Hypotension and Bradycardia Mid-Dental in a 3.4 kg Terrier
An 11-year-old spayed female Yorkshire Terrier weighing 3.4 kg is scheduled for a full-mouth dental prophylaxis with several extractions. Pre-anesthetic bloodwork is unremarkable except a urine specific gravity of 1.012. At today's pre-anesthetic examination you auscult a newly identified grade II/VI left apical systolic murmur; the owner reports the dog "slows down on walks" but has not coughed or fainted. The veterinarian decides to proceed and asks you to induce. Premedication is butorphanol 0.2 mg/kg with midazolam 0.2 mg/kg IM, induction is propofol to effect, and maintenance is isoflurane in oxygen through a non-rebreathing circuit. Ten minutes after induction: heart rate 58 bpm, blood pressure 78/45 mmHg (MAP 56), end-tidal isoflurane 1.9%, respiratory rate 8 breaths/min, SpO2 97%, rectal temperature 36.2 °C, and the veterinarian is actively extracting a mobile mandibular incisor.
1. Interpret these values and list, in order, the most likely contributors to the hypotension and bradycardia. What are your first three actions, and what would make you ask the veterinarian to stop?
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Interpretation: MAP 56 mmHg is below the ~60 mmHg target, so this is real hypotension, not a borderline reading. HR 58 in a 3.4 kg dog on isoflurane is bradycardic. SpO2 97% is reassuring but does not rule out hypoventilation (RR 8 is low). Temperature 36.2 °C is hypothermic. Before acting, verify the numbers: correct cuff size and limb/tail placement, palpate pulse quality, and check the capnograph trace.
Likely contributors, in order:
- Anesthetic depth - end-tidal 1.9% is high for this patient's needs, and isoflurane causes dose-dependent vasodilation and bradycardia.
- Vagal tone from dental traction - extraction of a mandibular tooth can produce a marked vagal bradycardia and drop in blood pressure.
- Hypothermia - worsens bradycardia, vasoconstriction, drug metabolism, and clotting.
- Volume depletion - fasting plus a small patient; consider blood loss from the extraction site.
- The new murmur - structural heart disease may limit compensation, but do not assume it is the sole cause.
First three actions: (1) ask the veterinarian to pause the extraction and reassess depth (jaw tone, palpebral reflex, eye position); (2) reduce the vaporizer setting and support ventilation if hypoventilating; (3) start active warming and give a crystalloid bolus. If bradycardia persists after depth is lightened, discuss an anticholinergic (glycopyrrolate ~0.01 mg/kg IV). Recheck HR and MAP after each intervention.
Stop signal: if MAP stays under 60 mmHg and HR under 50 despite lighter anesthesia, fluid, warmth, and an anticholinergic, or if the rhythm changes, the procedure should be paused and the patient stabilized or recovered.
2. Calculate a maintenance crystalloid rate at 10 mL/kg/hr and the drip rate using a 60 gtt/mL set. Then calculate a 10 mL/kg bolus given over 15 minutes and explain how you would deliver it.
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Maintenance: 3.4 kg × 10 mL/kg/hr = 34 mL/hr. With a 60 gtt/mL set: 34 mL/hr × 60 gtt/mL ÷ 60 min/hr = 34 gtt/min.
Bolus: 3.4 kg × 10 mL/kg = 34 mL, delivered over 15 minutes. That equals 34 mL × 4 = 136 mL/hr, or 136 gtt/min - about 2–3 drops per second, which is not something you can count reliably by hand.
How to deliver it: preferably with a syringe pump or fluid pump so the volume and rate are exact. If only gravity is available, use a burette or a labeled syringe with a three-way stopcock and push the bolus in aliquots while watching the monitor, or run it wide open with a stopwatch and a marked fluid bag, reassessing HR, MAP, and perfusion every few minutes. Do not simply "crank it up" and walk away: a 3.4 kg dog can be volume overloaded quickly, and this patient has a new murmur.
Reassessment: after the bolus check MAP, pulse quality, CRT, and respiratory rate/effort; repeat a smaller bolus if indicated rather than stacking large volumes. Recheck the endpoint: MAP ≥ 60 mmHg, pink mucous membranes, CRT under 2 seconds. Document the volume, rate, time given, and response.
3. The veterinarian asks whether to continue or recover the patient. What factors shape that decision, and what belongs in your post-operative plan and record?
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Who decides: the veterinarian does, but your monitoring data drive the decision and you should say plainly what you are seeing. Favorable to continuing: MAP and HR respond to lighter anesthesia, warmth, fluids, and an anticholinergic; blood loss is controllable; the extractions remaining are limited and pain-controlled with local blocks. Favorable to recovering: hypotension is refractory, the arrhythmia worsens, temperature keeps falling, or the remaining work is elective and can be staged. Staging dentistry is a legitimate clinical choice, not a failure.
The murmur: document the grade, location, timing, and that it is newly identified, and recommend an echocardiogram before any future anesthetic. That conversation belongs in the record and with the owner, not just between staff.
Post-operative plan to write down:
- Active warming (forced-air, blankets, warmed fluids) with temperature every 15 minutes until above 37 °C; avoid overshooting.
- Pain plan: opioid premedication already on board, local blocks, NSAID only after confirming renal values and no contraindication; reassess comfort with a pain score.
- Vital signs every 15 minutes through recovery; watch for re-narcotization from butorphanol, airway obstruction, and bleeding at extraction sites.
- Offer water and soft food once alert; discharge with written instructions on bleeding, swelling, appetite, medication, and a recheck.
- Record all drugs, doses, times, monitoring values, interventions, and the owner consent covering the extraction range.