Page 259 - Clinical Application of Mechanical Ventilation

Initiation of Mechanical Ventilation 225

between 80 and 100 mm Hg (lower for patients with chronic CO retention).
2
After stabilization of the patient, the F O is best kept below 50% to avoid oxygen-
I
2
induced lung injuries (Shapiro et al., 1994).
For patients with mild hypoxemia or patients with normal cardiopulmonary
functions (e.g., drug overdose, uncomplicated postoperative recovery), the initial
F O may be set at 40% or at the patient’s F O prior to mechanical ventilation. It
2
I
2
I
must also be evaluated and changed accordingly by means of subsequent blood gas
analyses and correlated with pulse oximetry trending.
PEEP

Positive end-expiratory pressure (PEEP) increases the functional residual capac-
ity and is useful to treat refractory hypoxemia (low PaO not responding to high
2
F O ). The initial PEEP level may be set at 5 cm H O. Subsequent changes of
2
2
I
Set the initial PEEP at PEEP should be based on the patient’s blood gas results, F O requirement, toler-
5 cm H 2 O and make changes I 2
based on the patient’s blood ance of PEEP, and cardiovascular responses. For other methods to titrate optimal
gas results, F I O 2 require- PEEP, see Table 12-4 (“Titration of optimal PEEP using PaO and compliance
ment, tolerance of PEEP, and 2
cardiovascular responses. as indicators”) and Table 15-4 (“Decremental recruitment maneuver (RM) to
determine optimal PEEP”).
I:E Ratio

The I:E ratio is the ratio of inspiratory time to expiratory time. It is usually kept
I:E ratio: A time ratio comparing
the inspiratory time and expiratory in the range between 1:2 and 1:4. A larger I:E ratio (longer E ratio) may be used
time, normally between 1:2 and on patients needing additional time for exhalation because of the possibility of air
1:4 in mechanical ventilation. This
ratio is regulated by the inspiratory trapping and auto-PEEP. Presence of air trapping during mechanical ventilation
flow rate, I time, or E time and is may be checked by occluding the expiratory port of the ventilator circuit at the
affected by the tidal volume and
respiratory rate. end of exhalation. Auto-PEEP is present when the end-expiratory pressure does not
return to baseline pressure (i.e., 0 cm H O or the PEEP level when PEEP is in use)
2
at the end of expiration. Presence of auto-PEEP should be apparent on ventilator
Auto-PEEP is present
when the end-expiratory waveforms (e.g., pressure-time waveform).
pressure does not return to Inverse I:E ratios have been used to correct refractory hypoxemia in ARDS patients
baseline pressure at the end
of expiration. with very low compliance. But it should not be the initial I:E setting since reverse
I:E ratio has its inherent cardiovascular complications. Inverse I:E ratio should be
tried only after traditional strategies have failed to improve a patient’s ventilation
Presence of auto-PEEP and oxygenation status.
should be apparent on Depending on the features available on the ventilator, the I:E ratio may be altered
ventilator waveforms (e.g.,
pressure-time waveform). by manipulating any one or a combination of the following controls: (1) flow rate,
(2) inspiratory time, (3) inspiratory time %, (4) frequency, and (5) minute volume
(tidal volume and frequency).

Effects of Flow Rate on I:E Ratio. Adjusting the flow rate is the most common method
to change an I:E ratio because the flow rate control is a feature available on almost all
ventilators. Table 8-9 shows the effects of flow rate change on the I time, E time, and
I:E ratio when the V and f are kept unchanged. Note that the I time and I:E ratio
T
are inversely related. A longer I time leads to a lower I:E ratio (Tejeda et al., 1997).

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