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.

Patient information

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.

Example combinations for premedication (all doses IV)

• Oxymorphone 0.1mg/kg + midazolam 0.1-0.2 mg/kg

• Hydromorphone 0.1mg/kg + midazolam 0.1-0.2 mg/kg

• Fentanyl 3ug/kg + midazolam 0.1-0.2 mg/kg

A safe induction is achieved by careful drug choices and close monitoring during the transition from consciousness to unconsciousness. Preoxygenation is recommended for all critical patients if not too stressful for them. ECG monitoring during induction can be helpful as changes can be detected rapidly. Almost all critical patients should be endotracheally intubated as it allows the anesthetist to provide high inspired oxygen concentrations (FiO2) and to provide positive pressure ventilation (PPV) as needed. It is important to try to minimize any undesirable effects of the induction agent chosen. Propofol is commonly used for induction as it provides a rapid transition to unconsciousness and generally a rapid recovery. However, propofol causes a significant decrease in blood pressure and cardiac output as well as hypoventilation. This is not tolerated well by most critical patients and it is recommended that the amount of propofol used is minimized as much as possible. Ketamine/ midazolam is an induction combination that can be helpful in critical patients as it supports heart rate and blood pressure. However, both of these drugs require hepatic metabolism and will have a prolonged effect in patients with hepatic dysfunction. Ketamine should be avoided in patients with increased intracranial pressure and those with severe cardiac disease. Etomidate is a good choice in patients with cardiac disease as it preserves cardiac output. However, it also causes adrenocortical suppression so should be used with caution in patients with severe systemic disease.

Examples for induction (all doses IV to effect)

     • Propofol 4 mg/kg (w/premedication)

     • Midazolam 0.1mg/kg + propofol 4 mg/kg

     • Ketamine/ midazolam 1cc/20#, 1:1 ratio

     • Fentanyl/ ketamine/ midazolam, 1cc/20# 3:1:1 ratio

     • Etomidate 1.5 mg/kg (w/premedication)

     • Midazolam 0.2mg/kg + etomidate 1.5mg/kg

     • Fentanyl 3ug/kg + Midazolam 0.1-0.3mg/kg

     • Hydromorphone 0.1-0.2mg/kg + midazolam 0.1-0.2mg/kg

The main goal for maintenance of anesthesia is to minimize the detrimental side effects of inhalant anesthetics while providing an adequate level of anesthesia for surgery. Inhalant anesthetics are commonly used for maintenance of anesthesia but they do have some major drawbacks. All of the inhalants used in veterinary medicine today cause a dose dependent decrease in blood pressure and cardiac output. This is generally tolerated poorly by critical patients. Addition of a constant rate infusion can be helpful to maintain an adequate anesthetic depth while lowering inhalant and decreasing the negative side effects of that inhalant.

Examples for maintenance of anesthesia

Isoflurane or sevoflurane ± CRI

     • Fentanyl CRI 0.3-0.7 ug/kg/min

     • Ketamine CRI 0.3-0.6 mg/kg/hr

     • Fentanyl CRI + Lidocaine /Ketamine CRI

     • Fentanyl CRI + Ketamine CRI

Fluid choice

Most patients will receive replacement fluids while under anesthesia to maintain blood pressure and offset losses. Crystalloids such as LRS, 0.9% NaCl, or Plasmalyte A are typically run at a rate of 10ml/kg/hr. If the patient is hypovolemic this rate may be increased or boluses may be given. Colloids are administered to patients requiring oncotic support. A low total protein or albumin would indicate a need for the addition of a colloid. If the patient is coagulopathic they may require fresh frozen plasma, cryoprecipitate or fresh whole blood to provide clotting factors and/or platelets. If the patient is anemic (generally < 20% PCV) packed red blood cells or whole blood should be provided to maintain oxygen delivery to tissues.

Vasopressors or inotropes

Many critical patients require the addition of vasopressors or inotropes to maintain an adequate blood pressure. Dopamine, dobutamine, or phenylephrine are a few possible drug choices to help address hypotension. It is essential that the mean arterial pressure be maintained above 60 mmHg in order to ensure adequate blood flow to vital organs. Having these drugs available and doses calculated out in advance can save time during a critical anesthesia.

Monitoring

Monitoring should start as soon as possible. ECG, SpO2, blood pressure (direct or indirect), and capnography should be used if available. Constant monitoring is essential to identifying potential life-threatening complications. Ideally, an anesthetist should be dedicated solely to the task of monitoring the critical patient. Depth of anesthesia should be carefully assessed by checking jaw tone, eye position, and palpebal reflexes. Pulse quality, mucous membrane color and capillary refill time should be assessed regularly as well. Effort should be made to maintain a normal body temperature as hypothermia has many detrimental effects on the body and commonly occurs during general anesthesia. Additional monitoring may include checking CVP's, arterial blood gases, or electrolytes throughout the surgery.

Most important part of critical anesthesia is to have a plan in place and anticipate problems that may occur. Be ready for possible complications and have whatever drugs or materials may be necessary in order to react quickly during a crisis and improve the outcome of the procedure.