Most critically ill veterinary patients will require anesthesia to perform diagnostic and/or therapeutic procedures at some point during their hospitalization.
Anesthesia - A reversible, controlled, drug-induced intoxication of the central nervous system in which the patient neither perceives nor recalls noxious or painful stimuli.
Critical - Being or relating to an illness or condition involving danger of death.
Most critically ill veterinary patients will require anesthesia to perform diagnostic and/or therapeutic procedures at some point during their hospitalization. Due to the patient's physical status, careful assessment and planning are needed to assure a safe course of anesthesia. Anesthesia is not without complications even under the best of circumstances; problems will arise. While not every patient is a critical case, physical status can rapidly change during anesthesia resulting in a patient that needs critical management. All patients that are to be anesthetized should be assessed and stabilized, when needed, prior to anesthesia. This principle is paramount in the anesthesia management of the critically ill patient.
Initial assessment is focused on the major body systems that are most often adversely affected during anesthesia: respiratory, cardiovascular, neurological and urogenital. The critically ill patient's basic requirements from the cardiopulmonary system are the same as the non-critical patient; the end-point is tissue perfusion that will facilitate oxygen delivery and carbon dioxide extraction. The ability of the patient to meet this basic need is the focus for the initial assessment of the patient. All anesthetic drugs can have negative effects on cardiopulmonary performance. The critically ill patient's physiology often is stressed; compensatory mechanisms may be unable to adequately respond and maintain homeostasis once anesthesia drugs are administered. Unstable patients usually deteriorate further with the addition of anesthetic drugs. The goal of proper stabilization prior to anesthesia is to produce a patient that is more likely to tolerate the anesthesia/operative period without substantial decompensation. Care must be taken to avoid rushing an unstable patient into an anesthesia and/or operative procedure.
Respiratory pattern and effort should be evaluated. Auscultation of the chest will offer information about both cardiac and respiratory function. Cardiac murmurs, arrhythmias or dull heart and lung sounds as well as pulmonary crackles or wheezes may be indications of abnormalities that, with the induction of anesthesia, could evolve into life threatening complications. If there is any doubt about cardiopulmonary function, further evaluation should be pursued. Oxygen carrying capacity should be included in the assessment. The patient must have adequate hemoglobin to carry enough oxygen to meet tissue needs. Respiratory abnormalities seldom improve and will most likely worsen with the administration of anesthesia drugs. Therefore, abnormalities that will affect respiration should be normalized as much as possible prior to anesthesia induction.
The cardiovascular system assessment will provide the anesthetist with information used to anticipate the patient's ability to adequately perfuse tissues. Mucous membrane color, capillary refill time, pulse rate, rhythm and quality coupled with the auscultation of the chest and measuring blood pressure will provide a baseline status of the cardiovascular system. The negative effects of anesthetic drugs are more likely to significantly depress the cardiovascular system in critically ill patients, which will further reduce the system's ability to perfuse tissues and meet oxygen delivery demands. Prior to induction of anesthesia, intravenous catheterization and fluid or blood products administration coupled with oxygen administration may be required to initially stabilize the cardiopulmonary system and assure adequate oxygen delivery.
Assessment of the neurological system is important for the critically ill patient. Anesthesia is the reversible depression of the central nervous system (CNS) and the anesthetist relies on the evaluation of this system to assess depth of anesthesia through reflexes and muscle tone. Decreased mental status and/or the inability to ambulate may indicate significant CNS problems. The development of a safe and effective anesthesia plan requires the anesthetist to know the baseline status of the patient's CNS prior to the administration of anesthesia drugs. The administration of various anesthetic drugs to a patient can result in significant changes in intracranial pressure that may exacerbate the effects of head trauma or brain disease. The patient with spinal injury may require additional musculoskeletal support from a backboard or other device to avoid further damage to the spine during or after anesthesia induction when the muscles have relaxed and no longer support the skeletal system.
For all patients anesthesia can affect renal function either through decreased glomerular filtration or decreased renal blood flow. Renal function can have an effect on the response to anesthetic agents. Azotemia can affect the CNS sensitivity to anesthetic agents. Renal insufficiency can affect acid-base status of the patient, resulting in a concurrent increase in serum potassium as well as a decrease in drug dosage required to produce anesthesia. Hyperkalemia is a possibility in patients with renal insufficiency, ruptured urinary bladder or urethral obstruction. Patients with serum potassium levels greater then 5.5 mEq/L should not be anesthetized until potassium levels can be reduced. If the patient already has some degree of renal disease and experiences hypotension and/or hypovolemia during anesthesia, the likelihood of exacerbating the renal disease is increased. Pre-anesthesia blood work should be evaluated to assess liver function, renal function, acid base status and electrolytes to establish a baseline.
Critically ill patients that are candidates for general anesthesia are often much more fragile than healthy patients. Their physiological reserves and compensatory mechanisms can be reduced resulting in a less stable patient that is more prone to complications and possibly unable to respond to the stresses added by anesthesia and/or an operative procedure. For any patient, anesthesia has the potential for deleterious effects, but the critically ill patient presents an increased risk of complications due to their often diminished reserves and propensity for rapid changes. Neural and hormonal responses will preserve circulation to the essential organs, but this response may make the patient more susceptible to the adverse vasoactive effects of most anesthetic drugs. Hypotension, hypovolemia, hypercapnia, hypoxia, hypothermia and pain are just a few of the potential problems for these patients.
The development of contingency plans to avoid or treat each anticipated problem will help to expedite the anesthetist's intervention when complications are detected, thereby improving the chances of a safe and uneventful course of anesthesia. Needed equipment and drugs should be gathered, organized, and made readily accessible prior to the induction of anesthesia.
The selection of drugs used to provide anesthesia care for the critically ill patient requires the anesthetist to have a good working knowledge of the risks generated by the anesthesia as well as those complications associated with the procedure to be performed on the patient. The anesthetist should be familiar with these operative risks, as well as the resources available to either avoid complications or intervene when complications arise. It is critical to understand the physiologic and pharmacologic effects of the anesthetic drugs to be used. New drug protocols or unfamiliar drugs should not be used on critical patients. A multi-modal approach to anesthesia and pain management in the critical patient is warranted due to the patient's sometimes-precarious physical status. The use of preemptive analgesics and balanced anesthesia techniques can provide very effective analgesia and often reduce or avoid many of the negative physiological changes associated with anesthesia.
Please see "Anesthesia Drugs For The Small Animal Patient" for more information on the various anesthetic drugs discussed in the presentation.
Adequate anesthesia and analgesia that produces the least detrimental effects on the cardiopulmonary system is only one goal of the anesthesia drug plan. The use of other support drugs such as crystalloids, colloids, and sympathomimetic drugs can aid the anesthetist in the support of the patient and counteract many of the harmful effects that anesthesia drugs can promote. The ultimate goal of the anesthesia drug plan is to maintain physiological homeostasis or a "normal stable" physiological environment while rendering the patient immobilized, unconscious, and pain free. The safest anesthesia drugs are often the ones you are the most familiar with; anesthetizing a critical patient is not the time to try new drugs and administration techniques. Anesthesia must be a reversible process, at the end of the procedure, anesthesia support will be withdrawn and the patient must be able to recover and support itself. Post procedural analgesia must be part of the recovery plan to assure patient comfort and support. In addition, care must be taken not to withdraw support too quickly and endanger the patient; rather we must realize the patient needs some time to adjust to many of the physiological changes that are taking place during the recovery period. The nursing team must provide support for the patient until it can adequately support itself.
The purpose of patient monitoring is to provide information that can be used to facilitate prudent decisions by the anesthetist. The focus of patient monitoring is to maximize patient safety and support. Monitoring should be designed to allow the anesthetist to recognize problems at the earliest stages and then provide a therapeutic response before small problems can evolve into an anesthesia crisis. Due to the often fragile nature of these patients, this point is especially important. How will you know if your patient is adequately supported during anesthesia?
When designing the monitoring plan, a systemic approach to monitoring is an effective design. Monitoring of the cardiopulmonary system is a priority since most anesthesia drugs produce significant depression of various components of the system. The anesthetist must be able to monitor performance and provide support as needed, to assure tissue perfusion and oxygen delivery demands are met. For the critically ill patient, continuous monitoring systems often are preferred over intermittent monitoring devices due to the increased likelihood of complications and potential for sudden changes. For this reason monitors such as pulse oximeter, doppler flow detectors, and direct arterial blood pressure transducers connected to continuous wave ECG monitors are favored over the monitors that give intermittent readings every few minutes. The continuous monitors show "beat to beat" cardiopulmonary performance as opposed to "snapshot" data readings from the intermittent monitors, where the information is not necessarily current. It is important to remember that each monitoring parameter has its specific meaning and value in time, but the true significance is better appreciated when correlating the measurement with the trend of previous measurements, the patient's recent history, and by looking at the patient as a whole.
Cardiovascular parameters that will assist the anesthetist include: mucous membrane color, CRT, heart rate and rhythm, pulse quality and arterial blood pressure as well as hematocrit and total solids. Urine production is often used as an indicator of renal perfusion. Extremity temperature is also an indicator of peripheral perfusion. If the core body temperature is close to normal but the extremities are cold to the touch, they are most likely not well perfused.
The other component of the cardiopulmonary system that requires the anesthetist's attention is the pulmonary system. Respiratory monitoring is more then just respiratory rate, effort and estimated volume. Critical patients frequently experience difficulty adequately delivering oxygen to tissues. Respiratory parameters to monitor include: respiratory rate and effort, estimated or measured tidal volumes, end-tidal carbon dioxide, percent arterial hemoglobin oxygen saturation (pulse oximetry), and blood gas analysis. Laboratory blood analysis during the course of anesthesia could also include hematocrit, hemoglobin content, blood glucose, electrolytes, colloid-oncotic pressure, and lactate threshold.
Anesthetists spend much of their time in direct contact with the patient. We use our senses to evaluate the patient and analyze the data we perceive through sight, sound and touch. We can augment that information with the addition of electrical and mechanical equipment to give the anesthetist additional information. When we use additional devices to assist our monitoring we must also ensure the information provided is accurate. In order to do this, we must ensure the equipment is dependable and in good working order. The anesthetist must also have a clear understanding of both the equipment and the significance of the information provided by the equipment. The additional information provided by the monitors is intended to make the anesthetist's job easier and provide a more complete picture of the patient's physiological status. If the data provided by the monitoring equipment is unreliable then it only serves to misinform, distract and confuse the anesthetist. When evaluating the patient, it is important to look at the whole and not just one or two parameters. This means the anesthetist must not only focus on the monitors attached to the patient, but we must also focus on the patient itself and evaluate all the data we have available. These points are true for any patient during the anesthesia period, but they are especially significant for the critically ill patient during anesthesia.
The critically ill patient that presents to veterinary clinics for treatment frequently requires anesthesia to facilitate their therapy. These patients often are experiencing pain and can suffer from various complications generated by trauma or their co-existing diseases. It is not only appropriate to provide pain therapy and stabilize these patients prior to the induction of anesthesia, but it is our obligation to optimize the care we provide to the patient. The stabilized patient is more likely to experience a smoother course of anesthesia and better tolerate the challenges of anesthesia and the operative procedure. A safe anesthesia plan for critically ill patients should be developed utilizing anesthetic drug protocols that will minimize the hemodynamic challenges and maximize patient support and comfort, as well as a monitoring plan that will alert the anesthetist to danger. One of the most important members of the patient care team for the critical patient is the anesthetist. The knowledge, skills, and diligence of the experienced anesthetist will provide the necessary care and support to significantly reduce morbidity and mortality for the critical patient.