Page 256 - Clinical Application of Mechanical Ventilation

222 Chapter 8

TABLE 8-6 Calculation of Predicted Body Weight

The predicted body weight (PBW) in pounds (lb) and kilograms (kg) can
be calculated as follows:
Male PBW in lb 5 106 1 [6 3 (Height in inches 2 60)]
Female PBW in lb 5 105 1 [5 3 (Height in inches 2 60)]
Convert the patient’s body weight from pounds to kilograms by dividing
pounds by 2.2.
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Tidal Volume

The initial tidal volume is usually set between 10 and 12 mL/kg of predicted
The initial tidal volume body weight. Usually the patient’s actual weight can be used for selecting the tidal
is usually set between 10 and
12 mL/kg of predicted body volume unless the patient is significantly underweight or overweight. Table 8-6
weight. shows a method to calculate the patient’s predicted body weight.
The lower end of the acceptable tidal volume range (i.e., about 10 mL/kg) may be
appropriate for certain patients. Tidal volumes as low as 6 mL per kg of predicted
body weight have been recommended for ARDS patients (de Durante et al., 2002).
The primary reason for using lower tidal volumes (i.e., permissive hypercapnia) is
to minimize the airway pressures and the risk of barotrauma (Feihl et al., 1994).
However, use of low tidal volume ventilation may lead to complications such as
acute hypercapnia, increased deadspace ventilation and work of breathing, dyspnea,
severe acidosis, and atelectasis (Kallet et al., 2001a, 2001b).
Decreasing the tidal COPD patients may also benefit from a reduced tidal volume setting. These pa-
volume by 100 to 200 mL in
COPD patients reduces the tients have reduced expiratory flow rates due to decreased alveolar elastic recoil. For
expiratory time requirements this reason, a longer expiratory time is needed for complete exhalation. If there is
and helps to prevent air
trapping. not enough time for complete exhalation, air trapping, V/Q mismatch, hypoxemia,
and hypercapnia may result. Decreasing the tidal volume by 100 to 200 mL in
COPD patients reduces the expiratory time requirements and helps to prevent air
trapping. A higher flow rate may also be used to shorten the inspiratory time and
flow rate: Peak flow during the
inspiratory phase. It determines lengthen the expiratory time.
how fast the tidal volume is For patients with a reduction of lung volumes due to lung resection, lower
delivered to the patient.
tidal volumes may also become necessary. Table 8-7 lists examples of clinical
conditions where lower tidal volume settings may be beneficial or necessary for
circuit compressible volume: the patient.
Expansion of the ventilator circuits
during inspiration leading to a Gas Leakage and Circuit Compressible Volume. The tidal volume actually delivered to
small “lost” volume of gas that
does not reach the patient, but the patient’s lungs is usually lower than the set tidal volume. This is mainly due to
is recorded as part of the expired (1) gas leakage in the ventilator circuitry, (2) gas leakage at the endotracheal tube
tidal volume.
cuff, and (3) circuit compressible volume loss.

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