Sedation

Excitement or stress caused by excessive restraint will induce high levels of circulating catecholamines, which will not be tolerated by severely compromised cats. Stress will lead to increased anesthetic requirements and consequently more severe cardiovascular depression. Pre-medication can help alleviate this stress. A full description of drugs that can be administered in cats and their indications is described in Feline Orthopedic Surgery and Musculoskeletal Disease.¹

Anesthesia

Injectable anesthetics can be used for induction of anesthesia followed by inhalational anesthesia for maintenance or they can be used as the sole anesthetic agent (total intravenous anesthesia [TIVA]). Inhalation anesthesia is still the preferred technique for producing general anesthesia for prolonged and invasive surgical procedures because the depth of anesthesia can easily be adapted and drug accumulation is not a problem with modern inhalation anesthetics. Modern protocols use the principle of balanced anesthesia, with a combination of different analgesic, sedative–hypnotic and muscle-relaxing drugs, with the aim of reducing the concentration of volatile anesthetics required (Table 2-1).

New drugs

Since the recent publication of the Feline Orthopedic Surgery and Musculoskeletal Disease textbook,^1,2 the variety of drugs available for anesthesia and analgesia in cats has changed very little. However, the use of tramadol in small animal practice has risen dramatically. It has the advantage of oral administration. A new non-steroidal anti-inflammatory drug (robenacoxib) is licensed for use in cats,and anti-epileptic agents (gabapentin and pregabalin) are being used more commonly in small animal practice. These drugs will be described below.

Fluid administration

During inhalational anesthesia a significant amount of fluids is lost via the respiratory tract, open wounds or hemorrhage. In addition, with most anesthetics vascular tone is reduced and intravascular volume has to be replaced to maintain an adequate perfusion pressure.

In a systemically healthy patient any isotonic crystalloid solution (i.e., lactated Ringer solution, saline) can be used at a rate of 5–10 mL/kg/hour depending on the expected fluid and blood loss during the planned procedure. For prolonged procedures without opening major body cavities the lower rate should be chosen in cats to avoid overzealous dilution and disturbance of colloid osmotic pressure and pulmonary edema formation. A large study on small animal perioperative mortalities found an increased anesthetic risk for cats receiving intravenous fluids.^6,7 This might be related to the fact that in practice only diseased, high-risk feline patients will be treated with infusions; however, it might also indicate overinfusion when the rate is not controlled properly which can occur when using large giving sets.

Emergency resuscitation

Cats with systemic disease may require emergency resuscitation. A list of recommended drugs and doses is included in Table 2-2.

Anemia

Arterial oxygen content depends on hemoglobin concentration (or packed cell volume (PCV)) and saturation with oxygen. A normal arterial oxygen content of 16–20 mL/dL is reached with 12–15 mg/dL hemoglobin. In anemia, compensatory increases in cardiac output are required to maintain adequate oxygen delivery, which increases myocardial work and in combination with low oxygen content can lead to myocardial hypoxia and death. Therefore, anemia should be treated before anesthesia and 100% oxygen given during anesthesia.

There are no standard, clear cut transfusion triggers available for cats. However, transfusion is recommended with a PCV of less than 13% in chronic anemia and 20% in acute anemia. Signs of hemorrhagic shock can become evident when 20% of the circulating blood volume (in cats approximately 7% of bodyweight) is lost. In cats with concomitant anemia and reduced cardiac performance a careful individual decision for transfusion has to be made and volume overload needs to be avoided.

Whole blood is recommended with acute hemorrhage and erythrocyte concentrate for treatment of chronic normovolemic anemia, to avoid volume overload. If stored blood is used in planned procedures the transfusion should be scheduled the day before surgery to allow for compensation for the changes in oxygen-carrying capacity of the stored products. In cats, blood transfusion requires blood typing or cross-matching even with the first transfusion because of preformed antibodies (Chapter 5).

Hypoproteinemia

Many drugs used for anesthesia are reversibly bound to transport proteins in the albumin fraction of blood. Only the unbound amount of drug is pharmacologically active. If the plasma protein (albumin) bound fraction in blood is decreased, then more unbound drug is available, resulting in an increased (anesthetic) effect. Therefore, there is the potential for an increased anesthesia effect in hypoproteinemic animals and the principle of giving anesthesia to effect and incremental dosing should be followed.

Decreased albumin concentrations lower plasma oncotic pressure and high amounts of crystalloid fluids are not well tolerated as they increase the risk of pulmonary edema. If the plasma protein concentration is below 35 g/L (3.5 g/dL) then giving colloidal fluids like hydroxyethyl starch or plasma transfusion should be considered.

Diabetes

Diabetic patients may present for a variety of diabetes-related or unrelated procedures. There are no specific indications or contraindications for a special type of anesthetic agent in diabetic cats. Whenever possible, anesthesia should be undertaken when the diabetes is adequately controlled; if this is not possible then short-acting agents with short recovery times allow early return to eating.

In diabetic animals the management of blood glucose levels in the perioperative period is the major consideration for the veterinary surgeon. Fasting times should be kept short; surgery should be scheduled for the morning to avoid long periods of fasting. In animals being admitted on the morning of surgery owners should be advised to feed the animal for the last time late at night instead of the early evening.

Insulin administration after fasting increases the risk of hypoglycemia. However, insulin activity is necessary to enable metabolism and uptake of nutrients by the tissues. In addition, preoperative stress with the release of catecholamines and corticosteroids increases glucose levels and insulin requirements. Therefore, insulin should not be withheld in the preoperative period. The usual recommendation on the proportion of the animal’s usual insulin dose varies from one half to one quarter of the usual dose. Ideally this is adapted according to the glucose levels on the morning of surgery (Table 2-3).

If the cat is on a twice-daily insulin regime it can often be returned to its usual feeding and insulin regime in the evening after surgery. If the animal is inappetent it can be maintained on a dextrose infusion and given regular/soluble insulin every six hours.

The glucose target range during surgery lies between 8.3 and 14 mmol/L; glucose monitoring during surgery is recommended every 30 to 60 minutes. A 2.5–5% dextrose infusion is given according to the cat’s systemic glucose concentrations (Table 2-3).

If the cat is on oral hypoglycemic agents, e.g., glipizide, then these should be withheld on the day of surgery; instead, the glucose concentration is monitored and stabilized with short-acting soluble insulin and dextrose infusion if necessary.

Obesity

There is an increasing incidence of obesity in domestic cats⁸ (Fig. 2-1). In obese animals the veterinarian is faced with a variety of general problems as well as specific anesthesia-related problems. Venous access as well as palpation of arterial pulses is impaired. Increased fat in the throat region may make endotracheal intubation more difficult and may also increase the risk of airway obstruction. Increased superficial fat obtunds thoracic auscultation and obscures landmarks for locoregional techniques and may act as a thermal insulator and increase the risk of heat stress.

Increased total body fat is accompanied by decreased total body water (on a mL/kg basis) resulting in decreased volume of distribution for water-soluble drugs and increased volume of distribution for lipid-soluble drugs. Most anesthetic agents are fat soluble. Excessive body fat in/on the chest and abdomen leads to reduced chest wall and lung compliance, reduced total lung capacity, reduced expiratory reserve volume, reduced functional reserve capacity (FRC) and increased work of breathing. Reduced FRC means reduced oxygen reserves and the patient can desaturate rapidly if it becomes apneic or intubation fails. The FRC may decrease below closing capacity and atelectasis and hypoxemia may occur during normal tidal breathing.

The goals of the anesthetic protocol in obese cats are listed in Box 2-1.

Brachial plexus block

A brachial plexus block in the cat is performed under general anesthesia. A brachial plexus block at the height of the shoulder is suitable for operations on the front limb within or distal to the elbow. A 22G 5 cm needle is inserted medial to the shoulder joint and advanced parallel with the spine directed towards the costochondral junction of the ribs. If no blood can be aspirated, lidocaine or bupivacaine diluted to a volume of 5 mL to facilitate spread is injected during withdrawal to reach the radial, ulnar, median and musculocutaneous nerves (RUMM block). The use of a nerve stimulator to locate the nerves and set specific depots improves the results (Fig. 2-2).

Femoral nerve block

Anesthetizing the femoral nerve desensitizes the medial part of the thigh, tibia and tarsus and achieves motor blockade of the quadriceps femoris muscle. After aseptic preparation of the inguinal area, a 22G needle is inserted 1–2 cm in a craniodorsal direction immediately cranial to the femoral artery, below the inguinal ligament.

Use of a nerve stimulator induces a twitch distal (extension of the knee) to the stimulating site and helps to verify appropriate needle placement. Ultrasound-guided nerve infiltration will also improve results.

Sciatic nerve block

The sciatic nerve block gives motor block to the caudal muscles of the thigh and desensitizes the surrounding tissue and skin. Landmarks for needle insertion in the laterally recumbent animal are the greater trochanter of the femur and the ischiatic tuberosity. At the level of one-third of the distance between the two points, a 22G needle is inserted in a cranial direction. Using a nerve stimulator will result in dorsal flexion of the foot.

Wound catheter infusion systems

Fenestrated wound catheters (Chapter 11), can be used for analgesia of defined wound regions, for example after ear canal ablation or large tumor excisions like fibrosarcoma removal (Chapter 17). Commercial systems come in different lengths with bacterial filters, a flow restrictor and a balloon pump (i.e., ON-Q PainBuster, Kimberly Clark or Diffusion/Wound catheter, MILA International Inc. Erlanger, KY, USA), but it is also possible to purchase the catheter and filter without the pump. The catheter can also be constructed from red rubber or polyethylene tubing (Fig. 2-3). The elastomeric reservoir (pump) delivers fluid at a rate of 0.5–5 mL/hour; alternatively, a syringe pump can be used in non-ambulatory patients. The catheter is usually placed subcutaneously during surgery in the desired area and can either be continuously perfused with a local anesthetic (lidocaine (1%) 0.2–0.4 mg/kg/hour) or repeated boluses of a long-acting local anesthetic can be given intermittently (1 mg/kg bupivacaine [0.25%] first dose followed by 0.5 mg/kg every six hours for one to two days).⁹

Epidural anesthesia

This provides useful adjunctive analgesia for both abdominal and thoracic procedures. Contraindications for epidural anesthesia are septicemia, coagulation disorders, hypovolemia, shock, skin infections at the puncture site and deformity, dislocations or fractures of the spine or pelvis. Possible complications include local skin infection, meningitis and trauma to the spinal cord, but these are rare. The procedure is described in Chapter 18 of Feline Orthopedic Surgery and Musculoskeletal Disease.²