Page 426 - Clinical Small Animal Internal Medicine

394 Section 5 Critical Care Medicine

potentially lead to respiratory muscle fatigue and exhaus- room. Modern therapeutic ventilators typically offer all
VetBooks.ir tion. Such patients may be able to maintain acceptable three formats.
In A/C ventilation, all breaths are completely machine
blood gas values for a period of time, but uncertainty lies
in how long this effort can be sustained. Determination
mode of ventilation, the clinician sets a minimum res-
of excessive respiratory effort is largely subjective and generated. These are termed mandatory breaths. In this
based on clinical judgment. Intermittent positive pres- piratory rate. If the trigger sensitivity (i.e., pressure or
sure ventilation (IPPV) is indicated in these patients to flow alteration required to trigger delivery of a breath
avoid abrupt exhaustion, asphyxiation, and subsequent that was not prescheduled to be delivered) is set appro-
arrest. Asphyxiation in experimental canine studies is priately then the patient can increase the respiratory
accompanied by an abrupt drop in heart rate of about rate above this minimum value, but all breaths delivered
50 beats per minute. Any comparable drop in pulse rate will be full ventilator‐delivered (mandatory) breaths.
in a dog laboring to breath should prompt immediate The tidal volume of these ventilator breaths will depend
patient evaluation and careful consideration of intuba- on the machine settings (and any patient–ventilator
tion and mechanical ventilation. In these aforementioned dyssynchrony if present). Ventilator breaths can either
canine studies, failure to prevent complete asphyxiation be pressure (PC) or volume controlled (VC). When VC
often leads to cardiac arrest in the form of pulseless is employed, the clinician presets the desired tidal vol-
electrical activity; this is a particularly challenging form ume; the resultant peak airway pressure will be depend-
of cardiac arrest to reverse and is often refractory to ent on the size of the tidal volume chosen and the
therapy. compliance of the respiratory system. More compliant
Patients with excessive respiratory effort usually have lungs will accept a given volume at a lower peak pres-
an excessive mechanical or chemical load imposed upon sure than less compliant lungs. In PC ventilation, the
their respiratory system (see also Chapter 39). clinician presets the peak airway pressure; in this set-
ting, the tidal volume generated will depend on the level
of peak airway pressure chosen and the compliance of
Ventilator Settings the respiratory system. Assist/control ventilation pro-
vides maximum support of the respiratory system and is
Positive pressure ventilators achieve gas flow into the used in patients with severe respiratory disease or those
lungs by generating positive airway pressure in a man- with healthy lungs but reduced respiratory drive or ven-
ner similar to that achieved by squeezing the rebreath- tilatory capacity.
ing bag on an anesthetic machine. Modern therapeutic In SIMV, the operator also sets the number of full
ventilators have an array of settings that can be altered ventilator (mandatory) breaths to be delivered.
to change the characteristics of the delivered breath. However, between these breaths the patient can
Despite the complexity of many therapeutic ventilators, breathe spontaneously, depending on their needs. The
there are only a few key settings that are essential to machine will try to synchronize the ventilator breaths
understand in order to provide effective ventilation in with the patient’s own respiratory efforts. This is
most cases. When choosing ventilator settings, the cli- accomplished by partitioning the periods between
nician must first select a mode of ventilation and then mandatory breaths into two periods. The first period
input initial ventilator settings based on general guide- immediately follows a mandatory breath. During this
lines (described below) and their own understanding of period, the patient may breathe spontaneously with-
the nature of the patient’s respiratory compromise. out triggering a machine‐delivered breath. The second
Following initiation of mechanical ventilation, the venti- period is immediately prior to the next scheduled
lator settings are then adjusted as required to achieve mandatory breath. Patient effort during this period
the blood gas, cardiopulmonary performance, and/or will trigger early delivery of the next mandatory
oxygen delivery goals desired. breath. This latter period is the “synchronized” por-
tion and is what distinguishes SIMV from intermittent
mandatory ventilation. As this mode combines full
Ventilator Breath Types
ventilator breaths with spontaneous patient breaths, it
There are three main ventilator volume delivery (breath) is often used for animals that need less than 100%
patterns commonly used in veterinary medicine: assist/ assistance from the ventilator or during the weaning
control (A/C) ventilation, synchronized intermittent period. Neurologically impaired animals with highly
mandatory ventilation (SIMV), and continuous sponta- labile respiratory drive or patients with resolving lung
neous ventilation (CSV). Some ventilators only have dysfunction may not need as much support as A/C
one breath pattern option, such as a compact volume‐ provides and SIMV can prove a useful option in these
control ventilator designed for use in the operating conditions.