Measurement of Respiratory Mechanics
Principles
Mechanical properties of the respiratory system include the passive
mechanical properties of resistance, elastance, and inertance, in addition to the
motor properties of the muscles of respiration. Resistance (R), in pulmonary mechanical
terms, is the increment in pressure (ΔP) applied to the system divided by the
increment in flow or rate of change of volume (ΔV):
Normal resistance is 1.5 cm H2
O · L-1
· sec,
but under anesthesia it may be as high as 9 cm H2
O/L/sec. Most of the
resistance of the respiratory system results from the flow resistance of the major
conducting airways. A small portion of the resistance is caused by the tissue viscosity.
Conductance (G) is the reciprocal of resistance:
Elastance (E) is the applied pressure divided by the resulting
static change in volume:
A more commonly used variable is compliance (C), the reciprocal
of elastance:
A commonly used unit for compliance is mL/cm H2
O.
Because the lung and chest wall are mechanically in series, the following relationships
apply:
In Equation 38, CTH, CL,
and CCW are the thoracic, lung, and chest wall compliances,
respectively.
Inertance (I) is the applied pressure divided by the gas acceleration
or the second derivative of volume with respect to time:
Inertance is analogous to the mass of tissue and gas within the lung. Normal thoracic
inertance is 0.02 to 0.04 cm H2
O · L · sec-2
.
Ordinarily, because acceleration of gas flow and mass of gas and tissue are low,
inertance plays a minor part in the mechanical behavior of the respiratory system.
However, under conditions of high-frequency ventilation or high inspired gas density
(e.g., in deep diving), inertance may play a major role in determining gas flow.
