Page 76 - JSOM Winter 2021

replacing disposables. Specific procedures can be found in the gases with an anesthesia machine is different from rebreathing
manufacturer guidelines. with an ICU ventilator. The Fio may differ from the set oxy-
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gen concentration in the fresh gas as the result of mixing fresh
gas with the rebreathed gas of the patient. Therefore, special
Alarm Management
attention to Fio values and to the Fio low-alarm limit is war-
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For alarm history management, one should be aware that in ranted. The difference between the fresh gas oxygen concen-
some anesthesia machines, alarm notifications are automat- tration and Fio can be reduced to a minimum by increasing
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ically removed when the alarming event has resolved. 6–11 In the fresh gas flow to at least 150% of the minute volume, so
general, the alarm design of ICU ventilators is completely dif- long as adequate O flow rates can be achieved (this may be
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ferent in this respect. Therefore, it is recommended that the difficult in patients with high minute ventilation).
user check periodically the alarm history/alarm log of the an-
esthesia machine to see whether any alarms have been gener- The measures above are important for keeping the installed
ated that may have been unnoticed by the user. base of ventilators functional for patients who need long-term
ventilation. Most anesthesia machines work with an electron-
Most anesthesia machines have basic ventilation alarm capa- ically driven ventilator. These devices do not consume any
bilities, including minute ventilation, low tidal volume, low/ driving gas, and the consumption of gases supplied by the cen-
high respiratory rate, O saturation, end-tidal CO monitor- tral gas supply or from cylinders equals the fresh gas flow set-
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ing, peak pressure, low inspiratory pressure, disconnect of tings. 6–11 For example, if the fresh gas setting is FG flow 9L/min
alarms, and pressure volume limit. Particular attention should and the FG–O concentration is 50%, this will lead to a con-
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be paid to the following default settings when used for ICU sumption of approximately 5.7L/min air and 3.3L/min O .
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ventilation: alarm volume, alarm limits, ventilation settings When using active scavenging, the consumption of centrally
(particularly the maximum pressure limit in volume-controlled supplied gases will be higher.
ventilation modes), fresh gas flow (FG flow ), and fresh gas oxy-
gen concentration. All alarms should remain active in manual/ In general, circuit leakages are not compensated by anesthesia
spontaneous (Man/Spon) modes of ventilation (i.e., the sup- machines. 6–11 This must be considered by the user, especially
pression of alarms in Man/Spon is configured with “No”; this during all volume-controlled ventilation modes. In the presence
setting can be configured in most anesthesia machines). We of a clinically significant circuit leak (as can be seen in patients
further recommend the deactivation of the N O alarm and, with a ruptured endotracheal tube balloon or in those with
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where possible, activation of alarm entries in the device log- pneumothorax), insufficient ventilation situations may occur.
book with set entry interval to ≤2 minutes. Most anesthesia If leakages cannot be avoided, the Pressure Control mode
machines do not have nurse call capabilities. 6–11 may be considered because the delivered inspiratory pressure
will be maintained independent of any leakage as long as the
In addition, the alarm and safety systems of anesthesia ma- capacity of the breathing bag is sufficient. Depending on the
chines are designed for the user to be within 4 meters (approx- device type, the PEEP may fall. In fresh gas–deficit situations
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imately 13 feet) of the machine at all times ; the alarm volume (e.g., leakage plus patient uptake higher than fresh gas flow),
must be adjusted to a sufficiently loud level, particularly in ventilation will be affected. Appropriate alarms such as “Fresh
noisy environments. For most anesthesia machines, the alarm gas low or leakage” will appear. The immediate reaction of
distribution via a serial interface is not designed in a redun- the operator is required (i.e., reduce leakage; increase FG flow ;
dant (i.e., failsafe) way. In situations where a user is not within add up to 70L/min O by pressing the O -flush to refill the
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direct proximity of the anesthesia machine, it must be ensured system with pure oxygen immediately). As an alternative, one
that the alarm volume is set to maximum (100%) to increase may disconnect the manual breathing bag to entrain ambient
the probability that potentially life-threatening situations are air. This prevents a low fresh gas situation and increases the
recognized and addressed in a timely manner. To enable the availability of ventilation. In this case, the resulting inspiratory
anesthesia machine to generate the necessary alarms, we rec- oxygen concentration will be between the Set fresh gas oxygen
ommend setting patient-specific limits for all alarms with the concentration and the 21% of the ambient air.
intention of adapting these limits to changing clinical situa-
tions. Alarm limits for the minute volume (lower and upper It is imperative for the operator to understand the Man/Spon
limits) and the expiratory CO (lower and upper limits) are mode, which is a unique ventilation mode that is not available
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particularly important for generating alarms when hypoven- in most intensive care ventilators. This mode can be lifesaving
tilation or hyperventilation occurs. In contrast to most ICU in case of a failure of automatic ventilation and in the ab-
ventilators, some anesthesia machines also have an adjustable sence of a resuscitator. The influence of the adjustable pres-
airway pressure (P ) low alarm limit. This alarm limit must be sure-limiting (APL) valve must be understood, as well. Users
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set either to automatic/AUTO (if available) or between peak with no anesthesia background may expect that it also limits
end expiratory pressure (PEEP) and inspiratory pressure/pla- airway pressure during mechanical ventilation; the APL valve
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teau pressure to detect unintentionally applied continuous air- has no influence on mechanical ventilation. It is active only in
way pressures as well as intrinsic PEEP situations. the Man/Spon mode. In the event of a ventilator failure, Man/
Spon becomes active automatically, and the fresh gas flow will
make the airway pressure rise to the APL setting. Therefore,
Fresh Gas Management
the APL valve always must be set to a value suitable for the pa-
The management of the oxygen concentration of the inhaled tient. When setting the APL valve to the desired PEEP level (or
gas (measured as the fraction of inspired oxygen, or Fio ) is alternatively SPONT [i.e., spontaneous], which equals zero),
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different from that of traditional ICU ventilators. Fio is ad- one prevents excessive airway pressures from being applied
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justed via manual admixture settings of supplied gas (typically to the patient in the event of a ventilator failure. For the sys-
O and pressurized air). The rebreathing of exhaled patient tem test, the APL valve must be set to a relatively high value.
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74 | JSOM Volume 21, Edition 4 / Winter 2021