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Mechanical Ventilation: Indications and Outcomes

virginia-sinnott-webby Virginia Sinnott, DVM, DACVECC
www.angell.org/emergency
617-522-7282

 

Exposure to mechanical ventilation, for many, is limited to one or two experiences in veterinary school. A lot of veterinarians graduate thinking of the ventilator as a “last resort” option because it is often used as a part of end-of-life care to spare suffering when owners are unable to contemplate euthanasia.  However, when ventilation is applied to the severest forms of reversible respiratory disease, recent literature has shown that outcomes can be quite favorable.  Thus we have learned over the past 15 years that it is the prognosis of the underlying disease which determines outcomes in mechanical ventilation cases not the fact that ventilation was needed.  There are two categories of disease that may require positive pressure ventilation: respiratory failure and pulmonary failure.  Respiratory failure implies that the patient’s thoracic excursions cannot adequately eliminate carbon dioxide while pulmonary failure implies that the lungs are damaged in some way and cannot exchange oxygen properly no matter how hard the pet breathes.

In pure respiratory failure, gas exchange of the lungs is normal, but the patient cannot breathe deeply or fast enough to move gases in and out of the lungs.  There are many disease processes that can cause this problem, and clearly the prognosis is not uniform among them.  For example, a dog that cannot breathe due to brain stem herniation from a large intracranial tumor does not have the same prognosis as a dog that is unconscious after ingestion of marijuana!  Although not common in this area, a recent Australian study revealed that 75% of dogs undergoing mechanical ventilation for tick paralysis or paralytic snake bite (Elapidae) envenomation survived to discharge (when those euthanized for cost reasons were excluded).  Tick paralysis as well as Elapidae envenomation are completely reversible diseases which lead to respiratory arrest.  Other reversible diseases that can lead to respiratory arrest may have similarly good outcomes with mechanical ventilation.  These include: severe marijuana intoxication, opioid drug overdose, botulism and tetanus, hypokalemia, and anesthetic overdose or extreme sensitivity.  Other more serious diseases that lead to respiratory arrest or hypoventilation may also benefit from mechanical ventilation.  These include cervical intervertebral disc disease and flail chest after thoracic trauma as well as post-operative hypoventilation following thoracotomy.  In the case of these diseases associated with severe pain, mechanical ventilation cannot only be life sustaining, but part of the analgesic plan as well.  This is because the ventilator can be set to breathe faster instead of deeper to spare the pain of movement with rib fractures and the opioids required to sedate animals to accept mechanical ventilation are analgesic themselves.

A more serious indication for mechanical ventilation is pulmonary failure.  In this case, the patient may have a normal CO2 level, but cannot oxygenate adequately.  The most reversible forms of pulmonary failure include pulmonary contusions, non-cardiogenic edema from drowning or strangulation, congestive heart failure and some forms of pneumonia.  An older study (2000) of dogs with pulmonary contusions severe enough to require mechanical ventilation showed that 50% of these dogs survived to discharge.  A more recent study looking at severe congestive heart failure cases requiring mechanical ventilation revealed that in looking at dogs after 2005, 75% of the dogs survived to discharge (67% for all dogs in the study).  Given that early mechanical ventilation studies looking at patients requiring mechanical ventilation for myriad diseases quoted 20% overall survival, it stands to reason that choosing potentially reversible diseases to ventilate as well as improvements in care have dramatically improved survival rates.

Angell patient, Rufus, on mechanical ventilation

A patient at Angell Animal Medical Center receiving mechanical ventilation after successful cardiopulmonary-cerebral resuscitation. He was later discharged after a full neurologic recovery.

Mechanical ventilation represents the most aggressive form of respiratory support and often is employed in diseases many vets consider terminal.  As veterinary ventilation techniques were developed in the 1990’s and early 2000’s, mechanical ventilation was reserved for such cases and often the patients did not survive.  This is likely due to a combination of severe disease and a necessary learning curve.  Acute respiratory distress syndrome, a severe form of lung disease, is often thought of as universally fatal in animals, but a recent case report documents two veterinary survivors raising the hope that as we select for reversible underlying causes (such as pneumonia or thoracic trauma) and our ability to ventilate very diseased lungs improves, survival rates will improve.  Recently at Angell Animal Medical Center in Boston, a cocker spaniel suffering a cardiopulmonary arrest was resuscitated, mechanically ventilated in the post-arrest period, and then discharged with a full neurologic recovery (figure 1). Cases such as these illustrate that this tool, once thought of as a terminal exercise, can be life-saving when cases are selected carefully and well-trained individuals prescribe and monitor ventilator care.

For more information, please contact Dr. Sinnott at emergency@angell.org or vsinnott@angell.org, or call 617-522-7282.

 

References

  1. Trigg NL, Leister E, Whitney J, McAlees TJ Outcomes of mechanical ventilation in 302 dogs and cats in Australia (2005-2013) Aust Vet Pract 2014, 44(4): 698-703.
  2. Campbell VK, King LG, Pulmonary function, ventilator management and outcome of dogs with thoracic trauma and pulmonary contusions: 10 cases (1994-1998) J Am Vet Med Assoc 2000: 217(10): 1505-9.
  3. Edwards TH, Coleman AE, Brainard BM et al. Outcomes of positive-pressure ventilation in dogs and cats with congestive heart failure: 16 cases (1992-2012) J Vet Emerg Crit Care 2014: 24(5): 586-93.
  4. Kelmer E, Love LC, DeClue AE, Cohn LA et al. Successful treatment of acute respiratory distress syndrome in 2 dogs. Can Vet J 2012 53(3): 167-173.
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