absorbent and CO2 are beneficial to the patient and to the proper function of the absorber.
However, one reaction might prove to be a hindrance in excess. Removal of carbon dioxide
is clearly beneficial and a primary function of the absorbent. In the process, a chemical
reaction produces a color change in the absorbent, which acts as a visual indicator of
the absorbent’s activity. The color of absorbent changes from off-white to violet as its
ability to absorb carbon dioxide diminishes. Under certain, but rare, conditions, the color
change reverts to the original. A more common problem is that users overlook, or forget
to notice, the color change. In addition, optical properties of the housing can change, thus
making the color change difficult to see. Channeling occurs when respiratory gases take
the path of least resistance and channel through a relatively narrow cross-section. This can
result in unwanted inspired carbon dioxide.
Heat and water are by-products of CO2 absorption. The heat generated is not excessive
and is beneficial to the patient because it helps to warm cool supply gases, which could
irritate the lung lining. Water produced is also beneficial because supply gases are dry,
and added moisture reduces lung irritation. If water production is neglected, it will build
to the point where it could contribute to problems in the breathing circuit. The most frequently
noted problems are sticking expiratory check valves from surface tension
between valve disc and housing, and water accumulation in hoses and tubing to the ventilator,
affecting proper function by inhibiting proper pressure sensing and gas flow.
Gas Scavengers
When the machine is set for an FGF rate of anything higher than the patient’s uptake (normally
200–300 ml of oxygen per minute for most adults), any excess must be removed to
avoid a build-up of volume as discussed above. Although PEEP is used for certain clinical
indications, its uncontrolled application is disastrous for the patient. The scavenging
system plays a vital role in conjunction with the breathing circuit to prevent this from
happening. During automated ventilation, the bellows pop-off valve opens at the end of
tidal expiration to divert excess volume to the scavenging system. During manual ventilation,
the user must open and close (as necessary) an adjustable pressure-limiting (APL)
valve to set the upper limit for breathing-circuit pressures. If the APL valve is open, or
any pressure occurs in excess of its setting, gas flows to the scavenger. The physical construction
of scavengers used on the machines varies and can require user interaction. The
various designs offer their own risks and benefits; some put greater demands on the hospital
infrastructure (vacuum pumps), while others require the user to make adjustments
according to FGF rate.
PEEP Valves
During spontaneous ventilation, the body maintains a residual volume in its lungs to prevent
them from collapse. During positive-pressure ventilation, the ventilator maintains a
slight PEEP (2–3 cm H2O) to accomplish the same. Clinical indications (e.g., adult respiratory
distress syndrome [ARDS]), PEEP valves are used to increase the resistance that
a patient encounters during exhalation, increasing the residual volume in their lungs.
Different models of PEEP valves can be permanently installed either on the
machine/absorber or temporarily on the expiratory limb. For the latter, the design is a variation
of a check valve, making it dangerous if installed backwards or in the inspiratory
limb, as it would result in little gas flow and inadequate oxygenation (Figure 90-4).
Humidification
As discussed previously, supply gases are dry and can irritate the lung’s lining, particularly
with susceptible patients who are on the machine for extended periods. Although the
use of active humidifiers has declined over the years, they may still be used for certain
patients. Two techniques are used to maintain moisture in the gas that the patient breathes:
The heated active humidifier and the heat moisture exchanger (HME).
Active heated units are generally more effective in adding both humidity and heat to
the breathing circuit. The operator must connect the unit to an electrical power source,
must monitor its temperature, and must clean and fill them. Many units incorporate check
valves to prevent inadvertent delivery of fluid to the patient if they are installed incorrectly.
Backward installation can result in significant flow restriction and inadequate oxygenation.
These units are installed in the inspiratory limb and have a predetermined “in”
and “out.”
HMEs are single-use, disposable units. They are connected to the distal end of the
breathing-circuit tubing that is proximal to the endotracheal tube. Their primary function
is to maintain as much heat and humidity as possible in the patient while preventing heat
and humidity from entering the absorber. HMEs are occasional