Page 456 - Clinical Application of Mechanical Ventilation

422 Chapter 13

DRUGS FOR IMPROVING VENTILATION

Airway narrowing is a common complication in patients receiving mechanical ven-
tilation. Increasing peak inspiratory pressure, wheezing, hypoxemia, and agitation
are some clinical signs that indicate the presence of airway distress.
For the ventilator patient, airway distress may be caused by (1) preexisting airway
disease (chronic bronchitis, asthma), (2) drug-induced bronchospasm, (3) accumu-
lated secretions, and (4) mechanical irritation. Whatever the cause, the distress must
be recognized and corrected quickly to prevent hypoxemia and further deterioration.
Along with bronchial hygiene procedures, bronchodilators and corticosteroids
play a critical role in achieving optimal airway patency and constitute the majority
of nebulized drugs used in respiratory care.
Autonomic Nervous System Agents

The smooth muscles of the airway are under autonomic (involuntary) nervous con-
trol. Patients do not have control over the patency of the affected airways. When
airways constrict due to different mechanisms, bronchodilators are commonly used
to provide airway dilation and patient relief. The following sections discuss the
bronchodilators that exert their effects on two major pathways (i.e., sympathetic
and parasympathetic) under the autonomic nervous system (Figure 13-1).
Sympathetic and Parasympathetic Branches. The autonomic nervous system (ANS)
comprises motor neurons that innervate tissues under involuntary control. Among
important functions regulated by the ANS are respiration, heart rate, blood pres-
sure, perspiration, and glandular secretions. The sympathetic and parasympathetic
fibers are the basic subdivisions of the ANS (the enteric pathway is a third subdi-
vision not of direct interest to this discussion), and, for the most part, they elicit
responses in an opposing manner (at the effector sites). For example, stimulation
of the sympathetic branch results in bronchodilation, whereas stimulation of the
parasympathetic branch causes bronchoconstriction (Tortora et al., 2002).
For example, the interaction of the sympathetic and parasympathetic branches
can be represented by two teams in a very well-balanced tug-of-war match. The goal
is to keep the flag tied to the middle of the rope from crossing the line on either
team’s side. Both teams always pull to keep tension on the rope (basal tone) but oc-
casionally the sympathetic team gives an extra surge of effort to pull the flag in its
direction, and the parasympathetic team quickly responds by increasing its effort
until the other team tires and the flag is once more pulled back toward the middle
ground. Bronchodilators produce appropriate changes in the sympathetic or para-
sympathetic branch.

Adrenergic and Cholinergic Responses. The sympathetic pathway terminal axon uses the
neurotransmitter substance epinephrine (adrenaline)/norepinephrine, from which the
term adrenergic response is derived.
The neurotransmitter substance released at the terminal axon of the parasympa-
thetic fiber is acetylcholine (ACh) and it elicits a cholinergic response.

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