Page 135 - Biomedical Engineering and Design Handbook Volume 2, Applications

114 MEDICAL DEVICE DESIGN

Similar to electrical resistance and capacitance, airway resistance and lung compliance together
impose a frequency-dependent impedance to ventilation. Thus, the normal lung emptying passively
follows an exponential decay with a single time constant equal to the product of the resistance and
the compliance. Because the lung is composed of millions of lung units, potentially having regional
differences in impedance, dynamic characteristics of ventilation affect how quickly the lung may
inflate or deflate, and may also result in portions of the lung being underventilated, while other por-
tions are either normally ventilated or even overventilated.
In addition to ventilating the alveoli, another important function of the respiratory system is gas
exchange, the process of moving oxygen into and carbon dioxide from the blood supply. Gas exchange
occurs through the passive diffusion of gases across the alveolar membrane. Diffusive gas exchange is
affected by the thickness and permeability of the alveolar membrane, the total surface area available for
diffusion, and the availability of blood flow to receive or deliver the diffused gases. One common pul-
monary test, the diffusion capacity, indirectly assesses the alveolar membrane by measuring the dif-
fusion of carbon monoxide from the lungs into the blood. Of course, gas exchange can occur only
with gas that has actually reached the alveoli, which is considerably less than the amount of gas mov-
ing through the nose and mouth. In fact, in a normal person at rest, only about two-thirds of the air
inspired in a single breath reaches the alveoli. The remaining one-third occupies the dead space, por-
tions of the lung, airways, and nasopharynx that do not participate in gas exchange. In certain con-
ditions, alveoli that are ventilated may not participate in gas exchange due to aberrations in diffusion
or to alterations in blood flow that reduce or eliminate perfusion. These nonexchanging alveoli
increase the physiologic dead space (as distinguished from anatomic dead space, which comprises
the mouth, nasopharynx, trachea, and other conducting airways) and reduce the alveolar ventilation.
Dead space ventilation, then, is “wasted” ventilation and must be subtracted from the total ventila-
tion in order to determine the effective alveolar ventilation.
Once the pulmonary circulation has undergone gas exchange with ventilated alveoli, the oxygen-
enriched blood circulates to body tissues, which consume the oxygen and replace it with carbon

dioxide. The net consumption of oxygen (V O 2 ) is not equal to the net respiratory production of car-
bon dioxide (V CO 2 ), owing to the fact that cellular metabolic pathways do not maintain a 1:1 balance
between the two, and also to the fact that carbon dioxide may be excreted in the urine as well as

through the respiratory system. At rest, the ratio of V CO 2 to V O 2 , the respiratory quotient (RQ), is
typically 0.7. During maximum exercise, this value may rise to well above 1.0. Measurements of

V O 2 , V CO 2 , and RQ are important in assessing the adequacy of nutrition of a critically ill patient, the
diagnosis and treatment of patients with various pulmonary and cardiovascular diseases, and the
training of elite athletes.
Most of the physiological parameters related to the respiratory system vary across age groups,
gender, and ethnicity, and most are affected significantly by the size of the individual being assessed
(large people tend to have large lungs). Therefore, interpreting the meaning of measured parameters
often relies on comparing results with those obtained in large population studies of “normal”
(disease-free) people of similar characteristics. There are many sources for the equations available
to predict these normal values. While many devices attempt to include information about the pre-
dicted values, the most useful devices will allow the user to choose from among many sets.

4.3 IMPORTANT PRINCIPLES OF GAS PHYSICS

Most measurements made in the context of the respiratory system assume that the gas or gas mix-
ture involved obeys the ideal gas law:
PV = nRT (4.1)

where P = pressure (atmospheres, atm)
V = volume (liters, L)
n = moles of gas (mol)
T = temperature (kelvin, K)
R = gas constant [= 0.082057 (atm • L)/(mol • K)]