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Initiation of Mechanical Ventilation 217

an increasing minute volume is a sign of improving lung functions, in actuality the
patient may not be able to sustain the increased work of breathing. Muscle fatigue
can occur over time and lead to eventual ventilatory failure. In addition, an increase
in minute volume achieved by an increased frequency and a decreased tidal vol-
ume lead to a larger percentage of deadspace (wasted) ventilation. This condition
increases the oxygen cost of breathing and carbon dioxide production, progressive
hypercapnia, and hypoxemia.
Vital capacity. If the patient’s vital capacity is less than 15 mL/kg, then im-
pending ventilatory failure is likely. An accurate measurement of vital capacity
requires patient cooperation, which may be difficult to achieve during impend-
ing ventilatory failure. The maximum inspiratory pressure measurement can
be used as its alternative if the patient is unable to perform the vital capacity
maneuver.
Maximum inspiratory pressure. The maximum inspiratory pressure (MIP) is a
maximum inspiratory pres-
sure (MIP): Also called negative measure of the inspiratory muscle strength reflecting the patient’s pulmonary re-
inspiratory force (NIF). MIP reflects serves. Patients with an MIP of greater than 225 cm H O obtained within 20 sec
a patient’s respiratory muscle 2
strength. MIP of less than –20 cm can be assumed to have a vital capacity of 15 mL/kg (Shapiro et al., 1991). When
H 2 O (e.g., –10 cm H 2 O) is one of MIP is less than 220 cm H O, it is one of the signs of impending ventilatory
the indications for impending 2
ventilatory failure. It is obtained failure.
by measuring the maximum
negative pressure during a forced The MIP is obtained by measuring the maximum negative pressure that the
inspiratory maneuver against a patient can generate with a forced inspiratory maneuver against a negative
closed manometer.
manometer (pressure measuring device). Although the MIP maneuver can be
performed using a face mask, it is easier to obtain with an endotracheal or tra-
cheostomy tube.
MIP can be measured by using a T-piece with one port attached to the endotra-
cheal or tracheostomy tube, one port attached to the negative pressure manometer,
and one port attached to a special unidirectional valve that allows exhalation only.
The patient is encouraged to exhale to residual volume and then inhale as forcefully
as possible. The unidirectional valve allows the patient to exhale so that the subse-
quent MIP maneuvers can be performed from the residual volume level (Caruso
et al., 1999).
PaCO trend. A gradual but persistent increase of the PaCO to more than 50 mm
2
2
Hg is indicative of impending ventilatory failure. The PaCO measurements should
2
be done over a period of time and on an as-needed basis. The PaCO should be
2
interpreted along with the patient’s breathing pattern since progressive tachypnea is
common during impending ventilatory failure.
Vital signs. Any clinical indicators that show a patient is under distress or is tir-
ing must also be considered when assessing for presence of impending ventilatory
failure. These indicators include tachycardia, arrhythmias, hypertension, tachypnea,
use of accessory respiratory muscles, diaphoresis, and cyanosis.

Severe hypoxemia is Severe Hypoxemia
present when the PaO 2 is less
than 60 mm Hg on 50% or
more of oxygen or less than
40 mm Hg at any F I O 2 . Hypoxemia is a common finding in lung diseases. When hypoxemia is severe,
mechanical ventilation may be necessary to support the oxygenation deficit. ALI,

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