Page 267 - Clinical Application of Mechanical Ventilation

Initiation of Mechanical Ventilation 233

cava, increasing the intravascular resistance and pressures associated with these
structures. The pressures in the vena cava and right atrium are approximately equal
and are collectively known as the central venous pressure (CVP). The usually low
central venous pressure creates an intravascular pressure gradient between the right
Since positive pressure
ventilation increases the atrium and the systemic venous drainage that augments venous blood return to
CVP, the pressure gradient the right atrium. If positive pressure ventilation increases the CVP, the pressure
between the right atrium and
the venous drainage will be gradient between the right atrium and the venous drainage will be decreased with
decreased with a resultant a resultant decreased venous return to the right atrium. If the venous return is
decreased venous return to
the right atrium. significantly reduced, this can result in a decreased cardiac output and arterial
hypotension.
If the patient has no preexisting cardiovascular disease and is not hypovolemic, a
competent cardiovascular system can compensate for a small drop in venous return
A competent cardiovas- and thus maintain cardiac output and blood pressure. The two primary compensa-
cular system can compensate
for a small drop in venous tory mechanisms include an increased heart rate and arterial vasoconstriction initi-
return by an increased ated by the cardiac baroreceptors.
heart rate and arterial
vasoconstriction. The magnitude of the increase in the CVP and the resultant decrease in venous
return depends on several factors, including the airway pressure, lung compliance,
and chest wall compliance. Higher airway pressures will more likely result in higher
pleural and central venous pressures. It is important to note that increases of the
mean airway pressures tend to depress venous return more than increases of the peak
inspiratory pressures.
The degree of increased pleural pressure for a given airway pressure is further af-
fected by the patient’s lung and chest wall compliance. If the patient’s lung compli-
ance is low (stiff lungs), then airway pressures are less readily transmitted into the
pleural space. Therefore in patients with low lung compliance, a given airway pres-
sure will result in a smaller increase in pleural pressure and a less dramatic decrease
in venous return. This does not mean that patients with low lung compliance can-
not have significant decreases in cardiac output due to positive pressure ventilation.
These patients must also be closely monitored for potentially significant decreases in
cardiac output and blood pressure.
Patients with more compliant lungs, such as COPD patients, tend to more readily
High airway pressures transmit a higher airway pressure into the pleural space. Therefore in these patients,
are more detrimental to the
cardiac output in patients a given airway pressure will tend to result in a more dramatic decrease in venous
with high lung compliance return and cardiac output.
than those with low
compliance. The effects of chest wall compliance on the transmission of airway pressure into the
pleural space are exactly opposite to the effects of lung compliance. A low chest wall
compliance (stiff chest wall) will tend to increase the pleural pressure more significantly
for a given airway pressure than a normal chest wall compliance. Conditions in which
the chest wall would be less compliant than normal include the application of tight
chest wall bandages that encircle the thorax and extensive chest wall burn injuries.
All ventilator patients must be monitored for signs of cardiovascular instability.
However, because ventilator patients with suspected or known preexisting cardiovascu-
lar disease are more likely to suffer clinically significant decreases in cardiac output and
blood pressure, these patients must be monitored with an extra measure of vigilance.

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