Exam 3, L5 Flashcards
(27 cards)
What happens to FRC when a patient moves from upright to supine?
It decreases due to abdominal contents pushing on the diaphragm.
What lung volume is primarily responsible for this FRC reduction?
Expiratory reserve volume (ERV) decreases.
How does total lung capacity change with position?
It doesn’t change much unless the patient is very obese or elderly.
What does the early part of expiration on a capnograph represent?
Dead space gas — no CO₂ present.
What does the plateau phase on a capnograph indicate?
Alveolar gas, showing gradual CO₂ increase due to ongoing capillary offloading.
What causes a normal slope in the capnograph plateau?
Continuous offloading of CO₂ into alveolar gas during expiration.
What causes an inverted slope in severe emphysema?
Early collapse of base alveoli → more CO₂ comes from apex (less perfused) → lower average CO₂ in late expiration.
What is alveolar dead space?
Alveoli that are ventilated but not perfused — no gas exchange.
How does alveolar dead space affect ETCO₂ readings?
It dilutes CO₂, making ETCO₂ lower than arterial PCO₂.
What does a large gradient between arterial PCO₂ and ETCO₂ suggest?
High alveolar dead space, possibly due to emphysema or overdistended alveoli.
What does the Bohr equation calculate?
Physiologic dead space (anatomical + alveolar).
What is the Bohr equation (using partial pressures)?
V_D/V_T = P_aCO₂ - Pˉ_ECO₂/P_aCO₂
What does PaCO₂ represent in the Bohr equation?
Arterial PCO₂, approximating alveolar PCO₂ if there’s no shunt.
What does Pˉ_ECO₂ represent?
Mixed expired PCO₂, from total exhaled gas.
How is anatomical dead space measured?
Fowler’s test, using nitrogen washout.
What is normal physiologic dead space in a healthy adult?
About 150 mL or 30% of tidal volume.
What is pulmonary compliance?
Change in volume per unit pressure: Compliance = ΔV/ΔP
What is normal pulmonary compliance?
~0.2 L/cm H₂O
What is the pleural pressure change during normal inspiration?
From –5 to –7.5 cm H₂O (ΔP = 2.5 cm H₂O)
How does emphysema affect pleural pressure and lung volume?
Higher lung volume but less negative pleural pressure due to lost elastic recoil.
How are the lung and chest wall arranged mechanically?
In series, so total compliance is less than either component alone.
What is the formula for total compliance of a system in series?
1/C_total = 1/C_lung + 1/C_chestwall
How is resistance calculated in series?
R_total = R_1 + R_2
How is resistance calculated in parallel?
1/R_total = 1/R_1 + 1/R_2
