Week 1: physiology part 3 Flashcards
FVC vs FEV1 definition
- Vital capacity: maximum volume of air that an individual can move in a single breath
- FVC: forced vital capacity that is timed
- FEV1: forced expiratory volume in 1 sec, the volume of air exhaled in the first second
dynamic compression of airways, limits of expiratory flow rates
- people can exhale only 80% of vital capacity in 1 sec
- there is compression of the airways by the lungs which limits the expiratory flow rates
PFT: obstructive pattern
- characterized by increase in airway resistance, measured as decreased expiratory flow rate: e.g. chronic bronchitis, asthma, bronchietasis, alpha-1 AT, emphysema
- decreased FVC and greatly decreased FEV
- increased FRC, TLC, RV
- FEV1/FVC ratio ~50%
PFT: restrictive pattern
- characterized by a decrease in lung compliance
- decrease in all lung volumes: FVC, FRC, TLC, RV, FEV
- FEV1/FVC=88% increased or normal
Flow Volume loops: obstructive vs restrictive
- Flow vs. volume graph with expiration above the axis and inspiration below
- restrictive: small loop since all lung volumes decreased
- obstructive: wider loop because increased TLC, steep drop then plateau on expiration. Inspiration relatively normal
Is spirometry good for detecting early COPD?
No. emphysema affects distal airways, which are in parallel (which means decreased airway resistance). Resistance only sightly increases. Need to damage a lot of alveoli to have an affect that can be measured by spirometry
- the contribution of small airways to airflow resistance is small of because of large cross sectional area in this zone
- early small airway changes of cold may not be measured by PFTs
Why is COPD a problem with exhalation?
- Normally: during inspiration, alveoli enlarge and squish the bronchiole, during expiration, the alveoli shrink and pulls bronchiole open (radial traction)
- in emphysema: lose elastic recoil strength and have impaired expiration. Alveoli can’t compress properly and bronchiole stays compressed. Airtrapping with increased residual volume
Factors that affect Alveolar Oxygen (PAO2)
PAO2=(Patm-47)FIO2 - PACO2/RER
- 47mmGH is the water vapor pressure since the air entering trachea is humidified
1. Atmospheric pressure - an increase in atm pressure will increase PAO2
- high altitude will decrease PAO2
2. FiO2 - an increased in O2 concentration will increase PAO2
3. PACO2 (minute ventilation) - an increase in PACO2 will decrease PAO2
- think of CO2 and O2 finding for surface in alveoli
4. RER-respiratory exchange ratio: CO2 produced/O2 consumed - normally 0.8
RQ vs RER
- RQ is respiratory quotient. Ratio between CO2 production and O2 consumption at the cellular level
- RER is ration of CO2 output and O2 uptake in the lung
- equal in the steady state
Factors affecting Alveolar PCO2
- Metabolic rate
- Alveolar ventilation (minute ventilation)
- if ventilation increases, PACO2 decreases because blowing off the CO2
- unless there is fever or hypothermia, CO2 production is relatively constant
Fick’s law of diffusion
- Structural
- Surface area: decreased in emphysema, increased in exercise
- Thickness: increased in fibrosis and other restrictive diseases - Physiologic
- P1- P2: gas partial pressure difference across the alveolar membrane
- Solubility: CO2>O2>N2
5 causes of hypoxemia
- V/Q mismatch
-main cause: e.g. PE, COPD, ARDS
-responds to O2
-widening of A-a gradient - Anatomical shunting
-Intra-cardiac:
L-R: ASD, VSD, PDA, PFO
R-L: Terrible Ts, Eisenmenger’s complex
-Intra-pulmonary - Hypoventilation
-A-a gradient is normal
-responds to supplemental O2
-drug overdose and neuromuscular disorders - high Altitude
-A-a gradient is normal, responds to supplemental O2 - Decreased diffusion: thickening of alveolar-capillary interface to decrease diffusion of O2
-widening of A-a gradient, responds to supplemental O2
-e.g. interstitial lung disease
A-a gradient
Calculate by
A-a= PAO2-PaO2
-normal gradient is (age+4)/4
