Page 429 - Clinical Small Animal Internal Medicine

40 Mechanical Ventilation 397

mode of ventilation chosen. In VC the tidal volume is sufficiently such that it will tolerate the endotracheal
VetBooks.ir preset by the operator, once the patient is connected to tube and PPV but to avoid excessive depth of anesthesia
in an effort to minimize the adverse effects of the anes-
the machine the operator should note the peak inspira-
tory pressure associated with this tidal volume and
monitoring and constant tinkering with the anesthetic
determine whether it is acceptable for this patient. In PC thetic drugs. This requires some experience in anesthetic
ventilation, the clinician presets the peak airway pres- drug infusion rates as drug requirements will change
sure. Once the patient is connected to the ventilator the with time.
operator will need to note the tidal volume and mean air- Typical intravenous anesthetic doses used to maintain
way pressure and determine whether they are acceptable patients on mechanical ventilators at the authors’ prac-
for this patient. tice include the following.
These settings can then be altered as needed once the
patient is connected to the ventilator. Whenever possi- ● Midazolam: loading dose 0.2–0.4 mg/kg IV; constant
rate infusion (CRI) 0.1–0.5 mg/kg/h IV.
ble, it is best to have the ventilator already running and Propofol: loading dose 0.5–2.0 mg/kg IV; CRI 0.05–
confirmed to be working properly before the patient is ● 0.4 mg/kg/min IV.
connected. When this is not possible, for example initiat- Fentanyl: loading dose 2–5 μg/kg IV; CRI 1–7 μg/kg/h IV.
ing mechanical ventilation for patients already on an ●
anesthetic machine, then it is best to start with quite The vast majority of patients receiving long‐term
conservative settings and titrate them upwards as mechanical ventilatory support in our practice are
required to avoid inadvertent over‐distension of the anesthetized with some combination of these three
lung. It is imperative to always have a method for per- agents. The occasional patient will also receive a dex-
forming manual ventilation (e.g. Ambu‐bag, Bain’s cir- medetomidine infusion (0.1–3.0 μg/kg/h) either as a
cuit) close at hand during IPPV in case of equipment replacement for one of those three agents or in addition
malfunction, power failure, or operator error. to them. Etomidate is rarely if ever given and has
resulted in marked suppression of adrenal function
when it was employed. Diazepam may be substituted
Mechanical Ventilation for Hypoxemic for midazolam as required. Pentobarbital, once a main-
Respiratory Failure stay in our practice, is rarely used now due to relative
expense, reduced availability due to shortages, and pro-
Patients suffering from hypoxemic respiratory failure longed recovery times.
often require inspiratory pressure settings that are sub- Anesthetic depth should be formally assessed every
stantially greater than normal. Pulmonary parenchymal hour and recorded on the patient’s flow sheet/medical
disease often reduces lung compliance and increases air- record, but caregivers should be continuously assessing
way resistance. The result is that higher pressures will be anesthetic depth as they work on the patient. It is impor-
needed to achieve the same tidal volume. It is now recog- tant to have drug dose charts that correctly convert the
nized that excessive distension of the lung (or volu- mg/kg/h drug doses to mL/h for the drug concentration
trauma) is a major cause of ventilator‐induced lung you are using. In addition, it is recommended to have pre-
injury. For this reason, it is recommended to deliver calculated some “rescue doses” of drugs in case animals
smaller tidal volumes when ventilating patients with sig- suddenly become light and try to extubate themselves.
nificant pulmonary parenchymal disease. In very severe
lung disease such as ARDS, it may be necessary to deliver Initial Stabilization on the Ventilator
tidal volumes as low as 6 mL/kg.
When patients are first connected to the ventilator cir-
cuit, an inspired oxygen fraction (FiO 2 ) of 100% is advised
Anesthesia
as a safety measure. The FiO 2 can be reduced later once
Unless the patient has significant neuromuscular or stabilization of the patient has been achieved. When
intracranial disease, anesthesia will be necessary to allow ventilating patients with hypoxemic respiratory failure, it
positive pressure ventilation and maintenance of an air- can be beneficial to keep them in sternal recumbency
way. Balanced anesthesia incorporating 2–3 drugs is during the initial stabilization period. These animals fre-
advised to reduce the dose required (and adverse effects) quently oxygenate better in sternal recumbency than in
of any single agent. Propofol or alfaxalone infusions are other body positions. After connection, the patient’s tho-
commonly used anesthetic agents in conjunction with a racic wall is observed for appropriate movement. When
benzodiazepine and/or an opioid and/or a dexmedetomi- insufficient or excessive thoracic wall motion is observed,
dine infusion. The aim is to keep the patient anesthetized the ventilator settings should be adjusted appropriately.