Overview and Clinical Importance
Preanesthetic assessment and preparation forms the foundation of safe anesthesia practice. This critical phase occurs BEFORE any drugs are administered and directly impacts patient outcomes. Studies demonstrate that animals with ASA Physical Status scores of III or higher have 3-11 times increased risk of anesthesia-related death compared to healthier patients.
The BCSE heavily tests this domain because proper patient evaluation, risk stratification, equipment preparation, and fluid selection represent core competencies expected of entry-level veterinarians. Questions often present clinical scenarios requiring you to identify appropriate fasting times, catheter sizes, fluid choices, and ASA classifications.
High-YieldThe preanesthetic period begins AT HOME with client instructions (fasting) and continues through equipment checks and IV catheter placement. Think of anesthesia as a journey that starts well before induction.
| ASA Class |
Definition |
Veterinary Examples |
| ASA I |
Normal, healthy patient |
Young healthy animal for elective OVH or castration. No detectable disease. Normal lab values. |
| ASA II |
Patient with mild systemic disease, no functional limitations |
Skin tumor removal. Mild obesity. Compensated heart murmur (grade I-II). Mild dental disease. Geriatric patient with no concurrent disease. Brachycephalic breeds (baseline). |
| ASA III |
Patient with severe systemic disease with functional limitations |
Compensated cardiac disease. Stable diabetes mellitus. Chronic renal disease (IRIS Stage 2-3). Moderate anemia (PCV 20-30%). Controlled hypothyroidism. Portosystemic shunt repair. PDA ligation. |
| ASA IV |
Patient with severe systemic disease that is a constant threat to life |
Gastric dilatation-volvulus (GDV). Uncontrolled diabetes with ketoacidosis. Severe anemia (PCV less than 20%). Decompensated cardiac failure. Ruptured bladder. Severe pneumonia. Dystocia with compromised patient. |
| ASA V |
Moribund patient not expected to survive 24 hours with or without surgery |
Ruptured splenic hemangiosarcoma with severe hemorrhage. End-stage multiorgan failure. Severe trauma with shock. Advanced sepsis unresponsive to therapy. |
| E (modifier) |
Emergency status - added to any class |
Any ASA class can have E added (e.g., ASA III-E for emergency C-section). Emergency status generally increases risk. |
| Patient Category |
Minimum Testing |
Consider Adding |
| Young, healthy (ASA I) less than 5 years |
PCV/TS, BUN, Glucose |
None required for routine procedures |
| Middle-aged (5-7 years) |
PCV/TS, BUN, Glucose, Chemistry panel |
CBC, Urinalysis |
| Senior (greater than 7 years) |
CBC, Full chemistry, Urinalysis |
T4, ECG, Blood pressure, Thoracic radiographs |
| ASA III-V (any age) |
CBC, Full chemistry, Urinalysis, Coagulation profile |
Blood gas, Lactate, Imaging as indicated by condition |
Section 1: Patient Evaluation and Risk Assessment
The Preanesthetic Assessment Process
Every patient undergoing anesthesia requires a systematic evaluation to identify risk factors and guide anesthetic planning. The preanesthetic assessment consists of several key components that must be completed before anesthetic drug administration.
Components of Preanesthetic Evaluation
- Complete patient history: Previous anesthetic episodes, current medications, known drug reactions, concurrent diseases
- Physical examination: TPR (temperature, pulse, respiration), body condition score, cardiovascular auscultation, respiratory evaluation, hydration status
- Minimum database: PCV/TS (packed cell volume/total solids), BUN, blood glucose; expanded testing based on age and health status
- Risk assessment: ASA classification, procedure-specific considerations, emergency vs. elective status
MEMORY AID - "HAPPY PETS" for Preanesthetic Assessment:
History (previous anesthesia, allergies)
Auscultation (heart, lungs)
Physical exam (TPR, BCS)
PCV/TS (minimum database)
Youth/age considerations
Plan (protocol selection)
Equipment preparation
Test for leaks (machine check)
Safety verification (ASA status)
ASA Physical Status Classification System
The American Society of Anesthesiologists (ASA) Physical Status Classification is the most widely used system for categorizing anesthetic risk in veterinary medicine. Originally developed for human patients, it has been adapted for veterinary use and provides a standardized method for communicating patient status and predicting anesthetic outcomes.
High-YieldResearch shows dogs with ASA III or higher have 3.26 times greater risk of anesthesia-related death, cats have 4.83 times greater risk, and rabbits have 11.31 times greater risk compared to ASA I-II patients. BRACHYCEPHALIC breeds should be assigned minimum ASA II even for routine procedures due to their increased perioperative risk.
MEMORY AID - ASA Classification Numbers:
Think of ASA as a "countdown to catastrophe":
I = Ideal (healthy)
II = mIld disease
III = serIous/severe disease (but compensated)
IV = Very sick (life-threatening disease)
V = Virtually dying (moribund)
Preanesthetic Laboratory Evaluation
The extent of preanesthetic testing depends on patient age, health status, and the nature of the procedure. A minimum database should be obtained for all patients, with expanded testing for higher-risk individuals.
| Patient Category |
Food Fast |
Water |
| Healthy adult dogs (AAHA 2020) |
4-6 hours |
Available until premedication; small amounts (up to 10 mL/kg) can be offered 2-3 hours before anesthesia |
| Healthy adult cats (AAHA 2020) |
4-6 hours |
Same as dogs; cats may tolerate shorter fasts (3-4 hours) |
| Puppies/Kittens (less than 8 weeks) |
1-2 hours or no fast |
Do NOT restrict water; high risk of hypoglycemia |
| Puppies/Kittens (8-16 weeks) |
2-4 hours |
Allow water until premedication; monitor glucose |
| Diabetic patients |
Feed small meal morning of procedure |
Water available; adjust insulin dose accordingly; monitor glucose frequently |
| Brachycephalic breeds |
6-12 hours |
Longer fast recommended due to increased GER and aspiration risk; increased regurgitation risk |
| GI disease/megaesophagus |
12+ hours or as tolerated |
Case-dependent; may need anti-emetics; increased aspiration risk |
| Species |
Food Fast |
Water/Special Considerations |
| Horses (adult) |
8-12 hours hay, 4 hours grain |
Water is NOT typically withheld from horses; fasting reduces gut motility and colic risk |
| Foals (nursing) |
No fasting required |
Very young foals not on solid feed do not require fasting |
| Cattle (adult) |
24-48 hours |
Water: 12-24 hours. Critical to reduce rumen distension and regurgitation risk |
| Calves (nursing) |
No fasting required |
Nursing calves not on solid feed do not require fasting |
| Sheep/Goats (adult) |
12-24 hours |
Water: 12 hours. Reduces bloat and regurgitation risk |
| Pigs |
12-24 hours |
Water: 4-6 hours. Pigs are highly prone to aspiration |
| Rabbits |
DO NOT FAST |
NEVER fast rabbits - hindgut fermenters; causes gut stasis, hepatic lipidosis. Feed until surgery. |
Section 2: Fasting Guidelines by Species
Preanesthetic fasting reduces the risk of gastroesophageal reflux (GER), regurgitation, and aspiration pneumonia. However, excessively long fasts can cause dehydration, hypoglycemia, and may actually increase GER rates. Modern guidelines favor shorter fasting periods than traditional "NPO after midnight" protocols.
[Include Image: Figure 2. Species-Specific Fasting Timeline Diagram - visual comparison of fasting recommendations across species]
Rationale for Fasting
Anesthesia affects normal digestive processes in several ways that increase aspiration risk:
- Relaxation of lower esophageal sphincter increases reflux likelihood
- Depression of protective reflexes (swallow, gag, cough) reduces ability to clear aspirated material
- Delayed gastric emptying keeps stomach contents present longer
- Positioning during surgery may promote passive regurgitation
Dogs and Cats: Current Fasting Recommendations
High-YieldRecent research shows SHORTER fasts (3-4 hours) may actually REDUCE gastroesophageal reflux compared to traditional 12-hour fasts. Studies demonstrate that 10-hour fasted dogs had 20-27% GER rates while 3-hour fasted dogs had only 0-5% GER rates. The key is balancing aspiration risk against hypoglycemia and dehydration.
Large Animal Fasting Guidelines
MEMORY AID - Fasting Exceptions ("NEVER FAST THESE"):
Rabbits - hindgut fermenters, gut stasis risk
Babies (neonates) - hypoglycemia risk
Foals/calves (nursing) - if not on solid feed
"LONGER FASTS NEEDED":
Brachycephalic breeds (6-12 hours)
Ruminants (24-48 hours for cattle)
| Catheter Gauge |
Patient Size |
Typical Use |
| 24 gauge (smallest) |
Puppies, kittens, very small dogs (less than 2 kg) |
Neonatal patients, exotic species, fragile veins |
| 22 gauge |
Small dogs (2-5 kg), adult cats |
Standard for feline patients and toy breeds |
| 20 gauge |
Medium dogs (5-20 kg) |
Most common size for average canine patients |
| 18 gauge |
Large dogs (20-40 kg) |
Allows rapid fluid administration |
| 16-14 gauge (largest) |
Giant breed dogs (greater than 40 kg), large animals |
Emergency fluid resuscitation, blood transfusion |
| Vein |
Advantages |
Considerations |
| Cephalic (most common) |
Easy access, visible, convenient for most procedures, allows patient mobility |
May kink with limb flexion; preferred site for dogs and cats |
| Saphenous (lateral or medial) |
Alternative when cephalic unavailable, good for surgical procedures involving forelimbs |
Can be challenging in obese patients; useful backup site |
| Jugular |
Large vessel, rapid flow rates, central venous pressure monitoring |
Higher complication risk; used for critical patients, large volume resuscitation, CVP monitoring |
| Auricular (ear vein) |
Accessible in rabbits and pigs |
Primarily for blood collection or short procedures; species-specific |
Section 3: IV Catheter Placement and Fluid Selection
Importance of IV Access
Intravenous catheterization is considered standard of care for all patients undergoing general anesthesia. IV access provides:
- Rapid drug administration - essential for induction agents and emergency drugs
- Fluid therapy delivery - maintains blood pressure and tissue perfusion
- Emergency access - critical for treating anesthetic complications
- Blood sampling - for intraoperative monitoring without additional venipunctures
[Include Image: Figure 3. IV Catheter Types and Sizes - showing over-the-needle catheters in different gauges with visual size comparison]
Catheter Size Selection
Catheter size is measured in gauge (G). IMPORTANT: The LARGER the gauge number, the SMALLER the catheter diameter. Select the largest catheter the patient's vein can accommodate to minimize flow resistance.
MEMORY AID - Catheter Gauge:
"Golf Score Rule" - Lower number = BETTER (larger catheter, more flow)
18G = Large dog
20G = Medium dog ("Twenty = Tweener")
22G = Small dog/cat ("Twenty-two = Tiny")
24G = Puppies/kittens ("Twenty-four = For babies")
Catheter Placement Sites
Catheter Placement and Care Principles
- Aseptic technique: Clip hair, clean with chlorhexidine and/or alcohol
- Bevel orientation: Insert with bevel UP for easier vessel entry
- Flush protocol: Every 4 hours with heparinized saline if not on continuous fluids
- Daily assessment: Check for redness, swelling, discharge, pain at insertion site
- Replacement: Change catheters every 72-96 hours or sooner if complications noted (thrombophlebitis)
Fluid Selection for Anesthesia
Balanced isotonic crystalloids are the standard fluid choice for routine anesthetic patients. The choice of specific fluid depends on patient electrolyte status, acid-base balance, and underlying conditions.
Isotonic Crystalloid Solutions
High-YieldLRS contains CALCIUM - NEVER administer LRS through the same IV line as blood products! The calcium chelates anticoagulant citrate in the blood product, causing clot formation. Use Normosol-R, Plasmalyte, or 0.9% NaCl when transfusing blood.
Anesthetic Fluid Rates
MEMORY AID - Fluid Selection:
"LRS = Life-Risking if with blood Products" (calcium causes clots)
"Normal Saline = No potassium, Needs to be used for hyperK"
"Normosol = No calcium = Nice with blood"
| Fluid |
Na+ (mEq/L) |
K+ (mEq/L) |
Clinical Notes |
| Lactated Ringer's (LRS) |
130 |
4 |
Most common choice. Alkalinizing. Contains calcium - do NOT give in same line as blood products. |
| Normosol-R / Plasmalyte |
140 |
5 |
Balanced, alkalinizing. Contains acetate and gluconate buffers. No calcium - safe with blood. |
| 0.9% NaCl (Normal Saline) |
154 |
0 |
Acidifying (high chloride). Use for hyperkalemia. No potassium - safe for Addisonian crisis. |
| Patient Type |
Initial Rate |
Notes |
| Dogs - routine anesthesia (AAHA 2024) |
5 mL/kg/hr |
Reduce for cardiac/renal disease |
| Cats - routine anesthesia (AAHA 2024) |
3-5 mL/kg/hr |
Lower end for cardiac patients; cats more prone to fluid overload |
| Dogs - maintenance (hospitalized) |
2-6 mL/kg/hr |
Based on physiologic needs |
| Cats - maintenance (hospitalized) |
2-3 mL/kg/hr |
Cats have lower maintenance requirements |
| Dogs - shock resuscitation |
Up to 90 mL/kg (give in 25% increments) |
Reassess after each 25% bolus; rarely need full dose |
| Cats - shock resuscitation |
Up to 45-55 mL/kg (give in 25% increments) |
Lower volume than dogs; high risk of fluid overload |
| Horses - anesthesia |
5-10 mL/kg/hr first hour, then 5 mL/kg/hr maintenance |
Average 450 kg horse: 4-5 L first hour |
Section 4: Equipment Preparation and Safety Checks
Anesthetic equipment is considered "life-critical" because malfunctions can result in patient hypoxia, anesthetic overdose, or death. A systematic equipment check should be performed DAILY and verified BEFORE EACH CASE.
[Include Image: Figure 4. Anesthesia Machine Diagram - labeled components including oxygen source, flowmeter, vaporizer, breathing circuit, pop-off valve, scavenger system]
Anesthesia Machine Components
Breathing Circuit Selection
High-YieldTwo ESSENTIAL safety features on EVERY anesthetic machine: (1) In-circuit manometer (pressure gauge) - allows safe manual ventilation and leak checking; (2) Safety pop-off valve - prevents dangerously high airway pressures and barotrauma.
Systematic Equipment Check ("SOVIET" Mnemonic)
MEMORY AID - "SOVIET" Equipment Check:
Suction - available and working
Oxygen - tank greater than 200 psi, backup available
Vaporizer - filled with CORRECT agent
Intubation equipment - ET tubes (multiple sizes), laryngoscope, cuff syringe
Emergency drugs - calculated, drawn up, labeled
Testing - LEAK TEST the circuit before each patient
Leak Test Procedure
- Connect breathing circuit and appropriate reservoir bag
- Occlude the Y-piece (patient connection) with your palm or thumb
- Close the pop-off (APL) valve completely
- Turn on oxygen flow and fill the circuit until manometer reads 20 cm H2O
- Turn OFF oxygen flow
- Pressure should hold at 20 cm H2O for at least 10 seconds. If pressure drops, there is a LEAK - identify and repair before using on patient
Airway Equipment Preparation
Always have MULTIPLE sizes of endotracheal (ET) tubes ready - the estimated size PLUS one size larger and one size smaller.
ET tube cuff check: Inflate cuff and submerge in water or apply external pressure to verify no leaks. After intubation, inflate cuff just until no air escapes when giving a positive pressure breath (15-20 cm H2O).
| Component |
Function and Check |
| Oxygen supply (E-tank or pipeline) |
Provides carrier gas for inhalant delivery. Check: Tank pressure greater than 200 psi (full E-tank = 2200 psi = ~700 L). Ensure backup tank available. |
| Pressure regulator |
Reduces high tank pressure (2200 psi) to working pressure (~50 psi). Check: Gauge reads appropriate pressure. |
| Flowmeter |
Controls oxygen flow rate (L/min). Check: Ball or bobbin moves freely, reads accurately. |
| Vaporizer |
Converts liquid anesthetic to vapor at precise concentrations. Check: Filled with CORRECT agent, sufficient level for procedure. |
| CO2 absorbent (soda lime) |
Removes exhaled CO2 from rebreathing circuits. Check: Color indicator (purple/blue = exhausted); change after 6-8 hours use. |
| Pop-off (APL) valve |
Releases excess pressure to prevent barotrauma. Check: Opens freely, closes when needed for manual ventilation. |
| Reservoir (rebreathing) bag |
Collects gas mixture, allows manual ventilation. Check: Appropriate size (10-15 mL/kg), no holes. |
| Breathing circuit |
Connects machine to patient. Check: Correct type (circle vs non-rebreathing), no kinks or leaks. |
| Scavenging system |
Removes waste anesthetic gases. Check: Connected, functioning (active preferred over passive). |
| Manometer (pressure gauge) |
Displays circuit pressure (cm H2O). Check: Reads zero when circuit open, responds to bag squeeze. |
| Circuit Type |
Patient Size |
Characteristics |
| Non-rebreathing (Bain, Jackson-Rees) |
Less than 5-7 kg (cats, small dogs, exotics) |
Low resistance, minimal dead space. Requires higher O2 flows (150-300 mL/kg/min). No CO2 absorbent needed. |
| Rebreathing (Circle system) |
Greater than 5-7 kg |
Lower O2 flows (20-40 mL/kg/min after stabilization). Requires functional CO2 absorbent. More economical for larger patients. |
| Patient |
ET Tube Size (ID in mm) |
| Cat (3-5 kg) |
3.5-5.0 mm |
| Small dog (5-10 kg) |
6-8 mm |
| Medium dog (10-25 kg) |
8-10 mm |
| Large dog (25-40 kg) |
10-12 mm |
| Giant dog (greater than 40 kg) |
12-16 mm |