Anesthesia and Analgesia: depth, monitoring, and multimodal pain control
Anesthesia and Analgesia
This domain tests pre-anesthetic assessment, drug selection, intraoperative monitoring, recognition of inadequate depth, and analgesic planning, including infusion calculations.
Core rules and concepts
- Risk assessment: ASA class I–V guides plan and monitoring intensity. Minimum database (PCV/TS, glucose, renal values) for sick, pediatric, or geriatric patients.
- Balanced anesthesia: premedication (opioid ± alpha-2 agonist or benzodiazepine), induction agent, then inhalant or TIVA maintenance. Lower inhalant needs mean less hypotension.
- MAC: alveolar concentration preventing movement in 50% of patients; approximately 1.3% isoflurane and 2.3% sevoflurane in dogs. Monitor end-tidal agent, not just vaporizer setting.
- Monitoring: HR, RR, SpO2 ≥95%, end-tidal CO2 35–45 mmHg, MAP ≥60 mmHg (systolic ≥90), temperature, palpebral reflex and jaw tone.
- Multimodal analgesia: opioid + NSAID ± local block. Avoid NSAIDs with hypotension, renal disease, or GI ulceration; never combine NSAIDs with corticosteroids.
Worked example
A 12 kg dog requires fentanyl at 5 µg/kg/h from a 50 µg/mL vial. Dose = 12 × 5 = 60 µg/h. Rate = 60 ÷ 50 = 1.2 mL/h. Set the syringe pump to 1.2 mL/h, label the line, confirm patency, and record the rate and response (HR, respiratory effort, pain score) on the anesthesia sheet.
Traps
A normal SpO2 does not exclude severe hypoventilation - check ETCO2 and chest excursion. Sudden ETCO2 drop may signal disconnection or hypotension, not "light" anesthesia. Do not deepen anesthesia to treat hypotension; lighten depth, reduce vaporizer, support blood pressure with fluids or pressors as directed.