Effect of Preexisting Respiratory
Dysfunction on the Respiratory Effects of Anesthesia
Anesthesiologists are frequently required to care for (1) patients
with acute chest disease (pulmonary infection, atelectasis) or systemic diseases
(sepsis, cardiac and renal failure, multiple trauma) who require emergency operations,
(2) heavy smokers with subtle pathologic airway and parenchymal conditions and hyperreactive
airways, (3) patients with classic emphysematous and bronchitic problems, (4) obese
people prone to decreases in FRC during anesthesia,[130]
(5) patients with chest deformities, and (6) extremely old patients.
The nature and magnitude of these preexisting respiratory conditions
determine, in part, the effect of a given standard anesthetic on respiratory function.
For example, in Figure 17-32
,
the FRC-CC relationship is depicted for normal, obese, bronchitic, and emphysematous
patients. In
Figure 17-32
The lung volume (ordinate) at which tidal volume is breathed
decreases (by 1 L) from the awake state to the anesthetized state. Functional residual
capacity (FRC), which is the volume of lung existing at the end of tidal volume,
therefore also decreases (by 1 L) from the awake to the anesthetized state. In healthy,
obese, bronchitic, and emphysematous patients, the awake FRC considerably exceeds
the closing capacity (CC). In obese, bronchitic, and emphysematous patients, the
anesthetized state causes FRC to be less than CC. In healthy patients, anesthesia
causes FRC to equal CC.
a healthy patient, FRC exceeds CC by approximately 1 L. In the latter three respiratory
conditions, CC is 0.5 to 0.75 L less than FRC. If anesthesia causes a 1-L decrease
in FRC, a healthy patient will have no change in the qualitative relationship between
FRC and CC. In patients with special respiratory conditions, a 1-L decrease in FRC
will cause CC to exceed FRC and will change the previous marginally normal FRC-CC
relationship to either a grossly low V̇A/
or an atelectatic FRC-CC relationship. Similarly, patients with chronic bronchitis,
who have copious airway secretions, may suffer more from an anesthetic-induced decrease
in mucus velocity flow than other patients. Finally, if an anesthetic inhibits HPV,
the drug may increase shunting more in patients with preexisting HPV than in those
without preexisting HPV. Thus, the effect of a standard anesthetic can be expected
to produce varying degrees of respiratory change in patients who have different degrees
of preexisting respiratory dysfunction.
