Thoracic surgery is associated with considerable

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1 3 CREDITS Article 3 Analgesia for Small Animal Thoracic Surgery Kyriaki Pavlidou, DVM Lysimachos G. Papazoglou, DVM, PhD, MRCVS Ioannis Savvas, DVM, PhD Georgios Kazakos, DVM, PhD Aristotle University of Thessaloniki Thessaloniki, Greece At a Glance Pathophysiology of Pain in Thoracic Surgery Page 432 Effect of Surgical Technique Page 432 Pharmacologic Pain Relief After Thoracotomy Page 433 Factors Contributing to Postthoracotomy Pain Page 433 Suggested Protocols for Pain Management Page 435 Abstract: Thoracic surgery in small animals is considered a painful procedure, resulting in alterations in pulmonary function and respiratory mechanics. Modifications in surgical approach and technique and selection of the appropriate analgesic protocol may improve outcomes in dogs and cats after thoracic surgery. Systemic administration of opioids and other agents, intercostal and intrapleural blocks, and epidural analgesia are among the most common options for pain management after thoracic surgery in small animals. Thoracic surgery is associated with considerable postoperative pain in small animals, leading to hypoventilation, increased morbidity, prolonged hospitalization, and delayed recovery Therefore, analgesia is indicated in all thoracic surgery patients to provide pain relief without interfering with respiration. The choice of surgical technique and analgesic protocol may affect the analgesic outcome in dogs and cats after thoracic surgery. Preemptive and multimodal analgesia are considered to be among the leading analgesic strategies for postthoracotomy pain in small animals. Most of the information presented in this article pertains to dogs due to the lack of research into pain associated with thoracic surgery in cats; however, information regarding cats has been included where available. Pathophysiology of Pain in Thoracic Surgery Noxious stimuli associated with thoracic surgery are carried via C and Aδ fibers and transmitted to the central nervous system (CNS) by the intercostal, vagus, and phrenic nerves. 11 Analgesic management during and after thoracic surgery is aimed at stopping the transmission of nociceptive stimuli between the area of incision and the CNS and prohibiting peripheral and central sensitization. Severe postthoracotomy pain in small animals is the most important factor responsible for alterations in pulmonary function and impairment of respiratory mechanics. 9,10 Anesthesia and thoracotomy in healthy dogs may result in ventilation/perfusion mismatch and hypoxemia, decreased lung compliance and tidal volume, and increased work of breathing and pulmonary resistance. 2,9,10 Effect of Surgical Technique Access to the thoracic cavity may be obtained in many ways. The choice of thoracic approach is mainly determined by the type of intrathoracic damage or disease. Postoperative pain may be affected by the surgical approach (e.g., open thoracotomy versus thoracoscopy), the type of incision, and the technique of rib or sternebrae approximation for thoracotomy closure. 2,9,10 15 Median sternotomy in dogs appears to be more painful than intercostal thoracotomy. 10 Thoracoscopic surgery for pericardiectomy in dogs has been reported to cause less postoperative pain and morbidity than intercostal thoracotomy, 15 possibly due to smaller incisions and reduced rib retraction resulting in less surgical trauma. Pain after intercostal thoracotomy is mainly associated with intercostal nerve irritation. 14 Muscle-sparing thoracotomy, which preserves the latissimus dorsi muscle, decreases morbidity without compromising exposure. 16 However, further studies regarding pain and morbidity in small animals should be implemented to compare muscle-sparing intercostal thoracotomy with the traditional technique. 432 Compendium: Continuing Education for Veterinarians September 2009 CompendiumVet.com

2 Analgesia for Small Animal Thoracic Surgery FREE Intense pain may develop during intercostal thoracotomy closure because the intercostal nerves are often trapped in the sutures that are placed circumcostal to the thoracotomy wound. 14 Transcostal suture placement for thoracotomy closure has been reported to avoid nerve entrapment and appears to be less painful than the standard technique of circumcostal closure in dogs. 14 Passage of the blunt end of the needle in close proximity to the rib during intercostal thoracotomy closure has been demonstrated to result in less nerve entrapment than other techniques. 14 Suture type (wire versus polybutester) has been shown to have no effect on the degree of pain after median sternotomy closure in dogs. 13 Surgical procedures and factors that contribute to pain after thoracic surgery in dogs and cats and the nerve supply involved are presented in Table 1. Pharmacologic Pain Relief After Thoracotomy Regional Analgesia Intercostal Nerve Block Selective intercostal block is employed before thoracotomy closure to alleviate pain and improve pulmonary function in small animals. Because of the overlapping nerve supply, two or three nerves on either side of the thoracotomy should be blocked. 1,2 4,7,17 Selective intercostal block with a solution of 0.5% bupivacaine provides analgesia for up to 12 hours, and compared with parenteral administration of morphine or oxymorphone has minimal effect on postoperative blood gas values and minute ventilation in dogs undergoing intercostal thoracotomy. 2,4 The bupivacaine 0.5% solution is injected caudal to the head of the rib near the insertion of the epaxial musculature and close to the intervertebral foramen. 4 Doses of bupivacaine for intercostal blocks are shown in Table 2. The total dose of bupivacaine should not exceed 5 mg/kg. 18 Intercostal nerve block has been recommended as an adjunct to systemic opioid therapy for the management of postthoracotomy pain, 18 but it is not recommended for pain control after median sternotomy. Intrapleural Analgesia Intrapleural analgesia is achieved by placing local anesthetic between the visceral and parietal pleura to produce ipsilateral somatic block of multiple thoracic dermatomes. Diffusion table 1 Factors Contributing to Postthoracotomy Pain Surgical Procedure Involved Skin incision Muscle incision in intercostal thoracotomy (division of latissimus dorsi, scalenus, external abdominal oblique, and intercostal muscles) Muscle incision in median sternotomy (division of pectoral muscles) Rib spreading or fracture Intercostal nerve stretching, compression by retractors, sutures, or thoracostomy tubes Thoracic wall excision Parietal pleura incision Thoracostomy tube placement Nerve Supply of the anesthetic across the parietal pleura allows intercostal neural blockade by prohibiting the generation and conduction of nerve impulses. 8 Local anesthetics can be given as a single injection, multiple injections, or a continuous infusion through an indwelling catheter placed intrapleurally. Intrapleural administration of bupivacaine is used for pain relief after intercostal thoracotomy and median sternotomy in dogs Localization of the block over dependent areas of the rib with changes in animal position suggests an influence by gravitational pooling. 19 Use of this block is gaining popularity because a larger volume of bupivacaine can be used and no spinal or central effects are seen after induction. 19 Bupivacaine 0.5% (1.5 mg/kg) is instilled through a thoracostomy tube; after instillation, the tube is flushed with saline solution. Because the bupivacaine is distributed by gravity, the animal is placed with the incision site down for up to 5 minutes. Intrapleural Thoracic and pectoral nerves Pectoral, thoracodorsal, intercostal, and cervical nerves Pectoral, cervical, and thoracic nerves Intercostal and thoracic nerves Thoracic, pectoral, thoracodorsal, and intercostal nerves table 2 Doses of Bupivacaine 0.5% for Intercostal Blocks in Dogs and Cats 1,17,a Animal Size Small dogs (<10 kg) and cats Medium dogs (<25 kg) Large dogs (>25 kg) Any size a Modified by the authors. Dose 0.25 ml/site 0.5 ml/site 1 ml/site 0.3 ml/site for five sites CompendiumVet.com September 2009 Compendium: Continuing Education for Veterinarians 433

3 FREE Analgesia for Small Animal Thoracic Surgery table 3 Analgesic Agents for Loading and Continuous Epidural Administration to Manage Postthoracotomy Pain in Dogs and Cats 5 7,21 23 Analgesic Agent(s) Bupivacaine 0.5% 1 QuickNotes Median sternotomy appears to be more painful than intercostal thoracotomy in small animals. Dose (mg/kg) Morphine Oxymorphone 0.1 Buprenorphine Morphine + bupivacaine ( ) + (0.6 2) Morphine + fentanyl ( ) + ( ) Bupivacaine + buprenorphine ( ) + ( ) Morphine + bupivacaine + fentanyl ( ) bupivacaine administration provides analgesia for up to 12 hours. Analgesia produced by intrapleural bupivacaine in dogs has been reported to be similar to that from morphine given systemically or intrapleurally 4,9,10 and that from selective intercostal block with bupivacaine 4 and superior to that from buprenorphine given systemically. 8 Intrapleural bupivacaine in dogs has been associated with fewer blood gas alterations and earlier normalization of certain pulmonary function values than morphine given systemically or intrapleurally 4,9 and produces significantly improved oxygenation compared with buprenorphine administered systemically. 8 Bupivacaine administered intrapleurally at a dose of 1.5 mg/kg has been found to have minimal effects on cardiac output and to cause clinically insignificant hemodynamic alterations in dogs, and it can be used safely in healthy dogs with a previous pericardectomy. 19,20 Epidural Analgesia Analgesic agents may be injected into the spinal epidural space, most commonly through the lumbosacral space and close to the site of action (e.g., spinal cord receptors, spinal nerves) to achieve regional analgesia. Epidural analgesics may be administered as a bolus or by multiple injections through an epidural catheter to provide preemptive, intraoperative, and postoperative analgesia for thoracic surgery in dogs and cats. Local anesthetics, opioids, and other agents and combinations have been employed to provide analgesia using this technique. 5,7,21 23 Local anesthetics primarily block spinal nerve roots via a gravity effect. 23 Bupivacaine is the most commonly used local anesthetic in small animals and has a longer duration of action than lidocaine or mepivacaine. Potentiation of the analgesic effect has been reported in dogs undergoing thoracotomy with combined epidural administration of bu - pi vacaine and morphine. 22 This synergy of opioids with local anesthetics may be the result of interaction with opioid receptors or diminished efferent nociceptive transmission facilitating the effect of opioids. 22 Epidural morphine administered preemptively has been reported to provide long-lasting analgesia in dogs and cats 6,22 and is at least as effective as intercostal bupivacaine after intercostal thoracotomy in dogs. 7 Morphine administered epidurally provides better analgesia than morphine given intravenously in dogs after intercostal thoracotomy. 6 Epidural oxymorphone administered intraoperatively has been reported to provide better and longerlasting analgesia in dogs than postoperative intramuscular oxymorphone. 5 The increased lipid solubility of oxymorphone compared with morphine may provide more segmental analgesia for surgical procedures involving the hindlimbs and caudal abdomen; by contrast, epidural morphine because of its more hydrophilic nature may diffuse cranially and is more appropriate for providing analgesia for thoracic and cranial abdominal surgery. 5,23 Analgesic agents and their epidural doses for the relief of postthoracotomy pain in small animals are presented in Table 3. Systemic Analgesia Systemic analgesics can be adjuncts to other, more invasive analgesic strategies, especially when the latter are discontinued. Opioids administered parenterally are the primary form of systemic analgesia for thoracic surgery. Central respiratory depression is a potential adverse effect of opioid administration; however, because postthoracotomy pain may cause hypoventilation, systemic opioids may actually improve respiratory function by relieving the pain. 2,4,5 Parenterally administered morphine (0.5 to 1 mg/kg SC, IM, or IV), oxymorphone (0.1 to 0.2 mg/kg IM or IV), or buprenorphine (10 μg/kg IV) can provide effective postoperative analgesia in dogs after intercostal thoracotomy or median sternotomy. 2,4 6,8 10 Morphine 434 Compendium: Continuing Education for Veterinarians September 2009 CompendiumVet.com

4 Analgesia for Small Animal Thoracic Surgery FREE table 4 Suggested Protocols for Pain Management in Small Animals Undergoing Intercostal Thoracotomy or Median Sternotomy 2,4 10,12,21 24 When to Administer Analgesics Intercostal Thoracotomy Protocol Median Sternotomy Protocol Preoperatively NSAIDs IV, SC Systemic opioids (fentanyl CRI) Ketamine IV loading dose Intraoperatively Intercostal bupivacaine block Intrapleural bupivacaine Epidural analgesia Fentanyl CRI Ketamine CRI Postoperatively Ketamine CRI NSAIDs Opioids (fentanyl CRI) Systemic opioids (fentanyl CRI) Intrapleural bupivacaine Epidural analgesia Fentanyl CRI Ketamine CRI Opioids (fentanyl CRI) CRI = constant-rate infusion and oxymorphone administration may cause hypoventilation, respiratory acidosis, and moderate hypoxemia in dogs, which (unlike humans) are relatively insensitive to the respiratory depressant effects of opioids. 2,4,5 Systemic administration of buprenorphine has shown no effect on respiratory function in dogs. 8 Fentanyl administered at a loading dose (2 to 5 μg/kg IV) before surgery and followed by continuous infusion intraoperatively (20 to 80 μg/kg/hr) and postoperatively (2 to 5 μg/kg/ hr) provides adequate analgesia and allows for quick titration to increase the analgesic effect and decrease excessive hypoventilation. 12,18 Low-dose ketamine infusion administered at a loading dose (0.5 mg/kg IV) before surgery and followed by continuous infusion intraoperatively (10 μg/kg/min) and postoperatively (2 μg/kg/min) has been employed as an adjunct to preanesthesia morphine (1 mg/kg SC). This protocol can also be used as an adjunct to continuous intraoperative fentanyl infusion (1 to 5 μg/kg) to augment analgesia and comfort after forelimb amputation in dogs 24 ; it may also be used for postthoracotomy pain relief in small animals. 12 Medetomidine (10 μg/kg IM) and other α 2 -adrenergic agonists given postoperatively are thought to produce a better analgesic effect than buprenorphine (20 μg/kg IM) in dogs after intercostal thoracotomy. 25 NSAIDs are particularly useful for pain relief after thoracic surgery because they have no respiratory depressant effects. They are employed as adjuncts to systemic opioids or as part of a multimodal approach to pain management after thoracotomy. 12 Postoperative ad - ministration of magnesium sulfate is reportedly associated with reduced morphine consumption for pain after thoracotomy in human patients. 26 An algorithm for postthoracotomy pain control is presented in Table 4. Conclusion Several analgesic modalities are available for use in thoracic surgery in small animals, providing a range of choices to suit practice requirements and surgeon preference. These protocols can not only increase patient comfort but also improve pulmonary function during the critical period immediately after surgery. QuickNotes Transcostal thoracotomy closure seems to result in less pain compared with circumcostal thoracotomy closure. References 1. Gilroy BA. Effect of intercostal nerve blocks on post thoracotomy ventilation and oxygenation in the canine. J Vet Crit Care 1982;6: Berg JR, Orton. Pulmonary function in dogs after intercostal thoracotomy: comparison of morphine, oxymorphone, and selective intercostal nerve block. Am J Vet Res 1986;47: Flecknell PA, Kirk AJB, Liles JH, et al. Post-operative analgesia following thoracotomy in the dog: an evaluation of the effects of bupivacaine intercostal nerve block and nalbuphine on respiratory function. Lab Anim 1991;25: Thompson SE, Johnson JM. Analgesia in dogs after intercostal thoracotomy. A comparison of morphine, selective intercostal nerve block, and intrapleural regional analgesia with bupivacaine. Vet Surg 1991;20: Popilskis S, Kohn D, Sanchez JA, et al. Epidural vs. intramuscular oxymorphone analgesia after thoracotomy in dogs. Vet Surg 1991;20: Popilskis S, Kohn D, Laurent L, et al. Efficacy of epidural morphine versus intravenous morphine for post-thoracotomy pain in dogs. J Vet Anaesth 1993;20: CompendiumVet.com September 2009 Compendium: Continuing Education for Veterinarians 435

5 FREE Analgesia for Small Animal Thoracic Surgery 7. Pascoe PJ, Dyson DH. Analgesia after lateral thoracotomy in dogs. Epidural morphine vs. intercostal bupivacaine. Vet Surg 1993;22: Conzemius MG, Brockman DJ, King LG, et al. Analgesia in dogs after intercostal thoracotomy: a clinical trial comparing intravenous buprenorphine and intrapleural bupivacaine. Vet Surg 1994;23: Stobie D, Caywood DD, Rozanski EA, et al. Evaluation of pulmonary function and analgesia in dogs after intercostal thoracotomy and use of morphine administered intramuscularly or intrapleurally and bupivacaine administered intrapleurally. Am J Vet Res 1995;56: Dhokarikar P, Caywood DD, Stobie D, et al. Effects of intramuscular or intrapleural administration of morphine and intrapleural administration of bupivacaine on pulmonary function in dogs that have undergone median sternotomy. Am J Vet Res 1996;57: Hughes R, Gao F. Pain control for thoracotomy. Contin Educ Anesth Crit Care Pain 2005;5: Perkowski SZ. Anesthesia of the patient with respiratory disease. In: King LG, ed. Textbook of Respiratory Disease in Dogs and Cats. St. Louis: Saunders; 2004: Pelsue DH, Monnet E, Gaynor JS, et al. Closure of median sternotomy in dogs: suture versus wire. JAAHA 2002;38: Rooney MB, Mehl M, Monnet E. Intercostal thoracotomy closure: transcostal sutures as a less painful alternative to circumcostal suture placement. Vet Surg 2004;33: Walsh PJ, Remedios AM, Ferguson JF, et al. Thoracoscopic versus open partial pericardiectomy in dogs: comparison of postoperative pain and morbidity. Vet Surg 1999;28: Dean PW, Pope ER. Modified intercostal thoracotomy approach. JAAHA 1992;28: Scarda RT. Local and regional anesthetic and analgesic techniques: dogs. In: Thurmon JC, Tranquilli WJ, Benson JG, eds. Lumb & Jones Veterinary Anesthesia, ed 3. Baltimore: Williams & Wilkins; 1996: Orton. Thoracic wall. In: Slatter D, ed. Textbook of Small Animal Surgery, ed 3. Philadelphia: Saunders; 2003: Bernard F, Kudnig ST, Monnet E. Hemodynamic effects of intrapleural lidocaine and bupivacaine combination in anesthetized dogs with and without an open pericardium. Vet Surg 2006;35: Kushner LI, Trim CM, Madhusudhan S, et al. Evaluation of the hemodynamic effects of intrapleural bupivacaine in dogs. Vet Surg 1995;24: Hansen BD. Epidural catheter analgesia in dogs and cats: technique and review of 182 cases ( ). J Vet Emerg Crit Care 2001;11: Troncy E, Junot S, Keroack S, et al. Results of preemptive epidural administration of morphine with or without bupivacaine in dogs and cats undergoing surgery: 265 cases ( ). JAVMA 2002; 221: Torske KE, Dyson DH. Epidural analgesia and anesthesia. Vet Clin North Am Small Anim Pract 2000;30: Wagner AE, Walton JA, Hellyer PW, et al. Use of low doses of ketamine administered by constant rate infusion as an adjunct for postoperative analgesia in dogs. JAVMA 2002;221: Vainio O, Ojala M. Medetomidine, an α 2 -agonist, alleviates post-thoracotomy pain in dogs. Lab Anim 1994;28: Ozcan PE, Tugrul S, Senturk NM, et al. Role of magnesium sulfate in postoperative pain management for patients undergoing thoracotomy. J Cardiothorac Vasc Anesth 2007;21(6): CREDITS Test 3 This article qualifies for 3 contact hours of continuing education credit from the Auburn University College of Veterinary Medicine. Subscribers may take individual tests online and get real-time scores at CompendiumVet.com. Those who wish to apply this credit to fulfill state relicensure requirements should consult their respective state authorities regarding the applicability of this program. 1. Which statement regarding the pathophysiology of postthoracotomy pain is false? a. Intercostal, vagus, and phrenic nerves carry impulses to the CNS. b. Postthoracotomy pain may cause alterations in pulmonary function. c. Provision of analgesia is aimed at interrupting stimulus transmission to the CNS. d. The basic goal of analgesic management is peripheral and central sensitization. 2. Postthoracotomy pain may be affected by the a. surgical approach. b. technique of rib approximation. c. surgical incision (muscle-sparing versus standard thoracotomy). d. all of the above 3. Which type of suture placement for thoracotomy closure avoids nerve entrapment and causes less postoperative pain? a. transcostal c. circumcostal b. subcostal d. wire 4. Thoracotomy in dogs has been associated with a. increased lung compliance. b. increased tidal volume. c. decreased tidal volume. d. decreased pulmonary resistance. 5. Intercostal nerve block is not recommended a. for pain control after median sternotomy. b. for pain control after intercostal thoracotomy. c. as an adjunct to opioids for the management of pain after intercostal thoracotomy. d. before thoracotomy closure. 6. Which statement concerning intrapleural bupivacaine administration in dogs is false? a. Analgesia achieved by intrapleural bupivacaine administration is similar to that produced by systemically administered morphine. b. Analgesia achieved by intrapleural bupivacaine administration is similar to that produced by intercostal bupivacaine block. c. Intrapleural administration of bupivacaine can be used safely in healthy dogs after pericardiectomy. d. Intrapleural administration of bupivacaine has been associated with more blood gas alterations than systemically administered morphine. 7. Which statement regarding epidural administration of bupivacaine for the management of postthoracotomy pain is correct? a. Bupivacaine has a shorter duration of action than lidocaine. b. Combined administration of bupivacaine and morphine potentiates the analgesic effect. c. Combined administration of bupivacaine and morphine decreases the analgesic effect. d. Epidural bupivacaine administration has a shorter duration of action than mepivacaine. 8. In human patients, postoperative administration of magnesium sulfate has been associated with a. increased morphine consumption. b. decreased morphine consumption. c. respiratory depression. d. small-airway collapse. 9. Which opioid(s), administered systemically, showed no effect on respiratory function in dogs? a. morphine c. buprenorphine b. oxymorphone d. all of the above 10. Which statement is false with regard to the use of NSAIDs in postthoracotomy pain management? a. NSAIDs cause respiratory depression. b. NSAIDs have no effect on respiratory function. c. NSAIDs are used as adjuncts to systemic opioids. d. NSAIDs may be used as part of a multimodal approach. 436 Compendium: Continuing Education for Veterinarians September 2009 CompendiumVet.com