Shock is the leading cause of death in veterinary emergency patients, and early recognition and aggressive treatment within the first hour (the "golden hour") significantly improves survival. Cats present very differently from dogs in shock, and failure to recognize feline decompensatory shock leads to delayed treatment and increased mortality. Use the Fluid Therapy Calculator for resuscitation volumes and the Triage/Emergency Specialist for stabilization guidance.
Shock is defined as a state of inadequate cellular oxygen delivery relative to demand, resulting in cellular dysfunction and, if untreated, organ failure and death. Regardless of type, the final common pathway involves: decreased tissue perfusion, anaerobic metabolism, lactate accumulation, cellular membrane dysfunction, inflammatory cascade activation, and ultimately multi-organ dysfunction syndrome (MODS).
Understanding the hemodynamic determinants of oxygen delivery (DO&sub2;) is essential: DO&sub2; = cardiac output (CO) × arterial oxygen content (CaO&sub2;). Cardiac output = heart rate × stroke volume. Stroke volume depends on preload, contractility, and afterload. Each shock type disrupts one or more of these determinants.
| Shock Type | Mechanism | Common Causes | Initial Treatment |
|---|---|---|---|
| Hypovolemic | Decreased circulating volume (reduced preload) | Hemorrhage, severe dehydration, GI losses (vomiting/diarrhea), third-spacing, burns | Aggressive IV crystalloid boluses; blood products if hemorrhagic |
| Distributive (including septic) | Inappropriate vasodilation (decreased SVR) and/or maldistribution of blood flow | Sepsis/SIRS, anaphylaxis, neurogenic shock (spinal injury) | IV crystalloids + vasopressors (norepinephrine); source control for sepsis |
| Cardiogenic | Decreased contractility or pump failure (reduced CO) | DCM, severe arrhythmias, myocarditis, drug toxicity (doxorubicin, propranolol overdose) | Positive inotropes (dobutamine); treat arrhythmias; AVOID aggressive fluids |
| Obstructive | Physical obstruction of blood flow | Pericardial effusion/tamponade, GDV, tension pneumothorax, pulmonary thromboembolism | Relieve obstruction (pericardiocentesis, gastric decompression, thoracocentesis) |
Multiple shock types can coexist. A dog with GDV often has both obstructive shock (gastric compression of the caudal vena cava) and hypovolemic shock (sequestration of blood in the splanchnic circulation). A septic patient may have distributive AND cardiogenic shock (sepsis-induced myocardial depression).
Compensatory (early) shock: The body activates sympathetic nervous system responses to maintain perfusion to vital organs. Clinical signs in dogs include tachycardia, bounding (hyperdynamic) pulses, injected (brick-red) mucous membranes, rapid CRT (<1 second), normal to increased blood pressure, and mild to moderate tachypnea. This stage is treatable and reversible with appropriate fluid resuscitation.
Decompensatory (late) shock: Compensatory mechanisms fail. Clinical signs include severe tachycardia (or paradoxical bradycardia in terminal stages), weak/thready pulses, pale or muddy mucous membranes, prolonged CRT (>2 seconds), hypothermia, altered mentation (obtundation or stupor), oliguria, and hypotension (systolic <90 mmHg). This stage carries high mortality and requires aggressive, multimodal intervention.
Warning: Cats in decompensatory shock present very differently from dogs. Cats typically present with bradycardia (heart rate <140-160 bpm), hypothermia (rectal temperature <98°F / 36.7°C), and profound obtundation. Hyperdynamic (compensatory) shock is rarely observed clinically in cats. If a cat presents bradycardic, hypothermic, and obtunded, assume decompensatory shock and treat aggressively.
For hypovolemic and distributive shock, IV fluid resuscitation is the cornerstone of initial treatment. Use the "shock dose" as a guideline, administered in incremental boluses rather than the full volume at once.
Dogs: Isotonic crystalloid (LRS or Normosol-R) shock dose is 90 mL/kg/hr. Administer in boluses of 20-30 mL/kg over 15-20 minutes, then reassess perfusion parameters. Repeat as needed up to the full shock dose.
Cats: Crystalloid shock dose is 60 mL/kg/hr. Administer in smaller boluses of 10-15 mL/kg over 15-20 minutes. Cats are more susceptible to volume overload and pulmonary edema.
Colloids: If perfusion does not improve after 2-3 crystalloid boluses, or if total protein is low (<3.5 g/dL), consider synthetic colloids (Vetstarch 5 mL/kg bolus in dogs, 2.5 mL/kg in cats) or plasma.
Blood products: For hemorrhagic shock, transfuse pRBCs when PCV drops below 20-25% in dogs or 15-20% in cats. Fresh frozen plasma provides clotting factors in cases of coagulopathy.
Warning: Do NOT aggressively fluid resuscitate patients with cardiogenic shock. Excessive fluids worsen pulmonary edema and congestive heart failure. Auscultate the chest before and during fluid resuscitation. If lung crackles develop, stop fluids immediately and consider furosemide and positive inotropic support.
When fluid resuscitation alone fails to restore adequate blood pressure (MAP <60-65 mmHg), vasopressors and inotropes are indicated:
Norepinephrine (0.1-1 mcg/kg/min CRI) is the first-line vasopressor for distributive/septic shock. It provides potent vasoconstriction with some positive inotropic effect.
Dopamine (5-15 mcg/kg/min CRI) provides dose-dependent inotropic and vasopressor effects. Higher doses (>10 mcg/kg/min) provide primarily vasoconstriction.
Dobutamine (2-20 mcg/kg/min CRI) is the first-line inotrope for cardiogenic shock, providing positive inotropy with minimal vasoconstriction.
Vasopressin (0.5-5 mU/kg/min CRI) is used as a second-line vasopressor in catecholamine-refractory shock.
Effective shock management requires serial reassessment of perfusion parameters. Key monitoring targets include:
Blood lactate: Normal <2.0 mmol/L. Serial lactate clearance is one of the best indicators of resuscitation adequacy. A 50% reduction in lactate within 6 hours of treatment is associated with improved survival. Persistent hyperlactatemia despite treatment indicates ongoing tissue hypoperfusion.
Blood pressure: Target MAP ≥65 mmHg (systolic ≥90 mmHg). Oscillometric or Doppler methods are acceptable; direct arterial monitoring is ideal for critically ill patients.
Urine output: Target ≥1-2 mL/kg/hr in dogs, ≥1 mL/kg/hr in cats. Oliguria (<0.5 mL/kg/hr) despite adequate volume resuscitation suggests acute kidney injury.
Central venous pressure (CVP): Guides fluid resuscitation in conjunction with other parameters. Target 5-10 cmH&sub2;O. CVP alone should not drive fluid decisions.
- Four types of shock: hypovolemic, distributive, cardiogenic, obstructive; they can coexist.
- Recognize compensatory shock early (tachycardia, bounding pulses, injected membranes in dogs).
- Cats in shock present with bradycardia and hypothermia; do not wait for hyperdynamic signs.
- Fluid resuscitate in boluses (dogs 20-30 mL/kg, cats 10-15 mL/kg) and reassess; never give the full shock dose at once.
- Do NOT aggressively fluid resuscitate cardiogenic shock; use inotropes (dobutamine) instead.
- Serial lactate clearance is the best indicator of resuscitation success; target 50% reduction in 6 hours.