Veterinary Fluid Therapy, Pain Management, and Anesthesia Pharmacology

Total body water in animals

Approximately 60% of body weight in adult animals and 80% in neonates.

Body water compartments

Intracellular fluid (inside cells), interstitial fluid (between cells), and intravascular fluid (within blood vessels).

Effect of body fat on total body water

Increased body fat decreases total body water; fluid requirements are better estimated using lean body mass.

Sensible fluid losses

Measurable fluid losses, including urination, vomiting (emesis), and diarrhea.

Insensible fluid losses

Fluid losses that are difficult to measure directly, including respiration and sweating.

Sweating and fluid loss

Dogs and cats lose negligible fluid through sweating compared with horses, in which sweat loss can be significant.

Hypovolemia

Loss of intravascular fluid volume.

Dehydration

Deficiency of body water resulting from fluid loss.

Subjective assessment of hydration

Skin turgor, mucous membrane status, sunken eyes, water intake, urine output, urine color, and tachycardia.

Skin turgor assessment

In horses and some dogs, assess skin turgor at the point of the shoulder because the neck may not be reliable.

Objective assessment of hydration

Packed cell volume (PCV), plasma or total protein, and urine specific gravity (USG).

PCV, plasma protein, and USG in dehydration

All three are usually elevated in dehydration.

Effect of anemia on dehydration assessment

Anemia can make a dehydrated patient appear adequately hydrated based on PCV alone because the PCV may remain low despite fluid loss.

Why check PCV with total protein?

Checking both helps identify dehydration that could be masked by anemia and provides a more accurate interpretation of hydration status.

Three components of fluid requirements

Maintenance requirements, existing fluid deficit, and ongoing fluid losses.

Dog maintenance fluid formula

132 × body weight in kg^0.75 = approximate maintenance fluid requirement in mL/day.

Simplified dog maintenance fluid rate

Approximately 60 mL/kg/day; the lecture also gives 2-6 mL/kg/hour as a rule of thumb.

Cat maintenance fluid formula

80 × body weight in kg^0.75 = approximate maintenance fluid requirement in mL/day.

Simplified cat maintenance fluid rate

Approximately 40 mL/kg/day; the lecture also gives 2-3 mL/kg/hour as a rule of thumb.

Dog maintenance calculation example

A 35 kg dog requires approximately 132 × 35^0.75 = 1,900 mL over 24 hours.

Fluid deficit formula

Body weight in kg × dehydration percentage expressed as a decimal = estimated kilograms of fluid lost.

Convert fluid deficit to mL

Multiply kilograms of fluid deficit by 1,000 mL/kg.

Fluid deficit example

A 35 kg dog at 8% dehydration: 35 × 0.08 = 2.8 kg fluid deficit; 2.8 × 1,000 = 2,800 mL total deficit.

Fluid deficit example: first 24 hours

Following the lecture's example, replace 80% of the 2,800 mL deficit during the first 24 hours: 2,800 × 0.8 = 2,240 mL.

Ongoing fluid losses

Additional losses from excessive vomiting, diarrhea, hemorrhage, wounds, or third spacing that must be included in the fluid plan.

Measuring ongoing fluid losses

Measure excess urine output or weigh fluid-soaked bandages; estimate losses from vomiting or wounds when direct measurement is not possible.

Calculate the total daily fluid requirement

Add maintenance fluids, the fluid deficit planned for correction, and estimated ongoing losses; calculate and administer the planned volume over 24 hours as directed.

Dog crystalloid shock rate

80-90 mL/kg according to the lecture.

Cat crystalloid shock rate

50-55 mL/kg according to the lecture.

Shock fluid administration

Calculate the shock volume, give 25% rapidly, and reassess the patient after each 25% bolus.

When to consider colloids during shock treatment

The lecture suggests considering a colloid if the patient shows no favorable response after receiving 50% of the calculated crystalloid shock volume.

Parenteral fluid therapy

Administration of fluids by injection rather than through the digestive tract.

Subcutaneous fluid therapy

SQ fluids are absorbed more slowly than IV fluids and may be useful in selected patients with relatively mild dehydration.

Intravenous fluid therapy

Provides faster, more precise fluid administration than SQ therapy.

Osmolality

Measure of the concentration of osmotically active particles in a solution, which influences water movement between compartments.

Crystalloids

Solutions containing small dissolved particles that can move between the intravascular and interstitial compartments.

Colloids

Solutions containing larger molecules that remain within the vasculature longer and expand plasma volume by drawing fluid from the interstitial space.

Crystalloid duration in vasculature

The lecture states that large crystalloid volumes remain within the vasculature for approximately 40-80 minutes.

Extracellular fluid osmolality in dogs and cats

Approximately 300 mOsm/L according to the lecture.

Isotonic crystalloids

Fluids with sodium content and osmolality close to extracellular fluid.

Examples of isotonic crystalloids

0.9% sodium chloride (saline) and Lactated Ringer's solution.

Isotonic crystalloid movement

Approximately 80% of administered crystalloid volume moves into the interstitial compartment within one hour, according to the lecture.

Uses of isotonic crystalloids

Vomiting and diarrhea (V/D), pancreatitis, shock, and inflammatory bowel disease (IBD).

Caution with isotonic crystalloids

Use caution in patients with cardiac or renal disease because sodium and fluid loading can worsen their condition.

Hypotonic crystalloids

Fluids with sodium concentration and osmolality lower than extracellular fluid; they promote movement of water out of the vasculature.

Examples of hypotonic crystalloids

0.45% saline and 2.5% dextrose.

Uses of hypotonic crystalloids

The lecture lists situations involving sodium retention and diabetic ketoacidosis (DKA).

Hypertonic crystalloids

Fluids with osmolality greater than extracellular fluid; they draw water into the intravascular space from extracellular and potentially intracellular compartments.

Example of a hypertonic crystalloid

7.5% sodium chloride.

Uses of hypertonic crystalloids

Hypovolemia and, in selected cases, intracranial edema; the lecture also discusses their use in very large dogs when isotonic fluids cannot be administered quickly enough.

Major risk of hypertonic crystalloids

They can cause or worsen dehydration by drawing water into the vascular compartment, so careful monitoring is required.

Colloid types

Synthetic colloids and natural colloids.

Synthetic colloids listed

Vetstarch, hetastarch, and TurboStarch.

TurboStarch

Combination of colloid and 23.4% saline in a 2:1 ratio, as described in the lecture.

Natural colloids listed

Albumin and blood products, including fresh frozen plasma (FFP), packed red blood cells (PRBCs), and whole blood.

Why colloids remain in the vasculature

Their large molecular weight limits movement out of the vascular space, allowing them to expand plasma volume for hours.

Albumin

Primary natural physiologic plasma colloid; canine or human albumin may be administered, with canine albumin described in the lecture as more expensive and less available.

Main uses of colloids

Hypovolemia not primarily caused by dehydration, hypoalbuminemia, septic shock, edema, and selected cases of head trauma.

23.4% hypertonic saline and intracranial pressure

The lecture explains that its high osmolality draws water from brain tissue, reducing intracranial pressure; it can also decrease blood viscosity and improve cerebral blood flow and oxygenation.

Vetstarch

Hydroxyethyl starch product described in the lecture as commonly used for hypovolemia and hypoproteinemia.

Vetstarch versus hetastarch

Vetstarch has a smaller molecular weight and can be administered at a higher rate than hetastarch; the lecture also notes potential reduction in vascular inflammation.

Vetstarch duration

Approximately 12-36 hours of plasma-volume expansion according to the lecture.

Dextran duration

Approximately 4-8 hours in the vasculature according to the lecture; it is described as expensive and shorter acting than Vetstarch.

Hetastarch: volume overload

Hetastarch can contribute to excessive intravascular volume.

Hetastarch: coagulopathy

Can impair coagulation by prolonging partial thromboplastin time (PTT), prothrombin time (PT), and bleeding time, and by lowering fibrinogen and factor VIII.

Hetastarch: hemodilution

Hemodilution can lower concentrations of clotting proteins, including fibrinogen.

Hetastarch: hypersensitivity

Can cause type III hypersensitivity reactions.

Hetastarch and acute kidney injury (AKI)

The lecture describes conflicting evidence regarding AKI and advises caution in critically ill patients, particularly those with underlying renal insufficiency.

Fresh frozen plasma (FFP)

Plasma product containing all blood clotting factors, as described in the lecture.

Frozen plasma (FP)

Plasma produced when FFP is thawed and then refrozen; the lecture states that FP lacks factors V and VIII.

Packed red blood cells (PRBCs)

Blood product used when the patient has primarily red blood cell loss.

Whole blood

Used for blood-loss anemia when replacement of red blood cells and other components of lost blood is needed.

When to consider blood transfusion

The lecture identifies rapid decreases in hematocrit or signs of anemia as reasons to consider blood products.

Transfusion-associated hypocalcemia

Stored blood contains anticoagulant that can bind or precipitate calcium; repeated transfusions over a short period can contribute to hypocalcemia.

Whole-blood transfusion volume formula

Desired PCV increase in percentage points × 2 mL/kg × body weight in kg = estimated transfusion volume in mL.

Whole-blood transfusion example

A 20 kg dog needing a 10-percentage-point PCV increase: 10 × 2 mL/kg × 20 kg = 400 mL.

Pain pathway: transduction

Conversion of a noxious physical stimulus into excitation of a pain receptor (nociceptor); local anesthetics can block this process.

Nociceptor

Sensory receptor that responds to potentially damaging stimuli.

Pain pathway: transmission

Movement of pain information from the stimulated receptor along sensory nerves toward the spinal cord and central nervous system; sensory nerve blocks can interrupt transmission.

Pain pathway: modulation

Processing of pain signals as they travel through the spinal cord toward the brain; signals can be amplified or suppressed.

Pain pathway: perception

Conscious recognition of pain when the signal reaches higher brain centers.

Four stages of the pain pathway

Transduction → transmission → modulation → perception.

Hyperalgesia

Increased sensitivity or response to a stimulus that is normally painful.

Hyperesthesia

Increased sensitivity to sensory stimuli.

Allodynia

Pain caused by a stimulus that does not normally provoke pain.

Pain after tissue trauma

Initially localized acute pain can become more diffuse or throbbing as inflammation develops and nearby pain receptors become more sensitive.

Reducing post-traumatic pain sensitivity

Reducing inflammation early after trauma can help limit increased sensitivity of nearby pain receptors.

Wind-up

Progressive increase in successful transmission of pain signals up the spinal cord, resulting in increased pain intensity over time.

Mechanism of wind-up

Initially, some pain signals are dampened; over time, this dampening decreases and more pain signals reach the brain.

Wind-up and anesthesia

Wind-up can occur while an animal is unconscious because spinal pain transmission continues below the brain.

Why prevent wind-up during surgery?

Pain transmission during anesthesia can contribute to increased postoperative pain when the patient wakes.

Preventing wind-up

Providing analgesics and anti-inflammatory drugs early can reduce wind-up and improve postoperative pain control.

Analgesic

A drug that relieves pain.

Opioids

Drugs used primarily for analgesia that bind to opioid receptors; the lecture identifies them as among the most potent analgesics used in veterinary medicine.

Opiates versus opioids

Opiates are derived from poppy products; opioids is the broader class of natural and synthetic drugs that act on opioid receptors.

Endogenous opioids

Opioid substances produced by the body in response to stimuli.

Endorphins

Endogenous opioid substances; the lecture describes them as the body's "endogenous morphine."

Exogenous opioids

Opioids introduced into the body through administration.

Examples of exogenous opioids

Morphine is a natural opium derivative; fentanyl and methadone are synthetic opioids.

Opioid administration routes

Oral, transdermal, subcutaneous (SQ), intramuscular (IM), and intravenous (IV).