Recovery after surgery requires some especial cares that, if not done, could induce failure of the whole work done during surgery. Three aspects should be especially considered: comfort, oxygenation, and sedation and analgesia.

Assessment and management of pain should be an important aspect in the postsurgical care of all our patients. Many patients need sedation and analgesia after surgery. Pain provokes lots of neurohumoral responses like stress, complement and cytokines activation, and arachidonic acid cascade. Tachycardia, hypertension, higher cardiac oxygen demand, changes in respiration rate and electrolyte disturbances are physiologic changes suggestive of pain. Pain can also cause a decrease in immune system function, stress, anxiety, anorexia, and a longer hospitalisation time. All these aspects can increase morbidity and mortality of our patients.

We need to maintain a permeable airway to administer high levels of oxygen to several patients, but this requires deep sedation. A good sedation is the key to control a lot of the problems seen in patients requiring mechanical ventilation.

Drugs to be used in postsurgical care

None of the available analgesic drugs can be considered ideal. Pain management and sedation has been accomplished in human medicine trough four classical drug combinations: opioids and neuroleptic drugs, opioids and benzodiazepines, opioids and propofol, ketamine and benzodiazepines.

Phenothiazines

Acepromacine is the most used. Its effect is multiplied when combined with opioids. Can cause hypotension in hypovolemic patients. Low doses can be used to improve cardiac output in selected patients due to selective arteriolar dilatation. Should not be used in patients with head trauma as could reduce excitation threshold.

Benzodiazepines

They are usually classified s tranquillisers, but are very good sedatives when combined with opioids. They lack of significant cardiopulmonary depressant effects. Midazolam and diazepam are excellent muscle relaxants.

Alfa-2 agonists

Their use in postsurgical care is limited due to their important cardiopulmonary depressant effect. Xylazine and medetomidine are good sedatives but poor analgesics. They better exert their analgesic properties when used in combination with opioids.

Propofol

Propofol is used to induce and maintain general anaesthesia. Constant rate infusion can avoid motor responses to somatic painful stimulus and can provide sedation without anaesthesia. It is rapidly metabolised in both the plasma and liver and can be used in eight hound without prolonging recovery. It can have a significant cardiopulmonary depressant effect.

Ketamine

Ketamine is a dissociative anaesthetic that provides good somatic analgesia but poor visceral analgesia. It shouldn't be used postoperatively in patients with poor oxygenation or with high myocardial workload.

Opioids

Morphine and meperidine are good analgesics, but do not produce sedation when used in CRI.

Fentanyl is 100 times more analgesic than morphine. Is highly liposoluble. It shouldn't be used in long term infusion. It can cause muscle contraction.

Buprenorphine is an opioid agonist-antagonist. It has a long onset of action (35-35 min), but a long duration of action (6-8 hours). Is a good analgesic for treating moderate pain. It is an excellent sedative when used in combination with acepromacine.

Route of ADMINISTRATION

Intramuscular, subcutaneous, and intermittent intravenous administration are easy to perform but don't provide a constant sedative and analgesic effect. Intravenous administration is aimed to achieve an optimal effect and a rapid recovery. This can be done keeping a constant plasmatic concentration with a constant rate infusion. Morphine and fentanyl can be used for this purpose. We should first administer a load dose.

Local analgesia is a good option to treat postsurgical pain. We can place an epidural polyethylene catheter if we want to provide long-term post-operative analgesia. This allows administration of analgesics for a long period of time without having to use a spinal needle when completing duration of action. Placing an epidural catheter also allows for administration of the agent at the most efficacious site, even at thoracic or lumbar sites.

Opioids can be administered epiduraly to provide good analgesia at pelvic, abdominal, and even thoracic regions. Epidural opioids reduce stress and provide better results than being administered parenteraly, with less respiratory depression. They have proved to provide good analgesia in acute abdomen, including pancreatitis, and when sedation is not desired. Doses have to be diluted to 0,26 ml/kg. Their onset of action is of 30 to 60 seconds because of their low lipid solubility (table 2).

Intrapleural route of administration is another useful technique, especially with those patients suffering thoracic trauma or after an intrathoracic surgical procedure. We can use an intrapleural catheter (fig.1b) to deliver analgesics constantly (Fig.3). Bupivacaine is used for this purpose and provides 6 hours of analgesia. .

Transdermal patches can also be used (figure 4). Fentanyl transdermal patch (Duragesic®) provides constant analgesia. Its onset of action is after 24 hours and, therefore, requires to be administered as a CRI until then. Serum concentrations return to basal levels after disengaging the patch. The patches release a constant amount of fentanyl per hour. Those patches releasing 25 µg/h are a good options for dogs weighting less than 10 kg and cats, 50 µg/h are adequate for dogs between 10 and 20 kg, 75 µg/h for dogs between 20 and 30 kg, and 100 µg/h for dogs over 30 kg. Patches should be placed over a depilated area of skin and covered with a bandage to avoid accidental ingestion.

Table 1. Opioids used as systemic analgesics in postsurgical care.

Table 2. Opioids used in epidural analgesia in dogs.

Figure 1. Necessary equipment to provide continuous epidural and intrapleural analgesia. 17- to 18-G Tuohi spinal needle (a), Polyethylene radiopaque catheter with metallic stylet. (b), antibacterial filter (c), Y stopcock.

Figure 2. Placing an epidural catheter. After performing epidural puncture with a 17- to 18-G Tuohi spinal needle, a polyethylene catheter is advanced through the needle to the desired position.

Figure 3. Placing an intrapleural catheter. After placing a 17- to 18-G Tuohi needle to the interpleural space, a polyethylene catheter is advanced through it to 3 to 5 cm into the interpleural space.

Figure 4. Transdermal analgesia using fentanyl patches. Mechanism of action, commercial availability, and areas of placement in our patients.

references

1.  Aitkenhead AR. Analgesia and sedation in intensive care. Br J Anaesth. 1989;63:196-206.

2.  Bragg CL. Interpleural analgesia. Heart Lung. 1991;20:30-8.

3.  Chambles CR and Anand KJ. Pain management in the pediatric intensive care unit. Curr Opin Pediatr. 1997;9:246-53.

4.  Crews JC. Epidural opioid analgesia. Crit Care Clin. 1990;6:315-42.

5.  Faggella AM. Managemente of pain in the critically ill patient. Seminars in Veterinary Medicine and Surgery. 1997;12(2):115-121.

6.  Kyles AE, Papich M and Hardie EM. Disposition of transdermally administered fentanyl in dogs. Am J Vet Res. 1996;57:715-719.

7.  Pascoe PJ. Opioid analgesia. Vet Clin North Am. 2000;30(1):757.

8.  Short CE. Pain, analgesics and related medications, In: Short CE. Ed. Principles and practices of veterinary anesthesia, Baltimore. 1987, pp. 28-46.

9.  Skarda RT. Local and regional anesthetic and analgesic techniques: dogs. In: Thurmon JC, Tranquilli WJ and Benson GJ. eds. In: Veterinary anesthesia. Williams & Wilkins, Baltimore. pp. 1996;426-447.

10. Stein C. Opioids in pain control. Basic and clinical aspects. In: Stein C. ed. Cambridge University Press, Cambridge. 1999.

11. Wetmore LA and Glowaski MM. Epidural analgesia in veterinary critical care. Clinical Techniques in Small Animal Practice. 2000;15(3):177-188.