A compliant 1 compartment lung with resistance to air flow can be driven by an external negative pressure surrrounding the chest (Pexhaust) or by intrapleural negative pressure (chest or diaphragmatic breathing (Pchest)) or a positive pressure ventilator (P or all three together).

Description

The equations governing airflow in and out of a one compartment lung are
given by the following analogy to electrical circuits:

      Airway pressure is analogous to voltage.
      Air flow is analogous to current flow.
      Volume is analogous to charge.
      Resistance to air flow is analogous to electrical resistance.
      Compliance, the relationship between pressure and volume, is
        analogous to capacitance, the relationship between charge 
        and voltage.

The model shows that flows and volume transients are exponential  
with time constant tau=Res*Com. 
 
The main assumption is that the human lungs can be approximated as a single 
compartment modeled by an RC circuit where the quantities of interest, air 
flow, volume of air, pressure, compliance, and resistance are analogous to 
current, charge, voltage, capacitance, and resistance respectively.

GENERAL RESULTS: The ventilator, any one of the three sources of power using to develop 
a driving pressure of 10 mmHg gives an approximately normal tidal volume of 500 ml/breath. 
Normally of course, the driving force is provided by expansion of the chest, creating a
negative pressure in the intrapleural space, just the oppposite of a positive pressure
ventilator. When the chest is paralyzed, often occurred with severe polio, the patient 
was put in an IRON LUNG tank, sealed at the neck with the head out, and the chest expanded 
by pulling air from the tank, lowering the intratank pressure and expanding the chest.
 

Equations

The equations for this model may be viewed by running the JSim model applet and clicking on the Source tab at the bottom left of JSim's Run Time graphical user interface. The equations are written in JSim's Mathematical Modeling Language (MML). See the Introduction to MML and the MML Reference Manual. Additional documentation for MML can be found by using the search option at the Physiome home page.