Page 228 - Clinical Application of Mechanical Ventilation

194 Chapter 7

complications and a shorter stay in an acute care setting are two additional benefits
of NPPV (Antonelli et al., 1998).
During the polio epidemics, negative pressure ventilators (i.e., iron lungs) were used
to provide ventilation by generating a pressure gradient between the atmosphere and
lungs. Air flows into the lungs and ventilation occurs when the pressure in the lungs
becomes subatmospheric. Disadvantages of negative pressure ventilators include upper
airway obstruction and lack of access for patient care. Modern ventilators generate the
pressure gradient between the atmosphere and lungs by positive pressure. With positive
pressure ventilation, the pressure in the airway opening is higher than that in the lungs.
Some of the disadvantages of positive pressure ventilation are tracheal injury, infection,
ventilator-associated pneumonia, barotrauma, and prolonged hospital stay (Antonelli
With NPPV, many et al., 1998; Diaz et al., 1997; Keenan et al., 1997; Kramer et al., 1995).
complications of negative and NPPV provides ventilation via the patient’s nose, mouth, or face (nose and
positive pressure ventilation
are minimized or eliminated. mouth) without an artificial airway. For this reason, many complications of nega-
tive and positive pressure ventilation are minimized or eliminated.

TERmINOlOgy

In noninvasive positive pressure ventilation, the meanings of some terms are slightly
different from traditional usage. These terms, abbreviations, and a brief description
for each term are outlined in Table 7-1.

PhySIOlOgIC EffECTS Of NPPV

As in traditional positive pressure ventilation, NPPV has two primary pressure
settings. One pressure setting is used during the inspiratory phase and the other
during the expiratory phase.
inspiratory positive airway During the inspiratory phase, the inspiratory positive airway pressure (IPAP)
pressure (IPAP): An airway works like any other positive pressure breathing (ventilation) device. The IPAP level
pressure that is above 0 cm H 2 O
during the inspiratory phase of a is similar to the peak airway pressure in traditional mechanical ventilation. In gen-
respiratory cycle.
eral, the set value is directly related to the IPAP level. A higher IPAP level would
result in a larger tidal volume and minute ventilation. The expiratory positive air-
expiratory positive airway way pressure (EPAP) is the same as PEEP during mechanical ventilation or CPAP
pressure (EPAP): An airway during spontaneous breathing. In addition to its ability to improve oxygenation by
pressure that is above 0 cm H 2 O
during the expiratory phase of a increasing the functional residual capacity, EPAP also relieves upper airway obstruc-
respiratory cycle.
tion with its splinting action.
The level of IPAP and EPAP can be titrated according to a patient’s oxygenation
and ventilation needs. Since two benefits of NPPV are improvement of PO and
PaO 2 , PaCO 2 , SpO 2 , and 2
P ET CO 2 may be used for titra- PCO (Brown et al., 1998; Nicholson et al., 1998), these two parameters can be
2
tion of IPAP and EPAP. used as titration endpoints. If arterial blood gas results are available, oxygenation
(PaO ) and ventilation (PaCO ) endpoints can be easily assessed. Alternatively,
2
2
pulse oximetry (SpO ) and capnography (P CO ) may be used for the titration of
ET
2
2
appropriate IPAP and EPAP levels.

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