Pharmacology

When performing IVCS, one must never forget the primary goal of providing patient comfort without compromising cardiopulmonary function or the patient's ability to react purposely to verbal commands and physical stimuli. When it is anticipated that required sedation will lead to loss of protective airway reflexes, such patients require a greater level of care than exists with IVCS. Deep sedation is a complication of IVCS and must be avoided. In deep sedation, one creates a state of depressed consciousness from which the patient is not easily aroused, accompanied by a partial or complete loss of protective reflexes, including the ability to maintain a patent airway independently and respond purposely to physical stimuli or verbal commands. In keeping this goal in mind, understanding those situations in which patients are at increased risk should be emphasized. In general, the elderly show increased sensitivity to the drugs used for IVCS, so the dose and frequency of administration should be reduced. In addition, patients with COPD appear to be more sensitive to the respiratory depressant effects of narcotics and benzodiazepines, especially when used in combination. Patients with low serum albumin concentrations show increased sensitivity to drugs that are highly protein bound such as thiopental because more free drug is available for therapeutic effect. To avoid hypotention, caution should be exercised in patients with poor left ventricular function or borderline volume status before the administration of IVCS. Understanding the metabolism and excretion of the agents used for IVCS is critical to avoid oversedation. Drugs such as diazepam, morphine, meperidine, and fentanyl have active metabolites, so the potential for drug accumulation and prolonged effect certainly exists. Patients with renal disease are particularly susceptible to CNS toxicity from normeperidine because of the accumulation of the active metabolite. Drugs like fentanyl, although short acting, have prolonged activity as a result of seepage of stored drug back into the systemic circulation. In contrast, thiopental is metabolized to water-soluble inactive metabolites. Careful titration to effect with dosage adjustments will avoid unnecessary oversedation with resultant respiratory and cardiovascular complications. Time should elapse between repeat doses to allow peak effects to occur. In addition, potential drug interactions that can prolong the effects should be recognized. Examples of the latter are the interaction between cimetidine and diazepam or the protease inhibitors and the benzodiazepines, in which the potential exists for excessive and prolonged sedation. The use of the narcotic antagonist naloxone and the benzodiazepine antagonist flumazenil should be scrutinized because they should be reserved for the unusual situation in which excessive cardiopulmonary depression occurs. Maintenance of a patent airway and stable cardiovascular function in a patient who can respond to verbal commands and physical stimuli is the primary goal of IVCS. With the agents discussed in this chapter, this goal is easily obtained, keeping the principles just mentioned in mind with all the appropriate monitoring guidelines discussed elsewhere in this text.