Today's small animal veterinarians are seeing a gradually increasing number of feline patients, and as our experience with this species evolves, it is becoming clear that they cannot be treated as 'small dogs'. There are several significant physiologic differences between the two species that must be borne in mind during their critical care management. In addition, their small size and fragile, often fractious, nature increases the complexity of providing critical care to this species. Despite these limitations, it is possible to provide extensive supportive care to critically ill cats using relatively little equipment.
Shock in Cats
Hypovolemic shock in cats has a similar appearance to the problem in dogs. As in dogs, acutely hypovolemic cats are usually pale, tachycardic, with first bounding and then weak pulses.
Septic shock appears clinically to be slightly different (and is poorly understood) in cats compared to dogs. Although the general pathophysiology of inflammation as a sequela to bacteria or endotoxin is the same, the clinical signs are different between the two species. The hyperdynamic or hyperemic stage of septic shock is very rarely seen in cats. Instead, septic cats usually appear to progress straight to the vasoconstrictive stage of septic shock. Thus they appear clinically with pale mucous membranes, weak pulses and generalized collapse, signs that are not easily distinguishable from hypovolemic shock. Septic cats are often hypothermic, and frequently have evidence of mild to moderate anemia and icterus.
Cardiac function is often dramatically, but temporarily, affected in cats with systemic inflammatory response syndrome, a factor that has great significance for decisions about fluid therapy. In these cats, cardiac contractility is often diminished, the heart may appear enlarged, and the left atrium may be large: all findings that mimic cardiomyopathy. From a clinical perspective, heart rate can be a useful clue about the presence of abnormal cardiac function in septic cats. Cats with myocardial changes due to sepsis are often bradycardic, with heart rates of 120-160 bpm. They may develop a heart murmur or gallop rhythm that was not previously present. If bradycardia or a heart murmur is present, fluids should be administered judiciously because there is a real risk that fluid therapy could result in pleural effusion and pulmonary edema. The cause of these cardiac changes is not well understood, and they appear to be reversible following resolution of sepsis.
Crystalloid Fluids in Feline Hypovolemic and Septic Shock
Fluid therapy remains the mainstay of treatment of shock in cats as well as in dogs. In animals with shock, by providing large volumes of intravenous fluids, we hope to improve circulating blood volume, decrease blood viscosity, and increase venous return, thus helping to improve cardiac output. Increased tissue perfusion therefore results, which begins to reverse cellular acidosis, and provides a supply of oxygen to the cells.
There are several distinct aspects of fluid therapy in feline patients that sets them apart from dogs. The first and most important difference between the two species is that the blood volume of the normal cat (about 60 ml/kg) is smaller than that of the normal dog (about 90 ml/kg). Since the shock fluid bolus is derived from the blood volume, this means that in general fluid therapy needs to be given much more conservatively in cats than in dogs. Cats are much more susceptible to fluid overload, and great care must be taken not to produce clinical signs of excessive volume, usually respiratory distress due to the presence of a pleural effusion. Therefore, not only are the volumes administered usually lower, but boluses of fluids, especially colloids, are given much more slowly.
Shock boluses of replacement crystalloid (Lactated Ringers, balanced electrolyte replacement solutions) of up to 90 ml/kg are administered over an hour in dogs. In cats, the intravascular blood volume is much smaller than in dogs and the total shock bolus of crystalloid is 45-60 ml/kg. Initially, 10-30 ml/kg is delivered rapidly intravenously (over 15-20 minutes) while the animal is carefully observed for a response, or for evidence that the fluid bolus is causing a problem. This dose can then be repeated if necessary till the total shock dose has been reached. If the animal begins to improve, administration may be slowed down before the total bolus has been given. The end point of resuscitation is an improvement in tissue perfusion, which is clinically recognized by an improvement in mucous membrane color, better quality pulses, a decrease in the heart rate towards normal, and improved mentation. Packed cell volume (PCV), total solids (TS), electrolytes and blood glucose should be monitored before, during, and after each fluid bolus.
Blood products can be very important in management of shock and severe anemia, and are often vitally helpful in management of critically ill cats. In the critically ill cat, if the PCV acutely drops below about 20%, transfusion of packed red blood cells or whole blood may significantly improve oxygen delivery to the tissues and result in a significant improvement in blood pressure. Blood transfusions are often well tolerated by critically ill cats, even if they do not tolerate other forms of fluid therapy. Plasma transfusion may be a useful source of albumin if severe hypoproteinemia occurs; especially in small patients when it is possible to administer relatively large volumes of plasma compared to their body weight. Fresh or fresh frozen plasma may be required for management of dilutional coagulopathies or DIC.
Specific blood groups exist in both dogs and cats, based on the presence of antigens on the surface of the red blood cells. Cats have three blood types, A, B, and AB, which is relatively rare. Most domestic short-hair cats are type A. Cats differ from dogs in that naturally-occurring antibodies exist against other blood types. Thus, a type B cat is born with antibodies against type A erythrocytes, and will have a serious transfusion reaction if given type A blood, even if he was never previously transfused. Similarly, type A cats have antibodies against type B erythrocytes, although the transfusion reactions seen here are not so severe. Thus, all cats must be blood typed prior to transfusion.
Transfusion reactions can be classified as immune-mediated and non-immune-mediated. Immune-mediated transfusion reactions are most often hemolytic, as antibodies in the recipient react with antigens on the donor cells. In type B cats inadvertently transfused with type A blood, sudden collapse and death can occur after administration of only a few drops of blood. Respiratory signs, including tachypnea and pulmonary edema, or sudden death, are the most common signs of a transfusion reaction in cats. Other signs of transfusion reactions include: anxiety, restlessness; urticaria, pruritus, facial edema; muscle tremors; nausea, salivation, vomiting; hemoglobinemia, hemoglobinuria; bilirubinemia, bilirubinuria; fever; anuria/renal failure; or seizures.
Cardiovascular Monitoring for Cats in Shock
Doppler Ultrasound for Measurement of Arterial Blood Pressure
Doppler pulse detection systems either detect the flow of blood through a vessel or detect motion in the wall of the artery. The most commonly used system in veterinary medicine detects the flow of blood through vessels, and is typically used to measure only the systolic blood pressure. Arterial flow is detected using ultrasound waves, and is heard as an audible signal from the Doppler amplifier speaker. A cuff is inflated around the limb, until it occludes arterial blood flow, and then gradually deflated. To obtain accurate readings, the cuff must be of the correct size. The pressure at which flow returns is the systolic pressure, measured on an attached sphygmomanometer. For accuracy, the measure should be repeated several times until consistent readings are obtained.
With a little practice, a systolic pressure reading is relatively easy to obtain using Doppler ultrasound. Readings can usually be obtained in cats and in moderately hypotensive animals, thus the Doppler can be expected to provide some indication of systolic blood pressure even in compromised animals in which other methods have failed. The measurement is non-invasive and well tolerated by all but the most aggressive patients.
Oscillometric Blood Pressure Measurement
A number of automated devices are available for measurement of arterial blood pressure using oscillometric techniques. Arterial blood pressure is measured using a compression cuff, which is wrapped around a limb or the tail, and automatically inflated to various pressures. At each cuff pressure, oscillations within the cuff caused by the pulse are detected. The pressure at which oscillations are first detected is the systolic pressure, that at which the amplitude of the oscillation is maximal is the mean pressure, and the pressure at which the oscillations decrease rapidly is the diastolic pressure. Thus, the mean arterial pressure is the most accurate reading obtained by this type of device.
Oscillometric instruments are useful for clinical assessment of patients with either hypotension or hypertension, but are clinically less effective in cats than in dogs. However, the cuff is well tolerated, non-invasive, and safe. The readings give trends that are accurate enough to provide clinically relevant information, especially if the machine is concurrently reading an accurate heart rate. The machine can be set to automatically cycle repeatedly, and can be set to alarm for low or high pressures. The operator does not need to exactly locate the artery for each reading, but merely to place a marker on the cuff in the general anatomic location of the artery. Range markers are printed on each size cuff for easy election of the appropriate size for each patient.
Central Venous Pressure Monitoring
Central venous pressure (CVP) monitoring provides very different information from that obtained by arterial pressure monitoring. The venous system is the capacitance system: that is, the part of the circulatory system that holds most of the reserve blood volume. Changes in CVP provide information about the degree of filling of the great vessels. The CVP is usually low (0-5 cm H2O), but it may increase if the capacitance of the great vessels is exceeded, either by absolute overload of the circulation (e.g., in oliguric renal failure) or by relative circulatory overload (e.g., in heart failure, where the heart is unable to pump forward all of the blood that is returned to it). This technique is therefore most useful for monitoring animals that have a risk of fluid overload, such as the animal with heart disease that requires fluid administration. In general, intravenous fluids can be safely administered as long as there is no rise in central venous pressure, although caution should be exercised, and other parameters such as lung auscultation should also be monitored. Once a rise in central venous pressure occurs, fluid administration should be decreased or suspended.
To measure central venous pressure, a catheter is placed in the jugular vein to the level of the cranial vena cava. The catheter should not reach all the way into the right atrium. It is bandaged in place, and is attached to a water manometer or a pressure transducer. The manometer is filled with saline via a 3-way stopcock, and zeroed at the level of the right atrium. Saline is allowed to flow from the manometer into the catheter until it equilibrates with venous pressure, and central venous pressure is read from the water manometer. Repeated measurements are then obtained.
If the cat does not respond to fluid therapy, or for those in which fluid therapy is contraindicated, continuous infusions of catecholamines are an important way to support the circulation and improve tissue perfusion. Numerous drugs are available, but the most widely used inotropic and pressor drug for cats is dopamine. Dopamine is well tolerated by the majority of cats and at low to moderate doses does not appear to have many negative effects in this species. Beta receptor agonist doses (5-8 μg/kg/min CRI) are often effective and helpful, while alpha receptor agonist doses (8-15 μg/kg/min) may be needed in severely affected patients. Other catecholamines such as dobutamine, epinephrine or norepinephrine may be needed occasionally, but should represent a 'second line' only if dopamine is ineffective.