Ventilation and Lung Mechanics Flashcards
47 mm Hg
vapor pressure at body temperature (37C)
Dalton’s Law
the pressure exerted by one gas in a gas mixture is the same pressure it would exert if it occupied the volume alone
fractional concentration of atmospheric O2
.21
fractional concentration of atmospheric N2
.79
fractional concentration of atmospheric CO2
0
VC
Vital Capacity- the max V that can be exhaled after max inspiration
VC= IRV + TV + ERV
FRC
Functional Residual Capacity- gas V in lungs at the resting expiratory level
Measured by one of three methods:
1) nitrogen washout
2) helium dilution
3) blody plethysmography
TV
Tidal Volume- volume of gas inspired/exhaled during normal breathing
RV
Residual Volume- volume of gas in the lungs at the end of maximal expiration
RV= FRC-ERV
TLC
Total Lung Capacity- volume of gas in the lungs at end of max inspiration
TLC= VC + RV
IC
Inspiratory Capacity- max V that can be inspired from the resting expiratory level
IRV
Inspiratory Reserve Volume- max V that can be inspired from end-tidal inspiration
ERV
Expiratory Reserve Volume- max V that can be exhaled from the resting expiratory level
volumes a spirometer can’t measure
1) RV
2) FRC
3) TLC
Lung Volume Relationships
For Age Range 15-34 (calculated by Fowler in 1950s)
RV/TLC= .2
RV=VC(.25)
TLC= VC(1.25)
FRC is .4-.5 TLC
FVC
Forced Vital Capacity- The volume of air exhaled during the forced expiration maneuver.
assume same as vital capacity
FEV 0.5
Forced Expiratory Volume- The volume of gas exhaled during the first 0.5 sec of a forced expiration.
FEV 1.0
The volume of gas exhaled during the first 1.0 sec of a forced expiration.
FEF 25-75
Forced Expiratory Flow- The average rate of gas flow measured between 25% and 75% of the forced vital capacity.
obstructive ventilatory defect
- FVC most often (but not always) less than 70% of the predicted value
- The value of FEV(1.0) divided by the FVC is less than 0.75 “disproportionate reduction of maximal airflow”
- The other lung volumes, such as total lung capacity, are normal or above normal.
restrictive ventilatory defect
- The forced vital capacity is always less than 70% of the predicted value
- The value of FEV1.0 divided by the FVC is 0.75 or greater
- Other lung volumes, in particular the total lung capacity, are less than 70% of the predicted value
normal ventilatory pattern
- FVC greater or equal to 70% predicted FVC for that person’s sex, age and height
- FEV(1.0)/FVC > 0.75
- TLC (calculated by 1.25 x FVC) greater or equal to 70% predicted
flow-volume loop
tracing air flow in liters/sec (y-axis) as a function of volume in liters (x-axis)
- The difference in volume between the left and right side of the loop is the forced vital capacity
- FEF(50) or V(.)50- the forced expiratory flow at 50% of the vital capacity is the index of expiratory flow commonly obtained from a flow-volume loop
anatomic dead space
volume of air in the conducting airways
equal to weight in mL
physiologic dead space
anatomic dead space, plus any alveolar regions of the lung that, due to disease, have lost their ability to perform gas exchange with the blood
VC - PhDS = max V air that can reach functioning alveoli with a single breath
measured by determining the partial pressure of CO2 in expired gas and comparing it to the partial pressure of CO2 in the blood
dead space equation
VD = VT x (PaCO2 - PECO2)/PaCO2
pressure difference
dP = P(in) - P(out)
transpulmonary pressure
PA - Ppl
dP across chest wall
Ppl - Patm -> Ppl bc Patm = 0
dP across resp. system
PA - Patm -> PA bc Patm = 0
ERlung
Elastic Recoil Pressure = PA - Ppl
when glottis is open and no air flow occurs, PA = 0
ERlung = -Ppl
working to collapse the lung at a given lung volume
same as transpulmonary pressure Ptp
ERcw
Elasticity of the chest wall = Ppl - Patm = Ppl
lung compliance
how easy or difficult it is to inflate the lung
= dV/dP (L/cmH20)
LaPlace equation
P = 2y/r
derived from:
(pressure volume work) = (surface tension work)
->(P x d(4/3PIr^3)) = (y x d(4PIr^2)
->(P x (4PIr^2)) = (y x (2PIr)
PA
Alveolar Pressure determines air flow PA = ERlung + Ppl 0 = no flow neg = flow into the lung pos = flow out of lung
V(.)
rate of flow V(.)= dP/R where V(.) = airflow (L/sec) dP = (PA -Patm) (cm H20) R = airway resistance in cm (H20/(L/sec)) normal resistance is .5-2 (H20/(L/sec))
Poiseuille equation
laminar:
R = 8ηl/PIr^4
turbulent:
R = 8ηl/PIr^5
η is the viscosity of the gas, l is the length of the airway, and r is the radius of the airway
laminar flow
Laminar flows are linearly related to the driving
pressures, such that V(.) = ΔP/R
resistance to flow is dependent upon the gas viscosity, and independent of gas density
turbulent flow
resistance developed by the gas in a given airway is not constant, but increases as the rate of flow increases
resistance developed by the gas in the airway is dependent upon the density of the gas, rather than the viscosity
V(.) = ΔP^.5/K
where K is a constant
that if flow is turbulent, in order to increase flow rate two-fold, driving pressure must increase four-fold.
equal pressure point
as air flows from the alveoli to the mouth, the air pressure in the airway will change from alveolar pressure (greater than pleural pressure) to atmospheric pressure (less than pleural pressure). Somewhere along the way, the air pressure in the airway will be equal to the pleural pressure, and this is called the equal pressure point
dynamic compression
only occurs during forced expiration
major cause of flow limitation
due to compression of the airways when the airway pressure is less than the surrounding pleural pressure
beyond (~10cm H20) no further increase in flow rate is possible
