Page 255 - Clinical Application of Mechanical Ventilation

Initiation of Mechanical Ventilation 221

of any mode of ventilation (single or dual) must be based on sound scientific basis
and controlled clinical trials. Furthermore, the patient must be monitored and eval-
uated on a regular frequency in order to ensure the safety and effectiveness of the
mode of ventilation.

Frequency

The initial ventilator frequency is the number of breaths per minute that is
The initial ventilator fre- intended to provide eucapneic ventilation (PaCO at patient’s normal). The
quency should be set between 2
10 and 12/min. initial frequency is usually set between 10 and 12/min. This frequency, coupled
with a 10 to 12 mL/kg tidal volume, usually produces a minute volume that is
sufficient to normalize the patient’s PaCO . Frequencies of 20/min or higher are
2
associated with auto-PEEP and should be avoided (Shapiro, 1994). In addition to
high ventilator frequency, inadequate inspiratory flow and air trapping contrib-
High ventilator ute to the development of auto-PEEP.
frequency, inadequate inspira-
tory flow, and air trapping An alternative method of selecting the initial frequency is to estimate the patient’s
contribute to the develop-
ment of auto-PEEP. minute volume requirement and divide the estimated minute volume by the tidal
volume.
Estimated minute volume
Frequency =
Tidal volume

The estimated minute volume for males is equal to 4.0 multiplied by the body
surface area (BSA) and for females is equal to 3.5 multiplied by the BSA. The BSA
(in square meters) can be obtained from a nomogram such as the Dubois body sur-
face area chart (Appendix 2).

Minute Volume (Male) 5 (4)(BSA)
Minute Volume (Female) 5 (3.5)(BSA)

Adjusting the Frequency. The initial frequency setting of 10 to 12/min and the cal-
Ventilator frequency
is the primary control to culation shown above are based on the assumption that both CO production and
2
regulate the PaCO 2 . physiologic deadspace are normal. If the CO production is elevated (e.g., due to
2
an increase of metabolic rate) or the physiologic deadspace is increased (e.g., due
to a decrease of pulmonary perfusion), the minute volume required to normalize
the PaCO will need to be increased. Since increasing the tidal volume results in
2
higher airway pressures on a volume-limited ventilator, it is usually more appro-
priate to increase the minute volume by increasing the ventilator frequency.
After placing the patient on a ventilator, blood gases should be obtained
frequency if the PaCO 2 within 15 to 30 min after the patient has stabilized, to assess both ventilation
➞
is too high; frequency if the
➞
PaCO 2 is too low. and oxygenation. Since the PaCO varies inversely with the alveolar minute
2
ventilation, a higher than normal PaCO (e.g., .45 mm Hg or .50 mm Hg
2
for patients with chronic CO retention) means the patient’s minute volume
2
should be increased, usually by increasing the frequency. On the other hand, a
lower than normal PaCO (e.g., ,35 mm Hg or ,40 mm Hg for patients with
2
CO retention) indicates that the minute volume should be decreased, usually
2
by decreasing the frequency.

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