Page 428 - Clinical Small Animal Internal Medicine

396 Section 5 Critical Care Medicine

better matched to the patient’s intrinsic efforts. The Inspiratory:Expiratory (I:E) Ratio and Respiratory
VetBooks.ir “trigger” or “sensitivity” setting determines what the Rate Settings
ventilator will acknowledge as a genuine inspiratory
An operator‐set respiratory rate is available on nearly all
effort by the patient. Appropriate trigger sensitivity is
essential to ensure the ventilator recognizes actual ventilators. A respiratory rate of 15–20 breaths per min-
ute with an inspiratory time of approximately one sec-
respiratory efforts made by the patient while not ond reflects typical initial ventilator settings in the
delivering breaths as a result of minor fluctuations in authors’ practice. These settings can then be adjusted as
circuit pressure or flow. Appropriate trigger sensitivity needed to meet the patient’s needs. The ratio of the dura-
increases patient comfort, reduces dyssynchrony tion of the inspiratory phase relative to the expiratory
between patient effort and machine delivery, and allows phase (I:E ratio) may be preset by the clinician or may be
the patient to increase the respiratory rate as required a setting that is determined automatically based on rate
by their needs. The trigger variable can be too sensitive and inspiratory time settings. Commonly, an I:E ratio of
such that nonrespiratory efforts such as patient han- at least 1:2 is utilized to increase the likelihood that
dling and cardiac contractions may initiate breaths. patients are able to fully exhale prior to the delivery of
This situation should be avoided as it leads to patient the next breath. As respiratory rates are increased, the
discomfort, excessive mean airway pressures, breath expiratory time will be reduced in order to accommodate
stacking, auto‐PEEP, and greater dyssynchrony. An air- the increased number of breaths. High respiratory rates
way pressure drop of –2 cmH 2 O or a gas flow change of can lead to a situation known as auto‐PEEP or intrinsic
2 L/min are reasonable trigger sensitivity settings to PEEP if the patient is not able to fully exhale before the
start with. These settings can then be altered as start of the next inspiration. Such an occurrence can eas-
required. Some ventilators (e.g., AutoTrak on the ily be detected by inspecting the flow scalar graphic if
Phillips Esprit line) have proprietary forms of trigger waveform displays are available. Failure of expiratory
sensing that combine the flow and pressure signals into flow to return to zero before the next breath is delivered
an integrated variable. Evidence of superiority of this is indicative of the presence of auto‐PEEP. To avoid this
means of sensing patient inspiratory effort is currently problem, it is recommended to keep an I:E ratio of 1:1 or
lacking in the veterinary literature.
higher whenever possible. If a higher respiratory rate is
required, a shorter inspiratory time will allow mainte-
Positive End‐Expiratory Pressure Settings nance of an acceptable I:E ratio and is often better toler-
ated by the patient.
Positive end‐expiratory pressure (PEEP) maintains
pressure in the breathing circuit above atmospheric
pressure during exhalation so that the patient cannot General Guidelines for Initial Ventilator Settings
exhale all the way down to functional residual capacity
(FRC). This increased expiratory pressure maintains As discussed above, the optimal ventilator settings for a
the lung at a higher volume, with less small airway col- given patient cannot be predicted reliably. It is likely that
lapse, and improves oxygenating efficiency. PEEP may animals with hypoxemic respiratory failure will require
also help to recruit alveoli collapsed due to atelectasis more aggressive settings than those with hypercapneic
or increased surface tension forces. Evidence suggests respiratory failure. One must choose some initial set-
that it may reduce ventilator‐induced lung injury by tings prior to connecting the patient to the ventilator.
altering cyclic stresses and strains on the alveoli and First the mode of ventilation must be selected; A/C,
peripheral airways. Typically, a small amount of PEEP SIMV or continuous spontaneous ventilation. If A/C or
(~2 cmH 2 O) is applied in all ventilator modes to reduce SIMV are selected then it may be necessary to next
atelectasis and to account for the increased work of decide between volume control (VC) or pressure control
breathing required simply due to the physical nature of (PC) ventilation. The more modern and advanced the
the ventilator circuit (e.g., valves, circuit resistance). In ventilator, the more options one is likely to have.
patients with significant lung disease, much higher lev- Ventilator manufacturers are creating new proprietary
els of PEEP may be required to improve oxygenating ventilation modes faster than clinicans can evaluate
ability. In continuous spontaneous ventilation, CPAP is them. Even the most renowned respiratory therapists in
functionally equivalent to PEEP although they are the world admit that they cannot keep up with every
often generated by different means (i.e., closure of the mode becoming available. This situation is unlikely to
expiratory valve at supraatmospheric pressures versus change as equipment manufacturers are often less than
continuous inflow to maintain a given circuit pressure forthright about how these novel modes are designed.
during the expiratory phase). The settings that need to be selected will vary with the