Respiratory physiology 2 Flashcards
The Vd/Vt ratio tells us the percentage of the
total tidal volume that is allocated to dead space
In the spontaneously ventilating patient, the Vd/Vt ratio approximates
0.33
In the mechanically ventilated patient, the Vd/Vt ratio approximates
0.50
Factors that increase the ratio of Vd to Vt include
a facemask
atropine
neck extension
Factors that decrease the ratio of Vd to Vt include
ETT
LMA
neck flexion
Physiologic dead space can be calculated with the
Bohr equation
compares the partial pressure of CO2 in the blood vs. the partial pressure of CO2 in exhaled gas
The most common cause of increased Vd/Vt under general anesthesia is a reduction in
cardiac output
What pathophysiology will increase Vd?
decreased cardiac output
COPD
PE (thrombus, air, amniotic fluid)
What surgical position will increase Vd?
sitting
What surgical position will decrease Vd?
supine
Trendelenburg (head down)
Does increasing the length of the circuit increase dead space?
No; in the circle system dead space begins at the y-piece and anything proximal does not influence dead space
What is the A-a gradient equation?
PAO2-PaO2
A patient is in the sitting position. When compared to the apex of the lung, which of the following are higher in the base?
a. blood flow
b. partial pressure of alveolar oxygen
c. V/Q ratio
d. partial pressure of alveolar carbon dioxide
a. blood flow
d. partial pressure of alveolar carbon dioxide
What is the normal V/Q ratio?
0.8
Alveolar compliance is defined as
a change in alveolar volume for a given change in pressure
Alveolar ventilation is greatest in the lung
base due to greater alveolar compliance
Alveolar perfusion is greatest in the lung
base due to higher pulmonary blood flow
In the sitting position, which lung region is the dependent lung and which is the non-dependent lung?
base- dependent
apex- non-dependent
Identify the statements that represent the MOST accurate understanding of V/Q mismatch. (Select 2)
a. bronchioles constrict to minimize zone 1
b. the A-a gradient is usually small
c. blood passing through under-ventilated alveoli tends to retain CO2
d. hypoxic pulmonary vasoconstriction minimizes dead space
a. bronchioles constrict to minimize zone 1
c. blood passing through underventilated alveoli tends to retain CO2
The normal ratio of ventilation to perfusion is
0.8
Dead space occurs when there is
ventilation but no perfusion (V/Q= infinity)
Shunt occurs when there is
perfusion but no ventilation (V/Q= 0)
The most common cause of hypoxemia in the PACU is
atelectasis
Atelectasis leads to
right to left shunt
V/Q mismatch
hypoxemia
Patients with V/Q mismatch tend to have more trouble with _________ than with ______________
oxygenation than with CO2 elimination
CO2 retention suggests a
severe V/Q mismatch
Treatment for atelectasis includes
humidified O2 & maneuvers designed to reopen the airways (mobility, coughing, deep breathing, and incentive spirometry)
What happens in under-ventilated alveoli?
blood passing through underventilated alveoli tends to retain CO2 and is unable to take in enough oxygen
What happens in over-ventilated alveoli?
blood passing through over-ventilated alveoli tends to give off an excessive amount of CO2; remember that CO2 diffuses 20 times faster than oxygen
To combat dead space is zone 1, the body
constricts the bronchioles to minimize ventilation of poorly perfused alveoli
To combat shunt in zone 3, the body engages
hypoxic pulmonary vasoconstriction to reduce pulmonary blood flow to the poorly ventilated alveoli
Variables described by the law of LaPlace include all of the following EXCEPT:
a. tension
b. pressure
c. radius
d. density
d. density
The law of Laplace describes the relationship between
pressure, radius, and wall tension
____ equalizes the effect of surface tension by keeping alveolar pressures constant
Surfactant
Surfactant prevents
smaller alveoli from collapsing and emptying into larger alveoli
What is the law of Laplace for spherical shapes?
P= 2T/R
The tendency of an alveolus to collapse is directly proportional to
surface tension (more tension= more likely to collapse)
The tendency of an alveolus to collapse is inversely proportional to
alveolar radius (smaller radius= more likely to collapse)
A thin layer of _____ coats the alveoli, increasing _______ & promoting alveolar collapse
water; surface tension
________________ produce surfactant
Type 2 pneumocytes
When do type 2 pneumocytes produce surfactant?
between 22-26 weeks with peak production occurring at 35-36 weeks
Fetal lung maturity (in the setting of premature labor) can be hastened by
corticosteroids
What is the law of Laplace for cylinder shapes?
tension = pressure x radius
think blood vessels
