West Chapter 2: Ventilation - How Gas Gets to the Alveoli

How does inspired gas get to the alveoli?

Ventilation

How do oxygen and carbon dioxide cross the blood-gas interface?

Diffusion

How do oxygen and carbon dioxide moved to and from the lung by the blood?

Blood flow

Draw the Lung Volumes

Tidal Volume

Normal breathing

Functional Residual Capacity (FRC)

Volume of gas in the lung after a normal (or tidal) expiration

Vital Capacity

Maximal inspiration followed by maximal expiration

Residual Volume

Gas remaining in the lung after maximal expiration

What can’t be measured with a simple spirometer?

Functional residual capacity

Residual volume

Total lung capacity

Total Ventilation or Minute Ventilation

Total volume leaving the lung each minute

• The volume of air entering the lung is very slightly greater because more oxygen is taken in than carbon dioxide is given out

Alveolar Ventilation

Volume of fresh gas entering the respiratory zone each minute, the amount of fresh inspired air available for gas exchange

Dead Space Ventilation Equation

Dead space ventilation = dead space volume x respiratory frequency

Tidal Volume Equation

V_T = V_D + V_A

V_T - tidal volume

V_D - anatomic dead space

V_A - alveolar gas

How can alveolar ventilation be increased?

By raising either the tidal volume or respiratory frequency (or both)

• Increasing tidal volume is often more effective because this reduces the fraction of each breath occupied by anatomic dead space (dead space fraction)

Alveolar Ventilation Equation

VA = (VCO2/PCO2) x K

VCO2 - CO2 output or production

PCO2 - partial pressure of CO2

K - constant, total pressure

What can be used to determine alveolar ventilation in healthy individuals?

In healthy individuals, the PCO2 of alveolar gas and arterial blood are virtually identical so arterial PCO2 ca be used to determine alveolar ventilation

What factors increase CO2 production?

Exercise, fever, infection, nutritional intake, and seizures

What factors decrease CO2 production?

Hypothermia and fasting

Anatomic Dead Space

Volume of the conducting areas

What increases anatomic dead space?

Large inhalations because of the traction or pull exerted on the bronchi by the surrounding lung parenchyma

Ratio of Dead Space Volume to the Tidal Volume Equation

V_D/V_T = (PaCO2 - PECO2)/PaCO2

• PaCO2 - partial pressure of arterial CO2

• PECO2 - partial pressure of mixed expired CO2

Basic Alveolar Ventilation Equation

V_A = V_T - V_D

V_T - tidal volume

V_D - dead space volume

What does Fowler’s method measure?

Anatomic dead space

What does Bohr’s method measure?

Physiologic dead space

Measuring FRC Using the Helium Dilution Technique

• Helium is a very insoluble gas, when the subject breathes it almost none of it is taken up by the blood

• C1 - given concentration of spirometer

• V1 - volume of spirometer

• Subject breathes from the spirometer until there is equilibration of the He between the spirometer and the alveolar gas

  • Starts to do this at FRC

• No helium was lost so the concentration x the volume that you started with will be the concentration that you finished with plus the volume of the lung (FRC)

Difference Between Anatomic and Physiologic Dead Space

In normal subjects, the volumes are nearly the same, but in patients with either acute or chronic lung disease, the physiologic dead space may be considerably larger because of inequality of blood flow and ventilation within the lung

• The larger the physiologic dead space, the greater the total ventilation an individual must generate to ensure an adequate amount of air enters the alveoli to participate in gas exchange

• Plethysmograph measures the total volume of gas in the lung

• Helium dilution only measures the volume of gas that communicates with the mouth

  • Sometimes smaller if there's lung disease

Regional Difference in Ventilation

• The lower regions of the lung ventilate better than the upper zones

• When the subject is in the supine position, this difference disappears and apical and basal ventilations are the same

  • In this posture, the ventilation of the lowermost (posterior) lung, exceeds that of the uppermost (anterior) lung

  • In the lateral position, the dependent lung is best ventilated

Measuring FRC with a Plethysmograph

• In an airtight box

• Measure pressure in both with a manometer

• Subject has airway occluded and tries to inhale against the closed airway

  • Small increase in volume of the lung

  • Volume of gas in the box decreases and pressure increases

• Boyle's law - pressure x volume is a constant at a constant temperature