Page 148 - Clinical Application of Mechanical Ventilation

114 Chapter 4

Addition of Auto-PEEP. Since IRV provides a longer I time and shorter E time, breath
The presence of auto-
PEEP during IRV may help to stacking with an increase of end-expiratory pressure is likely when there is not
reduce shunting and improve enough time for complete expiration (Duncan et al., 1987; Kacmarek et al., 1990).
oxygenation in ARDS patients.
The presence of auto-PEEP during IRV may help to reduce shunting and improve
oxygenation in ARDS patients (Shanholtz et al., 1994).

Adverse Effects of IRV

During IRV, the increase in mPaw and the presence of auto-PEEP both contribute
to the increase of mean alveolar pressure and volume, and the incidence of baro-
trauma may be as high as that obtained by conventional ventilation with high levels
of PEEP (Tharratt et al., 1988).
Another potential hazard of IRV is a higher rate of transvascular fluid flow or
flooding induced by an increased alveolar pressure (Permutt, 1979). This condition
may induce or worsen preexisting pulmonary edema.
Patients receiving IRV are often agitated. They may require sedation and neu-
romuscular blocking agents to facilitate ventilation. The associated complications
with these drugs can be serious and they should be monitored carefully when used
in conjunction with IRV (Hansen-Flaschen et al., 1993).

Pressure Control-IRV (PC-IRV)

Since IRV may increase the mPaw, create auto-PEEP, and increase the incidence
When PC is used with of barotrauma, it is sometimes used in conjunction with pressure-controlled ven-
IRV, the peak airway pressure
may be kept at a safe level. tilation due to its pressure-limiting capability. By using pressure control, the peak
airway pressure may be kept at a safe level. This strategy helps to minimize pressure-
induced lung injuries. When an inverse I:E ratio is used with pressure-controlled
ventilation, it is called pressure control-inverse ratio ventilation (PC-IRV).
Several studies compare the outcomes of ARDS patients before and after the
implementation of PC-IRV. The changes that may occur when positive pressure
ventilation with PEEP (PPV 1 PEEP) is switched over to the PC-IRV mode of
ventilation are summarized in Table 4-11.

TABLE 4-11 Observed Changes after Switching from PPV 1 PEEP to PC-IRV

Increase Decrease No Change
Mean airway pressure PEEP requirement F O requirement
2
I
Central venous pressure Peak airway pressure Intrinsic PEEP

Pulmonary artery pressure Cardiac output Blood pressure
PaO 2 PaCO 2

(Data from East et al., 1992; Lain et al., 1989, 1990.)
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