Page 64 - Clinical Application of Mechanical Ventilation

30 Chapter 2

end-inspiration is higher under conditions of low compliance or high airway resis-
tance. On the other hand, the PIP is lower under conditions of high compliance or
low airway resistance.
In positive pressure ventilation, airway pressures (including PIP and mean airway
mean airway pressure (mPaw):
Average pressure within the pressure [mPaw]) are directly related to the tidal volume, airway resistance, and
airway during one complete respi- peak inspiratory flow rate and inversely related to compliance.
ratory cycle. It is directly related to
the inspiratory time, respiratory These airway pressures (and lung volumes) have a direct impact on the intratho-
frequency, peak inspiratory pres- racic pressure, blood flow, and blood pressure. Indirectly, they can affect the func-
sure, and positive end-expiratory
pressure (PEEP). tions of major organ systems as they depend on adequate blood flow and perfusion.

Compliance

In lungs with normal compliance, about 50% of the airway pressure is transmit-
ted to the thoracic cavity. In noncompliant or stiff lungs (e.g., atelectasis, ARDS),
the pressure transmitted to the thoracic cavity is much less due to the dampening
effect of the nonelastic lung tissues. For this reason, high levels of PIP or positive
end-expiratory pressure (PEEP) may be required to ventilate and oxygenate patients
with low compliance. The decrease in cardiac output due to excessive PIP or PEEP
is less severe than that if the same pressures are applied to lungs with normal or high
compliance (Perkins et al., 1989).

CARDIOVASCULAR CONSIDERATIONS

Mechanical ventilation creates airflow by generating a pressure gradient. In turn,
the pressures in the airways, thoracic cage, and pulmonary blood vessels are altered.
In a clinical setting, the cardiovascular functions should be evaluated and moni-
tored to prevent the adverse effects of positive pressure ventilation on the heart and
blood vessels.

Mean Airway Pressure and Cardiac Output

Positive pressure ventilation increases mPaw and decreases cardiac output. Regard-
positive end-expiratory pres-
sure (PEEP): PEEP is an airway less of the mode of ventilation, a higher mPaw usually results in a lower cardiac
pressure strategy in ventilation
that increases the end-expiratory output (Perkins et al., 1989). Since mPaw is a function of inspiratory time, respira-
or baseline airway pressure to tory frequency, peak inspiratory pressure, and positive end-expiratory pressure
a value greater than atmo-
spheric pressure. It is used to treat (PEEP), these four parameters should be kept to a minimum in order to keep the
refractory hypoxemia caused by mPaw at the lowest level possible.
intrapulmonary shunting.
In comparing continuous positive airway pressure (CPAP) and PEEP, PEEP
exerts a more negative effect on the cardiac output as it raises the mPaw (and PIP)
continuous positive airway proportionally. The effect of PEEP can be detrimental to the cardiac output be-
pressure (CPAP): The end-
expiratory pressure applied to cause PEEP is the end-expiratory pressure used in addition to positive pressure
the airway of a spontaneously ventilation—whereas, in CPAP, the pressure includes only the positive airway pres-
breathing patient.
sure during spontaneous breathing (Figure 2-1).

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