Page 429 - Clinical Application of Mechanical Ventilation

Management of Mechanical Ventilation 395

Circuit Compliance

The compliance of ventilator circuits should be as low as possible. High circuit com-
pliance leads to a higher compressible volume in the circuit during inspiration, and
this condition reduces the effective tidal volume delivered to the patient. For example,
at a peak inspiratory pressure of 40 cm H O, a ventilator circuit with a compliance
2
of 5 mL/cm H O would expand and hold 200 mL (40 cm H O 3 5 mL/cm H O)
2
2
2
of the set tidal volume. At the same peak inspiratory pressure, a ventilator circuit
with a compliance of 3 mL/cm H O would have a compressible volume of only
2
120 mL (40 cm H O 3 3 mL/cm H O). Unless a tidal volume adjustment is made
2
2
to account for the circuit compliance factor, the effective (delivered) tidal volume to
the patient would be reduced substantially when high compliance circuits are used
(Burton et al., 1997).
Circuit Patency

Condensation imposes the most common threat to the patency of ventilator cir-
cuits. Gas temperature drops as it travels from the heated humidifier to the patient.
As the temperature drops along the circuit, water vapor condenses and water col-
lects in the tubing. This condition leads to significant airflow obstruction. A heated-
wire circuit (Figure 12-3) and an inline water trap (Figure 12-4) have been used
successfully to reduce condensation and the amount of water in the circuit.

Heat and Moisture Exchanger (HME). Figure 12-5 shows a heat and moisture exchanger
(HME) that may be used as a temporary humidification device. The HME is placed
between the patient’s artificial airway and the ventilator circuit. During exhalation,
moisture and heat from the patient are absorbed by the condensation surface of the
HME impregnated with CaCl or AlCl . The moisture and heat are transferred back
2
2
to the patient during the next inhalation. The efficiency of HME units ranges from
70% to 90% relative humidity and 30°C to 31°C (White, 2004). Compared to the
heated humidifier, ventilator circuits with a bacterial-viral filtering HME cost less to
maintain and are less likely to colonize bacteria (Boots et al., 1997; Kirton et al., 1997).

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Figure 12-3 A heated-wire circuit.

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