Ventilation Flashcards
The volume of air that never reaches gas exchange surfaces is ____
The air tht does reach the alveoli for gas exchange is ____, which is directly related to the ____.
Dead space (not the same as anatomical dead space): volume that never reaches alveoli
Alveolar ventilation: air that does reach alveoli for gas exchange; directly related to amt of carbon dioxide in the blood
Functional residual capacity (FRC)
The amount of air in your chest when resting, relaxed, and with your mouth open
Includes the RV, which cannot be measured simply
Tidal volume
The volume of air that you breathe in and out normally
Usually ~0.5L
Expiratory reserve volume (ERV)
The amount of air that you can exhale from the FRC (air in chest when resting)
Residual volume (RV)
The amount of air left in your chest after exhaling your ERV
Changes in FRC (the starting point of the respiratory cycle) can be changed due to changes in
the ERV (can be measured directly)
&
the RV (more complicated to measure)
Inspiratory reserve volume (IRV)
The amount of additional air that can be inspired with effort from the FRC.
This is what we increase during exercise.
Vital capacity (VC)
ERV + TV + IRV
If we take a deep breath in to max inspiratory capacity and exhale the entire amount and measure it, that’s the vital capacity.
Total lung capacity
RV + VC
Spirometry
Measures the flow rate & amt of exhaled air
- Usually wanna know forced vital capacity (FVC) & forced expiratory volume in one sec (FEV1)
Static lung volumes
Measurement of residual volume and total lung capacity
Measuring RV requires a plethysmograph/body box!
Diffusion capacity (DLCO)
Measurement of the transfer of a gas, CO, across the alveolar-capillary membrane and into the blood.
Total ventilation / minute ventilation
The total amt of air we breathe in and out in a minute.
Volume of exhaled air
X
Respiratory rate in breaths-per-minute
The portion of air in each breath that fills the conducting airways and never reaches the alveoli for gas exchange
anatomic dead space; FIXED even if you take bigger breaths
~150mL
Alveolar ventilation equation
Amt of air that reaches the alveoli with each breath
X
Respiratory rate
Why is alveolar ventilation always less than the total ventilation?
Anatomic dead space - the volume of air that fills the conducitn gariways and never reaches the alveoli
Alveolar dead space
Air that enters alveoli that has no blood flow (due to pulmonary embolism or low bp) –> no gas exchange
Physiological dead space
Anatomic dead space + Alveolar dead space
Note: anatomic is fixed, alveolar is variable depending on the disease
Alveolar pCO2 = ____
Alveolar pCO2 = Blood pCO2
because there’s basically no CO2 in the inhaled air and CO2 diffuses efficiently across biological membranes
What is the alveolar ventilation equation?
The volume of alveolar ventilation (L/min)
=
Volume of CO2 exhaled / PCO2 in the alveoli
How can we determine the fraction of each breath that is dead space?
Bohr Equation
The fraction of each breath that is dead space is
(Parterial CO2 - Pexpired CO2)
Parterial CO2
Remember that ParterialCO2 = Palveolar CO2
The lungs are elastic, so how are they held open in the rib cage?
The negative pressure in the pleural space holds it open.
The visceral pleura of the lungs is in direct contact with the parietal pleura of the ribcage
Because of gravity, there is a more negative (lower) pleural pressure in the pleural space at the __ of the lungs.
What implication does this have for ventilation?
More negative (less) pressure at the top of the lungs
–> higher expansile force in the upper lung than the lower lung
–> the bottom of the lungs has more potential to increase with each breath, so there is more ventilation at the bottom of the lungs than the top.
Acinus
portion of the lung supplied by a terminal bronchiole
- all the oxygen uptake occurs in the acini
- the change in volume of the acini during breathing is greater than that of the whole lung (because the conducting airways stays constant)
- volume of the acini is about 95% of the total volume of the lung at FRC (FRC is about 3 liters, conducting airways are about 150 mL)
- ventilation of the acini is greater at the base than the apex of the upright lung at FRC
In a measurement of FRC by helium dilution, the original and final helium concentrations were 10% and 6%, and the spirometer volume was kept at 5 liters. What was the volume of the FRC in liters?
If the volume of the FRC is denoted as V,
the amount of helium initially in the spirometer is 5 x 0.1,
the amount after dilution is (5+V) x 0.06.
Therefore, V=0.5/0.06 - 5 or 3.3 liters.
A patient sits in a body plethysmograph (body box) and makes an expiratory effort against his closed glottis. What happens to the following: pressure in the lung airways, lung volume, box pressure, box volume?
up, down, down, up
When the patient makes an expiratory effort, he compresses the gas in the lung so that the airway pressure increases and lung volume decreases slightly.
The reduction of volume in the lung means that the box gas volume increases and therefore, it’s pressure decreases according to Boyle’s law.
If CO2 production remains constant and alveolar ventilation is increased threefold, the alveolar PCO2 after a steady state is reached will be what percentage of its former value?
The alveolar ventilation equation states that if CO2 production is constant, the alveolar PCO2 is inversely related to the alveolar ventilation. Therefore, if the ventilation is increased 3 times, the PCO2 will be reduced to a third of its former value, that is, 33%.
A woman is started on mechanical ventilation. The ventilator is set to deliver a tidal volume of 750 mL 10 times per minute. After transfer to the ICU, the physician decreases her tidal volume to 500 mL and raises her respiratory rate to 15 breaths per minute. Total ventilation is fixed.
Which changes would you expect to occur as a result of the physician’s intervention?
Increase in dead space fraction of total ventilation
- The Dr increased respiratory rate, but the volume of the anatomic dead space remains ~constant –> dead space ventilation is increased.
- Because minute ventilation is unchanged, the dead space fraction of the total ventilation is increased
Tidal volume has decreased, meaning alveolar ventilation has decreased–> arterial PCO2 increases
A man is receiving mechanical ventilation after severe respiratory failure. The ventilator settings include a tidal volume of 600 mL and respiratory rate of 15. The patient is in a deep coma and cannot increase his total ventilation beyond what the ventilator is set to deliver. On his fifth hospital day, he develops high fevers and is determined to have a new blood stream infection. Which of the following changes would be expected?
With fever and a bloodstream infection, CO2 production increases.
Because minute ventilation is fixed, the patient cannot raise alveolar ventilation to compensate for the increase in CO2 production and, as a result, arterial PCO2 increases.
