Specialty Spotlight: Classifications of Shock

Specialty Spotlight: Classifications of Shock
Madeline Libin, DVM, DACVECC

There are several types of shock, but ultimately the definition of shock is consistent. Shock is the result of any condition in which the metabolic demand for oxygen exceeds uptake and utilization of oxygen. Most cases of shock are due to decreased delivery of blood to tissues.

Shock is typically divided into categories that help explain why oxygen delivery is not matching demand. The box below outlines five broad categories of shock (Silverstein and Hopper 2015). Each classification of shock results from a different physiologic mechanism, and many patients suffer from multiple types of shock simultaneously.

There are three stages of shock: compensated reversible, uncompensated reversible, and uncompensated irreversible.

In the compensated stage, the patient is able to maintain oxygen delivery to tissues to preserve cellular metabolism. Without intervention, compensation fails, and progression to decompensated shock occurs. The terminal stage of decompensated shock is irreversible and leads to cardiovascular collapse and death.

Shock is based largely on physical examination findings. Hypovolemic shock is most frequently encountered, and rapid assessment of volume status is necessary. A hypovolemic patient is not necessarily a dehydrated patient.

 Hypovolemia refers to a reduction in intravascular volume, which affects six key PE parameters. These “perfusion parameters” are: mentation, heart rate, pulse quality, mucous membrane color, capillary refill time, and extremity temperature (Table 1).

Heart rate is often the first parameter to change. Tachycardia is a consistent feature of compensated shock in dogs. Cats often exhibit bradycardia, even with mild to moderate shock.

Pulse quality should always be assessed in suspected shock patients. Femoral and distal limb pulse should be palpated, as changes in pulse amplitude typically occur distally first. Patients with fever and severe anemia may have hyperdynamic pulse quality, mimicking “strong” pulses.

Tachypnea and increased respiratory effort typically accompany shock, although they are not consistent indicators of decreased perfusion. Cats may display profound tachypnea that can resemble respiratory distress.

Blood pressure is often maintained in mild to moderate hypovolemic shock but is reduced as shock progresses. In severe shock, PE parameters should be clearly abnormal, and measurement of BP should not delay diagnosis of or treatment of shock.

Compensated shock is more frequently present than realized. Early identification and intervention prior to referral is vital to a patient’s ultimate survival.

When possible, suspected shock patients should have an IV catheter placed. Appropriate IV catheter size is important, and ideally the largest catheter gauge should be placed.

At the time of IV catheter placement blood glucose and PCV/TS should be assessed, and ideally electrolytes. CBC, chemistry panel, and other tests (such as cPL/PLI, T4 and urinalysis) are important and helpful, but are not absolutely essential. If urination is observed, or urinalysis has been obtained, urine specific gravity provides essential information regarding the patient’s volume status and kidney function. The utility of this test decreases once IV fluids are administered.

Once an IV catheter is placed and after obtaining a minimum database (BG, PCV/TS, electrolytes +/-USG), a bolus of an isotonic crystalloid, such as LRS, Plasmalyte-A or Normosol R should be administered. Cardiogenic shock (with the exception of cardiac tamponade) is the only situation in which a fluid bolus would be deleterious.

In dogs, a bolus of 10-20 ml/kg delivered over 15 to 20 minutes is appropriate. In cats, a bolus of 5-10 ml/kg delivered over the same time frame is appropriate. It is important to monitor perfusion parameters, especially heart rate, during and immediately after this intervention, as even a transient improvement in perfusion parameters is supportive of a diagnosis of hypovolemic shock.

Regardless of a patient’s shock etiology, excess isotonic crystalloid boluses should be avoided. Only 25% of the administered dose of fluids remains in the intravascular space within 1 hour, and excess isotonic crystalloid fluid administration can lead to interstitial edema and organ edema, especially in patients who are euhydrated, hypoproteinemic and/or are experiencing vasculitis.

Following initiation of a fluid bolus, antibiotics should be administered to any patient in which sepsis is suspected. Once a fluid bolus and potentially antibiotics have been administered, re-assess the patient’s perfusion parameters. It is important to remember that other conditions aside from intravascular volume status will affect some perfusion parameters. Nausea and pain are frequent causes of tachycardia and tachypnea which can be easily overlooked in a shock patient.

Managing client expectations is crucial to ensuring the best possible outcome of any case. Most patients in shock, regardless of etiology, will require at least 18-24 hours of close monitoring. If overnight monitoring is not a possibility, and the owners are willing and able to afford more intensive care, refer shock patients to the closest 24-hour facility. Further diagnostics, such as CBC and chemistry panel, UA and culture, diagnostic imaging (abdominal/thoracic radiographs, ultrasound), baseline cortisol/ACTH stim, thyroid testing, and/or echocardiography should all be discussed and estimated for prior to transfer.

-Madeline Libin, DVM, DACVECC