Pediatric Anesthsia.pdf

CHAPTER 42 ■ Modern Modes of Ventilation in the Operating Room 723
but because those areas of atelectasis, whatever the age of the
patient, contain lung units with prolonged time constants, it is
likely that long periods of high pressure will be required. Direct
observation in neonates and infants undergoing thoracotomy
suggests that long and repeated inspiratory pressures in excess of
20 cmH 2
O are frequently required to reverse areas of atelectasis
from lung compression.
The use of continuous positive airway pressure (CPAP) during
induction has also been shown to prevent atelectasis in adults; in
children, its continued use during spontaneous ventilation may
minimize the formation of atelectasis. 46,47 Patients with tracheo -
malacia are particularly prone to airway collapse and will also
benefit from CPAP. 48 The routine use of CPAP during spontaneous
ventilation with either an ETT or an LMA should also assist in
minimizing the work of breathing by increasing lung volumes
and restoring these to the more efficient part of the pressurevolume
curve.
DESIGN PRINCIPLES OF
MECHANICAL VENTILATORS
Anesthesia ventilators have undergone significant advances since
the late 1990s, changing from being adjuncts or add-ons to the
anesthesia machine to their current position as a prominent and
integral part of the modern anesthesia work station. In parallel,
advances in electronics and microprocessing have led to a number
of advanced features that were limited to intensive care unit (ICU)
ventilators now being available and integrated into anesthesia
ventilators. Thus, more modes of ventilation are available to the
practicing anesthetist in the operating room setting. The speed
of these changes afforded by advances in technology mean that
software updates can be applied to existing anesthesia systems
to improve their function without necessarily requiring replace -
ment of the whole anesthesia machine. Different manufacturers
incorporate different technology into their anesthesia machines
and these models frequently change, so general principles rather
than details of specific machines are discussed whenever possible.
At the same time, practice in ICUs has been moving toward pro -
viding respiratory support augmenting or assisting patients’
breathing rather than completely replacing it, as is the case with
controlled ventilation. Philosophically, the same approach can
now be applied to anesthetized patients if paralysis is not required
for the surgical procedure.
There is no single classification system for ventilators, but
they can be classified according to their power source, drive
mechanism, cycling mechanism, and bellows type. In addition,
anesthesia ventilators may be described as single-circuit or doublecircuit
depending on whether there is a separate gas supply to
power the bellows independent from the gas flowing in the patient
circuit. The power source provides the energy necessary to operate
a mechanical ventilator and is usually electrical or pneumatic.
Many older pneumatic ventilators required only a source of
compressed gas to function. Most contemporary ventilators,
however, require an electrical or electrical plus pneumatic power
source. The drive system provides the actual force required to
generate the gas flow, and in the operating room setting, this
requires a pressure gradient to be developed between the ventilator
and the lungs. They also require a mechanism to provide
variable PEEP.
Double-circuit and Single-circuit Systems
Most anesthesia machine ventilators can be classified as doublecircuit,
pneumatically driven ventilators, but this is changing.
In the double-circuit system, the driving force (i.e., compressed
gas) compresses a bag or bellows, which then delivers gas to the
patient. The driving gas can be either 100% oxygen or a mixture
of oxygen and air.
Some newer anesthesia systems are classified as singlecircuit,
piston-driven, and electronically controlled with fresh gas
decoupling (FGD). The important difference with these systems from
traditional anesthesia circle systems is that, rather than having
separate circuits for the patient gas and drive gas, there is only a single
gas circuit for the patient. The piston functions in the same way as
the plunger of a syringe and can be set to deliver a predetermined
VT or airway pressure to the patient. This type of ventilator uses a
computer-controlled stepper motor rather than compressed gas to
generate gas flow within the circuit. These systems also incorporate
a feature known as fresh gas decoupling (FGD), which is required
for the safe functioning of the systems and is described in Monitor -
ing the System: FGF Compensation, Compliance Compensation.
Although both types of system are widely used in pediatric
practice, the single-circuit, piston-driven anesthesia breathing
systems may perform more reliably when used at high frequencies
and in pressure-control mode likely to be used with neonates and
infants. 49
Bellows
Bellows can be classified by the direction of movement during the
expiratory phase into ascending and descending bellows. As the
names suggest, ascending bellows ascend during the expiratory
phase, whereas descending (hanging) bellows, descend during the
expiratory phase. Most modern electronic ventilators using a
bellows system have an ascending design that is inherently safer
because these bellows do not fill if a total disconnection occurs.
Descending bellows, however, may continue to fill during discon -
nection because, although the driving gas pushes the bellows
upward during the inspiratory phase, during the expiratory phase,
the weight of the bellows as they descend may entrain air into the
breathing system. The situation has become more complicated
recently because some of the new anesthesia systems have incor -
porated descending bellows as part of their FGD system. The
operating principles of ascending bellows in a traditional system
are shown in Figure 42–8, with the ventilator relief valve shown to
the right of the bellows. During the inspiratory phase, the driving
gas entering the bellows chamber causes the pressure within it to
increase, which closes the ventilator relief valve and forces the
anesthetic gas within the bellows into the lungs. During the
expiratory phase, the driving gas exits the bellows chamber and
the pressure within declines to zero. This allows the ventilator
relief valve to open and the gas exhaled by the patient to fill the
bellows and be exhaled via the ventilator relief valve. Gas flows
preferentially to fill the bellows before being scavenged through
the ventilator relief valve, by utilizing a weighted ball in the
ventilator relief valve that produces 2 to 3 cm of back pressure.
This design means that all ascending bellows ventilators produce
2 to 3 cm of PEEP within the circuit, whether PEEP is set on the
machine or not. This is not enough, however, to prevent atelectasis
with controlled ventilation. Scavenging occurs only during the
expiratory phase when the ventilator relief valve is open. Oxygen