Page 588 - Clinical Application of Mechanical Ventilation

554 Chapter 17

called the compressible volume, and it is partly caused by the positive pressure ap-
plied to these devices. Higher inspiratory pressures would cause the circuit and hu-
midifier to expand more than those under low pressures. This compressible volume
is also dependent on the characteristics of the circuit and humidifier. More compli-
ant circuits and humidifiers would expand more under positive pressure than those
with low compliance.
Circuit Compression Factor. Neonatal ventilator circuits should have a low compres-
compression factor: The amount
of expansion of the ventilator circuit sion factor. This provides minimal circuit expansion when pressure is applied dur-
or humidifier during the inspiratory
phase measured in mL/cm H 2 O. ing inspiration. A highly compliant circuit would expand under pressure and hold
This volume is considered “lost” and a larger portion of the volume of gas that should be delivered to the patient.
unavailable to the patient.
Humidifiers. Humidifiers that are used in a neonatal circuit should possess a low
compressible volume. A higher compressible volume allows more expansion to
occur under pressure. A humidifier with a high compressible volume may hold
more volume than the neonate’s lungs and thus greatly reduce alveolar ventilation.
The ideal humidifiers for neonatal ventilation are types that incorporate a wick-
To minimize volume loss type system, as they provide excellent warming and humidifying properties and
the circuit and humidifier
used in a neonatal ventilator maintain a low compressible volume.
should have a low compress- Ideally, the temperature of the gas at the trachea should be 37°C with a water con-
ible factor or volume.
tent of 44 mg/L. In a standard circuit (nonheated wire circuit), when inspired gas
temperature is measured at the patient connection, the humidifier must heat the gas
3 or 4 degrees above the desired temperature to overcome the loss of heat after the
gas exits the humidifier. One problem that occurs in this type of ventilator circuit is
condensation or “rain-out” inside the tubing.
Rain-out occurs when the warmed and humidified gas exits the humidifier and
makes contact with the cooler walls of the tubing. This causes the gas temperature
to decrease and condensation to occur on the tubing wall. The water accumulated
in the ventilator tubing may result in increased airway resistance, a higher risk of
contamination, and the potential of accidentally draining the water into the pa-
tient’s lungs. A water trap placed inline with the ventilator circuit helps to prevent
these hazards.

Heated Wire Circuits. To counter this problem, many ventilator circuits have a heated
Heated wire inside the
inspiratory tubing reduces wire inside the inspiratory tubing that runs from the humidifier to the patient con-
condensation. nection, shown in Figure 17-1. The heated wire is attached to a servo-controller
before its entry into the circuit. The temperature of the gas is measured as it exits
the canister and is controlled by the humidifier. The temperature is again measured
at the patient connection by the servo-controller.
The servo heats the gas flow in the inspiratory tubing by heating the wire, which
then heats the inspired gas. Both the humidifier and servo work from a negative
feedback mechanism. The desired temperature becomes the set point, and as the ac-
tual temperature drops below the set point, power is increased until the temperature
returns to the desired level. Newer circuits heat the expiratory gas in addition to the
inspiratory flow, thus minimizing condensation in the expiratory line.
One potential problem with this system is found when the distal temperature
probe is placed at the patient connection inside a heated incubator that is set at a

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