Premedication Volume and Bradycardia in a Cat Castration
RT is a 6.4 kg, 2-year-old male cat admitted for castration. The veterinarian orders premedication with dexmedetomidine 0.01 mg/kg IM and ketamine 5 mg/kg IM in the same syringe. Stock concentrations are dexmedetomidine 0.5 mg/mL and ketamine 100 mg/mL. The technician uses a 3 mL syringe with a needle hub dead space of approximately 0.08 mL. Ten minutes after injection, the cat's heart rate is 78 beats/min, mucous membranes are pale pink, and Doppler systolic blood pressure is 85 mmHg. The cat is laterally recumbent and minimally responsive. The surgeon is ready and asks for the cat to be induced with propofol 4 mg/kg IV. The technician has a 22-gauge catheter in the cephalic vein and is unsure whether to reverse the dexmedetomidine.
1. Calculate the intended dose volume of each drug and the total volume. Explain how the syringe dead space affects accuracy and how the technician should prepare the premedication.
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Dexmedetomidine: 6.4 kg x 0.01 mg/kg = 0.064 mg; 0.064 mg divided by 0.5 mg/mL = 0.128 mL. Ketamine: 6.4 kg x 5 mg/kg = 32 mg; 32 mg divided by 100 mg/mL = 0.32 mL. Total intended volume = 0.448 mL. A 0.08 mL hub dead space is about 18% of the total, so combining the drugs in one 3 mL syringe can underdeliver one or both drugs and makes the mixture difficult to verify. Use separate syringes: a 1 mL syringe for dexmedetomidine and a 1 mL syringe for ketamine, or use a validated technique that accounts for dead space. Label both syringes. Do not induce with propofol until the premed effect and airway plan are assessed.
2. Interpret the heart rate and blood pressure. What should the technician do about induction and whether to reverse the dexmedetomidine?
Read the worked answer
A heart rate of 78 beats/min and systolic pressure of 85 mmHg after dexmedetomidine are consistent with its alpha-2 effects: bradycardia and vasoconstriction. Assess perfusion, including pulse quality, capillary refill time, lactate, and mentation, rather than treating a single number. Provide preoxygenation, check the catheter, and consider a crystalloid bolus of 10 to 15 mL/kg over 15 to 20 minutes if hypotension persists. Do not give propofol 4 mg/kg as a rapid bolus to a bradycardic, hypotensive cat; that can cause apnea and further hypotension. Reduce to 1 to 2 mg/kg IV titrated to effect, or use alfaxalone to effect, and be ready to intubate. If perfusion remains poor, reverse dexmedetomidine with atipamezole 0.1 mg/kg IM. For this cat that is 0.64 mg, or 0.13 mL of 5 mg/mL atipamezole. Anticipate loss of sedation and have alternative sedation and analgesia ready. The surgeon should wait until the cat is stable.