1.3 Canine Treatment, Anesthesia & Surgery

1.3 Canine Treatment, Anesthesia & Surgery

Mastery of basic surgical techniques, anesthetic planning, and pharmacological mechanisms is essential for a veterinary graduate entering canine clinical practice.

Surgical Principles

1. Ovariohysterectomy (OHE)

Ovariohysterectomy is the surgical removal of the ovaries and uterus.

• Key Steps: A ventral midline celiotomy is performed. The suspensory ligament of the ovary is manually broken down (torn) to exteriorize the ovary. A three-clamp technique is applied to the ovarian pedicle, followed by double ligation with absorbable suture (typically 2-0 or 3-0 polydioxanone). The broad ligament is broken down, and the uterine body is double-ligated at the level of the cervix.
• Complications: Acute hemorrhage from the ovarian pedicle (the most common cause of post-operative death), ovarian remnant syndrome (recurrent estrus due to incomplete removal of ovarian tissue), stump pyometra (if progesterone is present due to ovarian remnant), and urinary incontinence.

2. Castration (Orchiectomy)

Orchiectomy is the surgical removal of both testicles.

• Open vs. Closed Castration:
  • Open Castration: The parietal vaginal tunic is incised, exposing the testis and spermatic cord directly. This allows for direct ligation of the vas deferens and testicular vessels, which is preferred in larger dogs.
  • Closed Castration: The parietal vaginal tunic is left intact, and the entire spermatic cord enclosed in the tunic is ligated and transected. This carries less risk of post-operative scrotal hematoma or peritoneal contamination.
• Complications: Scrotal hematoma (common in older, larger dogs), hemorrhage, and scrotal dermatitis.

3. Gastropexy for Gastric Dilatation-Volvulus (GDV)

GDV is a life-threatening emergency where the stomach dilates with gas and rotates, typically in a clockwise direction when viewed cranially.

• Pathophysiology: Gastric rotation compresses the portal vein and caudal vena cava, severely reducing venous return to the heart and leading to obstructive shock. It also causes gastric wall necrosis and cardiac arrhythmias.
• Surgical Intervention: Following decompression and anatomical repositioning of the stomach, a permanent gastropexy must be performed. The goal is to create a permanent adhesion between the right antrum of the stomach and the right lateral abdominal wall. The most common technique is an incisional gastropexy, where a 4–5 cm incision is made through the seromuscular layer of the gastric antrum and sutured to a matching incision in the transverse abdominis muscle of the right body wall.

4. Splenectomy

Splenectomy is indicated for splenic masses (such as hemangiosarcoma, hematoma, or leiomyosarcoma), splenic torsion, or trauma.

• Key Steps: Carefully exteriorize the spleen. Ligations can be performed using suture or a vascular sealing device. The surgeon can ligate the main splenic artery and vein (which requires fewer ligations but risks compromising blood supply to the left lobe of the pancreas via the pancreatic branches) or perform multiple ligations of the individual hilar vessels close to the splenic parenchyma.
• Complications: Ventricular arrhythmias are highly common during and within the first 48 hours post-operatively. Continuous ECG monitoring is mandatory. If the dog is hemodynamically stable, the arrhythmia should be monitored; treatment with lidocaine is reserved for sustained ventricular tachycardia or hemodynamic instability.

5. Cranial Cruciate Ligament (CCL) Repair

CCL rupture is the most common cause of hindlimb lameness in dogs, resulting in stifle instability, osteoarthritic progression, and meniscal injury.

• Diagnostics: Confirmed by demonstrating cranial drawer (cranial translation of the tibia relative to the femur) or tibial compression test (tibial thrust).
• Surgical Options:
  • Extracapsular Stabilization (Lateral Fabellar Suture): A heavy nylon suture is placed around the lateral fabella and through a tunnel in the tibial tuberosity. Primarily indicated for small or inactive dogs (< 15 kg).
  • Tibial Plateau Leveling Osteotomy (TPLO): A radial cut is made in the proximal tibia, and the tibial plateau is rotated to reduce the tibial plateau angle (typically to 5 degrees). This eliminates cranial tibial thrust during weight-bearing, rendering the cranial cruciate ligament unnecessary for stability. Preferred for large, active, or working dogs.

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Anesthesia Protocols

Anesthetic protocols must be tailored to the individual patient’s physical status, breed, and co-morbidities.

1. Pre-medications

Pre-medications provide sedation, muscle relaxation, and preemptive analgesia, which reduces the required doses of induction and maintenance agents.

• Acepromazine: A phenothiazine derivative. Causes vasodilation and hypotension via alpha-1 adrenergic antagonism. It provides excellent tranquilization but no analgesia. It must be avoided or dose-reduced in hypovolemic, shocked, or geriatric patients.
• Dexmedetomidine: An alpha-2 adrenergic agonist. Provides profound sedation and analgesia. It causes severe systemic vasoconstriction, leading to reflex bradycardia (HR may drop to 30–40 bpm). It is reversible with atipamezole. Avoid in patients with pre-existing cardiovascular disease.
• Midazolam / Diazepam: Benzodiazepines. Have minimal cardiovascular and respiratory effects, making them excellent choices for sick, young, or geriatric patients. However, they can cause paradoxical excitement/dysphoria in young, healthy dogs when used alone.
• Opioids: The cornerstone of analgesia. Classifications include pure mu-agonists (e.g., morphine, hydromorphone, methadone, fentanyl) for severe pain, partial mu-agonists (buprenorphine) for mild-to-moderate pain, and agonist-antagonists (butorphanol) primarily for sedation with minimal analgesia.

2. Induction Agents

• Propofol: An ultra-short-acting GABA agonist. Can cause transient apnea if administered too rapidly, and causes vasodilation and myocardial depression.
• Alfaxalone: A neuroactive steroid anesthetic. Similar to propofol but can be administered intramuscularly in fractious patients.
• Ketamine: An NMDA receptor antagonist, typically combined with diazepam or midazolam. It maintains sympathetic tone, resulting in increased heart rate and blood pressure, making it useful for hemodynamically unstable patients.

3. Maintenance and Monitoring

Anesthesia is typically maintained using inhalant anesthetics like isoflurane (MAC ~1.3%) or sevoflurane (MAC ~2.36%) in oxygen. Inhalants cause dose-dependent respiratory depression and vasodilation, leading to hypotension.

• Monitoring Parameters:
  • Mean Arterial Pressure (MAP): Must be maintained > 60 mmHg to ensure adequate perfusion to vital organs (kidneys, brain).
  • Pulse Oximetry (SpO2): Reflects arterial oxygen saturation; must be maintained > 95%.
  • Capnography (ETCO2): Measures ventilation. Normal range is 35 to 45 mmHg. Values > 45 mmHg indicate hypovolemic or anesthetic-induced respiratory depression, necessitating positive pressure ventilation.

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Pharmacology

• Pimobendan: An inodilator. It acts as a positive inotrope by sensitizing cardiac troponin C to calcium, and as a balanced systemic vasodilator by inhibiting phosphodiesterase III. Used in the management of congestive heart failure secondary to dilated cardiomyopathy (DCM) or myxomatous mitral valve disease (MMVD).
• Carprofen: A non-steroidal anti-inflammatory drug (NSAID) that selectively inhibits COX-2 over COX-1, reducing prostaglandin synthesis. Used for osteoarthritis and post-operative pain. Potential side effects include idiosyncratic hepatotoxicity (particularly in Labrador Retrievers), gastrointestinal ulceration, and renal perfusion impairment.
• Ivermectin & MDR1 (ABCB1) Mutation: A deletion mutation in the ABCB1 gene (formerly MDR1) leads to a non-functional P-glycoprotein efflux pump at the blood-brain barrier. Affected dogs (typically Collies, Australian Shepherds, Shetland Sheepdogs) accumulate substrates like high-dose ivermectin, loperamide, acepromazine, and vincristine in the central nervous system, causing severe neurotoxicity (tremors, ataxia, blindness, coma).
• Prednisone: A synthetic corticosteroid. Doses are divided into:
  • Physiologic replacement: 0.1 to 0.2 mg/kg/day
  • Anti-inflammatory: 0.5 to 1.0 mg/kg/day
  • Immunosuppressive: 2.0 to 4.0 mg/kg/day Side effects include polyuria, polydipsia, polyphagia, muscle wasting, hepatomegaly, and increased susceptibility to infection.