Perioperative Fluid Administration

Perioperative Fluid Administration

“One can drink too much, but one never drinks enough.”

GOTTHOLD EPHRAIM LESSING

Overview

Perioperative fluid therapy is a vital adjunct to any anesthetic plan. The term perioperative is defined as the period of time extending from when the animal goes into the hospital, clinic, or veterinarian’s office for surgery until the time the animal is discharged home. Almost all drugs used to produce anesthesia decrease the force of cardiac contraction and cause vasodilatation thereby increasing intravascular volume and resulting in relative hypovolemia. These effects generally lead to decreased cardiac output (blood flow) and arterial blood pressure (BP). It is important to remember that intravenous (IV) fluids are drugs and that their administration during anesthesia helps maintain an adequate and effective circulating blood volume, improve cardiac output, and enhance tissue perfusion. Additionally, perioperative fluid therapy is frequently administered to treat electrolyte and acid-base disorders. The choices for acute fluid therapy frequently include balanced electrolyte solutions (crystalloids), colloids, plasma, cell-free blood substitutes, and blood.

General Considerations (Table 16-1)

TABLE 16-1

Normal Electrolyte Composition of Serum (mEq/L)

Animal	Na⁺	K⁺	Ca⁺⁺	Mg⁺⁺	HPO₄	Cl⁻	HCO₃
Dog	143-153	4.2-5.4	5.0-6.1	0.43-0.60	3.2-8.1	109-120	18-25
Cat	146-156	3.2-5.5	4.9-5.5	0.43-0.70	3.2-6.5	114-126	18-22
Horse	132-142	2.4-4.6	6.0-7.2	0.46-0.66	1.2-4.8	97-105	23-31
Cow	133-143	3.9-5.2	8.9-5.4	0.47-0.65	3.8-7.7	98-108	23-31

I Anesthesia, surgery, and many of the diseases for which surgical intervention is required disturb cardiac function, smooth muscle vascular tone, blood flow distribution, plasma electrolyte concentrations and acid-base balance potentially altering the effective circulating blood volume (the volume circulating).

A Many diseases produce changes in the relative or absolute blood volume, electrolyte concentrations, and acid-base balance (Table 16-2)

TABLE 16-2

Common Diseases And Expected Electrolyte Abnormalities

Disease Syndrome	Water	Na⁺	K⁺	Ca⁺⁺	Mg⁺⁺	HPO₄	Cl^—	HCO₃
Gastric loss, vomiting	Loss	↓	↓		↓		↓	—↑
Pancreatic or intestinal fluid loss	Loss	↓	↓		↓		↓	↓
Diarrhea	Loss	↓	↓		↓		↓	↓
Starvation	Loss		↓↑	↓	↓
Acute hemorrhagic pancreatitis	Loss	—↓	—↓	↓			↓
Malabsorption syndrome	Loss			↓	↓	↓
Acute renal failure (oliguric)	Excess	—↑↓	↑	↓	↓	↑	↑	↓
Renal tubular dysfunction	Loss	↓	—↓					↓
Chronic renal disease	Loss	—↓	—↓↑	—↓	↑	—↑	—↓	↓
Diabetes insipidus	Loss	↑				↑
Burns	Loss	↓	↑	↓
Primary aldosteronism	Excess	↑—	↓		↓		↓	↑
Stress, surgery, including antidiuretic hormone	Excess	↓						↓
Hypoadrenocorticism (Addison’s disease)	Loss	↓	↑	—↑	↑	—↑	—↓	↓
Hypopituitarism	Excess	↓
Hyperadrenocorticalism (Cushing’s disease)	Excess	—↑	↓	↓				↑
Excess citrated blood				↓
Hyperparathyroidism			↑	↑	↓	↓
Excess lactation (milk fever)				↓	↓	↓
Acidosis (metabolic)				↑			—↑	↓
Alkalosis (metabolic)				↓			↓	↑

↑, Increased serum concentration; ↓, decreased serum concentration; —, normal serum concentration.

B Common side effects of anesthesia

1. Inhalant and/or IV anesthesia

a. Decrease myocardial contractility, cardiac output and tissue perfusion

b. Produce vasodilation; relative hypovolemia; hypotension is one of the most common, if not the most common side effects of general anesthesia in most species

c. Depress minute ventilation; respiratory acidosis may occur

d. Decrease urine production and renal concentrating ability

e. Predispose to hypothermia

f. Depress autoregulatory compensatory mechanisms

2. General anesthesia usually depresses sympathoadrenal and compensatory reflex responses to hypotension, hypercapnia, and hypoxemia

C Imbalances associated with or caused by surgery

1. Vasodilatation (relative hypovolemia)

2. Blood loss (absolute hypovolemia)

3. Evaporative fluid loss from exposed tissues

4. Pain-induced redistribution of fluids

II Fluid loss leads to a decrease in the “effective” circulating blood volume eventually resulting in a decrease in cardiac output, a redistribution of blood to vital organs (heart, brain, lung), and the development of metabolic acidosis

III Fluid administration in young or small animals (<3 kg) should be supplemented with a source of calories (dextrose) and monitored closely to prevent hypothermia and/or overhydration

IV The administration of large quantities of room-temperature fluids to any animal can produce hypothermia and hemodilution, resulting in coagulation abnormalities

Normal Body Water Distribution (Fig. 16-1)

I Total body water represents 55% to 75% of body weight (use 60%), primarily depending on the animal’s age and percentage of body fat

II Extracellular water (ECW): the water outside cells and constituting the blood (plasma; red blood cells [RBCs]) volume and interstitial fluid volume. ECW ranges from 23% to 33% of body weight (use 30%); the percentage is greater in very young animals

III Interstitial water constitutes 15% to 25% of body weight

IV Plasma water constitutes approximately 5% of body weight

V Extracellular water equals plasma water plus interstitial water

VI Intracellular water constitutes 35% to 45% of body weight

VII Blood volume constitutes 8% to 10% of body weight (approximately 90 mL/kg of body weight), depending on the hematocrit; hematocrit measures how much space in the blood is occupied by RBCs and is also referred to as the packed red cell volume or packed cell volume (PCV) and abbreviated as Hct or Crit

A Blood volume equals plasma volume plus RBC volume

Electrolyte Distribution

I Extracellular water contains large quantities of salt: sodium and chloride ions. The extracellular fluids (plasma and interstitial fluid) also contain smaller quantities of other solutes

A Blood contains RBCs and plasma. Plasma is a complex fluid that contains water and other solutes including electrolytes (see Table 16-1), proteins (albumin), large glycoproteins (antibodies: immunoglobulins), glucose and many other substances

1. Plasma osmolarity is a measure of the concentration of substances such as sodium, chloride, potassium, urea, glucose, and other ions in blood. It is calculated as the osmoles of solute per kilogram of solvent.

a. plasma osmolarity = 2[Na+] + [Glucose]/18 + [Blood urea nitrogen]/2.8

2. The plasma osmolarity is sensitive to changes in hydration status during dehydration and rehydration. The normal plasma osmolarity ranges from 280 to 300 mOs/kg

II Intracellular water contains large quantities of potassium ions

Principles of Fluid Administration

I Correct dehydration and electrolyte and acid-base imbalances 12 to 24 hours before anesthesia when possible

II Do not attempt to replace dehydration or chronic fluid losses acutely; severe dilution of plasma proteins, RBCs, and electrolytes may be produced (see Table 16-2)

A Normal daily fluid maintenance rates vary as follows:

1. Mature animal: 30 to 60 mL/kg/day

2. Young animal: 40 to 80 mL/kg/day

B Calculation of replacement fluid volume in dehydrated animals

1. Body weight (kg) × % dehydration (figured as a decimal) = fluid deficit (L) (e.g., 20 kg × 10% dehydration = 2 L fluid deficit); reassess periodically to determine the response to volume administered

2. Fluid administration to correct dehydration should be administered over a 6- to 12-hour period or added to the maintenance fluid volume and administered over a 24-hour period

Dehydration in Animals

Organ	Minimal (4%)	Moderate (6% to 8%)	Severe (10% to 12%)
Skin resiliency	Pliable	Leathery	Absolutely no pliability
Skin tenting	Tented skin immediately disappears and tent persists up to 2 sec	Tented skin slowly disappears and tent persists up to 3 sec or more	Twisted and tented skin persists indefinitely
Eye	Bright Slightly sunken	Duller than normal Obviously sunken	Cornea dry Deeply sunken, 2- to 4-mm space between eyeball and bony orbit
Mouth	Moist, warm	Sticky to dry, warm	Dry, cyanotic

III Monitor hemodynamics, pulmonary, and renal function when administering fluids rapidly

A Cardiac function

1. Monitor arterial BP; systolic BP >90 mm Hg; mean BP >60 mm Hg

2. Auscultate the chest; if the total protein (TP) drops below 3.5 g/dL from fluid administration, pulmonary edema may occur

3. Central venous pressure (0.5 to 5.0 cm H₂O) or pulmonary capillary wedge pressure (5 to 10 mm Hg) can be monitored, although their predictive value for determining rates and volumes of fluid administration is low unless fluids are administered in excess or the animal develops right heart failure

4. Measures of systolic arterial pressure variation (SPV) or pulse–oximeter-derived plethysmographic variability index (PVI) provides valuable information regarding fluid requirements and the effectiveness of fluid administration (see: Chapter 14)

a. PVI: pleth variability index, is a noninvasive method for optimizing fluid administration and predicting fluid responsiveness in mechanically or manually ventilated animals during general anesthesia

b. PVI improves fluid management and helps to detect hypovolemia

B Renal function

1. Improve or reestablish renal perfusion and urine production (0.5 to 2.0 mL/kg/hr) before anesthesia

2. Administer mannitol at 1 to 2 g/kg IV over a 30-minute period to induce urine production in animals with normal hydration

C Pulmonary function—overhydration can lead to pulmonary edema initially exemplified by increases in respiratory rate, increased breathing effort (abdominal contraction) and increased lung (bronchovesicular) sounds (Box 16-1).

Box 16-1 End Points for Monitoring Fluid Resuscitation

Variable	Resuscitation End Point
Mentation	Alert
Heart rate (beats/min)	Dog: 70-130 Cat: 120-180
Mucous membrane color	Pink
Capillary refill time (seconds)	1-2
Rectal temperature (°F)	99-101
Mean arterial blood pressure (mm Hg)	60-80
Systolic arterial blood pressure (mm Hg)	80-100
Central venous pressure (cm H₂O)	5-10
Urine output (mL/kg/h)	>0.5
SpO₂ (%)	>95
SCVO₂ (%)	<60
PCV (%)	>21
Hemoglobin (g/dL)	>7
Lactate (mmol/L)	<2
pH	>7.32
Base deficit (mEq/L)	−2 to +2
COP (mm Hg)	14-20

COP, Colloid oncotic pressure; PCV, packed cell volume; SpO₂, arterial oxygen saturation; SCVO₂, central venous oxygen saturation.

Fluid Administration During Anesthesia (Table 16-3; Boxes 16-2 and 16-3)

Box 16-2

Guidelines for a “Recipe-Based” Approach for Crystalloid Fluids Administration

Start IV fluids, replace insensible (feces, saliva, skin, respiratory tract ≅ 20 mL/kg/day) loss with maintenance-type solutions, 2 to 3 mL/kg/hr, during interval since last oral intake. Example:

10 kg dog; 3 hr since oral H₂O intake

10 kg × 3 (hr) × 3 mL = 90 mL added to first hour of basal fluids

Basal fluid administration solution for intraoperative losses. Administer lactated Ringer’s solution or Normosol-R

Estimate surgical trauma and add appropriate volume of replacement-type solution to that given in Step 2:

Minimal trauma, add 5 mL/kg/hr

Moderate trauma, add 10 mL/kg/hr

Extreme trauma, add 15 mL/kg/hr

Replace 1 mL of blood loss with 3 mL of crystalloid

Administer appropriate fluid; when in doubt give balanced electrolyte solution

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Tags: Handbook of Veterinary Anesthesia

Sep 6, 2016 | Posted by admin in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off

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