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Management of Shock in Cats: They’re Not Just Small Dogs Marie K. Holowaychuk, DVM, Diplomate ACVECC [email protected] Introduction There are many aspects of critical care that are unique to the cat. The physiologic response to shock, monitoring techniques used, and methods of resuscitation are different in cats in comparison to dogs. Although the underlying etiologies and pathophysiology of shock are similar in the two species, care must be taken to treat cats appropriately in order to avoid life-threatening consequences. Definition The medical dictionary defines shock as “a medical emergency in which the organs and tissues of the body are not receiving adequate blood flow, thus depriving them of oxygen and allowing the buildup of waste products”. Essentially, shock occurs when the delivery of oxygen to the tissues does not meet their oxygen requirement. The body compensates by diverting blood flow to the heart and brain. Other organs including the gastrointestinal (GI) tract, pancreas, kidneys, and lungs sustain ischemic injury as a result. Ultimately, there is death of renal tubular cells resulting in kidney insufficiency, GI ischemia leading to bacterial translocation, and acute lung injury and impaired pulmonary gas exchange. In cats, the “shock organ” or organ that is most affected during shock is the lung. Thus, cats will often present with signs localizing to the respiratory system including tachypnea, respiratory distress, and pale or cyanotic mucous membranes. Cytokines (i.e., tumor necrosis factor-α, interleukin-1) are also released and cause a systemic inflammatory response. Anaerobic metabolism occurs leading to excess lactate production and subsequent metabolic acidosis. If left untreated, eventually shock will lead to cellular dysfunction, organ failure, and ultimately death. Classification There are several classification schemes used to categorize shock including those based on underlying etiology (e.g., cardiogenic, hypovolemic, hemorrhagic, septic, traumatic, anaphylactic), anatomic location affected (e.g., heart, blood [loss], vascular), and identification of the hemodynamic defect (e.g., hypovolemic, cardiogenic, distributive, obstructive). Classification by the hemodynamic defect (Table 1) is helpful to understand the pathophysiologic mechanisms of shock and to consider therapies that will reverse the dysfunctions. However, shock is complex and it is important to realize that most clinical forms of shock have many underlying mechanisms affecting the animal simultaneously. Therefore, there is no universal treatment for each category of shock that will suit every patient. Hypovolemic shock is a common cause of shock in cats. It occurs due to an actual (absolute) or perceived loss (relative) of intravascular volume. Hemorrhage is considered an absolute cause of hypovolemic shock because it involves an actual loss of intravascular (blood) volume. Examples of clinical presentations for cats with absolute hypovolemic shock due to hemorrhage include trauma, coagulopathies, or GI hemorrhage. Absolute hypovolemic shock can also be caused by non-hemorrhagic events that involve a large loss in plasma volume. Examples include severe dehydration, polyuria, vomiting/diarrhea, and 1 third space losses (e.g., abdominal or pleural effusion). Anaphylaxis also has characteristics of absolute hypovolemic shock as endothelial damage can enable the leakage of large volumes of plasma from the intravascular to the interstitial spaces, resulting in peripheral edema. Relative hypovolemic shock occurs when there is no loss of intravascular (blood or plasma) volume; however, because of venous or arterial dilation, there is a relative inadequacy of intravascular volume. This term is not used very commonly anymore as most people categorize this type of shock as distributive. Table 1: Common Causes of Shock in Cats Hypovolemic Cardiogenic Hemorrhage (trauma, Cardiomyopathy coagulopathy) (dilated, Profuse vomiting hypertrophic, and/or diarrhea unclassified) Severe polyuria Severe arrhythmias Marked internal fluid loss (pleural, peritoneal, interstitial) Severe dehydration Distributive Sepsis Pancreatitis Severe tissue trauma Anaphylaxis Obstructive Pericardial tamponade Restrictive pericarditis Pulmonary thrombo-embolism Cardiogenic shock occurs due to a decrease in cardiac output and is most commonly seen in cats with cardiomyopathy, arrhythmias, or other conditions that adversely affect myocardial contractility. Many of these cats are concurrently in congestive heart failure. Because of this, the presenting clinical signs are not just characteristic of shock, but can also include additional signs such as pulmonary crackles, decreased lung/cardiac sounds due to pleural effusion, jugular distension, or a heart murmur. Distributive shock occurs when there is dysfunction of the microcirculation due to inappropriate arterial or venous dilation resulting in abnormal blood flow. Most often vasodilation is a consequence of inflammatory mediators that are released and lead to the production of nitric oxide, a potent vasodilator. This can occur in cats with systemic inflammatory response syndrome (SIRS), sepsis, or severe pancreatitis. Conversely, anaphylaxis is another cause of distributive shock resulting in the IgE-mediated release of vasoactive substances (e.g., histamine, amine, tumor necrosis factor-α, prostaglandin, leukotriene) from mast cells, which cause massive vasodilation. Obstructive shock happens when there is an obstruction of blood flow from the heart or returning to the heart. Clinical examples of obstructive shock include pericardial tamponade due to pericardial effusion or pulmonary or aortic thromboembolism. The clinical signs associated with obstructive shock vary depending on the location of obstruction. For instance, pericardial tamponade can cause pulses paradoxus (i.e., a weaker pulse on inspiration and a stronger pulse on expiration), whereas aortic thromboembolism can result in an absence of femoral pulses. Clinical Stages and Signs Shock has been traditionally separated into three clinical stages that illustrate the progression: compensatory, early decompensatory, and decompensatory shock. Cats are different than dogs in that they rarely present with signs of compensatory shock, but are 2 much more likely to show signs of early decompensatory or decompensatory shock. During the compensatory stage of shock, the body is able to maintain adequate cardiac output despite a mild to moderate loss of intravascular volume. However, the compensatory stage cannot last indefinitely without treatment and ultimately systemic vascular resistance will decrease, cardiac dysfunction will occur, and decompensation begins. Eventually, complete circulatory collapse occurs and death can occur. Compensatory stage: The signs of compensatory shock are often overlooked or mistaken as “normal”. These signs include a normal to mildly increased heart and respiratory rate, injected (bright pink to reddish) mucous membranes, shortened capillary refill time (< 1 sec), normal mentation, normal blood pressure, and normal to increased (bounding) pulses. Injected mucous membranes and increased pulse pressure are rarely detected in cats as opposed to dogs. Early decompensatory stage: Cats and dogs most commonly present during the early decompensated stages of shock. Signs noted during this stage include tachycardia, normal to decreased pulse pressure, hypotension, pale mucous membranes, prolonged capillary refill time, decreased mentation, and decreased body temperature. Tachycardia does not occur in cats as often as dogs during this stage and should not be an expected finding. Cats that have all of the other signs should be considered in shock and treated accordingly. Decompensatory (terminal) stage: Clinical signs of decompensatory shock include a low heart rate, severe hypotension, pale or grey mucous membranes, absent capillary refill, weak or absent pulses, decreased heart sounds, low body temperature, stuporous or comatose mentation, and decreased or absent urine production. If aggressive resuscitation is not provided, cardiopulmonary arrest and death will occur. Cats typically present with a trilogy of signs during shock regardless of the stage that include hypothermia, hypotension, and bradycardia. It is important to note that a heart rate < 160 bpm in a visibly sick cat is a concerning finding and the possibility of shock should be considered, especially if other signs of shock are seen. Monitoring and Quick Diagnostic Tests Thorough and frequent physical examinations focusing on perfusion parameters are important when monitoring patients during resuscitation from shock. Monitoring cats can be particularly difficult given that changes in their physical perfusion parameters (i.e., pulse quality, capillary refill time, mucous membrane color, extremity temperature) are often subtle compared to those in dogs. For example, while femoral and dorsal pedal pulses are easily detected in most dogs, dorsal pedal pulses can be difficult to palpate even in normal healthy cats. Likewise, detecting subtle changes in femoral pulse quality is a skill that only those with excellent tactile sensation possess. Nevertheless, it is important that attempts be made to palpate both dorsal pedal and femoral pulses in cats during shock resuscitation. A change in pulse strength or an absence or gain of a pulse can help to gauge response to resuscitation efforts. Similarly, vital signs and other perfusion parameters should be monitored frequently to ensure an appropriate response to resuscitation (Table 2). Table 2: Perfusion Parameters and the Desired Change during Resuscitation Parameter During Shock Desired Change Presence of pulse/quality Absent, poor/weak Present, normal Heart rate < 160 bpm > 160 bpm 3 Respiratory rate Extremity temperature Blood pressure Mucous membrane colour Capillary refill time Rectal temperature Urine production Mentation Lactate Tachypneic Cool Systolic < 90 mmHg Pale/grey/white Prolonged or absent < 37oC Minimal or absent Depressed, stupor, coma > 2 mmol/L Normal Normal Systolic > 110 mmHg Pale pink or pink 1-2 seconds > 37oC Normal (1-2 mL/kg/hour) Alert and responsive < 2 mmol/L Blood pressure monitoring should be used to confirm the physical examination findings. It is important when measuring blood pressure using either Doppler or Oscillometric devices to ensure that the cuff size is appropriate (width 30-40% of limb circumference) as small cuffs might over-estimate blood pressure and large cuffs might under-estimate blood pressure. Normal pressure is 120/80 mmHg (mean arterial pressure [MAP] 90 mmHg) and hypotension is typically defined as a systolic blood pressure [SBP] 90 mmHg or a MAP 70 mmHg. Doppler ultrasonographic blood pressure devices measure the systolic blood pressure when the inflated cuff is released and blood flow and arterial wall motion occur creating the characteristic “whooshing” sound. Research studies have shown that in anaesthetized cats Doppler measurements more closely reflect the mean arterial blood pressure; however, this is debatable in awake or ill cats. A study investigating Doppler blood pressure in critically ill cats revealed that cats with an increase in Doppler blood pressure ≥ 20 mmHg were more likely to survive. Oscillometric blood pressure devices detect pulsatile arterial wall vibrations by an automatically inflating/deflating cuff. The cuff is inflated until no vascular vibrations (oscillations) are detected and then deflated. Throughout the deflation cycle, the average oscillations and corresponding cuff pressures are computer analyzed to determine the systolic, diastolic, and mean arterial pressures. Oscillometric devices are most appropriate for monitoring trends (they can be set to measure every 1-30 minutes). Oscillometric devices generally underestimate systolic blood pressure in cats but are fairly accurate for mean and diastolic blood pressures. Unfortunately, their accuracy in measuring blood pressure in small or hypotensive animals is decreased. Therefore, they are most accurate in large cats (> 5 kg) but may not be as accurate with tachycardia, rapidly changing heart rates, or shock/vasoconstricted states. Lactate: When perfusion and oxygen delivery are inadequate, anaerobic metabolism increases resulting in increased lactate production and a metabolic acidosis. Normal blood lactate levels are < 2 mmol/L. Mild hyperlactatemia is 2-4 mmol/L, moderate hyperlactatemia is 5-8 mmol/L, and severe hyperlactatemia is > 8 mmol/L. Lactate measurements can also be trended over time to assess the adequacy of fluid resuscitation or improvement in oxygen delivery during treatment for shock. Blood lactate can be easily measured in practice by the use of an iSTAT® or other handheld devices. When handheld devices were compared to standard laboratory analyzers using blood samples from cats, the iSTAT® and LactatePro® (http://www.vetlab.com/Lactate%20Pro.htm) were found to be the most accurate. ECG monitors are becoming more frequently used in small animal practice. Many blood pressure and pulse oximetry machines have an ECG as well. An ECG is helpful for assessing cats with bradycardia, tachycardia, pulse deficits, or a history of heart disease, and 4 for enabling moment-to-moment monitoring of heart rate during resuscitation from shock. If hyperkalemia is measured or suspected (e.g., urethral obstruction, uroabdomen, kidney failure), an ECG should be performed to assess for associated conduction disturbances such as small to absent P-waves, tall T-waves, or a widened QRS complex. Severe bradycardia (HR < 140 bpm) in critically ill cats is an emergency that requires immediate intervention. Pulse oximetry is a non-invasive method for assessing oxygenation in cats with shock and respiratory signs. It measures the peripheral oxygen saturation (SpO 2), which is an indirect measure of SaO2 and thus PaO2 using the oxygen-hemoglobin dissociation curve. A SpO2 of 90% corresponds to a PaO 2 of 60 mmHg (severe hypoxemia) and a SpO2 of 95% corresponds to a PaO2 of 80 mmHg (normal). Electrolyte and Glucose Measurements: Electrolytes can be measured on a chemistry analyzer or blood gas machine to rule out severe abnormalities (potassium < 3.0 or > 6.0 mEq/L, sodium < 140 or > 170 mEq/L, ionized calcium < 1.0 mmol/L). A blood glucose measurement should always be measured for cats in shock using a chemistry machine or glucometer. Hyperglycemia is common in cats due to the stress of hospitalization or illness; however, hypoglycemia can occur in cats with shock. A blood glucose measurement < 80 mg/dL (< 4.5 mmol/L) in a cat in shock necessitates treatment with dextrose supplementation. Focused Assessment Using Sonography for Trauma (FAST): Abdominal FAST is a simple and rapid ultrasound examination that can be performed emergently in cats to detect intra-abdominal free fluid suggestive of hemorrhage or organ rupture (e.g., uroabdomen). Veterinarians with limited previous ultrasound experience can perform a FAST examination in less than five minutes. It can be used with a scoring system (score 0-4) to evaluate for the presence of fluid (anechoic) in four areas. With the cat in lateral recumbency (preferably right lateral), use the ultrasound probe to assess for fluid: 1) just caudal to the xiphoid process, 2) on the midline over the urinary bladder, and at the 3) left and 4) right flank regions. During the FAST examination, the urinary bladder and gallbladder can also be visualized. Note that even though an organ may be seen on radiographs or ultrasound and appear intact, organ rupture (e.g., urinary bladder, urethra, gallbladder) can still be present. FAST can be performed on initial presentation and serially thereafter to monitor for the presence of abdominal fluid. If abdominal fluid is found, an abdominocentesis can be performed to obtain a sample of fluid for analysis. If the sample appears bloody, place it in a red top blood tube to ensure that it does not clot. If the fluid sample clots, it is possible that a blood vessel or organ (e.g., spleen) was inadvertently aspirated. Otherwise, perform a PCV and TS on the fluid; a PCV > 10% is suggestive of hemorrhage. If uroabdomen is suspected, measure the potassium or creatinine concentration of the fluid and compare it to the periphery (serum). A fluid:serum ratio of potassium > 1.4:1 or creatinine > 2:1 is consistent with a uroabdomen. Thoracic FAST can also be performed in cats presenting in shock with increased respiratory effort or respiratory distress, to quickly determine if fluid is present in the thoracic cavity. Differentials for the fluid will depend on the history and presenting complaint, but could range from hemorrhage (i.e., traumatic shock) to pyothorax (i.e., septic shock) or chylothorax (i.e., cardiogenic shock). PCV and TS (if bloody) or fluid cytology will be helpful if a sample of fluid can be obtained for analysis. Treatment 5 Early recognition and resuscitation are very important for the successful treatment of shock in cats. There are many facets of the treatment of shock in cats that are unlike that for dogs. Cats can be difficult to resuscitate and often require more conservative management to avoid complications such as fluid overload or pulmonary edema. Fluid Resuscitation: Vascular access is a must when providing appropriate fluid resuscitation to cats in shock. Subcutaneous fluids are NOT appropriate in cats with evidence of shock and poor perfusion. Vascular access can be obtained via the cephalic or medial saphenous veins. The volume of fluid required for treating shock is based on the cat’s weight and estimated blood volume. If crystalloid fluids are used, one full blood volume might need to be administered over one hour. The blood volume of cats is estimated to be 45-60 mL/kg and is less than that of dogs. The response to fluid resuscitation should determine the volume of fluid that is administered. The recommended regimen for fluid resuscitation during shock is to divide the total volume to be given over one hour into smaller volumes and use frequent monitoring to assess the cat’s status. Divide the total fluid volume into four aliquots (10-15 mL/kg) and recheck the cat’s perfusion parameters every 15 minutes to determine the response. Not all cats in shock will require replacement of a full blood volume to treat their shock. If the clinical signs of shock resolve after the first 15 minutes, there is no need to proceed with another bolus of fluid. Consider giving a fluid bolus by drawing the amount of fluid up into a syringe and manually “pushing” it into the cat over 10-15 minutes. Conversely, a fluid pump can be set to deliver the volume over 15 minutes. Balanced electrolyte solutions (i.e., LRS, Plasmalyte-148, Plasmalyte-A) are preferred over normal saline (0.9% NaCl); however, any isotonic fluid that is available can be used. Because cats in shock are typically hypothermic, it is beneficial to warm fluids prior to administration if possible. Hypertonic saline (3-7%) is another fluid that can be considered during fluid resuscitation of shock. It is not commonly used in cats because they are small enough that isotonic fluids can be given rapidly and in large enough volumes to achieve the desired effect of intravascular volume expansion. However, hypertonic saline has other benefits including the reduction of intracranial pressure in cats with head trauma. In that case, a dose of 2-4 mL/kg is recommended over 15-20 minutes. Care should be taken not to administer hypertonic saline too quickly as it can cause bronchoconstriction. Hydroxyethyl starch (HES) solutions (e.g., Voluven®, Vetstarch®) are tetrastarch colloid solutions containing hydrolyzed amylopectin dissolved in a crystalloid solution (typically NaCl 0.9%). They are administered for volume resuscitation in patients with hypoalbuminemia secondary to sepsis or trauma that are not responsive to crystalloid therapy alone. Potential complications of HES solution administration include volume overload, kidney injury, and coagulopathies. Typically, boluses are given in 2.5-5 mL/kg volumes over 10-15 minutes, not exceeding a total daily dose of 20-40 mL/kg/day. Cats are especially sensitive to boluses of HES solutions and will vomit if they are given too quickly. Blood Transfusions: Cats in shock due to traumatic episodes might be anemic and hypoproteinemic secondary to blood loss. If the cat is indeed in hypovolemic shock due to hemorrhage and cannot be stabilized with fluid resuscitation alone, a whole blood transfusion might be necessary. Remember that cats MUST be blood typed (or cross matched) prior to receiving a blood transfusion of any kind, as type A-B incompatibilities in cats can result in lethal transfusion reaction. 6 Analgesia: Providing adequate pain relief to cats that are in shock is also extremely important. Cats that are in shock will not be able to elicit the same signs of pain that are recognized in otherwise normal cats (e.g., tachycardia, restlessness, irritability, withdrawn demeanor). Therefore, if the cat has experienced a traumatic event (e.g., dog attack, HBC) it must be assumed that the cat is painful and its pain should be treated accordingly. Ideally, reversible opioid medications should be used, but it is best to titrate them to effect. If additional pain relievers are needed, ketamine (IV) or gabapentin (PO) can be considered. Non-steroidal anti-inflammatory drugs (NSAIDs) should be avoided in cats with shock due to the risk of kidney injury. Table 3: Suggested Analgesics Pain-relieving Medication Fentanyl Hydromorphone Buprenorphine Ketamine Dose 1-2 mcg/kg IV bolus followed by 2-5 mcg/kg/hr IV continuous rate infusion 0.02-0.05 mg/kg IV or IM every 4-6 hours 0.005-0.02 mg/kg IV or IM every 6-8 hours 0.5-1.0 mg/kg IV bolus followed by 0.1-1.0 mg/kg/hr IV continuous rate infusion Heat Support: It is very common for cats in shock to be hypothermic. The low body temperature is often exacerbated by the low heart rate and subsequent low cardiac output. Unfortunately, the hypothermia will also heighten the bradycardia by depressing the sinus node. Therefore, it is very important that attempts be made to warm cats during shock resuscitation. This can be effectively achieved by applying external warming sources such as a circulating war water blanket, warm air blower, or heated towel to the cat. Additionally, administering warmed (rather than room temperature) intravenous fluids can be helpful. Oxygen Therapy: Oxygen supplementation should also be administered to cats that are in shock and whose SpO2 is < 95%. This will help to improve oxygen delivery to tissues. It should be provided in such a way that does not cause stress to the cat. The easiest methods during the resuscitation period are flow-by (blow-by) or mask oxygen. Mask oxygen will enable a higher concentration of oxygen to be provided to the cat; however, not all cats will tolerate the mask. In those cases, directing the oxygen flow toward or near the cat’s nose and mouth is sufficient. If the cat is being housed in a cage during its resuscitation, an oxygen cage or oxygen hood may be used if available. Electrolyte and Blood Glucose Disturbances: Hypoglycemia should be immediately treated with an IV bolus of dextrose. Dilute 0.5-1 mL/kg of 50% dextrose to a concentration of 12.5-25% and administer it over 2-3 minutes. Recheck the blood glucose 10-15 minutes after the bolus to ensure that it is normal. If it is low again, administer another IV bolus and add 2.5-5% dextrose to the IV fluids. Calcium gluconate 10% 0.5-1 mL/kg IV should be administered over 10-15 minutes in cats with ionized hypocalcemia or hyperkalemia with associated ECG abnormalities. Hyperkalemia should also be treated with IV dextrose (as above), as well as sodium bicarbonate (1 mL/kg IV over 10-15 minutes) or regular insulin (0.5 U/kg) and IV dextrose 50% (2 mL/kg) concurrently. 7