Anesthesia of the critical patient (Proceedings)
The main considerations for anesthesia of the critical patient really are the same as for most other patients. Blood pressure and cardiac output must be maintained, oxygenation of tissues must be maximized, temperature must be regulated, ventilation must be assisted as needed, and an appropriate level of analgesia and anesthesia must be provided. The main difference with anesthesia of the critical patient is that great care must be taken to prevent abrupt or radical changes in the patient's physiologic parameters. Changes that occur during anesthesia can be significantly more detrimental as the critical patient has a drastically decreased ability to compensate for these changes.
The patient's signalment is the first piece of pertinent information. Species, breed, and age must be taken in account. Pediatric, neonatal and geriatric patients will have special needs compared to the previously normal adult patient. The TPR is the next step. Hyperthermia or hypothermia may indicate sepsis or septic shock as can tachycardia and/ or increased respiratory rate or effort. Recent blood work should be evaluated. Packed cell volume, total protein, BUN, creatinine, electrolytes, lactate, and blood pH should be evaluated. A complete blood count will tell the anesthetist if the patient is anemic or thrombocytopenic. If a CBC cannot be performed, a PCV should be checked as well as a manual platelet count from a blood smear. A blood chemistry can give the anesthetist information about hepatic and renal function. Both the liver and kidneys are important to drug metabolism and excretion. Elevations in ALT, alk phos, and GGT may indicate liver dysfunction. Even if these values are relatively normal, liver disease may still be present. A low BUN, albumin, total protein, and blood glucose or a prolongation of coagulation times (PT, PTT) all can signal the inability of the liver to function properly. An elevated BUN, creatinine, and/or elevated K+ can indicate that the kidneys are not functioning and that drugs that require elimination through the kidneys must be avoided.The key to providing safe anesthesia for the critical patient is to provide a balanced anesthetic plan. The first step is to decide on a premedication protocol. Most patients require some kind of premedication, even those with critical illness. The goal of premedication is to provide pre-emptive analgesia, to decrease stress and anxiety in the pre-operative patient, and to decrease the amount of induction drug needed to produce unconsciousness. Some patients are so obtunded that premedication may be unnecessary. However, using drug combinations judiciously can be helpful in decreasing the amount of individual drugs needed and therefore decreasing the risk of side effects from excessive administration of individual drugs.
A full agonist opioid such as fentanyl, hydromorphone, or oxymorphone is most often recommended for analgesia over a partial agonist such as buprenorphine in a critical patient. The reason for this is that buprenorphine binds very tightly to its receptors and can be extremely difficult to reverse once administered. If a crisis were to occur under anesthesia, a full agonist can be reversed easily with naloxone while buprenorphine generally cannot be. It is common to combine an opioid with a benzodiazepine (diazepam, midazolam) as part of the premedication. Generally, acepromazine should be avoided in critical patients undergoing anesthesia. While an excellent tranquilizer, acepromazine has some undesirable side effects. Hypotension, hypothermia, splenic enlargement, and lack of reversibility make this drug unsuitable for critical patients undergoing anesthesia.