As cats become more debilitated or aged, we use butorphanol 0.2 mg/kg or hydromorphone 0.2 mg/kg with 0.2 to 0.4 mg/kg midazolam.
Ketamine is added 1 to 5 mg/kg as needed if fractious and not HCM cats. [HCM cats get a touch of medetomidine (0.005 mg/kg)
as the next step.]
Induction agents (ket/val, propofol, etomidiate) are given intravenous after catheter placement.
Anesthesia is generally maintained with isoflurane or sevoflurane and oxygen. Little isoflurane or sevoflurane is metabolized,
the insolubility of the inhalants allows for a speedier induction and recovery. Patient temperature is controlled in many
ways including blankets, warm intravenous bottles placed next to the patient under the blankets, and a warm air Bair Hugger.
Intraoperative local anesthesia blocks are used to decrease pain, and the need for excessive anesthesia when painful procedures
The third leg of the anesthetic safety trilogy is evaluating the patient while anesthetized and after. Anesthesia monitoring
varies from observing respiration and noting mucous membrane color refill, to arterial blood gas evaluation. American Animal
Hospital Association (AAHA) anesthesia guidelines require that one or more of the following monitors must be used on the anesthetized
- Electronic respiratory monitor
- Pulse oximeter
- Blood-pressure monitor
- Continuous electrocardiograph (ECG) monitor
- Esophageal stethoscope
- End-tidal CO2 monitor
As with any complicated endeavor, it is far better to know more. Some manufacturers incorporate multiple monitors into a master
ECG evaluations before and during anesthesia give the veterinarian information regarding heart rate, rhythm and abnormal complexes.
Lead 2 is primarily used to monitor rate and rhythm in patients under anesthesia. Handheld units used as part of the pre-operative
patient evaluation can also perform single-lead continuous readings during the dental procedure.
Electrocardiograms can also be generated using esophageal probes. While anesthetized, the probe is inserted into the esophagus
until the distal electrode reaches the area dorsal to the heart base. If the ECG tracing appears small, the probe may not
be inserted far enough. If inserted too deep, the tracing can appear inverted.
The electrocardiogram gives minimal information on cardiac contractility and tissue perfusion. Presence of normal-appearing
complexes does not indicate the patient's tissues are adequately perfused. The ECG should be used with another form of monitoring
(end tidal CO2 and/or blood pressure) for patient evaluation during anesthesia.
Respiratory depression from anesthetic premedication, induction agents and inhalant anesthetics occur. The effects of these
medications are dose-dependent and, when multiple agents are used, may become synergistic.
Apnea monitors alert the clinician when the patient's respiratory rate is depressed or stops. Most respiratory monitors detect
exhaled airflow. The sensor is attached between the endotracheal tube and anesthesia machine's delivery tubes. Every time
the animal exhales, the monitor emits an audible sound. When choosing an apnea monitor, it is important that the signal is
loud enough to easily hear over the ambient noise and that an alarm sounds when breathing stops.
Hemoglobin travels through the blood in two forms: oxyhemoglobin and reduced hemoglobin. Most oxygen transported to the tissues
is carried on the hemoglobin molecule. The pulse oximeter estimates the patient's oxygenation via light absorption measurement
of arterial hemoglobin oxygen saturation. One of the most effective placements of the oximeter probe is on the tongue. Dental
procedures by their nature involve movement and instruments in the mouth, which often dislodge the tongue oximeter probe.
Other areas for probe placement include the pinna, toe, prepuce, vulva, metacarpus (tarsus), digits, tail and rectum.
Oxygen saturation should be maintained between 95 percent and 100 percent, particularly if the animal is breathing 100-percent
oxygen. Saturation readings of 90 percent or less indicate marked desaturation, hypovolemia, shock or anemia.