Page 412 - Clinical Application of Mechanical Ventilation

378 Chapter 12

volume and frequency are obtained. The increase in spontaneous tidal volume
improves the minute ventilation. It is important to note that PSV is only active
during spontaneous breathing. PSV is only available in modes of mechanical venti-
lation that allow spontaneous breathing (e.g., SIMV).
Low levels of PSV (,10 cm H O) are titrated and used to overcome the airflow
2
Pressure support ventila- resistance of the ventilator circuit and endotracheal tube. At high levels of PSV
tion increases spontaneous
tidal volume, and therefore (.20 cm H O), the breathing pattern resembles pressure-controlled ventilation
2
the minute ventilation.
(Burton et al., 1997; Nathan et al., 1993).
c Minute Ventilation 5 (Ventilator V 3 Ventilator f) 1 (c Spontaneous V
T
T
3 Spontaneous f )
Increase Ventilator Tidal Volume

The ventilator tidal volume is usually set according to the patient’s body weight, and
its range available for adjustments is rather narrow. Excessive ventilator tidal volume
may increase the likelihood of ventilator-related lung injuries. On the other hand,
inadequate ventilator tidal volume may lead to hypoventilation and atelectasis.
Before a decision is made to increase the ventilator tidal volume, one must first
consider the detrimental side effects of excessive volume and pressure. Increasing the
volume should be implemented only when the ventilator frequency is too high and
exceeds the patient’s ideal breathing pattern and I:E ratio.

Other Strategies to Improve Ventilation

Other strategies to improve the minute ventilation may involve use of ventilator cir-
cuits with low compressible volume. This helps to reduce the mechanical deadspace
and volume loss due to the circuit internal pressure and tubing compression factor.
The endotracheal tube is sometimes cut shorter to facilitate tube management,
to clear secretions, and to reduce deadspace. Tracheostomy also improves ventila-
tion by enhancing tube management and secretion removal. In addition, it provides
easier access for oral care and lower deadspace volume than an endotracheal tube.
High frequency jet ventilation has been used primarily in the neonatal popula-
tion. It is effective to improve ventilation in neonates but its usefulness in adult
patients shows mixed results.

Permissive Hypercapnia
permissive hypercapnia:
Intentional hypoventilation of a
patient by reducing the ventila-
tor tidal volume to a range of In volume-controlled ventilation, peak inspiratory pressure creates the pressure
4–7 mL/kg (normally 10 mL/kg). gradient necessary to deliver a predetermined tidal volume. Occasionally the peak
It is used to lower the pulmonary
pressures and to minimize the risk inspiratory pressure can be excessively high in the presence of high airflow resis-
of ventilator-related lung injuries. tance and low compliance. This high level of pressure and volume in the lungs may
The patient’s PaCO 2 is significantly
elevated and the resulting acidotic lead to ventilator-related lung injuries.
pH is neutralized by bicarbonate or Permissive hypercapnia is a strategy used to minimize the incidence of ven-
tromethamine.
tilator-induced lung injuries caused by positive-pressure ventilation (Hickling,

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