Dräger Instructional CD: Mechanical Ventilation - VentWorld

Key Idea
Mechanical Ventilation
This equation is sometimes expressed in terms of compliance instead of elastance.
muscle pressure + ventilator pressure = ( volume/compliance) + (resistance × flow)
Pressure, volume and flow are variable functions of time, all measured relative to their end
expiratory values. Under normal conditions, these values are: muscle pressure = 0, ventilator
pressure = 0, volume = functional residual capacity, flow = 0. During mechanical
ventilation, these values are: muscle pressure = 0, ventilator pressure = PEEP, volume =
end expiratory volume, flow = 0. Elastance and resistance are constants.
When airway pressure rises above baseline (ie, when ventilator pressure increases),
inspiration is assisted. The pressure driving inspiration is called transrespiratory system
pressure. It is defined as the pressure at the airway opening (mouth, endotracheal tube or
tracheostomy tube) minus the pressure at the body surface. Transrespiratory system pressure
has two components, transairway pressure (defined as airway opening pressure minus lung
pressure) and transthoracic pressure (defined as lung pressure minus body surface
pressure). We may occasionally use the term transpulmonary pressure, defined as airway
opening pressure minus pleural pressure.
Figure 3-1. Models of the ventilatory system. P = pressure. Note that compliance = 1/elastance.
Note that intertance is ignored in this model, as it is usually insignificant.
resistance =
∆ transairway pressure
∆ flow
compliance =
flow
volume
∆ volume
∆ transthoracic pressure
transairway
pressure
transthoracic
pressure
elastance =
transrespiratory
pressure
Equation of Motion for the Respiratory System
∆ transthoracic pressure
∆ volume
P vent + P muscles = elastance x volume + resistance x flow