Lecture 2: Ventilation and Metabolism Flashcards
What is FRC and how is it measured?
FRC= functional residual capacity refers to the lung volume after a normal exhalation.
Measured with body plethysmograph or through Helium dilution method
What is the helium dilution method and what does it measure?
measures functional residual capacity.
How does it work?
you have a bag of helium with a known conc. and vol.
c1, v1
equilibrium is reached, spirometer is attached after a normal breath, lung vol is at frc. He concentration is measured C2
know conc C2 *vol (bag + FRC) which is
C1Vbag = C2 (bag vol +FRC).
- not useful in people with lung disease because of their ventilation problems
What is the body plethysmograph used for and what does it measure and how?
measures functional residual capacity
a subject makes an inspiratory effort against a closed airway, she slightly increases the vol of the lung, airway pressure decr. and box pressure incr
using boyles law, volume is obtained.
why are there differences in ventilation in the lung?
because of the weight of the lung, the intrapleural pressure of the lung is less negative at the base than the negative and thus at resting state is compressed but expands more on inspiration than the apex.
forced vital capacity
volume of gas that can be expired from the lungs with MAXIMUM EFFORT in a given time (1 min).
usually expressed as a fraction of forced vital capacity. (max inhalation to max exhalation)
how can forced vital capacity confirm lung pathology?
high FEV= 90% rigid lungs lungs want to recoil in a restrictive lung ( reduced flow rate, reduced total volume exhaled but since flow rate is related to absolute lung volume, the flow rate is abnormally high in the later part of expiration due to high lung recoil)
low FEV= 42% floppy lungs aka obstructive disease
(total lung capacity if large but expiration ends prematurely)
obstructive lung disease
hard to exhale all the air out of the lungs
ex: asthma
COPD (includes chronic bronchitis, emphysema)
bronchiectasis
restrictive lung disease
hard to fill lungs with air
ex: interstitial lung disease
sarcoidosis
pneumoconiosis
normal FEV?
80%
pulmonary ventilation rate
the volume of gas inspired/expired per unit time
Ve = frequency * tidal volume
ex: 12 breaths/min * 0.5L = 6L/min
alveolar ventilation
aka effective ventilation because it represents the gas that reaches the alveoli and can participate in gas exchange
Va = (tidal volume - anatomic dead space) * frequency
= (500mL - 150mL) *12 breaths/min = 4.2 L/min
What is fowlers method and what does it measure
Fowlers method measures anatomic dead space.
step 1: subject takes a breath of 100% N2. nitrogen sampled at lips reaches zero
step 2: subject exhales and nitrogen sampled at lips increases and plateaus
step 3: plot % [N2] conc as a function of expired lung vol and the area under the curve that is half to max n2 concentration approximates the anatomic dead space
how is inspired air (100% O2) diluted?
1) 75% N2 gas in lungs
2) inspire 500mL of 100% O2
3) 150mL O2 trapped in dead space and 350mL reaches alveoli
4) now we have pure O2 mixed with pre-existing N2 in lung –>diluted N2 but 150mL of pure O2 in dead space
5) exhalation of a tidal volume: 350mL of diluted N2 and 150mL of pure O2
anatomic dead space vs physiologic dead space
Vd is the volume of the lungs which is VENTILATED but not perfused by pulmonary capillary blood
the physiologic dead space is the sum anatomic dead space and the space occupied by non-perfused alveoli
