Critical Care in Cats:  Nutrition and Respiratory Support

The cat is unique in the way it responds to disease.  As previously discussed, many times, cats are not presented to their Doctor until later in the disease process because they are masters at hiding their problems.  Compound this with the fact that they are obligate carnivores and cannot efficiently utilize glucose for energy, many patients are presented in a negative energy balance.  Any condition which causes an unwillingness or inability to eat such as anorexia, head or neck trauma, or neurologic abnormalities combined with stress mediators can lead to an override of the body's normal response to fasting (decreased metabolic rate and protein conservation).  Instead, there is a significantly increased rate of protein turnover to assist with the current disease process.  While this occurs, other proteins such as albumin and transferrin are decreased.  After 2-3 days of anorexia, muscle proteins begin to break down for a source of amino acids needed for synthesis of acute phase protein synthesis, energy for leukocytes and wound healing.  The increased use of amino acids for glucose production is reflected by an increased nitrogen excretion in the urine.  For a senior cat patient, who may have already have loss of lean mass, the protein catabolism of critical illness accelerates this loss even further.

The consequences of severe stress of illness are that the cat may be unlikely to provide sufficient amino acids for high priority protein synthesis.  This results in compromised immune function, decreased wound healing and erythropoiesis (red cell production), and increased morbidity and mortality.  Cats with chronic disease may be protein depleted at presentation due to decreased food intake concurrent with ongoing nitrogen loss from the disease process.

Whew, what does this mean?  In a nutshell, it means ALL patients could benefit from nutritional support.  Additionally, body mass (underweight or with evidence of muscle wasting, despite body fat stores (overcoat syndrome), weight monitoring, appetite summary and diet evaluation need to be performed on a regular basis.  Increased nutrient losses from vomiting, diarrhea, wounds, and acute exacerbations of chronic disease are all risk factors for malnutrition and indicators for nutritional support if intake remains inadequate.

During hospitalization, the patient’s nutritional needs are evaluated daily after the patient is stabilized by correcting hydration deficits, electrolyte and acid base disorders.  Even for patients managed at home, it is imperative to know the nutrient requirements (both in protein and calories), to measure and record nutrient intake and to intervene with nutritional support when intake does not meet the requirements.

The golden rule for nutrition is "If the gut works, use it."  It is preferred to feed by the gastrointestinal tract.  No matter which method is used, it is imperative that total calorie intake is known.  It is not acceptable for the kitty to eat 1 tsp when it needs 1/4 cup.

Voluntary:  The first attempt should be oral or coax feeding.  Nursing care such as hand feeding, petting, stroking and verbal soothing will go a long way with many cats.  Owner visitation with familiar food may also assist in this process.  Syringe feeding or the use of an oral feeding tube are not well tolerated and do not provide enough food.  Additionally, in the cat, these methods of feeding can promote the formation of dietary food aversion, a serious issue in the cat.  Appetite stimulants (short term), behavioral medications, pain control and Feliway may also assist.

Involuntary:  Feeding tubes are commonly used to provide nutritional support to cats.  They can be used in the short term (i.e. after oral surgery/trauma), long term (i.e. chronic kidney disease patients who may need assistance), and medium range (i.e. hepatic lipidosis, neoplasia, pancreatitis).  Tubes are easy to place and once the patient is stabilized, owners can easily feed at home.  If the patient desires, it can still eat and drink.  In fact, many patients will actually need to be weaned off of their tubes due to preference.

The most common types of tubes used are nasoesophageal (NE), Esophogostomy (e-tube) or Gatrostomy tubes (PEG or G-tube). Each tube has advantages and disadvantages, which can be discussed with the Doctor.  The type of food used to feed the patient will be determined by the underlying disease process.  Each cat is an individual, so feeding schedules are tailored to the patient.  Common issues include vomiting/regurgitation, general anesthesia for placement, cellulitis at the insertion site, or patient removal.  Successes are very common even with these risks, and feeding tubes should always be viewed as a bridge to health, unlike an end of life option as seen in humans.

The respiratory system encompasses the nasal passages, pharynx (referred to as upper airways) and the trachea and lungs (referred to as upper airway).  The two overall functions of the respiratory system are to transfer oxygen from the environment to the blood and to remove carbon dioxide from the blood to the atmosphere.  In order to achieve this there are two major processes occurring.  The first is the movement of gas in and out of the lungs (ventilation).  This delivers fresh gas containing oxygen to the alveoli and removes carbon dioxide.  The second process is the gas exchange across the alveolus (oxygenation).  To accomplish these functions, the following mechanisms must be functioning:

• Muscles and nerves--draw air into the lungs by the ventilator apparatus of the chest
• Alveoli--site of the gas exchange
• Red Blood Cells--must be available to carry oxygen molecules
• Circulation--necessary to carry the oxygenated blood to the peripheral tissues and uptake of carbon dioxide to take back to the alveoli

In order for oxygenation to occur, the patient must have enough red blood cells and hemoglobin, sufficient cardiac output and blood flow to the peripheral cells.  If one of these factors is not adequate, hypoxemia will occur.  The definition of hypoxemia (too low of oxygen in blood) is the failure of the lungs to provide adequate oxygen to meet the metabolic needs of the patient.  Oxygenation is estimated by looking at mucous membrane color, using a pulse oximeter or by analyzing oxygen levels in the blood.  Each has its limitations yet when used, can yield a lot of information.

Ventilation is the movement of air in and out of the lungs, and carbon dioxide levels are used to determine if this process is working.  In order to obtain normal ventilation, the patient must have an open airway, normal brain functions, intact neurological pathways from the brain through the high cervical spinal cord to the diaphragm and intercostals muscles, and normal respiratory function.  If ventilation is the movement of air in and out of the body, then hypoventilation (increased carbon dioxide) is the failure of the lungs to eliminate carbon dioxide.  (Aka hypercapnea.)

In order to treat hypoxemia, one must give oxygen.  This is easier said than done in the cat.  Methods used include a mask, oxygen hood, nasal canula, an oxygen cage, transtracheal catheter, hyperbaric chamber and volume/pressure ventilator.  The chosen therapy will depend on the stress to the patient, disease process, other examination findings and willingness of owner to allow expensive treatments to be performed.

Treatment of hypercapnea/hypoventilation is to ensure an open airway, provide artificial ventilation, correct nerve and muscle disorders, correct dead space and provide pain control in order to encourage the patient to breathe deeper.

Critical care monitoring is vital to patients with respiratory or gastrointestinal compromise.  Treatment can be very rewarding with many successful outcomes.  Owner commitment is very important during the entire process as much of the care is provided at home.