Recognizing shock
Merck's triage chapter lists complaints that should send an animal straight to the treatment area [1]. These include collapse, loss of consciousness, severe change in mental state, and pale or weak presentations. The chapter says "All members of the veterinary medical staff should be able to recognize these common complaints" [1]. Beyond that, the cited sources do not assign specific tasks to veterinarians, technicians or reception staff.
Merck describes shock as moving through three stages [1]:
| Stage | Dogs | Cats |
|---|
| Early compensatory | Tachycardia, pink to red membranes, rapid CRT, bounding pulses, usually alert | Rarely identified |
| Middle (early decompensatory) | Pale membranes, prolonged CRT, weak pulses, tachycardia, duller responsiveness | Gray membranes, slow CRT, weak or absent pulses, hypothermia, normal or low heart rate |
| Terminal (late decompensatory) | Heart rate slows, loss of consciousness, severe hypotension, absent pulses, gray membranes | Same pattern as dogs |
Merck adds that tachycardia may be the only early sign in dogs, so "persistent tachycardia must be considered a sign of altered perfusion" [1]. It also cautions that pain or anxiety can mimic compensatory shock, which is why the chapter recommends analgesia and acclimation time [1].
Its triage table notes that cats in shock develop bradycardia below 120 bpm [1]. The Canadian review similarly describes feline shock as hypotension, hypothermia and bradycardia, "in contrast to tachycardia in dogs" [3].
For a collapsed, unstable animal, the Canadian review lists steps that include the following [3]:
- Evaluate an ECG and blood pressure.
- Use point-of-care ultrasound to look for free fluid.
- Run baseline bloodwork including at least glucose, lactate, packed cell volume and total protein (PCV/TP), sodium and potassium.
Planning the resuscitation
Merck's fluid chapter frames the plan as four steps: determine where the deficit lies, select fluids, determine end points, and choose a technique [2].
Locating the deficit
According to Merck, the type of deficit shows in different signs [2]:
- An intravascular deficit shows up as poor perfusion.
- Interstitial and intracellular deficits show up as dehydration.
- Most animals with poor perfusion also have extravascular deficits.
The chapter estimates dehydration from physical findings [2]. The range runs from 4-5% (semidry membranes, normal skin turgor) to 12% or more (very dry membranes, complete loss of turgor, severely retracted eyes, thready pulses). The chapter flags these signs as misleading in emaciated or geriatric animals, very young animals, nauseated animals, and those with rapid third-space losses [2].
The deficit in liters is body weight in kilograms multiplied by the dehydration fraction. Merck's example is a 12-kg dog at 8% dehydration with a 0.96 L loss [2].
Selecting fluids
Merck states that more than 75% of an IV isotonic crystalloid moves out of the vessels within 1 hour in a healthy animal [2]. It also states that hypotonic fluids such as 5% dextrose in water "should not be used as resuscitation fluids" [2].
Per Merck, most patients benefit from buffered, balanced isotonic crystalloids such as lactated Ringer's solution or Plasma-Lyte A [2]. Merck says 0.9% saline is used first for specific problems, including hyponatremia, hypercalcemia, head trauma and oliguric renal failure [2]. Adding colloids to crystalloids, Merck says, reduces crystalloid requirements by 40-60% and shortens resuscitation [2].
The sources attach cautions to synthetic colloids:
- The Canadian review notes human and canine studies suggesting that synthetic colloids can cause acute kidney injury, coagulopathy and platelet dysfunction [3].
- Merck reports that more recent evidence does not suggest kidney injury with 7% or lower solutions, while advising against synthetic colloids in already azotemic patients [2].
- The Frontiers review notes that the 2016 human sepsis guidelines carry "a strong recommendation/high quality of evidence against hydroxyethyl starches" [4]. That is human guidance.
On hypertonic saline, Merck's triage chapter says 7% saline at 4 mL/kg IV should not be used in dehydrated patients or those at risk of hypernatremia [1].
Resuscitation end points
Merck's fluid chapter sets out two sets of targets [2]:
| Parameter | Supranormal (high volume) | Hypotensive (low volume) |
|---|
| Systolic pressure | 90-120 mm Hg | 80-90 mm Hg |
| Mean arterial pressure | 80-90 mm Hg | 60-80 mm Hg |
| Central venous pressure | 6-8 cm H2O | 3-5 cm H2O |
| Heart rate, dog | < 140 bpm | < 140 bpm |
| Heart rate, cat | 160-200 bpm | 160-200 bpm |
| CRT | 1-2 seconds | 1-2 seconds |
| Femoral pulse | Strong | Strong |
Per Merck, the two sets of targets apply to different conditions [2]:
- Supranormal targets: SIRS diseases, cortisol deficiency and anaphylaxis.
- Hypotensive targets: heart failure, closed cavity or ongoing hemorrhage, brain disease, lung edema and oliguric renal failure.
Merck's triage chapter gives a slightly different large-volume target of 80-100 mm Hg mean arterial pressure [1].
The fluid chapter also cites resolution of raised lactate as supporting adequate tissue oxygenation [2]. It lists supplementary end points that require additional instrumentation, including central venous oxygen saturation above 70% and urine output of at least 1-2 mL/kg/h [2].
The Canadian review lists its own goals [3]:
- Restored respiratory rate, membranes, CRT and pulse quality.
- Heart rate ideally falling 10 to 15% from baseline.
- Blood glucose above 80 mg/dL.
- Systolic pressure above 90 mmHg and/or a 10 to 15% rise.
- Lactate trending toward normal.
It notes septic animals often stay tachycardic despite adequate resuscitation [3].
Large-volume and small-volume techniques
Merck's fluid chapter describes giving each dose as a rapid infusion over 15-30 minutes, then reassessing perfusion, with continual reassessment and titration [2].
- Large-volume (dogs needing supranormal end points): per Merck, an initial 20-50 mL/kg of buffered, balanced isotonic crystalloid IV, followed by 5-15 mL/kg of an HES solution if indicated [2].
- Small-volume: per Merck, an initial 10-15 mL/kg for dogs and 5-10 mL/kg for cats of balanced isotonic crystalloid IV [2]. A colloid (dogs 5 mL/kg; cats 2-5 mL/kg) can also be given IV over 5 minutes; perfusion is then reassessed and the initial bolus repeated as needed until end points are reached [2].
The triage chapter gives a different bolus range: low-volume boluses of 10-15 mL/kg IV and high-volume boluses of 20-30 mL/kg IV, each over 10-15 minutes [1]. Readers comparing protocols should note that the two Merck chapters do not use identical numbers.
The Canadian review calculates boluses from a shock volume, 90 mL/kg in dogs and 45 to 60 mL/kg in cats, giving 1/4 to 1/3 IV [3]. Its example is a 300 mL bolus for a 10-kilogram dog, ideally over less than 10 minutes, with cat boluses over 10 to 15 minutes [3].
Its synthetic colloid boluses are [3]:
- Dogs: 5 mL/kg (up to 20 mL/kg total).
- Cats: 3 mL/kg over 10 to 15 minutes (up to 10 mL/kg total).
On equipment, the authors write that "Fluid pumps cannot give more than 250 mL within 15 min" [3]. They recommend a pressure bag or squeezing the bag in medium-to-large dogs [3].
For suspected sepsis, the Canadian review cautions against large volumes that can leak and cause edema, and quotes the maxim "Give them as much [fluid] as they need and not a drop more" [3].
Warming
Merck's fluid chapter recommends active external warming alongside resuscitation in hypothermic patients [2]. It says hypothermia, especially in cats, blunts the response to fluids [2].
The triage chapter describes slow warming until rectal temperature exceeds 36.5 C (98 F) [1]. It specifies that surface warming starts only after initial volume resuscitation [1].
Hemorrhage and permissive hypotension
Merck classifies hemorrhage in four classes [1]:
- Class I: under 15% of blood volume (under 13 mL/kg dog, under 9 mL/kg cat).
- Class IV: 40% or more (at least 35 mL/kg dog, 24 mL/kg cat).
The chapter's damage control principles, which it notes have been associated with improved outcome in human trauma patients, include permissive hypotension, early blood products and limiting large crystalloid volumes [1].
The Canadian review states that hypovolemia is a more emergent issue than anemia [3]. It gives systolic pressures of 60 to 90 mmHg as an example of permissive targets, while noting that "No consensus exists as to the exact blood pressure goals" [3].
The same review states that red cell or whole blood transfusion is rarely used for resuscitation and reserves it for PCV/TP below 15 to 20% with persistent hypotension, tachycardia or weakness despite fluids [3]. It also notes that acute traumatic blood loss can warrant transfusion at higher PCV values [3].
Refractory hypotension
Merck's triage chapter states that pressors should be added next when volume resuscitation fails [1]. It names dopamine, norepinephrine, epinephrine, phenylephrine and vasopressin, given at the smallest dose that keeps systolic pressure above 90 mm Hg [1].
The Frontiers review by Valverde summarizes the 2016 human sepsis guidelines [4]. These favor norepinephrine first, with vasopressin or epinephrine as add-ons. The review also notes that dopamine was associated with more arrhythmias and higher mortality than norepinephrine in human patients [4].
In healthy, normovolemic, deeply isoflurane-anesthetized research dogs, the review reports two findings [4]:
- 80 mL/kg of isotonic crystalloid over up to 1 hour did not raise blood pressure.
- Hetastarch restored systolic pressure to within 10% of baseline values in 66% of cases.
The author cautions that findings from healthy research dogs "should be extrapolated cautiously" to patients in refractory shock [4].
