Critical care anesthesia (Proceedings)

Due to the fact that critical patients are more fragile than stabile patients, they require special nursing care. The same is true of these special patients under anesthesia. Using drugs that are reversible and drugs that have a short half life are a couple of specific ways to care for these patients. Unstable patients have a lower minimum alveolar concentration (MAC); therefore, close attention needs to be paid to the level of inhalant with which a patient is maintained.

The inclusion of multimodal anesthesia is helpful to decrease the volumes of particular agents such as inhalants. Multimodal anesthesia is the combination of multiple agents that have different mechanisms of action to achieve ideal analgesia. For example, combining hydromorphone dosed every four hours with ketamine as a constant rate infusion. Hydromorphone works at the mu opioid receptor, and ketamine works as an NMDA receptor antagonist. They work on different parts of the pain pathway.

Work Up

A thorough pre-anesthetic work up is important in order to provide safe anesthesia for all patients. Critical patients especially benefit from an evaluation of both past medical history and current physical status. By working up patients pre-anesthetically, a greater sense of the patient's requirements can be achieved. Key components to a good work up include the following: history, physical exam, current blood work, and a plan for anesthesia.

The history may be as brief as HBC, previously healthy. Sometimes the history is much longer and contains medical conditions or diseases the patient has on top of the presenting complaint. The following could be included in a list of important questions to ask when working up a patient: what happened, when did it happen, how long has this been going on, and does the patient have any significant previous medical conditions. Has the patient received any medications? If so, what, how much, and when?

Current physical exam findings play a large role in the pre-anesthetic work up. What is the current TPR? Is the animal tachycardic? Why? Any arrhythmias? The patient should be auscultated for the presence of a murmur. If the patient is obese, drug calculations could be done based upon what the animal's lean body mass would be. Is the animal bright and alert or dull, recumbent and non-responsive? When touching the animal, does it seem painful somewhere? Was this pain response expected based upon the presenting complaint?

Because critical patients require such special handling, sometimes extra diagnostic testing is necessary before anesthesia. Current blood work for healthy patients usually means at least a big four (BUN/Azo, BG, PCV, TS) within the last month. The extra testing performed on critical patients typically depends upon a number of different criteria, the age of the patient, the current status of the patient, the procedure being performed, the tests that were previously performed, and the time elapsed since the last test. In house lab work is obviously preferred for critical patients in order to get fast results.

Included in an anesthetic plan should be the different anesthetic agents (injectable versus inhalant), and dosages required for premedication, induction, maintenance, and recovery. Additionally, the plan should cover any necessary monitoring equipment, whether a ventilator is indicated, type and rate of intravenous fluid administration, and catheters (intravenous and arterial). Emergency drugs should be calculated before anesthesia begins. When a patient arrests, it is difficult to think quickly and calculate the appropriate volume of drug; if these calculations were done prior to the arrest, it would save time and effort. For situations where an arrest is probable, having emergency drugs drawn up and ready is recommended. Preparation is the key to critical care anesthesia. Drugs and supplies should all be ready before inducing a critical patient.

Induction

Frequently used injectable induction agents include ketamine and diazepam (or midazolam), propofol, etomidate, thiopental, and opioids administered in conjunction with sedatives. Each has pros and cons. Other induction techniques could include mask or chamber inductions. These are not recommended for critical patients due to the stress response they cause, the inability to monitor during the induction (chamber), and the time it takes for the inhalant to wear off (in the event of a crisis).

The combination of ketamine and a benzodiazepine (diazepam or midazolam) adds analgesia and can be cardio-supportive. Ketamine, however, will commonly increase the heart rate so not the best induction agent when a patient is already tachycardic.

Propofol has a very fast distribution time. After administration, propofol has a rapid onset of action and then it is redistributed to the tissues. Clinical effects of propofol are much shorter than the time it takes to clear from the body. Vasodilation and severe respiratory depression are some of the effects seen with large bolus doses of propofol. Propofol contains egg lecithin but does not have preservatives so it can support microbial growth. Extreme care should be used to ensure that the bottle of propofol is not contaminated. Iatrogenic sepsis is thought to be possible if contaminated propofol is administered. Because of this, some people avoid propofol in septic patients when possible.

Etomidate is an induction agent with limited availability. It has no cardiovascular or respiratory effects so it is a very safe choice for patients in heart failure. It inhibits adrenal steroidogenesis in dogs. A single dose may depress adrenal function for up to three hours. Long term administration of etomidate may cause an Addisonian crisis.

Thiopental is a thiobarbiturate with marked cardiovascular effects. Some of the arrhythmias seen after thiopental administration are bigeminy, ventricular tachycardia, and ventricular fibrillation. Elimination of this drug is performed by the body moving the drug to lean body tissues from the brain. From lean body tissues, thiopental is then redistributed to fat tissue. Care should be taken with especially lean dogs without a lot of body fat (greyhounds) because they will have a prolonged recovery. Also, obese animals have difficulty eliminating the drug due to the high lipid solubility. Thiopental is caustic when administered perivascularly and can cause sloughing of tissues. It also causes splenic engorgement which will decrease the peripherally circulating RBCs Thiopental is short acting, but has a longer duration of action than propofol. Thiopental is not ideal for critical patients due to the long list of side effects.

An opioid induction combines a sedative or tranquillizer (midazolam) with an opioid (fentanyl) (the addition of ketamine is also acceptable). This induction technique works best on critically ill or debilitated animals. Other than some occasional bradycardia, the cardiovascular system is relatively unaffected. Sometimes animals will not lose jaw tone or palpebral reflexes, they may even still be swallowing, but they tend to accept the endotracheal tube.

Monitoring during the anesthetic induction may be recommended, depending on the patient's status. If the patient has been having arrhythmias, an ECG should be monitored before and during any anesthetic drug administration. The number of different monitors a patient requires depends on their status and the procedure being performed. At some locations the induction period covers the time before the animal is moved to the surgical suite for surgery. If this is the case, once the patient is under anesthesia whichever monitors were not on the patient already should be attached.

During this induction period, while the patient is being prepped, any additional catheters should be placed. Having more than one intravenous catheter is important in order to appropriately administer more than one different type of fluid and or drugs as necessary. If a blood transfusion becomes necessary, it cannot be administered through the same line as LRS. Some critical patients will require many different infusions such as lidocaine, ketamine, reglan, dopamine, fentanyl, LRS, plasma, and pRBCs. Arterial catheters are desirable for many critical patients. Invasive blood pressure measuring is the primary reason for placing an arterial catheter, but they also allow for blood sampling during a procedure. Patients with significant difficulties under anesthesia sometimes benefit from an arterial blood gas, to evaluate their acid-base status and electrolytes (when available).

Maintenance

Maintaining animals under general anesthesia typically means they will be on an inhalant anesthetic. Since inhalant anesthetics cause significant vasodilation, critical patients especially should be maintained as low as possible. In some cases the addition of a constant rate infusion (CRI) can help to decrease the amount of inhalant necessary. Fentanyl is a great example of an opioid used for CRIs; there are different rates at which this CRI can be run to meet the patient's needs, and boluses can be given on top of the CRI to provide additional analgesia when necessary. There are other constant rate infusions that can help to decrease a patient's MAC such as ketamine, or MLK (morphine, lidocaine, and ketamine).

Some CRIs are not intended to help decrease MAC, but are necessary to help support the cardiovascular function of patients under anesthesia. Positive inotropes, such as dopamine, are typically started when the blood pressure is low (MAP <60mmHg) and not responsive to decreasing inhalant, or fluid therapy.

Fluids should be administered to all critical patients under anesthesia. The type of fluid, amount and rate of administration are largely dependent on the patient and the procedure performed. If the patient has heart disease, the tendency is to choose a lesser rate of fluids to prevent fluid overload. The typically rate of crystalloid (LRS, NaCl) administration for small animals under anesthesia is 10mL/kg/hr, patients with heart disease requiring less fluids are usually run at 5mL/kg/hr. Blood loss should be replaced with either a crystalloid or a colloid. Crystalloid replacement should be about two to three times the volume of blood lost. A shock dose of fluids for a dog is 90mL/kg. If an anesthetist is bolusing fluids and is getting near the shock dose, a recheck PCV and TS should be performed as well as considering switching to a colloid. Colloids (hetastarch, voluven, blood, plasma) are solutions with larger molecules than crystalloids. These larger molecules help to keep or draw water into the vascular space which helps maintain oncotic pressure. These solutions can help to maintain vascular volume for hours to days, which is significantly longer than crystalloids. Reasons for giving colloids are rapid volume expansion (hypovolemia, hemorrhage), and oncotic support (hypoproteinemia, hypoalbuminemia).

Monitoring under anesthesia is vital with critical patients. Watching for trends in the patient's vital signs can allow for the anesthetist to predict when a crisis might happen. Access to monitoring equipment is often limited, but with critical patients it is important to monitor as many different vitals as possible. Even without fancy multiparameter units, monitoring of heart rates and respiratory rates is still possible. Blood pressure monitoring is possible with a Doppler even when oscillometric is not available or it will not work. Capnographs are great for critical patients because they can also help with the patient's vascular status during an arrest, or even immediately before one.

Monitoring of temperature is another important vital because critical patients are sometimes in shock and therefore hypothermic, this may or may not have been corrected prior to anesthesia. Warming of patients under anesthesia is usually necessary. Even febrile patients will frequently become hypothermic under anesthesia. Methods for warming could include forced air warmers (Bair Huggers), heated pads, warm IV fluids, wrapping with blankets or bubble wrap, heating lamps, and warm bags of fluids around the patient or the anesthetic breathing circuit. As patients get colder, their MAC is decreased and the inhalant gas should be as well.

Cardiopulmonary cerebral resuscitation is sometimes necessary under anesthesia. Emergency drugs should be already calculated if not drawn up. Typically during an arrest situation there are several people that can perform different tasks simultaneously. Any inhalant anesthetic and CRIs of anesthetic drugs should be discontinued. Emergency drugs should be administered as needed starting with atropine; chest compressions should begin if necessary (along with defibrillation if available and indicated). Monitoring the patient's capnograph for CO2 production will help determine the adequacy of CPCR. Anesthetic drugs already administered may be hindering the patient's ability to respond to CPCR so reversal of anesthetic drugs should be considered when possible.

Recovery

Monitoring should continue through much of the recovery period, this is still a critical period, especially for critically ill patients. Depending on the patient's needs and the procedure performed, different vitals may need to be monitored continuously past the immediate post-op period. Patients with arrhythmias pre or intra-op should be monitored with continuous ECG when available. Patients with significant blood loss under anesthesia should have a PCV and TS checked at some point during their recovery.

Some patients require analgesia in the recovery period. This should be done carefully in the critically ill patient. Special attention should be paid to what the patient has gotten already in terms of analgesic drugs, and what he will be getting after the recovery period. Giving a large dose of an opioid like hydromorphone is not recommended when the patient will be going on a fentanyl CRI right away. Also if the patient is not yet extubated, giving additional analgesia may prolong the recovery. In fact, some critically ill patients require reversal of some or all of their anesthetic drugs before extubation is possible.