Page 410 - Clinical Application of Mechanical Ventilation

376 Chapter 12

STRATEGIES TO IMPROVE VENTILATION

Hypoventilation causes respiratory acidosis (ventilatory failure) and hypoxemia
PaCO 2 .45 mm Hg is if supplemental oxygen is not provided to the patient. The best measure of
indicative of hypoventilation
(the normal PaCO 2 for COPD a patient’s ventilatory status is the PaCO level. The normal PaCO is 35 to
2
2
patients is about 50 mm Hg). 45 mm Hg; PaCO greater than 45 mm Hg is indicative of hypoventilation.
2
For COPD patients, however, the acceptable PaCO should be the patient’s
2
normal value upon last hospital discharge, and generally it is about 50 mm Hg.
When the PaCO level goes above this value, significant hypoventilation may
2
be present.
Strategies for improving a patient’s ventilation are summarized in Table 12-2.
Increase Ventilator Frequency

The most common approach to improve minute ventilation is to increase the ventila-
tor frequency (f ). This may be the control frequency in assist/control, the mandatory
frequency in synchronized intermittent mandatory ventilation, or other modes of ven-
tilation that regulate the frequency of the ventilator. However, the ventilator frequency
should not exceed 20/min as auto-PEEP may occur at or above this frequency, especially
auto-PEEP: Unintentional PEEP
associated with pressure support during pressure support ventilation (MacIntyre, 1986; Shapiro, 1994). The following
ventilation, high tidal volume and
frequency, inadequate inspiratory
flow, excessive I-time, inadequate
E-time, and air trapping. TABLE 12-2 Strategies to Improve Ventilation

Priority Methods
1 Increase ventilator frequency
Control frequency in assist/control mode
Intermittent mandatory ventilation (IMV) frequency
Synchronized IMV frequency

2 Increase spontaneous tidal volume
Nutritional support and reconditioning of respiratory muscles
Administer bronchodilators
Initiate pressure support ventilation (PSV)
Use largest endotracheal tube possible

3 Increase ventilator tidal volume
Tidal volume in volume-controlled ventilation.
Pressure in pressure-controlled ventilation.

4 Reduce mechanical deadspace
mechanical deadspace: Volume
of gas contained in the equipment Use low-compliance ventilator circuit
and supplies (e.g., endotracheal Cut endotracheal tube to appropriate length
tube, ventilator circuit) that does Perform tracheotomy
not take part in gas exchange.
5 Consider high frequency jet or oscillatory ventilation
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