Ventilator Therapy for the Critical Patient

Julien Guillaumin, Columbus, Ohio

In humans, modern mechanical ventilation (MV) took off during the Copenhagen poliomyelitis epidemic of 1952. Survival rates in the 1960s were 30% to 40% but dramatically improved to almost 90% in the early 1980s. MV in veterinary medicine is lagging behind its human counterpart, with less than 250 dogs and 100 cats reported in the clinical literature.

Indications

The indications for MV in veterinary medicine include (1) ventilatory failure characterized by hypercarbia, with a cutoff of PaCO₂ greater than 60 mm Hg or PvCO₂ greater than 65 mm Hg; (2) hypoxemic failure, with objective cutoffs including a PaO₂ lower than 60 mm Hg or pulse oximetry lower than 90% despite oxygen therapy (FiO₂ ≥ 60%); and (3) increased work of breathing with a risk of respiratory arrest. This last criterion is a subjective assessment of a patient showing evidence of respiratory fatigue or substantial distress, even if the blood gas values are still acceptable. Under these circumstances, delayed intervention is unethical and medically inappropriate.

There are only three large retrospective studies of MV in veterinary medicine (Table 11-1). Another set of four studies researched specific populations of canine patients (see Table 11-1). The prognosis for survival to discharge varies greatly depending on the primary disease process. Survival to discharge is reported to be up to 86% for animals with toxicoses, who are usually younger patients with reversible causes and no pulmonary parenchymal disease; 40% for those with pulmonary contusions; and 11% for post–cardiopulmonary resuscitation (CPR) patients (Hopper, 2007). The overall prognosis ranges from 21% to 71% in dogs and from 15% to 42% in cats (see Table 11-1).

TABLE 11-1

Available Retrospective Studies or Case Series of Mechanical Ventilation in Small Animal Clinical Patients

MV, Mechanical ventilation; LMN, lower motor neuron.

Placing a Patient on the Mechanical Ventilator

Selection of drugs for anesthetic induction and sedation depends on the clinician’s preference and the patient’s status (see Chapter 13). Some unconscious patients may not require drugs, but most will not tolerate MV without sedation. Various options are available. Most clinicians use a combination of benzodiazepine and an opioid and then add propofol, barbiturates, ketamine, or dexmedetomidine as needed. For the benzodiazepines, midazolam is preferred over diazepam because diazepam binds to plastic syringes and infusion lines, can precipitate with other drugs, and is less available. Fentanyl is usually the opioid of choice, at “anesthetic doses” of 6 to 20 µg/kg/hr. Propofol is generally a safe anesthetic but its use generates some concerns, notably involving volume, cost, hyperlipidemia, and bacterial contamination because of the soy-based carrier. Long-term propofol use in cats is discouraged because it can cause Heinz body anemia. Pentobarbital was most widely used (65% to 80%) in veterinary medicine in the 1990s but has become obsolete. Although induction and plane of anesthesia are smooth on barbiturates, tremors and seizures can potentially occur at the time of weaning. Ketamine has some cardiovascular sparing effects and requires small volumes when administered as a constant-rate infusion. Dexmedetomidine has cardiovascular side effects including bradycardia, hypotension, and vasoconstriction but can be used at “microdoses” to enhance analgesia and sedation and to decrease the opioid and propofol requirement in some patients.

Paralytics such as nondepolarizing neuromuscular blocking agents (NMBAs) are controversial in human medicine. NMBAs should only be used as a last resort and the clinician should first attempt to troubleshoot both the machine and the patient when patient-ventilator dyssynchrony (PVD) occurs.