Treating cats in shock: they are not small dogs
The physiologic response to shock, monitoring techniques used, and methods of resuscitation are different in cats compared 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
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 tract, pancreas, kidneys, and lungs sustain hypoxic injury as a result. In cats, the “shock organ” that is most affected during shock is the lung. Thus, cats will often present with respiratory signs including tachypnea, respiratory distress, and pale or grey mucous membranes.
Classification
Hypovolemic shock is a common cause of shock in cats; examples include hemorrhage due to trauma or coagulopathies. Cardiogenic shock occurs due to a decrease in cardiac output and is most commonly seen in cats with cardiomyopathies or arrhythmias. Distributive shock occurs when there is dysfunction of the microcirculation due to inappropriate arterial or venous dilation resulting in abnormal blood flow. This can occur in cats with sepsis or anaphylaxis. Obstructive shock happens when there is an obstruction of blood flow to or from the heart. The most common example of obstructive shock in cats is aortic thromboembolism (i.e., saddle thrombus), which can result in the absence of femoral pulses.
Clinical stages and signs
Shock has been traditionally separated into three clinical stages: compensatory, early decompensatory, and decompensatory. Cats are different than dogs in that they rarely present with signs of compensatory shock, but are much more likely to show signs of early decompensatory or decompensatory shock. Signs noted during early decompensatory shock 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. 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. It is important to note that cats typically present with a trilogy of signs during shock regardless of the stage including hypothermia, hypotension, and bradycardia. A heart rate less than 160 bpm in a visibly sick cat is a concerning finding and is often a sign of shock.
Monitoring and diagnostic tests
Monitoring cats in shock can be particularly difficult given that changes in their physical perfusion parameters are often subtle compared to dogs. Therefore, blood pressure monitoring ideally should be used to confirm the physical examination findings. 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. Oscillometric devices generally underestimate systolic blood pressure in cats but are fairly accurate for mean and diastolic blood pressures.
Lactate measurements can 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 handheld devices, which have been validated for accuracy in cats. An ECG is helpful for assessing cats with bradycardia, tachycardia, pulse deficits, or a history of heart disease, and for enabling moment-to-moment monitoring of heart rate during resuscitation from shock. Pulse oximetry is a non-invasive method for assessing oxygenation and should also be measured in cats with shock. Additionally, electrolytes and blood glucose concentrations should always be measured if possible.
Ultrasound is another useful diagnostic tool, enabling performance of a FAST (focused assessment using sonography for trauma) exam to detect peritoneal or pleural effusion.
Treatment
Early recognition and resuscitation are very important for the successful treatment of shock in cats. Cats can be difficult to resuscitate and often require more conservative management than dogs to avoid complications, such as fluid overload or pulmonary edema. 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 four aliquots (10-15 mL/kg) and recheck the cat’s perfusion parameters every 15 minutes to determine the response. 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. 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%) can be considered during fluid resuscitation of shock in cats with head trauma. Hypertonic saline reduces intracranial pressure, increases blood pressure, and improves cerebral perfusion. Care should be taken not to administer hypertonic saline too quickly as it can cause bronchoconstriction. Hydroxyethyl starch (HES) solutions (e.g., Vetstarch®) are administered for volume resuscitation in cats with shock and hypoproteinemia, such as those with sepsis or trauma. Potential complications of HES solution administration include volume overload, coagulopathies, and kidney injury. Cats are especially sensitive to boluses of HES solutions and will vomit if administered too quickly.
For cats in hypovolemic shock due to hemorrhage, 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 reactions.
Cats 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, hit by car) it must be assumed that pain relief is warranted. Due to the critical nature of cats that are in shock, as well as cats’ sensitivity to opioids, it is best to titrate pain-relieving medications to effect. Ideally, opioids such as hydromorphone, fentanyl, or buprenorphine should be used. Non-steroidal anti-inflammatories should be avoided in cats due to the risk of acute kidney injury.
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, hypothermia will also heighten bradycardia by depressing the sinus node. Therefore, it is very important that attempts be made to warm cats during shock resuscitation by applying external warming sources such as a circulating warm water blanket, forced warm air blower, or heated towel to the cat.
Oxygen supplementation should also be administered in cats whose SpO2 is less than 95% to improve oxygen delivery to tissues, but in such a way that will not cause stress to the cat. The easiest methods during the resuscitation period are flow-by (blow-by), mask oxygen, or in an oxygen hood, cage, or incubator.
Conclusions
If shock is detected quickly and treated appropriately in cats, the outcome can be successful. Just remember that cats are unique and must be diagnosed and managed differently than dogs.
This article is based on Dr. Holowaychuk’s presentation at the Canadian Veterinary Medical Association Convention in St. John’s, NL.CVT
