Pulmonary Mechanics and PFTs

Question 1

Functional Residual Capacity

Characteristics

Answer 1

• “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 P_O2 and P_CO2 that occurs with alveolar ventilation
  
  • Prevents wide pH swings

Question 2

Inspiratory Muscles

Answer 2

• Diaphragm
  
  • ↑ vertical volume of chest cavity
  • ↑ AP diameter ⇒ bucket-handle effect
• External intercostals
  
  • Raises rib cage
  • ↑ AP diameter

Question 3

Accessory Respiratory Muscles

Answer 3

Sternocleidomastoid and strap muscles

Raises rib cage

↑ AP diameter

Question 4

Normal Tidal Breathing

Answer 4

1. Start at FRC
2. Inspiratory muscles ↑ thoracic volume above FRC
   
   • Pleural pressure ⇒ ⊖
   • Alveolar pressure ⇒ ⊖
   • Inspiration by negative pressure
3. Relaxation of inspiratory muscles
4. Elastic forces passively return lung/chest wall back to FRC ⇒ expiration
   
   • Expiratory muscles used for cough, exercise, respiratory distress

Question 5

Lung Compliance

Answer 5

Compliance = ∆V / ∆P

• High compliance @ low to moderate lung volumes
• Low compliance @ very high volumes or when lungs deflated
• Max compliance @ FRC

Question 6

Lung Recoil

Answer 6

Due to:

• Elastic and collagen fibers of the lung
• Surface tension

Question 7

Pulmonary Mechanics

Effects of Emphysema

Answer 7

• 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

Question 8

Pulmonary Mechanics

Effects of Bronchitis and Asthma

Answer 8

• Bronchitis
  
  • Airway narrowing due to inflammation
• Asthma
  
  • Airway narrowing due to inflammation & bronchoconstriction

Question 9

Pulmonary Mechanics

Effects of Interstitial Lung Disease

Answer 9

• ↑ 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

Question 10

Obstructive vs Restrictive

Pulmonary Changes

Answer 10

Question 11

Pulmonary Function Tests

Answer 11

• Spirometry
• Lung volume studies
• Diffusion capacity

Question 12

PFT

Indications

Answer 12

• 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

Question 13

Spirometry

Answer 13

Measurement of lung volumes as a means of detecting disease.

Question 14

Tidal Volume

(TV or V_T)

Answer 14

Volume inspired with a normal tidal breath.

Normal ~ 5 ml/kg ideal body weight

Question 15

Residual Volume

(RV)

Answer 15

Volume remaining after a maximal expiration

Question 16

Inspiratory Reserve Volume

(IRV)

Answer 16

Max volume that can be inhaled above an inhaled tidal volume.

Question 17

Expiratory Reserve Volume

(ERV)

Answer 17

Max volume that can be exhaled below FRC.

Question 18

Lung Capacities

Answer 18

Defined as one or more lung “volumes”

Question 19

Inspiratory Capacity

(IC)

Answer 19

Max volume inspired from resting volume @ FRC.

IC = VT + IRV

Question 20

Vital Capacity

(VC)

Answer 20

Max volume that can be exhaled after a maximal inspiration.

VC = IRV + VT + ERV

Question 21

Functional Residual Capacity

(FRC)

Answer 21

Volume remaining in the lung after a normal expired tidal volume.

FRC = RV + ERV

Question 22

Total Lung Capacity

(TLC)

Answer 22

Max volume that the lungs can contain.

TLC = VC + RV

TLC = FRC + IC

Question 23

Slow Vital Capacity

Answer 23

VC measured with a slow expiratory maneuver

Question 24

Forced Vital Capacity

Answer 24

VC performed with a maximal forced exhalation

Question 25

Combined Vital Capacity

Answer 25

Calculated by adding inspiratory capacity to expiratory reserve volume

Question 26

Inspired Vital Capacity

Answer 26

VC calculated by measuring complete inhalation from RV

Question 27

FEV-1

Answer 27

Volume of gas exhaled over 1 sec during the performance of a forced expiration

Question 28

FEV-1%

Answer 28

FEV-1% = ( FEV-1 / FVC ) x 100 • FEV-1% ≤ 70 or lower limit of normal ⇒ obstruction

Question 29

FEF 25-75%

Answer 29

**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

Question 30

Peak Flow

Answer 30

**Max flow obtained during FVC maneuver.** • Tangent of volume-time curve at start of flow

Question 31

Peak Expiratory Flow Meters

Answer 31

* **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

Question 32

Bronchodilator Response

Answer 32

_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%**

Question 33

Pharmacological Bronchoprovacation Testing

Answer 33

* **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**

Question 34

Cold Air Bronchoprovocation Testing

Answer 34

* Inhalation of dry 32°F air * Sensitivity overlaps methacholine * Neither is 100% sensitive * **May be more sensitive and specific for exercise induced asthma**

Question 35

Exercise Challenge Bronchoprovocation Testing

Answer 35

* Less sensitive than methacholine * More specific at detecting exercise induced asthma * **20% ↓ FEV-1 at 5-30 mins post-exercise ⇒ positive**

Question 36

Helium Dilution Method

Answer 36

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)

Question 37

Body Box Method

Answer 37

* 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

Question 38

Obstructive Lung Disease PFT Changes

Answer 38

* FEV-1 \< 80% predicted * ↔︎ or ↓ FVC * FEV-1% \< 70% or 75% predicted **Obstructive lung disease defined by FEV-1% \< 70%.**

Question 39

Obstructive Diseases

Answer 39

Question 40

Restrictive Lung Disease PFT Changes

Answer 40

Spirometry suggestive but not diagnostic of restriction. ↓ FEV-1 ↓ FVC ↔︎ or ↑ FEV-1% **Restrictive diseases defined by ↓ TLC.**

Question 41

Restrictive Diseases

Answer 41

Question 42

Summary

Answer 42

* 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