Lung volumes

From Iusmphysiology

• started here on 02/14/11 at 9AM.

Contents 1 Lung Volumes 1.1 Lung capacities 1.2 Lung volume and capaicity factors 1.2.1 Position 1.3 Fxnal residual capacity 1.4 Anatomic dead space 1.5 Partial pressures during expiration 1.5.1 Fowler's principle 1.6 Physiologic dead space 1.6.1 Bohr's method 1.6.2 Emphysema pt 1.6.3 Pulmonary embolism 1.7 Obstructive 1.8 Restrictive diseases 1.9 REstrictive and obstructive volumes 1.10 Dynamic functional measurements 1.11 Pattern of forced expirations 1.12 Question example

Lung Volumes

Lung capacities

Lung volume and capaicity factors

Position

• Gravity pulls the diaphragm down, so it starts at a lower position.
• So inspiratory capacity is lower.
• But when standing FRC is increased.
• Opposite is true when supine: FRC is down and IRV (inspiratory reserve) and IC are up.

Fxnal residual capacity

• FRC set by chest wall wanting t expand and the lungs wanting to contract and the balance they meet.
• Position can also matter.
• Normal FRC is about 2.5l
• Normal tidal volum eis about 0.5l
• This ratio mitigates fluctuation of gasses in alveoli.
• Use helium dilution to find the FRC b/c the pt can't blow the air out.
• FRC (V2) = V1(C1 - C2) / C2
• Body plethysmography is more accurate than helium dilution.

Anatomic dead space

• Dead space is where there is no gas exchange.
• Generations 0-16:
  • Nose trhough bronchioles.
  • Increased if breathing trhough a hose
• Estimated via Fowler's method
• Normal = 150ml of air

Partial pressures during expiration

• Air in dead space is exactly the same concentration of the air just consumed.
• So as we exhale, the concentration of the first air coming out is the same as the atmosphere (high oxygen, low CO2).
• The next stuff coming out will be mixed, PCo2 goes up, PO2 goes down.
• Then they flatten off because the last air is from the alveoli.

Fowler's principle

• Uses curve from last slide to estimate dead space.
• Normally, N concentration is high in the atmosphere and our lungs
• Pt breathes only oxygen.
• Hold breath, then exhale.
  • N in lungs will be nearly zero
• First part of the air will havce no nitrogen (because it is dead air and no exchange occurred).
• Then N goes up until it reaches Alveolar gas.
• The midpoint between zero N and alveolar N concentration gives an estimate of dead space.

Physiologic dead space

• Physio dead space = anatomic dead space + any volume of lung not exchanging gas (not getting air -- alveoli collapsed, or not getting blood flow)
• Physio dead space is wasted ventilation.
• We measure physio dead space with Bohr's method.

Bohr's method

• Vd = volume of dead space
• Vt = tidal volume
• PaCO2 = alveolar PCO2
  • Typically 40
• PeCO2 = mixed expired CO2
  • Collect all of the air coming out of the pt,
  • Typically 28.
• Normally only about 165 ml of physio dead space (that includes 150 ml of anatomic dead space)

Emphysema pt

• Tidal volume = 700
• Bohr's equation = Vd / Vt = PaCO2 - PeCO2 / PaCO2
• This pt has a ratio of Vd / Vt = 44%
• That means 44% of air breathing in is going to dead space.
• PeCO2 will be lower because less exchange is happening with atmospheric air so less CO2 comes out.

Pulmonary embolism

• PeCO2 is down because air from alveoli that are exchanging gas is mixed with alveoli that are not exchanging gas.

Obstructive

• COPD, emphysema, asthma (like an obstructive disease), CF, airway obstruction
• Breathing when there is an obstruction to air flow
• Can't get the air out.
• Residual and total capacity go up
• Loss of alveoli
• Loss of interdependence
• Elastic recoil is low
  • Old balloon.
• Chest Xray
  • Over inflation
  • Flattening of diaphragm
  • Narrow mediastinum
• Flow volume curve:
  • Good at bringing air in, high compliance
  • But blowing out is hard
    • Pleaural pressure needs to go really high, which collapses the airways b/c of dynamic compression.
    • The harder they push, the more their airways collapse.
    • Can't get the air out.

Restrictive diseases

• Restrictive: interstitial lung disease, pulmonary edema, pulmonary fibrosis
  • Edema dilutes sufactant to make breathing harder
• Typically involves pathologies with lungs or diaphgragm:
  • Muscular dystrophy: can't expand lung cage.
• Can't get air in!
• Show reduced total lung capacity.
• Increased recoil, decreased compliance.
• Lower gas exchange in something like pulmonary fibrosis because there is a thicker membrane.
• Chest x-ray:
  • Reduced lung volume

REstrictive and obstructive volumes

• Restrictive:
  • Can't get air in.
  • TD is normal but takes lots of effort
  • RV is lower because elastic recoil is higher
• Obstructive:
  • Can't get air out.
  • RV is really high b/c can't get air out.

Dynamic functional measurements

• Forced vital capacity maneuver.
• FEV1 = forced vital capaicty at 1 second
  • Around 3.5 L
  • About 80% of the VC (4500 ml)
• FEF25-75 is the slope of the line
  • Flow rate of the line.

Pattern of forced expirations

This will definitely be on the exam.

Question example

• Wheezing, dyspnea
• 15 yo, m
• No cyanosis
• cXRAy = over inflation
• give bronchodialator, albuterol
  • b/c could be asthma
• a: no, would increase
• b: no
• c: no, already high
• d: no, "you lung capacity is your lung capacity"
• e: yes

• stopped here on 02/14/11 at 10AM.