recovery and special techniques notes

Overview of Anesthetic Recovery

• Anesthetic recovery is defined as the time interval from when the anesthetic agent is discontinued until the animal is able to stand and walk on their own.

• Recovery is recognized as one of the most dangerous periods for the patient, regardless of whether induction or maintenance was problematic.

• Statistical data indicates that $47-60\%$ of all anesthetic-related deaths in dogs and cats occur during the first 3 hours of the post-operative period.

• Factors that contribute to longer recovery times include:     * Longer overall anesthetic times.     * Presence of underlying disease in the patient.     * Administration of Intramuscular (IM) anesthetics versus Intravenous (IV) anesthetics.     * Hypothermia.

The Role of the Anesthetist During Recovery

• The anesthetist must perform the following tasks continuously:     * Discontinue the administration of all anesthetic agents.     * Monitor the patient on a continual basis.     * Administer oxygen as necessary.     * Administer reversal agents, depending on the specific protocol used.     * Maintain a patent airway and perform extubation at the appropriate time.     * Provide general nursing care.     * Provide analgesia or administer other required medications.

Signs and Monitoring of Recovery

• Signs of Recovery:     * The animal progresses back through the anesthetic stages in reverse order.     * Patients passing through Stage II may exhibit dysphoria and delirium.     * Steps must be taken to prevent self-trauma or the disruption of surgical wounds.     * Tranquilizers may be necessary to manage recovery signs.     * Administration of preanesthetic medications prior to the procedure often prevents or moderates these signs.

• Monitoring Standards:     * Patients must be watched continuously at close range; evaluation from across a room is considered unacceptable.     * Vital signs must be evaluated at least every $5$ minutes.     * Abnormal vital signs or a delayed return to consciousness may signal serious conditions requiring prompt treatment.

Oxygen Therapy and Extubation Procedures

• Oxygen Therapy:     * Oxygen should be administered for $5$ minutes after the discontinuation of the anesthetic.     * Delivery methods include: endotracheal tube, mask, flow-by, nasal catheter, or an oxygen cage.     * An E-collar can also be converted into a makeshift oxygen chamber.

• Extubation Protocol:     * The cuff must be deflated and the tube ties undone for rapid removal.     * The neck should be kept in a natural but extended position to protect the airway.     * The tube is removed using a slow, steady motion as soon as signs of imminent arousal appear.     * Signs of imminent arousal include voluntary limb, head, or chewing movements; patients showing these should be extubated even if swallowing is not yet observed, as they typically swallow during removal.     * If fluid is present in the oral cavity, the cuff should be left partially inflated during removal to sweep out fluid and prevent aspiration.

• Species-Specific Considerations:     * Brachycephalic breeds: Require additional monitoring. Extubation should be delayed as long as possible. A laryngoscope, various endotracheal tubes, and an IV induction agent (e.g., propofol) must be nearby for emergency reintubation.     * Cats: Extubation occurs when signs of impending arousal (swallowing, active palpebral reflex, voluntary movement of limbs, tail, or head) are observed. Delaying extubation in cats is not advisable due to the risk of laryngospasm.

Postanesthetic Nursing Care

• Positioning:     * After extubation, patients should be placed in lateral or sternal recumbency with the neck extended.     * Recovering patients should be turned every $10$ to $15$ minutes to prevent hypostatic congestion (pooling of blood in the dependent lung and tissues).     * Patients should never be left unattended on a table due to the risk of falling.

• Comfort and Safety:     * The environment should be quiet and calm to assist disoriented or painful patients.     * Restraining ties must be removed before the animal regains consciousness.     * Bandaging, chest tubes, and urinary catheters should be completed before consciousness returns.     * Food and water must not be left in the cage during the recovery period.

• Equipment and Temperature:     * Monitoring probes, cuffs, and electrodes should be removed.     * Hypothermic patients require heat support until their temperature is normal.     * The IV catheter should remain in place until the patient is normothermic (>99^\circ\text{F}) and able to walk, providing venous access for potential complications.

• Analgesia:     * Analgesics must be administered as requested by the veterinarian before the onset of pain to ensure the patient can sleep comfortably.

Local Anesthesia: Agents and Mechanisms

• Definition: The use of a chemical agent on sensory neurons to produce a temporary loss of sensation.

• Benefits:     * Effective, practical, and inexpensive.     * Low cardiovascular toxicity.     * Minimal recovery time.     * Can be used with general anesthesia to enhance pain control and reduce the required amount of general anesthetic.

• Mechanism of Action:     * Local anesthetics block sodium channels in sensory neurons.     * This prevents the generation of electrical impulses and results in a loss of nerve conduction.     * Effect reversal occurs as the drug is absorbed into local circulation and metabolized by the liver.

• Local Anesthetic Agents:     * Lidocaine (Xylocaine): Immediate onset; duration is $1-2$ hours with epinephrine and $1$ hour without. IV administration must be slow ($15-20$ minutes).     * Mepivacaine (Carbocaine): Immediate onset; duration is $90-180$ minutes.     * Tetracaine (Pontocaine): Onset in $5-10$ minutes; duration is $2$ hours.     * Bupivacaine (Marcaine): Onset in $20$ minutes; duration is $4-6$ hours.     * Bupivacaine in Liposomes (Nocita): Slow-release onset; duration lasts for $72$ hours.

• Characteristics:     * They do not affect the brain or produce unconsciousness.     * Minimal effects on cardiovascular or respiratory systems.     * Effects are localized to the site of injection.     * Not transferred across the placenta (useful for C-sections).

Techniques and Routes of Local Administration

• Topical Administration:     * Provides less pain relief and shorter duration than infiltration.     * Examples: Lidocaine on arytenoids for laryngospasm, ophthalmic proparacaine for tonometry, lidocaine gel on urinary catheters, topical sprays on wounds, and lidocaine patches.

• Infiltration:     * Injected into tissues (intradermal, subcutaneous, or between muscle planes) near target nerves.     * Used for skin biopsies, small tumor removal, and minor laceration repair.

• Adding Epinephrine:     * Causes blood vessel constriction and decreases drug absorption rate.     * Prolongs effects by approximately $50\%$.     * Contraindications: Do not use at incision sites (delays healing), on ears, tails, or digits (compromised circulation), or in patients with cardiac disease.

• Nerve Blocks:     * Injecting near a nerve to desensitize a body part.     * Common uses: Dental blocks (infraorbital, mental, mandibular), intercostal blocks for chest surgery, brachial plexus blocks for limb amputation, cat declaws, paravertebral blocks for ruminant abdominal surgery/C-sections, and lameness exams in horses.

• Line Blocks and L-Blocks:     * A continuous line of anesthetic in SQ tissues proximal to the target area.     * Ring block: A line block that completely encircles an anatomic part.     * L-block: A specific type of line block used for laparotomy in ruminants.

• Intra-articular Blocks:     * Injection directly into a joint (e.g., bupivacaine into the stifle joint after arthroscopy).

• Intravenous Regional Anesthesia (Bier Block):     * Used for short-term anesthesia of an extremity (e.g., digit amputation).     * Lidocaine is injected into a superficial vein distal to a proximal tourniquet.     * Bupivacaine is never used for Bier blocks due to intravenous cardiotoxicity.

• Systemic Administration:     * Lidocaine via Constant Rate Infusion (CRI) reduces general anesthetic requirements, prevents cardiac arrhythmias, and aids postoperative analgesia.

Epidural Anesthesia and Spinal Anatomy

• Anatomy:     * Spinal cord membranes: pia mater, arachnoid, and dura mater.     * Subarachnoid space: Between the arachnoid and pia mater; contains cerebrospinal fluid (CSF).     * Spinal cord termination (cauda equina): Terminates around $L6/L7$ in dogs and $S1$ in cats.

• Epidural Procedure:     * Injection occurs in the epidural space between the dura mater and the vertebrae (a potential space often filled with fat).     * Injection site: Between the last lumbar vertebra ($L7$) and the sacrum.     * If injected into the subarachnoid space, it is called a spinal block.

• Effects:     * Blocks sensation and motor control of the abdomen, pelvis, tail, pelvic limbs, and perineum.     * Lidocaine ($2\%$) duration: $1.5$ to $3$ hours.     * Bupivacaine ($0.5\%$) duration: $4$ to $6$ hours.     * Opioids (e.g., morphine, buprenorphine) can be used alone or mixed with local anesthetics for analgesia with minimal motor impairment.

Adverse Effects of Local Anesthesia

• Loss of voluntary motor control.

• Temporary or permanent loss of function if injected directly into a nerve.

• Tissue irritation and paresthesia (tingling, pain, or irritation).

• Allergic reactions or anaphylaxis.

• Systemic toxicity.

• Trauma to the spinal cord or cauda equina.

• Respiratory difficulty or paralysis.

• Sympathetic Blockade: Leads to vasodilation and hypotension; treated with a bolus of IV fluids.

Assisted and Controlled Ventilation

• Definitions:     * Assisted Ventilation: The anesthetist provides oxygen/inhalant, but the patient initiates inspiration.     * Controlled Ventilation: The anesthetist delivers all air/gases; the patient has no spontaneous effort.     * Positive Pressure Ventilation (PPV): Control of delivery via bagging or a mechanical ventilator.

• Ventilation Dynamics in the Awake Animal:     * Driven by increased arterial carbon dioxide ($PaCO_2$).     * Inhalation (active phase): Thorax expands, creating negative pressure.     * Exhalation (passive phase): Lasts twice as long as inspiration.     * Tidal Volume ($V_T$): Air per single breath; normal awake $V_T = 10-15\,\text{mL/kg}$.     * Respiratory Minute Volume: $V_T \times \text{Respiratory Rate}$.

• Ventilation in the Anesthetized Animal:     * Tranquilizers and anesthetics decrease RR and $V_T$, leading to decreased minute volume.     * Risks: Hypercarbia/hypercapnia (leads to respiratory acidosis), hypoxemia (low $PaO_2$), and atelectasis (collapsed alveoli).     * Predisposing factors: Anesthesia >90 mins, obesity, neuromuscular blockers, lung disease, head trauma, chest surgery, and specific species (horses/ruminants).

Mechanical Ventilation and Risks

• Manual Ventilation (Bagging/Sighing):     * Pressure should not exceed $20\,\text{cm H}_2\text{O}$ in small animals.     * Pressure should not exceed $40\,\text{cm H}_2\text{O}$ in large animals.     * Squeeze duration: $1$ to $1.5$ seconds.

• Mechanical Ventilator Types:     * Pressure-cycled: Supplies air until a preset pressure is reached.     * Time-cycled: Supplies gas based on a set inspiratory time.     * Volume-cycled: Delivers a preset $V_T$ regardless of pressure.

• Settings:     * Respiratory rate: $6-12$ breaths/min.     * Inspiratory/Expiratory (I:E) ratio: $1:2$ to $1:4$.     * $EtCO_2$ target: $35-45\,\text{mmHg}$.

• Risks of PPV:     * Ruptured alveoli leading to pneumothorax or pneumomediastinum.     * Decreased cardiac output.     * Respiratory alkalosis (if RR is too high).     * Increased depth of anesthesia.

Neuromuscular Blocking Agents

• Characteristics: Cause paralysis without analgesia, sedation, or anesthesia. Use in conscious animals is inhumane.

• Types:     * Depolarizing agents: Succinylcholine (causes muscle fiber resistance to ACh).     * Nondepolarizing agents: Gallamine, pancuronium, atracurium besylate, cisatracurium (block receptors at the endplate).

• Effects:     * Affects voluntary (skeletal) muscles only; does not affect cardiac or smooth muscle.     * Paralysis order: Face/neck first, then tail, limbs, abdomen, and finally intercostals/diaphragm.

• Reversal:     * Nondepolarizing agents are reversed with anticholinesterase agents: Edrophonium, Neostigmine, or Pyridostigmine.     * Reversal agents have no effect on depolarizing agents.