Term
What do each of the following terms mean in relation to ventilation?
1) Tidal Volume (Vt)
2) Residual Volume (RV)
3) Expiratory reserve volume (ERV)
4) Inspiratory reserve volume (IRV) |
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Definition
1) Volume of gas inhaled during a breath (500ml baseline)
2) Gas remaining after maximal expiration (1.5L)
**determines by inward pressure of expiratory muscles and outward elastic recoil**
3) Gas that can be forcibly expelled at the end of normal tidal expiration
VC= FRC + IRV= IRV + ERV + RV
FRC= ERV + RV
4) Gas that can be maximally inhaled at the end of normal tidal inspiration |
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Term
| How does Functional residual capacity (FRC) relate to expiratory reserve volume? |
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Definition
FRC is gas left in lungs after passive expiration.
RV is volume left after maximal/active expiration.
FRC= ERV + RV
**ERV is amount of additional gas that can be expelled from FRC** |
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Term
| How does vital capacity (VC) relate to functional reserve volume (FRC)? |
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Definition
Remember, total lung capacity (TLC) is the volume in the lungs at the end of maximal inspiration (6L).
- FRC is the gas left after passive expiration (ERV + RV)
**FRC (3L) + inspiratory capacity (3L)= TLC**
- VC is the volume that can be exhaled during maximal effort beginning from maximal inspiration.
VC (4.5L) = TLC (6L)- RV (1.5L)= FRC (3L) + IC (3L) - RV (1.5L) |
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Term
| What is inspiratory capacity (IC) and how does it relate to functional residual capacity (FRC)? |
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Definition
TLC= 6L, and it is a combination of the gas that can be enhanced during maximal inspiration at the end of normal tidal expiration (IC), and the gas in the lungs at the end of passive expiration (FRC)
IC (3L) + FRC (3L)= TLC
IC= IRV + Vt
FRC= ERV + RV |
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Term
| What happens to the partial pressure of oxygen in air has it moves down the respiratory tree? |
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Definition
As air is humidified and heating, the partial pressure of H2O rises, and the partial pressure of O2 falls.
When air reaches alveoli, diffusion occurs and PO2 falls further, as PCO2 increases. |
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Term
| What is the alveolar air equation and what can it tell you about ventilation-perfusion mismatching, shunt and diffusion impairment? |
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Definition
PaO2= (Pb- Ph20) FiO2 - (PaCO2/R)
Where R represents the respiratory exchange ratio (0.8 normally), or vCO2/vO2
1) This equation estimates the average alveolar PO2.
Generally, oxygen delivered across alveoli exceeds CO2 extracted from the blood, making R <1 and meaning that PO2 will fall by more than the increase in PaCO2.
**PO2 will decrease by PaCO2/R**
2) Alveolar PaO2 is used to derive the PA-aO2 gradient, which is the difference between estimated PaO2 and measured arterial PO2 (normally 8-12 mmHg).
An increase in this ratio can symbolize ventilation-perfusion mismatching, shunt and diffusion impairment. |
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Term
| How does dead space volume (Vd) relate to alveolar volume (Va)? |
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Definition
1) Va is the volume of gas that actually reaches the alveoli
2) Some gas never reaches the alveoli (anatomic dead space), or reaches ill-perfused alveoli (alveolar dead space), and this volume is the "physiologic dead space," which is filled by dead space volume (Vd)
Va= Vt-Vd |
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Term
| What is minute ventilation (Ve)? How does it relate to alveolar ventilation? |
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Definition
Ve is the total volume of gas that enters/leaves the lungs each minute.
Ve= Vt X RR= VA + VE
- Dead space ventilation (VD)= Vd X RR
- Alveolar ventilation (VA)= Va X RR
- VA= VE- VD |
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Term
| How does PaCO2 relate to the rate of tissue CO2 production (VCO2) and to alveolar ventilation (VA)? |
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Definition
PaCO2 is directly proportional to VCO2 and inversely related to VA (the higher the VA, the more quickly CO2 is removed from alveoli).
PACO2= K X VCO2/VA= PaCO2
PACO2= K X VCO2/ (Ve - VD)***
PACO2= K X VCO2/ Ve [1-(Vd/Vt)] |
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Term
| How does PACO2 relate to dead space volume? |
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Definition
If dead space increases, minute ventilation must increases in order to maintain PACO2.
SOOO, if less air reaches alveoli, you breathe faster (RR increases).
1) PACO2= K X VCO2/VA
2) PACO2= K X VCO2/ (Ve x VD)
3) PACO2= K X VCO2/ Ve [1-(Vd/Vt)] |
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Term
| What is the influence of alveolar ventilation on PAO2? |
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Definition
Indirectly (unlike PACO2, which directly decreases with increasing VA).
1) PAO2= [Pb-PH2O] * FiO2 - [PACO2/R]
- so PAO2 moves opposite PACO2.
2) Therefore, as VA increases, PACO2 decreases, and PAO2 must increase. |
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Term
| Why is the amount of fresh gas entering the alveoli greater in the base of the lung than at the apex during inspiration? |
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Definition
Apical lung fields are already expanded at end-expiration, while Dependent fields are collapsed and can then inflate.
At end-expiration, alveolar volume is high at the lung apex and progressively falls in the more dependent regions of the lungs.
The apical regions are therefore less compliant during inspiration, and the amount of fresh gas entering the base will be greater. |
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Term
| What structures are perfused by the bronchial and pulmonary arterial systems, respectively? |
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Definition
1) high-pressure bronchial arteries take oxygenated blood from aorta and intercostal arteries, supplying the CONDUCTION AIRWAYS, VISCERAL PLEURA and ESOPHAGUS.
2) Low-resistance pulmonary arteries take mixed venous blood from body to distal airways and alveoli for oxygenation, before returning the blood to the left heart via the pulmonary veins. |
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Term
| What major active and passive factors affect pulmonary vascular resistance (PVR)? |
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Definition
Passive forces dominate PVR
Active= neural and humoral
Passive= lung volume, CO and gravity.
ACTIVE
1) Neural
- sympathetic= constriction
- parasympathetic= dilation
2) Humoral
- Alveolar Hypoxia, catecholamine, histamine, PGE2, GF2, TBX= constriction
- ACh, PGE1 and PGI2= dilation
PASSIVE
1) Lung volume (increasing)
- Alveolar vessel resistance increases (Palv)
- Extra-alveolar vessel resistance decreases (Ppl)
** PVR is lowest near FRC because of this balance**
2) CO (increasing)
- PVR falls due to recruitment and distention
3) GRAVITY
- vascular distention in lung base and narrowing at apices. |
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Term
In what regions of the lungs is pulmonary blood flow the greatest?
How does this relate to John West's lung zones? |
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Definition
1) Dependent (bases) areas due to distention and decreased PVR because of gravitational forces.
2) Zones are based upon relationship between Pa, Pv and PA pressures. Alveolar pressures are lowest in the bases of the lungs, so they stay the most patent.
- Zone 1= PA>Pa>Pv- No BF
- Zone 2= Pa>PA>Pv- BF driven by Pa:PA gradient
- Zone 3= Pa>Pv>PA- BF driven by Pa:PV gradient
These zones depend upon body position (supine will make Zone 1 minimal) |
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Term
| What starling variables can produce interstitial pulmonary edema? |
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Definition
***1) Increased capillary hydrostatic pressure
***2) Increases capillary permeability (increased Kf-permeability/decreased sigma-less solute retention)
3) Impaired lymphatic drainage
4) Decreased interstitial hydrostatic pressure or plasma oncotic pressure and increased interstitial oncotic pressure. |
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Term
| How would PAO2 change in the context of a fully obstructed airway. |
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Definition
In this case, ventilation would be 0 and perfusion would drive PAO2, so it would match PaO2 (40 mmHg).
This explains why a decrease in V/P in a given alveoli will decrease PAO2 and increase PACO2. |
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Term
| How does ventilation-perfusion balance vary by lung region? |
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Definition
Both ventilation and perfusion increase going from apex to base (due to gravitational force), but perfusion increases MORE.
Therefore, blood leaving dependent regions has a relatively low PaO2 and a relatively high PaCO2 (lower V/Q ratio). |
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Term
How does mismatching of ventilation and perfusion lead to impairment of overall gas exchange in the lungs?
Use a two compartment model as a thought experiment, where A receives 3/4 of ventilated air and B receives 1/4. |
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Definition
1) V/Q in A will be 3X greater than B, so PAO2 and PO2 will be high in A and low in B.
2) Average alveolar PO2 will increase slightly, but mixed arterial PO2 will DECREASE, because of Hb saturation kinetics (You must average content or saturation and NOT PO2).
**High V/Q units CANNOT compensate for low V/Q units, and low V/Q units will cause a fall in arterial PO2 and an increase in PA-aO2 (ratio of avleolar to arterial O2 pressure) |
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Term
| What does a PA-aO2 of 18 mmHg mean? |
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Definition
Normal is 8-12 mmHg.
PA-aO2 is ratio of partial pressure of alveolar and arterial oxygen, so an increased ratio means that their is an imbalance between ventilation and perfusion.
This is due to a low V/Q lung unit, which cannot be compensated for by other high V/Q units, due to the nonlinear Hb saturation kinetics. This leads to a fall in PO2 and ultimately may produce hypoxemia.
**this is not true for PA-aCO2, which can be compensated for** |
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Term
| How does the effect of low V/Q lung units differ between PaO2 and PaCO2? |
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Definition
Only high PA:PaCO2 can be compensated for by high V/Q units, due to nearly linear shape of CO2 dissociation curve (compared to Hb oxygen saturation curve)
Remember, partial pressure gradient for CO2 is relatively small (40:46) |
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Term
| What is the effect of a right to left shunt on (arterial) PaO2? |
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Definition
Similar to the effect of a low V/Q unit, a right to left shunt, where ventilation is completely occluded to a given lung unit, causes PAO2 to increase and PaO2 to decrease
**the occluded compartment is an alveolar dead space, which increases the Ve required to maintain PaCO2** |
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Term
| True or False: High V/Q lung units decrease alveolar and physiological dead space. |
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Definition
False, they INCREASE IT.
Over-ventilation or under-perfusion of a lung unit will create a dead spaces |
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Term
| What are the relative effects of high and low V/W lung units on PaCO2? |
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Definition
Increased PCO2 arises from blood leaving LOW V/Q regions. Here, PCO2 has not had to equilibrate with delivered PACO2 (low ventilation), so it remains high.
**This is true even though high V/Q units create dead space** |
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Term
True or False:
Any disturbance in ventilation or perfusion leads to the development of both low and high V/Q units. |
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Definition
True
High V/Q units will increase dead space and Low V/Q units & Shunts will decrease PaO2 and increase PaCO2. |
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Term
| How can the Bohr equation be used to calculate the amount of physiological dead space? |
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Definition
physiological space= anatomic + alveolar dead space.
VD/Vt= (PaCO2- PeCO2)/PaCO2
In other words, the amount of dead space increases with decreased exhaled CO2 (PeCO2)
The difference between arterial and exhaled CO2 increase with the proportion of dead space! |
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Term
| What is the effect of increasing VCO2, Ve and VD on PaCO2? |
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Definition
Remember, PaCO2= K X VCO2/ (Ve-VD)
Increases volume of produced CO2 or volume of dead space (VD) will increase PaCO2.
Increasing minute ventilation will decrease PaCO2, getting rid of more in lungs.
Fast (Ve), Deep (reduce VD) breaths get ride of CO2 the best |
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Term
| What is the PA-aO2 ratio of perfect lungs? |
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Definition
0
Remember, normal is a standard distribution, which is stretched out in the context of lung disease |
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Term
| Why would someone want to breathe quickly and deeply when breathing through a long tube? |
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Definition
Increased dead space (similar to high V/Q ratio, which creates low V/Q units in lung disease), so you want to make sure that you can get rid of CO2.
PACO2= K * VCO2/ (Ve)(1-vd/vt)
Breathing deeply with decrease Vd/Vt, by increasing tidal volume.
Breathing quickly will increase Ve. |
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Term
| How does lung disease that creates a high V/Q unit cause an increase in minute ventilation? |
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Definition
High V/Q unit means that there is ALSO a low V/Q area (more blood will flow to other unit, increasing perfusion), which means that PACO2 will increase.
Ve increases to compensate (PACO2= K * VCO2/ (Ve-Vd) |
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Term
| How does a pulmonary embolus affect V/Q balance? |
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Definition
Embolic decreases PeCO2 and increases Vd/Vt
Remember, by Bohr, Vd/Vt= (PaCO2-PeCO2)/PaCO2.
If you can't exhale CO2 from the lungs, the amount of dead space will increase relative to tidal volume |
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