Septic cats often present with a very different clinical picture than that seen in septic dogs. This can make rapid recognition of the septic cat challenging. Rapid identification of these patients, determination of the underlying cause and aggressive treatment is essential to the outcome of these cases. Sepsis is defined as a systemic inflammatory response syndrome (SIRS) secondary to an infectious process. Septic shock is a progression of this inflammatory response resulting in cardiovascular dysfunction and compromise. SIRS can result from a number of different disease processes, and the clinical definition of the syndrome is widely debated.
In cats, the following criteria for the diagnosis of SIRS has been proposed, extrapolated from definitions in humans and dogs (Brady et al. J Am Vet Med Assoc. 2000):
Temperature > 103.5°F, < 100°F
Heart rate > 225, < 140 beats/minute
Respiratory rate > 40 breaths/minute
White blood cell count > 19,500, < 5,000, or > 5% bands
Although the goals of resuscitation in septic cats are similar to any other patient, the unique hemodynamic manifestations in the feline patient necessitate a slightly different approach. The following will discuss the clinical manifestations of sepsis in the cat, as well as potential sources of the sepsis, treatment of the septic cat and recommendations for monitoring these critical patients.
Clinical Manifestation
While many septic dogs present with the "classic," or hyperdynamic, manifestation of sepsis, this phase is rarely seen in cats. Instead, cats often present with pale mucous membranes, prolonged capillary refill time, and poor to absent pulses.
Numerous studies have documented an inappropriately low heart rate in cats with severe systemic illness and sepsis. This response to critical illness appears to be unique to the feline species, and the mechanism is unknown. Cats should be evaluated for the presence of hypoglycemia or hyperkalemia and these electrolyte disturbances treated if present. Fluid resuscitation and warming generally result in an increase in the heart rate. If the heart rate remains below 100 beats per minute despite these treatments, only then should an anticholinergic or positive inotropic/chronotropic agent be considered.
Septic cats are frequently hypothermic, with temperatures often below 99°F (37°C) and occasionally too low to register on conventional digital thermometers. The systemic effects of this severe hypothermia are widespread, including compromise of cardiovascular, respiratory, central nervous system and hemodynamic functions. Rewarming of these cats is initially done by administering warm intravenous fluids. After an initial bolus, external warming should be instituted.
The clinicopathologic abnormality in cats with severe sepsis is consistent with the changes seen in any critically ill cat, and generally related to underlying organ dysfunction.
The general pattern of glucose derangements in acute sepsis consist of an early and transient hyperglycemia, followed by hypoglycemia. Cats are well known to become hyperglycemic in response to stress, but hypoglycemia is often seen in septic cats.
Etiology
In some cases of feline sepsis, the source of infection is readily apparent. This is true in cases with severe bite wounds, penetrating trauma, or infected wounds. However, determining the source of infection can be more challenging in other cases. In a study of severe sepsis in cats, many showed clinical signs of diffuse abdominal pain without significant intraabdominal pathology. In contrast, in a study of septic peritonitis in cats, 38% of the cats had no notation of pain on abdominal palpation in the history. Potential sources of infection in cats include pneumonia, pyothorax, septic peritonitis, septic pancreatitis, pyelonephritis, bacteremia secondary to severe gastrointestinal disease, pyometra, hepatic abscesses, endocarditis, or meningitis. Initial diagnostics in these cases generally consist of a complete blood count, serum biochemistry profile, urinalysis and culture, thoracic and abdominal radiographs, and abdominal ultrasound. Additional diagnostics may include blood cultures, endotracheal wash and culture, echocardiography, CSF tap or diagnostic peritoneal lavage.
Treatment
Septic patients are often fluid depleted. The shock bolus in cats is 50–60 ml/kg of crystalloids or 5 ml/kg of colloids. However, care must be taken when administering fluids to these cats as fluid overload, pulmonary edema and pleural effusion are common in this patient population. It is recommended that clinicians start with small boluses of 10–20 ml/kg given to effect.
In cats that remain hypotensive despite adequate volume replacement and normothermia, exogenous catecholamine therapy may be necessary. Supplemental oxygen is often necessary in these cats. In addition, the administration of red blood cells to an anemic patient will improve the oxygen carrying capacity of the blood and therefore improve oxygen delivery. In addition, replacement of coagulation factors with plasma therapy minimizes further blood loss and provides colloidal support. Red blood cell transfusions should be considered in any septic cat with a PCV < 20, and plasma should be administered in cats with prolonged coagulation times.
Appropriate antibiotic therapy is an integral part of the treatment of septic cats. Culture and sensitivity should be used to identify effective antibiotics whenever possible. Broad spectrum antibiotics should be instituted pending sensitivity results.
Nutritional support is extremely important in the septic patient. Nutritional needs should be addressed early in the course of hospitalization. This support can be provided enterally by a nasoesophageal, esophagostomy or PEG tube if the patient is not vomiting and is normothermic. In cats that cannot tolerate enteral feeding, total parenteral nutrition may be necessary. Recent studies in humans have shown that maintaining normoglycemia significantly improves outcome in critically ill patients. We have previously discussed that many of these cats are hypoglycemic on presentation. However, during hospitalization, particularly in patients receiving TPN, the clinician must carefully monitor for hyperglycemia and treat with low doses of insulin when appropriate.
Pain management is also important in these cats, and appropriate use of analgesics is an essential part of appropriate management of the septic cat.
Corticosteroid use in the treatment of sepsis and septic shock has been an area of controversy since the 1950s. Although placebo-controlled trials failed to document beneficial effects of high dose corticosteroids for the treatment of septic shock in people, recent studies have suggested septic human patients may benefit from physiologic doses of steroids. Several case reports in veterinary medicine indicate that this may also be true in our veterinary patients.
Monitoring
Careful monitoring is vital to the management of critically ill cats. Due to their small size, this can be challenging, despite these facts, there are numerous methods to monitor the critically ill cat. Central venous pressure is a useful tool for monitoring volume status in cats, but care must be taken when interpreting this data in cats with pleural effusion, uncertain lusitropic function or those on positive pressure ventilation with PEEP. An elevated CVP indicates the patient is at high risk for developing pulmonary edema or pleural effusion. In septic cats, the goal is to not exceed a CVP of ≤ 6 mm Hg (8 cmH2O) in an attempt to prevent these respiratory complications. Assessment of vascular tone in cats is challenging, as monitoring of blood pressure is difficult. Doppler measurements of blood pressure have been shown to underestimate the systolic blood pressure, but these studies were performed in healthy cats. No studies investigating the accuracy of Doppler measurements in septic cats have been performed. In our clinical cases, the absolute Doppler reading should be interpreted in conjunction with physical exam findings, and trends monitored. In general, the goal in septic cats is a Doppler of ≥ 90 mm Hg. Careful monitoring of oxygen content of the blood of the critically ill cat is a challenge. Arterial blood gases are rarely possible, but pulse oximetry is a noninvasive method to measure the oxygen carried by hemoglobin. It is important to remember that hypothermia, poor perfusion, severe anemia, movement and pigmented mucosa can lead to inaccurate results. With that understood, pulse oximetry can be very helpful in monitoring the respiratory status of many critically ill cats.
Placement of ECG patches is a noninvasive way to continuously monitor the electrical activity of the heart. The ECG should be monitored for changes such as bradycardia or arrhythmias. Monitoring of urine output should be attempted in septic cats to indirectly monitor volume status as well as renal perfusion and function. This is of particular importance in cats receiving exogenous catecholamines. Urine output can be measured directly via an indwelling urinary catheter, or indirectly by weighing diapers or using nonabsorbent litter. Venous blood gas measurements, PCV and total solids measurement, and lactate measurements should be done on a regular basis, but due to the small size of these patients, the frequency at which these are monitored should be determined by weighing the benefits of close monitoring against the detriment of repeated blood sampling.
Septic cats are a challenge to the emergency and critical care clinician. Although often extremely frustrating, these cases can also be extremely rewarding. With careful attention to the unique physiology and requirements of these cats, as well as a lot of patience, treating these cats can be one of the most rewarding aspects of our specialty.
References
References are available upon request.