Pulmonary Mechanics and PFTs Flashcards
Functional Residual Capacity
Characteristics
-
“Resting” position of lungs and chest wall
- Inward elastic recoil of lungs = outward forces of chest wall
- Where breathing begins
- Volume where gas exchange mostly occurs
-
Buffers changes in PO2 and PCO2 that occurs with alveolar ventilation
- Prevents wide pH swings
Inspiratory Muscles
-
Diaphragm
- ↑ vertical volume of chest cavity
- ↑ AP diameter ⇒ bucket-handle effect
-
External intercostals
- Raises rib cage
- ↑ AP diameter
Accessory Respiratory Muscles
Sternocleidomastoid and strap muscles
Raises rib cage
↑ AP diameter
Normal Tidal Breathing
- Start at FRC
-
Inspiratory muscles ↑ thoracic volume above FRC
- Pleural pressure ⇒ ⊖
- Alveolar pressure ⇒ ⊖
- Inspiration by negative pressure
- Relaxation of inspiratory muscles
-
Elastic forces passively return lung/chest wall back to FRC ⇒ expiration
- Expiratory muscles used for cough, exercise, respiratory distress
Lung Compliance
Compliance = ∆V / ∆P
- High compliance @ low to moderate lung volumes
- Low compliance @ very high volumes or when lungs deflated
- Max compliance @ FRC
Lung Recoil
Due to:
- Elastic and collagen fibers of the lung
- Surface tension
Pulmonary Mechanics
Effects of Emphysema
- Secretion of proteases, elastases, etc ⇒ destruction of elastic and collagen fibers
- High compliance ⇒ easy to inflate
- Low elastic recoil ⇒ low flow during expiration
- Equal pressure point moves into the lung
- Can lead to collapse of airway
- “Paper bag” model
- Leads to obstructive lung disease
Pulmonary Mechanics
Effects of Bronchitis and Asthma
-
Bronchitis
- Airway narrowing due to inflammation
-
Asthma
- Airway narrowing due to inflammation & bronchoconstriction
Pulmonary Mechanics
Effects of Interstitial Lung Disease
-
↑ Collagen or fibrotic tissue
- Low compliance ⇒ hard to inflate
- High elastic recoil ⇒ high flow during expiration
- Ex. pulmonary fibrosis
- Model ⇒ rubber band around the chest
- Result ⇒ Restrictive lung disease
Obstructive vs Restrictive
Pulmonary Changes
Pulmonary Function Tests
- Spirometry
- Lung volume studies
- Diffusion capacity
PFT
Indications
-
ID cause of dyspnea
- Obstructive vs Restrictive
- Detect/assess reversible airway diseases
- R/O pulmonary cause of dyspnea
- Measure effect of disease on pulmonary function
- Follow course of disease progress or treatment
Spirometry
Measurement of lung volumes as a means of detecting disease.
Tidal Volume
(TV or VT)
Volume inspired with a normal tidal breath.
Normal ~ 5 ml/kg ideal body weight
Residual Volume
(RV)
Volume remaining after a maximal expiration
Inspiratory Reserve Volume
(IRV)
Max volume that can be inhaled above an inhaled tidal volume.
Expiratory Reserve Volume
(ERV)
Max volume that can be exhaled below FRC.
Lung Capacities
Defined as one or more lung “volumes”
Inspiratory Capacity
(IC)
Max volume inspired from resting volume @ FRC.
IC = VT + IRV
Vital Capacity
(VC)
Max volume that can be exhaled after a maximal inspiration.
VC = IRV + VT + ERV
Functional Residual Capacity
(FRC)
Volume remaining in the lung after a normal expired tidal volume.
FRC = RV + ERV
Total Lung Capacity
(TLC)
Max volume that the lungs can contain.
TLC = VC + RV
TLC = FRC + IC
Slow Vital Capacity
VC measured with a slow expiratory maneuver
Forced Vital Capacity
VC performed with a maximal forced exhalation
Combined Vital Capacity
Calculated by adding inspiratory capacity to expiratory reserve volume
Inspired Vital Capacity
VC calculated by measuring complete inhalation from RV
FEV-1
Volume of gas exhaled over 1 sec during the performance of a forced expiration
FEV-1%
FEV-1% = ( FEV-1 / FVC ) x 100
• FEV-1% ≤ 70 or lower limit of normal ⇒ obstruction
FEF 25-75%
Average rate of flow during the middle half of the volume expired during forced expiration.
- Measure of small airways obstruction when FEV-1% is normal
- May be the first signs of COPD
Peak Flow
Max flow obtained during FVC maneuver.
• Tangent of volume-time curve at start of flow
Peak Expiratory Flow
Meters
- Measures initial flow from a forced expiration
- Provides info about daily variability
- Used to detect early signs of deterioration in asthma
- Can be repeated without triggering bronchospasm
- Portable, inexpensive, and reproducible
Bronchodilator Response
A positive response defined as any of the following:
- 12% improvement in FEV-1 and at least 200 cc absolute value improvement
- 12% improvement in FVC and at least 200 cc absolute value improvement
- 30% improvement in REF 25-75%
Pharmacological
Bronchoprovacation Testing
- Non-specific ⇒ produces rxn in most people
- Agents
- Methacholine
- Histamine
- Carbachol
- Specific challenge testing ⇒ oral ASA or inhaled antigen
- Start with low doses (0.03 mg/ml)
- Double until concentration > 16 mg/ml or FEV-1 ↓ ≥ 20%
- If there is < 20% ↓ FEV-1 @ 16 mg/ml ⇒ asthma unlikely
Cold Air
Bronchoprovocation Testing
- Inhalation of dry 32°F air
- Sensitivity overlaps methacholine
- Neither is 100% sensitive
- May be more sensitive and specific for exercise induced asthma
Exercise Challenge
Bronchoprovocation Testing
- Less sensitive than methacholine
- More specific at detecting exercise induced asthma
- 20% ↓ FEV-1 at 5-30 mins post-exercise ⇒ positive
Helium Dilution Method
Pt inhales a known [He] at end of forced expiration.
[He] determined after air allowed to equilibrate with lungs.
Can be used to calculate residual volume.
C1 x V1 = C2 x (V1 + FRC)
Body Box Method
- Patient placed in a closed box
- Inhalation taken at pressure valve starting from FRC
- V1 = FRC
- V2 = V1 + ∆V
- Pressure in box ↑
- Pressure at valve ↓
- Based on Boyle’s Law ⇒ P1V1 = P2V2
- P1V1 = P2(V1 + ∆V)
- P1 and P2 are measured
- ∆V can be calculated from change in box pressure
Obstructive Lung Disease
PFT Changes
- FEV-1 < 80% predicted
- ↔︎ or ↓ FVC
- FEV-1% < 70% or 75% predicted
Obstructive lung disease defined by FEV-1% < 70%.
Obstructive Diseases
Restrictive Lung Disease
PFT Changes
Spirometry suggestive but not diagnostic of restriction.
↓ FEV-1
↓ FVC
↔︎ or ↑ FEV-1%
Restrictive diseases defined by ↓ TLC.
Restrictive Diseases
Summary
- Narrowed airways or overly compliant lungs ⇒ obstruction
- Interstitial disease or other processes reducing compliance ⇒ restriction
- Spirometry ⇒ accurate assessment of obstruction
- Lung volumes ⇒ denies presence of restriction
- Presence of restriction or obstruction on PFTs may give clues to etiology of pulmonary sx
