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24 Cards in this Set
- Front
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resistance and pleural pressure |
NEED TO GO OVER THIS WITH WEST |
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negative pressure breathing |
this is the normal way that we breath; we generate negative pressure in the pleura in order to inhale and then we relax to let the air out; when we start the inhalation we have the largest negative pressure at the alveoli, an intermediate pressure in the tubes, and a 0 pressure in the trachea; this difference in pressure is due to the difference in resistance; the air goes from higher pressure to lower pressure; we generate a more negative intrapleural pressure (from -5 to -7) which pulls open the airway; at the end of the inhalation all of the pressures in the airway are again 0 but the intrapleural pressure is ever more negative (-8) because the lung was fully expanded which required this pull; the greater the breath the more negative this will become |
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positive pressure breathing |
like if you were connected to a ventilator; during inspiration there is a positive pressure in the trachea and it goes down to 0 in the alveoli (so again the air is moving down the pressure gradient); the intrapleural pressure is less negative (-4); at the end of the inspiration all the pressures in the airways are the same positive values and the intrapleural pressure is a positive value because it is getting pushed against |
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3 types of flow |
laminar, transitional, and turbulent |
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laminar flow |
the movement of fluid is the greatest in the center of the tube and is 0 at the boundary of the tube; so the velocity is in a parabolic shape through the tube |
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turbulent flow |
the flow is chaotic; higher resistance to a given flow |
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transitional |
overall laminar except for at branching points of the tube were you get turbulent eddies from bumping into the division point |
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resistance definition |
R=deltaP/deltaV where V is flow; flow can be considered flux of either volume or mass; in general R is a function of time and position; special cases where this is different include steady flow (resistance is the same and doesn't depend on time) and incompressible flow (density everywhere is the same and does not change) |
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equations for resistance |
deltaP=k1V+k2V^2 or R=k1+2k2V; the k1 term represents laminar flow where energy loss is due to viscous drag; the k2 term represents everything due to acceleration= inertia for oscillatory flow, inertia to change in direction around corners, and turbulence where mass flux is chaotic; know that as the radius becomes smaller the resistance becomes much higher and that the resistance is proportional to tube length |
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distribution of airway resistance |
majority of the resistance is in the trachea and main bronchi; it is almost 0 after the terminal bronchioles and this is because the cross sectional area becomes so large that it super diminishes resistance (this is total cross sectional area); flow in the large airways is turbulent; flow in the small airways is laminar; cross section of the small airways is much larger leading to a smaller axial velocity; these effects combine to place most of the airway resistance in the large airways; disease states can change this situation |
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forced exhalation and airway collapse |
intrapleural space has a positive value; at some point in the airway there is going to be a spot where the pressure outside the airway is larger than the pressure inside the airway and the airway is going to want to collapse; so this can only happen in the thorax (because that is where the lungs are that cause the pleural pressure) so this disappears in the trachea; this is called dynamic compression of the airway; the distribution of resistance will determine the anatomic location of where pleural pressure equals airway pressure (EPP or equal pressure point) and this becomes the choke point for the air |
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airway resistance and dynamic compliance |
this concept is due to a misuse of the term compliance; the literature discusses static compliance and dynamic compliance; static compliance measures pressure at the beginning and end of a breath where flow is zero and ten calculates Cs=deltaV/deltaP; compliance is a partial derivative and, of course, is always dynamic |
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this is an exam question: deltaPO2= |
PCO2/R; R=respiratory exchange ratio; usually this becomes 40 torr/0.8=50 torr |
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what is the expected arterial O2 content for a pt with 10% shunt? assume there are no other V/Q problems. assume that the O2 content of mixed venous blood is 14.6 and that the O2 content of blood passing through the normal lung is 19.5 a. 14.6 b. 18.5 c. 19.0 d. 19.5 e. 20.0 |
if there is shunt then it stands to reason that the answer will be somewhere between 14.6 and 19.5; CaO2=(0.9)(19.5)+(0.1)(14.6)=1.46+17.55=19.01; BE PREPARED FOR A QUESTION LIKE THIS ON THE TEST |
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a pt decreases ventilation due to use of narcotics. the normal pCO2 was 40 and increases to 80 following the narcotic. assume no other changes in conditions. what is the ratio of the new alveolar ventilation compared to the previous alveolar ventilation? a. 1/2 b. 1 c. 3/2 d. 2 e. not enough info. the dead space was not given |
1/2; pCO2 is directly proportional to VCO2 and inversely proportional to VA; VAnew/VAold=pCO2old/pCO2new=40/80=1/2 |
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a young and otherwise healthy 20 yr old presents to the ER with diabetic ketoacidosis. an arterial blood gas (ABG) demonstrates pH=6.92 and pCO2=20. assuming room air with FIO2=150 a normal respiratory exchange ratio (R) of 0.8 and no V/Q imbalances, what would the expected pO2 of this pt be? a. 75 b. 80 c. 100 d. 120 e. 125 |
pCO2 has gone down so PO2 goes up; normal pCO2 is 40 and normal pO2 is 100; if the ratio were 1 and pCO2 went to 20 then pO2 must have gone up by 20 but there is a difference in the ratio; so 20/0.8=25 so 100+25=125 |
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chest wall neutral |
we talked about the chest wall trying to expand outward but keep in mind that if you take a giant breath at some point you're going to go past the limit that it wants to expand to and then it is trying to collapse back; the limit is called chest wall neutral; same thing with the lung collapsing (it won't form a black hole so there is some stopping point) |
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the respiratory system curve |
adding together the pressures of the lung and chest wall curves to see what the final pressure would be; FRC is 0 pressure on the respiratory system curve (where the chest wall pull and lung pressure are in exact opposition); RV is the lower asymptote of the chest wall curve (determined largely by the chest wall); TLC is the upper asymptote of the lung curve (determined largely by the lung) |
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a helium dilution apparatus uses 1% helium mixed with air. the apparatus volume is 1L. a pt is tested and reaches an equilibration value of 0.50% helium. what is the value for FRC? a. 1 b. 2 c. 3 d. 4 e. not enough info |
1(1)=0.5(1+x) so 2=1+x so x=1 |
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a pt with ARDS (adult respiratory distress syndrome) requires mechanical ventilation. the respiratory compliance is severely impaired at 30 mL/cmH2O. the pt requires positive end expiratory pressure (PEEP) of 15 cmH2O to maintain an adequate pO2. what is the max tidal volume that will keep end inspiratory pressure from exceeding 30cmH2O? a. 400 mL b. 450 mL c. 500 mL d. 600 mL e. 800 mL |
we are solving for the change in volume; C=deltaV/deltaP and we know C=30 and deltaP=30-15=15 so 30(15)=deltaV=450 |
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a normal person is standing upright. which of the following is true? a. pleural pressure is equal at apex and base b. pleural pressure is greater (less negative) at apex than base) c. apical alveoli will receive more ventilation than basilar alveoli d. apical alveoli will receive less ventilation than basilar alveoli e. at FRC and 0 flow alveolar pressure is higher at apex than at the base |
apical alveoli will receive less ventilation than basilar alveoli; pleural pressure increases from apex to base due to the weight of the pleural fluid and the lung; the air inside alveoli is the same pressure everywhere at zero flow; the transmural pressure distending alveoli decreases from apex to base; so there is more pressure at the top but less ventilation because they are stiffer; apical alveoli are more inflated, stiffer (so less ventilation), receive less blood flow, and VQ is greater |
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a ventilator delivers a 600 mL tidal volume at a rate of 20 breaths per minute. the I/E ratio (inspiration/expiration) is 1. the respiratory compliance is determined to be 100 mL/cmH2O. the resistance is 10 cmH2O/(L/sec). what is the max airway pressure measured at the mouth? a. 60 cmH2O b. 40 cmH2O c. 30 cmH2O d. 10 cmH2O e. 4 cmH2O |
10; compliance=deltaV/deltaP 100=600/deltaP deltaP=6; 20 breaths/min so 3sec/breath and I/E=1:1 so an inspiration=1.5 sec; 600ml/1.5sec=400ml/sec flow rate; dynamic pressure to drive flow is flow(resistance)=0.4x10=4; so the static pressure to inflate the lungs (6) plus the dynamic pressure to drive flow (4)=10 cmH2O |
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an anaphylactic reaction to a drug causes the resistance to airflow to increase. there is no change in compliance. tidal volume, rate, and I/E ratio are all unchanged. which of the following statements about peak airway pressure (PAP) is true? a. PAP is unchanged b. PAP is decreased c. PAP is increased |
PAP is increased; this can increase by a decrease in compliance or an increase in resistance |
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a pt with tracheal stenosis is having severe dyspnea. a mix of 79% helium and 21% oxygen would have which effect? a. increase flow due to lower density b. decrease flow due to higher viscosity c. kill the pt due to hypoxemia d. kill the pt due to oxygen toxicity |
increase flow due to lower density; heliox replaces the N in air with He; helium is less dense and more viscous than nitrogen; viscosity determines the resistance to flow in LAMINAR flow; density determines the resistance to flow in TURBULENT flow; flow is laminar in small airways; flow is turbulent in large airways like the trachea; MAKE SURE YOU KNOW THIS WELL BECAUSE THE VERSION ON THE TEST IS MORE DIFFICULT |
