Page 411 - Clinical Application of Mechanical Ventilation

Management of Mechanical Ventilation 377

equations show that an increase in ventilator frequency (f ) leads to a higher minute
ventilation.
c Minute Ventilation 5 (Ventilator V 3 c Ventilator f) 1 (Spontaneous V
T
T
3 Spontaneous f )
It is generally not desirable to increase the ventilator tidal volume beyond a
The most common level that is appropriate to the patient’s body weight, generally 10 mL/kg (Burton
approach to improve minute
ventilation is to increase the et al., 1997). In volume-controlled ventilation, a larger tidal volume requires a
respiratory frequency of the higher peak inspiratory pressure. This high-pressure condition increases the in-
ventilator.
cidence of ventilator-related lung injuries such as cardiovascular impairment and
barotrauma.
To estimate the ventilator frequency needed to achieve a certain PaCO , the fol-
2
lowing formula may be used, assuming the ventilator tidal volume and deadspace
volume stay unchanged (Feihl et al., 1994; Barnes (Ed.), 1994; Burton et al.,
1997).

(Frequency * PaCO )
2
New frequency =
Desired PaCO 2
New frequency: Ventilator frequency needed for a desired PaCO
2
Frequency: Original ventilator frequency

2
See Appendix 1 PaCO : Original arterial carbon dioxide tension
for example.
Desired PaCO : Desired arterial carbon dioxide tension
2

Increase Spontaneous Tidal Volume or Frequency

In most modes of mechanical ventilation, minute ventilation is the sum of the
volume delivered by the ventilator and the volume achieved by a spontaneously
breathing patient. For this reason, the patient can contribute to the minute ven-
tilation by increasing either the spontaneous tidal volume or the spontaneous
frequency.

c Minute Ventilation 5 (Ventilator V 3 Ventilator f ) 1 (c Spontaneous V
T
T
3 c Spontaneous f )
It is more advantageous for a patient to increase the spontaneous tidal volume
since increasing the frequency usually results in shallow breathing (i.e., rapid shal-
low breathing pattern) and promotes deadspace ventilation. V /V ratio is increased
T
D
because of an unchanged anatomic V in concurrence with a reduced V . (V /T
D
D
T
V 5 higher V /V ratio).
D
T
T
In some patients, the respiratory muscles are not sufficient to maintain prolonged
spontaneous ventilation: Vol- spontaneous ventilation or to overcome airflow resistance imposed by the ventila-
ume of gas inspired by a patient. It
is directly related to the patient’s tor circuit and endotracheal tube. This condition may be compensated by using
spontaneous tidal volume and pressure support ventilation (PSV). The level of pressure support is usually started
frequency.
at 10 to 15 cm H O (Shapiro, 1994) and titrated until a desired spontaneous tidal
2
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