelectasis?
It helps reduce atelectasis because it is inert, and therefore maintains sufficient gas tension in the alveoli to resist collapse
What role does nitrogen play in the use of O2 therapy during pneumothorax?
Because nitrogen is inert, it’s important to ‘wash out’ the nitrogen component of ectopic gas so as to facilitate bloodstream absorption and decrease the volume of the pneumothorax
