Step 4: Anesthetic Protocols
Multimodal Analgesic Drug Considerations for the Four Phases of Anesthesia
Preanesthesia: NSAIDs, opioids, alpha-2 agonists, ±maropitant, +/- gabapentin
Induction: Sometimes opioids, potentially ketamine (induction dose = loading dose for continuous rate infusion [CRI])
Maintenance: Local/regional blocks, CRI (opioid, lidocaine, ketamine, alpha-2 agonists, combinations), boluses of opioids or alpha-2 agonists
Recovery: NSAIDs, boluses of opioids or alpha-2 agonists, continue CRI, ±maropitant, ±gabapentin or other adjunctive drugs
Step 4a. Pain Management
Effective analgesia throughout the entire anesthesia continuum is an integral component of patient health and welfare. Analgesia has numerous advantages as a component of general anesthesia.21 First, analgesia improves anesthetic safety by decreasing the dose of inhalant drugs required for anesthesia maintenance, thus decreasing the likelihood of inhalant-mediated, dose-dependent adverse effects such as hypotension and hypoventilation.22 Second, provision of analgesia optimizes patient outcome with fewer pain-related adverse effects such as tachycardia, hypertension, slowed gastrointestinal motility, delayed wound healing, upregulation of pain, changes in behavior, etc.23,24 Third, although not yet proved in animals, provision of perioperative analgesia may decrease the development of acute pain-related chronic pain.25
Pain is a complex multidimensional sensation with multiple sources. Using one drug or drug class for treatment of pain is unlikely to provide adequate pain relief, at least in moderate to severe pain states. Using multiple drugs and modalities, each with activity at different sites of the pain pathway, alleviates or eliminates pain at multiple sites and from multiple sources. An example is the combination of an anti-inflammatory drug that decreases nociceptor activity and a local anesthetic that blocks pain signal transmission, plus an opioid or alpha-2 agonist to decrease receptor response in the central nervous system. This concept, known asmultimodal or balanced analgesia, provides greater pain relief and promotes anesthetic safety by further decreasing required anesthetic drug dosages.23 The use of multimodal analgesia also decreases the impact of drug unavailability, as with the recent opioid shortage. In addition, a balanced protocol includes preemptive, or preventive, analgesia and analgesia for a duration appropriate for the type/degree of pain. Preemptive administration of analgesic drugs decreases intra- and postoperative analgesic requirements.23 Although not thoroughly researched in animals, the appropriate duration of analgesic therapy in animals can be extrapolated from the duration needed to control pain in humans suffering similar pain insults. Extrapolation is appropriate because of the similarities of the mammalian pain pathway across species.26 Because animals conceal pain, treatment duration should be based on scientific knowledge of pain duration and not on presence/absence of pain signs.
Building an Analgesic Protocol:
This is an overview of perioperative analgesia. For more in-depth information, see the AAHA/ American Association of Feline Practitioners (AAFP) Pain Management Guidelines and World Small Animal Veterinary Association Analgesia Guidelines.2,27 Analgesic drugs can be distributed into the four phases of anesthesia (preanesthesia, induction, maintenance, and recovery). Opioids are often the first class considered when designing protocols. Although this strategy is efficient for anesthetic plan development, when considering the sources of pain and the mechanisms of action of analgesic drugs, building an analgesic protocol is more effective if centered on anti-inflammatory and local anesthetic drugs. Inflammation is generally a major component of acute pain. Because inflammation is also the pathology that produces most acute pain syndromes, control of inflammation decreases further tissue damage and speeds healing. An anti-inflammatory drug (traditional NSAID or grapiprant) should be administered to all appropriate patients.
Local anesthetic drugs block sodium channels and provide complete pain relief from the nerves that are blocked. This fact led to the recommendation “...local anesthetics should be utilized, insofar as possible, with every surgical procedure.”2 The task force recommends the use of local anesthesia, including these simple techniques for castrations, ovariohysterectomies, and perineal procedures (i.e., urinary catheter placement.).
Although opioids do not block pain at its source or stop the transmission of pain, they are potent and rapidly acting, making them excellent for acute pain relief. Full mu-opioid receptor agonists (morphine, for example) are the most potent analgesics but also the most impacted by regulatory control. Buprenorphine is moderately potent but has a longer duration (4–6 hr) than most full mu-opioid agonists, with the FDA-approved buprenorphine for cats providing 24 hr of analgesia. Butorphanol is only mildly to moderately potent and has a short duration of action (<60 min in the dog and 90 min in the cat).27 See opioid selection considerations.
After an anti-inflammatory drug, a local block, and an opioid have been chosen, adjunctive drugs should be added. Dexmedetomidine and medetomidine provide both sedation and analgesia, and their analgesic effects are synergistic with those of the opioids, thus enhancing the effects of the less potent opioids.28 Ketamine, administered as a subanesthetic dose infusion in a multimodal protocol, prevents or decreases the development of central sensitization, a condition that significantly amplifies pain intensity.27
Adjunctive drugs with less proof of efficacy include maropitant and gabapentin. Although minimum alveolar concentration reduction does not necessarily indicate analgesia,22 maropitant decreased minimum alveolar concentration in cats (administered at label dose) and dogs (administered as infusion).29,30 If not an analgesic, the potential for increased patient comfort secondary to decreased vomiting makes maropitant a valid addition to an anesthetic protocol. Gabapentin is used for treatment of chronic neuropathic pain but is unlikely to provide analgesia for acute inflammatory pain. However, gabapentin might be appropriate in patients with pre-existing neuropathic pain if dosed at a minimum of 10 mg/kg q 8 hr.31,32 Tramadol appears to provide minimal acute pain relief in dogs.33 Although it is perhaps more efficacious in cats, especially used multimodally, cats are particularly averse to the drug’s taste.34,35 Tramadol is controlled and linked to human diversion, so dispensing for home use should be limited. Other adjunctive drugs and nonpharmacologic therapy are covered in the 2015 AAHA/AAFP Pain Management Guidelines for Dogs and Cats.2
Step 4b. Preanesthetic Anxiolytics and Sedatives
- Nonmedication strategies such as low-stress handling, pheromones, and environmental considerations (such as cat-only wards) play an important role in reducing patient fear and anxiety in the preanesthetic period.
- Fractious/fearful patients should be managed with appropriate preanesthetic anxiolytics and sedatives, generally at higher dosages than those required by calmer patients, to allow IV catheterization (if possible) and induction while ensuring staff safety.
In addition to preanesthetic analgesic drugs, preanesthetic sedatives and anxiolytics are an important component of the continuum of anesthesia care. Benefits include decreased stress/anxiety and dose reduction of induction and maintenance drugs, which have dosedependent adverse effects. Reduced patient stress can reduce risk of harm to staff members who are restraining/handling patients. As stated, providing appropriate oral medication at home can alleviate anxiety and pain prior to the pet’s admission to the hospital.36,37 In hospital, preanesthetic medications can be administered by the intramuscular (IM) or subcutaneous route to achieve effects well in advance of anesthesia induction, or intravenously just prior to anesthesia for acute dose-sparing effects of induction drugs. Specific drugs/drug combinations should be chosen based on desired effects (e.g., reversible sedation) and individual patient needs (e.g., degree of analgesia). After premedication, physiologic monitoring and support are conducted as indicated by the patient’s health status. Early support of body temperature should be initiated in all patients. Thermal supplementation is critical as hypothermia can cause numerous adverse events (more information in the Troubleshooting Anesthetic Complications section).
Premedicant drugs are also useful for sedation alone, remembering from previous information that greater patient safety is often achieved with general anesthesia, even for short procedures, especially if they are painful (biopsy, laceration repair, etc.). A crucial point is that sedated patients, just as those under general anesthesia, require appropriate monitoring and supportive care. Administer supplemental oxygen to the deeply sedated patient using a facemask. The patient may require airway management, so be prepared to intubate if necessary.
Step 4c. Anesthetic Induction
Induction and Intubation Tips
- Mask or chamber inductions can cause stress, delayed airway control, and environmental contamination and are not recommended by the authors.
- Intubation tip: The capnograph adapter can be placed on the end of the ETT to confirm proper endotracheal tube placement during intubation.
- Cats can be particularly difficult to intubate. See tips in the 2018 AAFP Feline Anesthesia Guidelines.3
Preoxygenation should be considered part of the preanesthetic/induction sequence. Delivery of 100% oxygen for only 3 min provides almost 6 min of adequate saturation of hemoglobin with oxygen.38 This is especially critical in patients with airway disease (e.g., pneumonia, asthma) and breathing difficulty (e.g., upper airway dysfunction, limited thoracic movement [e.g., thoracic injury, impingement on diaphragm from dilated stomach or gravid uterus]) and in patients with expected difficult intubation (e.g., upper airway collapse or airway foreign body). All pregnant patients should be preoxygenated to ensure adequate fetal oxygen delivery.
A patient’s sedation level following preanesthetic drugs will influence the dose of induction drug, which should always be dosed “to effect.” Typically, appropriate premedication will result in lower doses of induction drugs. In addition, sick, debilitated, or depressed patients may require lower doses than healthy, alert patients. Anesthetic induction is most effectively and efficiently achieved by IV administration of fast-acting drugs (dosages and specific protocols), such as propofol, alfaxalone, etomidate, diazepam- or midazolamketamine, or tiletamine-zolazepam. IV induction allows rapid airway control. IM administration of a combination of a sedative and either ketamine or tiletamine-zolazepam combined in the same syringe can be used to both premedicate and induce patients whose venous access is limited by size (i.e., cats and very small dogs) or temperament (i.e., fractious or aggressive). This does not allow rapid control of the airway, and patients should be observed closely as they are becoming unconscious. Supplemental oxygen administration during this period should be considered in manageable (i.e., not fractious or aggressive) patients.
A patent airway should be secured using placement of an ETT or supraglottic device as soon after induction as possible. Tracheal intubation is an essential part of maintaining an open and protected airway. The length of the tube should be assessed prior to intubation. The proper length will allow the proximal end of the tube to be at or just external to the incisors and the distal tip of the tube to lie midway between the larynx and the thoracic inlet. The largest-diameter ETT that will easily fit through the arytenoid cartilages without trauma should be used. This will minimize resistance and the work of breathing. Correct placement can be confirmed by direct visualization of the tube between the arytenoids, movement of the rebreathing bag, condensation in the ETT during exhalation, and/or observation of a definitive end-tidal carbon dioxide (ETCO2) tracing. A properly inflated cuff on a conventional ETT is necessary to create a seal for adequate positive pressure ventilation (PPV) and avoid inhalant leakage, being aware that overinflation may cause tracheal damage.39 Applying a light coating of sterile lubricating jelly improves the cuff’s ability to seal the airway.40 Once the patient is intubated and connected to a breathing circuit, ensure that O2 is flowing, close the pop-off valve or push down the button on the safety pop-off valve, and administer a manual breath to 15–20 cm H20 while listening at the patient’s mouth for a slight hissing sound, audible if the cuff is under-deflated. If detected, slowly inflate the cuff while simultaneously administering a manual breath. Once the sound has disappeared, immediately open the pop-off valve or release the button on the safety pop-off valve and secure the ETT to the patient’s head. Self-sealing baffled ETTs may be more leak resistant, but they do not allow air to escape around them if the airway pressure is excessive. When changing the patient’s position after intubation, disconnect the ETT from the breathing tube so that the ETT does not rotate within the trachea as tube rotation may cause tracheal tears, especially if the cuff is relatively overinflated. Tracheal tears are a significant issue in anesthetized intubated cats.39,41
Final considerations: Padding and appropriate positioning (especially for cachectic, geriatric, or large patients) should be provided. Apply corneal lubricant postinduction to protect the eyes from corneal ulceration after induction and every 2–4 hr.
Step 4d. Anesthetic Maintenance
Anesthesia is typically maintained using inhalant anesthetics delivered in O2 and dosed “to effect.”19,38 Maintenance can also be achieved using continuous infusions or intermittent doses of injectable agents, or a combination of injectable and inhalant drugs. Short-duration maintenance can be achieved with IM administration of sedatives plus ketamine or tiletamine/zolazepam. IM alfaxalone can be effective for short-duration, deep sedation in cats and small dogs, but the high dose required for anesthesia maintenance in healthy cats can cause excitement and hypermetria in recovery. 42
Regardless of the drugs used for anesthesia maintenance (i.e., inhalant or injectable), vigilant monitoring, interpretation of physiologic changes, and response to patient physiologic status by well-trained and attentive staff are critical. Monitoring decreases the odds of anesthetic death,18 whereas lack of monitoring increases the odds of anesthetic death by a factor of 5–35.5 Both multiparameter electronic monitors and hands-on assessment of the patient by the anesthetist should be used. Treatment decisions should be made based on information from both the electronic monitors and the anesthetist’s assessment. Monitoring respiratory function includes respiratory rate, oxygenation (percentage of hemoglobin saturated with oxygen [Sp O2]), and ventilation (ETC O2). BP, heart rate (HR) and rhythm (ECG), capillary refill time, mucous membrane color, and pulse oximetry (Sp O2) provide the best indices of cardiovascular function. Anesthetic depth is monitored, and a surgical plane of anesthesia is typically defined as a patient with absent palpebral reflex, mild jaw tone (i.e., muscle relaxation), and lack of purposeful movement. Body temperature monitoring is critical, with heat supplementation starting early; see section on anesthetic complications.
Regardless of the drugs used for anesthesia maintenance (i.e., inhalant or injectable), O2 should be delivered to the patient. The O2 flow rates depend on the breathing circuit (see section on equipment preparation). For an RC, use a relatively high flow rate (2–3 L/min) when rapid changes in anesthetic depth are needed, such as during the transition from injectables to inhalants (induction) or when discontinuing inhalants at the end of the procedure. Because of the high oxygen flow, increased flow at induction and after discontinuing inhalants is not necessary when using an NRC. Increased inspired CO2 suggests an inadequate O2 flow rate. Following induction and intubation, the patient may be apneic or have a low or shallow respiratory rate, requiring intermittent (1–4 breaths/min) PPV breaths delivered by the anesthetist to maintain anesthesia until the respiratory depression of the induction drugs subsides. If PPV is excessive, ETC O2 levels will decrease below the level that stimulates ventilation and the patient may not begin spontaneously breathing. Balanced crystalloid fluids should be administered for most patients undergoing anesthesia. The basal fluid rate for healthy dogs and cats is 5 mL/kg/hr and 3 mL/kg/hr, respectively. Additional volume should be added to the basal rate for correction of hypovolemia, including dehydration, and replacement of ongoing fluid losses. Fluid volume should increase or decrease depending on the patient’s health status and fluid needs. See the 2013 AAHA/ AAFP Fluid Therapy Guidelines for Dogs and Cats for more information.43
Step 4e. Recovery from Anesthesia
Although many complications occur throughout anesthesia, between 47 and 60% of all anesthetic-related dog and cat deaths, respectively, occur during the postoperative period of anesthesia, with most occurring within the first 3 hr.4–6 Thus, patient care and monitoring of the recovering patient by trained personnel is critical and should be maintained with the same vigilance as during the maintenance phase of anesthesia. The anesthetist should continue monitoring specific patient physiologic parameters such as HR/RR (respiratory rate), SpO2, BP, and body temperature. Patients should be closely observed until they are alert, normothermic, and ambulatory (unless nonambulatory preoperatively). An optimal recovery time (within 10–30 min of the end of anesthesia) for dogs and cats will depend on the patient health status, type of anesthetic technique used (i.e., inhalant versus injectable), duration of anesthesia, and body temperature.
In case sedatives, analgesics, or emergency resuscitative drugs are needed, intravenous catheters should be left in place until the patient is extubated and in sternal recumbency with physiologic parameters returned to normal. Extubate when the patient’s RR and SpO2 are within normal limits and the patient can adequately protect its airway by vigorously swallowing. Deflate the cuff immediately before removing the ETT. With patients who have undergone a dental procedure or rhinoscopy, it is beneficial to position the nose slightly lower than the back of the head and leave the ETT cuff slightly inflated during extubation. This will help clear blood clots and debris from the trachea and deposit any fluid or debris into the pharyngeal region where it can drain from the mouth or be swallowed, thereby reducing the risk of aspiration. Respiratory depression, potentially with resultant hypercapnia and hypoxemia, often persists during early recovery from anesthesia. Severe hypercapnia can lead to cerebral impairment and potentially to respiratory arrest. The capnograph adapter can remain on the end of the ETT to assess ventilation until the patient is extubated.53 In addition, a pulse oximeter should be used throughout recovery to assess the degree of oxygen saturation.
Numerous factors can impact the quality of recovery and should be addressed to aid the patient’s smooth emergence from anesthesia. Environmental stress, bright lights, excessive noise, and a cold environment can attribute to the patient’s discomfort following anesthesia. Bladder distension can be very uncomfortable. Express the bladder to minimize any discomfort, especially for those patients who may be nonambulatory and unable to urinate on their own in the immediate postoperative period.
Delayed recovery, dysphoria, and emergence delirium are common complications in the postoperative period. Delayed recovery can be caused by excessive anesthetic depth during the maintenance phase. This can not only prolong the recovery phase but also negatively impact the respiratory, cardiovascular, and thermoregulatory systems. Hypothermia (body temperature >98°F [36.7°C]) can lead to multiple physiologic complications, including delayed drug metabolism, further prolonging the patient’s recovery.46 Patient warming devices should be used throughout the recovery phase.46,53 Certain drugs can cause peripheral vasoconstriction (e.g., alpha-2 agonists) or vasodilation (e.g., inhalants), modifying the heat loss from the patient and influencing the effectiveness of external warming devices.46,47 Hypoglycemia, especially in small or neonatal patients, can lead to a prolonged recovery, so monitor blood glucose frequently. Judiciously titrated drug antagonism may be considered if the patient’s recovery is concerningly prolonged. Alpha-2 agonists should be reversed only if the patient is excessively sedate or rapid recovery is needed. Opioid effects should be antagonized only if other analgesics have been administered; otherwise, the patient could experience intolerable pain.
Dysphoric recoveries and emergence delirium can often be difficult to differentiate. Emergence delirium often presents as an uncontrolled, uncoordinated thrashing of the patient, often encountered when the patient has regained partial consciousness quickly after the discontinuation from maintenance anesthetic drugs. Dysphoric recoveries can result frommany issues, including uncontrolled pain, and hypoxemia due to airway obstruction. Administer an analgesic if pain is suspected. If pain management is adequate, administer an anxiolytic sedative, such as dexmedetomidine (which also provides analgesia), generally 0.001–0.005 mg/kg IV or IM (up to 0.01 mg/kg). If respiratory complications are the cause of the dysphoria/delirium, provide supplemental oxygen and assess the respiratory function of the patient. In some cases, the patient will need small doses of alfaxalone 1–2 mg/kg or propofol 1–2 mg/kg IV administered slowly until calm. Be prepared to reintubate if necessary.
Final considerations: Provide adequate padding for nonambulatory patients. Reapply eye ointment every 2–4 hr during the recovery period until an adequate blink refiex is present.
Postoperative Pain Assessment
Regardless of the perceived efficacy of the analgesic protocol, assessment of the patient’s pain level during recovery from anesthesia is imperative as adequate pain relief is unlikely to be achieved in all patients. Using pain scoring systems, although not perfect, increases the likelihood that pain will be recognized and treated appropriately. Pain scoring resources are available online.