VT131: Recovery and Special Anesthetic Techniques

Anesthetic Recovery: Definitions and Risk Factors

• Definition of Anesthetic Recovery: The period beginning when the anesthetic is discontinued until the animal can stand and walk independently.
• Factors Leading to Prolonged Recovery:
  • Lengthy anesthetic procedures.
  • Presence of underlying disease in the patient.
  • Administration of Intramuscular (IM) anesthetics as opposed to Intravenous (IV) anesthetics.
  • Hypothermia (low body temperature).
• Critical Risk Period:
  • Recovery is considered one of the most dangerous periods for the patient, regardless of how smooth the induction or maintenance phases were.
  • Statistics indicate that $47 - 60\%$ of all dog and cat anesthetic-related deaths occur within the first 3 hours postoperatively.
• The Anesthetist’s Primary Responsibilities:
  • Immediate discontinuation of all anesthetic agents.
  • Continuous monitoring of the patient at close range.
  • Administration of oxygen as needed.
  • Administration of specific reversal agents depending on the protocol used.
  • Maintenance of a patent (open) airway and performing extubation at the correct time.
  • Provision of general nursing care and analgesia (pain management).

Clinical Signs and Monitoring During Recovery

• Progression of Recovery:
  • As an animal recovers, it progresses backward through the anesthetic stages.
  • Stage II Recovery: Patients may experience dysphoria or delirium. Precautions must be taken to prevent self-trauma or the disruption of surgical wounds/incisions.
  • Tranquilizers may be administered if needed, though preanesthetic medications given before the procedure can moderate these signs.
• Monitoring Standards:
  • The patient must be watched continuously at close range; checking from across the room is insufficient.
  • Vital signs must be evaluated at least every $5\,\text{minutes}$.
  • Delayed return to consciousness or abnormal vital signs can indicate serious underlying conditions requiring prompt treatment.

Post-Anesthetic Oxygen Therapy and Extubation

• Oxygen Therapy:
  • Oxygen should be administered for $5\,\text{minutes}$ after the anesthetic is discontinued.
  • Delivery methods include: Endotracheal (ET) tube, mask, flow-by oxygen, oxygen cage, nasal catheter, or converting an E-collar into an oxygen chamber.
• Extubation Procedure:
  • Deflate the cuff of the ET tube and untie the tube for rapid removal.
  • Maintain the patient's neck in a natural, extended position to protect the airway.
  • Remove the tube using a slow, steady motion as soon as the patient shows signs of imminent arousal (voluntary limb, head, or chewing movements).
  • Swallowing is not always observed before extubation; many patients swallow as the tube is removed.
• Species-Specific Extubation Considerations:
  • Brachycephalic Breeds: Delay extubation as long as possible. Keep a laryngoscope, ET tubes, and an IV induction agent (like propofol) ready in case re-intubation is required.
  • Cats: Extubate as soon as signs of arousal (swallowing, active palpebral reflex, voluntary limb/tail/head movement) appear. Delaying extubation in cats can cause laryngospasm.
• Safety Measures:
  • If fluid is present in the oral cavity, leave the cuff partially inflated during removal to "sweep" fluid away from the airway.
  • Post-extubation positioning should be lateral or sternal recumbency with the neck extended.

Postoperative Nursing Care and Patient Comfort

• Environment and Comfort:
  • Patients are often disoriented or painful; providing quiet, calm care is essential.
  • Remove surgical restraints (ties) before the animal regains consciousness.
  • Complete bandaging, chest tube placement, and urinary catheterization while the patient is still unconscious.
  • Remove monitoring probes and electrodes post-op.
• Medical Management:
  • The IV catheter should remain until the patient is normothermic ($>99^\circ\text{F}$) and able to walk, providing access for emergency medications.
  • Hypostatic Congestion: To prevent blood pooling in the dependent lung and tissues, turn the patient every $10 - 15\,\text{minutes}$.
  • Provide heat support until the body temperature is normal.
  • Administer analgesics as requested by the veterinarian before the onset of pain.
• Safety Caveats:
  • Never leave a recovering patient unattended on a table due to fall risks.
  • Do not leave food or water in the cage during the immediate recovery phase.

Local Anesthesia: Agents and Mechanisms

• Overview: Local anesthesia involves chemical agents that act on sensory neurons to cause a temporary loss of sensation. It is effective, inexpensive, and has low cardiovascular toxicity.
• Common Clinical Uses: Dental extractions, leg amputations, orthopedic surgery, thoracic or abdominal surgery.
• Mechanism of Action:
  • Local anesthetics block sodium channels in sensory neurons, preventing the generation of electrical impulses and nerve conduction.
  • Reversal occurs as the drug is absorbed into the circulation and metabolized by the liver.
• Sympathetic Blockade: If the drug affects the sympathetic nervous system, it can cause vasodilation and hypotension, which is treated with IV fluid boluses.
• Types of Agents:
  • Lidocaine (Xylocaine): Immediate onset; duration is $1 - 2\,\text{hours}$ with epinephrine and $1\,\text{hour}$ without. Slow IV administration ($15 - 20\,\text{minutes}$).
  • Mepivacaine (Carbocaine): Immediate onset; duration $90 - 180\,\text{minutes}$.
  • Tetracaine (Pontocaine): Onset $5 - 10\,\text{minutes}$; duration $2\,\text{hours}$.
  • Bupivacaine (Marcaine): Onset $20\,\text{minutes}$; duration $4 - 6\,\text{hours}$.
  • Nocita (Liposomal Bupivacaine): Slow-release; duration up to $72\,\text{hours}$.

Routes of Administration for Local Anesthetics

• Topical Administration: Provides shorter, less intense relief than infiltration. High risk of overdose in small patients. Examples: Lidocaine on arytenoids (intubation), ophthalmic proparacaine (tonometry), lidocaine gel for urinary catheters, and lidocaine patches.
• Infiltration: Injected into tissues (intradermal, subcutaneous, or between muscle planes) near target nerves. Used for biopsies, small tumor removals, and laceration repair.
• Epinephrine Additive:
  • Causes vasoconstriction, decreasing drug absorption and prolonging effects by approximately $50\%$.
  • Contraindications: Do not use at incision sites (delays healing), on distal extremities like ears, tails, or digits (compromises circulation), or in patients with cardiac disease.
• Nerve and Line Blocks:
  • Nerve Blocks: Injected near a specific nerve (e.g., dental blocks, intercostal blocks, brachial plexus blocks for amputation, declaw blocks, and paravertebral blocks for ruminant laparotomy).
  • Line Blocks: A continuous line of anesthetic in SQ tissue targeting superficial nerves. A "Ring Block" encircles an entire anatomic part. An "L-block" is used specifically for ruminant laparotomy.
• Intra-articular Blocks: Injection directly into a joint (e.g., bupivacaine into a stifle joint after surgery).
• Intravenous Regional Anesthesia (Bier Block): Lidocaine is injected into a superficial vein distal to a tourniquet for extremity surgery. Note: Bupivacaine is never used for Bier blocks due to IV cardiotoxicity.
• Systemic Administration: Lidocaine can be given as a Constant Rate Infusion (CRI) to reduce general anesthetic doses, prevent arrhythmias, or provide postoperative analgesia.

Regional and Epidural Anesthesia

• Anatomy Review:
  • Spinal cord membranes: Pia mater (innermost), Arachnoid (middle), Dura mater (outermost).
  • Subarachnoid space: Between the arachnoid and pia, contains Cerebrospinal Fluid (CSF).
  • Cauda Equina: Terminal group of neurons located at $L6/L7$ in dogs and $S1$ in cats.
• Epidural Procedure:
  • Local anesthetic is deposited in the epidural space (between the dura mater and vertebrae).
  • Injection site: Between the last lumbar vertebra ($L7$) and the sacrum.
  • Spinal Block: Occurs if the drug is accidentally injected into the subarachnoid space.
• Drugs Used in Epidurals:
  • Lidocaine ($2\%$): Duration $1.5 - 3\,\text{hours}$. Blocks both motor and sensory neurons (patient cannot walk).
  • Bupivacaine ($0.5\%$): Duration $4 - 6\,\text{hours}$. Blocks both motor and sensory neurons.
  • Opioids (Morphine or Buprenorphine): Primarily for analgesia. Minimal effect on motor neurons; movement is usually unimpaired.

Assisted and Controlled Ventilation

• Definitions and Comparison:
  • Assisted Ventilation: The patient initiates inspiration, but the anesthetist provides extra oxygen/inhalant.
  • Controlled Ventilation: The anesthetist delivers all air/gas; the patient makes no spontaneous effort.
  • Positive Pressure Ventilation (PPV): Manual ("bagging") or mechanical delivery of gas.
• Normal Physiology in Awake Animals:
  • Inhalation (active) is triggered by increased arterial $CO_2$ ($P_aCO_2$). The brain stimulates intercostal muscles to expand the thorax, creating negative pressure.
  • Exhalation (passive) occurs as muscles relax. It typically lasts twice as long as inspiration.
• Vocabulary:
  • Tidal Volume ($V_T$): Air moved in one breath. Normal awake $V_T$: $10 - 15\,\text{mL/kg}$.
  • Respiratory Minute Volume: The total air moved in one minute ($V_T \times \text{Respiratory Rate}$).
• Anesthetic Effects on Ventilation:
  • Tranquilizers and anesthetics decrease both respiratory rate and tidal volume.
  • Hypercarbia/Hypercapnia: Rising $P_aCO_2$ leading to respiratory acidosis.
  • Hypoxemia: Falling $P_aO_2$.
  • Atelectasis: Partial collapse of alveoli.
• Ventilation Management:
  • Factors increasing risk: Obesity, prolonged anesthesia ($>90\,\text{minutes}$), chest surgery, or neuromuscular blocks.
  • Pressure Monitoring during Manual Ventilation: Max pressure for small animals is $20\,\text{cm}\,H_2O$; for large animals, it is $40\,\text{cm}\,H_2O$.

Neuromuscular Blocking Agents

• Function: These agents result in total muscle paralysis.
• Key Consideration: They do not provide analgesia or anesthesia; the patient remains conscious and able to feel pain unless other anesthetics are provided simultaneously.
• Classifications:
  • Depolarizing Agents (e.g., Succinylcholine): Cause a single surge of activity followed by paralysis. No reversal agent.
  • Non-depolarizing Agents (e.g., Pancuronium, Atracurium, Vecuronium): Block receptors directly without initial stimulation. Can be reversed with an acetylcholinesterase inhibitor (e.g., Edrophonium, Neostigmine).
• Monitoring: Since the patient is paralyzed, the anesthetist must use a peripheral nerve stimulator to assess the degree of blockade and ensure the patient is breathing via regular PPV.