Premedication and Monitoring

II. Drugs

A. Opioids. The opioid class of drugs produce analgesia and sedation without loss of consciousness. Therefore, they are excellent in enhancing patient cooperation and in reducing the discomfort associated with needle insertion or paresthesias. They also possess the desirable feature of easy reversibility with naloxone. Respiratory depression is the main drawback of the opioids, and doses must be individually titrated and the patient monitored appropriately. All of the opioids share the propensity to stimulate the chemoreceptor trigger zone and induce nausea. This is dose related and rarely occurs in the sedative dose range. Some respiratory depression will occur, and pulse oximetry and supplemental oxygen are appropriate.

1. Fentanyl is the most popular opioid sedative because of its rapid onset, short but adequate duration, and easy titratability. It is most appropriate for the ambulatory surgery setting, but is also effective for sedation while performing blocks in an inpatient induction area. Increments of 25 to 50 μg give rapid analgesia for 20 to 30 minutes, usually waning as the block is completed. Dosage is a function of patient vitality, not body size; microgram per kilogram schedules should be avoided.

2. Derivatives of fentanyl are available. Sufentanil is similar, although roughly ten times as potent; suitable dilution is advisable. Alfentanil is similar in effect but shorter in duration and less potent than fentanyl. Although perhaps ideal as an intravenous infusion anesthetic for outpatients, its sedative properties after a bolus injection may be too short to facilitate regional techniques. Remifentanil is even shorter in duration and is suitable only as an infusion. The expense and need of a pump for these two drugs make bolus fentanyl doses more often the drug of choice.

3. Morphine and meperidine are long acting and provide good sedation as well as analgesia, but their longer duration and slower onset make them less useful.

4. Several opioids with both agonist and antagonist properties are intended to reduce the potential for respiratory depression, but they have not been shown to have significant advantages in efficacy or safety.

B. Benzodiazepines are extensively used as premedicants or sedatives. They are effective, centrally acting anxiolytics, and they have the additional potential to produce amnesia. These drugs are effective in the treatment of local anesthetic toxicity and are therefore useful to have in induction areas in the event of a toxic reaction. The amnestic property, especially of the longer-acting lorazepam (1- to 2-mg doses), is advantageous for inpatients desiring to be unaware of procedures. Like morphine, it may require 30 to 60 minutes to reach peak effect, and care must be used when titrating intravenous supplemental sedation during this period of increasing blood levels. Prolonged sedation in the recovery room is seen frequently, particularly in the elderly patient. Because of concern about prolonged postoperative effects, there is less use of the long-acting drugs.

1. Midazolam is the most popular benzodiazepine, and has essentially replaced diazepam for parenteral use because of its predictable dose response and absence of venous irritation. Midazolam is rapid and short acting and produces less respiratory depression than opioids. It is useful for intraoperative sedation once an adequate block has been achieved. It is also useful in producing amnesia for the block itself, although it is not analgesic in this situation and profits from opioid supplementation. The amnestic effect can be a disadvantage by producing unwanted confusion and lack of cooperation in patients if excessive doses are used. Sedation can be long lasting, so the dosage should be kept to a minimum (1–3 mg intravenously, titrated intravenously in 0.5- to 1-mg increments). Its duration of sedation is short with these doses, lasting approximately 30 minutes. The amnestic effect is unpredictable, and occurs at doses lower than those required to produce sedation, which may present a problem in outpatients. Intraoperative observations and postoperative instructions may not be remembered by an apparently alert outpatient. Nevertheless, midazolam is an excellent anxiolytic, and the amnestic effect is useful both during performance of blocks and for intraoperative sedation. Larger doses may prolong recovery, and will also increase the potential for respiratory depression when used in conjunction with opioids.

2. Antagonists. The availability of the specific antagonist drug flumazenil has increased the safety margin of benzodiazepines, but it is more reasonable to shift to a shorter-acting infusion of propofol or dexmedetomidine if prolonged sedation is necessary.

C. Propofol is not only primarily a general anesthetic drug but also an excellent sedative in lower doses. Although it (like the benzodiazepines) does not have analgesic properties, it is not an antianalgesic like the barbiturates. It also lacks the subhypnotic amnestic properties of the benzodiazepines, but it provides more rapid recovery and an antiemetic effect that is beneficial, especially in outpatients.

1. It can be used as a bolus for brief deep sedation during performance of some selected blocks (such as retrobulbar block) where consciousness is not necessary.

2. As an infusion during surgical procedures, it provides anxiolytic, sedative, and amnestic properties, with the best results in the dose range of 30 to 60 μg/kg/min (4). Its rapid recovery, antiemetic effect, and easy titratability make it ideal for sedation in short outpatient procedures.

3. The combination of small doses of midazolam and fentanyl to enhance the performance of a regional block, followed by a propofol infusion for sedation, provides an ideal formula for patient satisfaction and rapid recovery.

D. Barbiturates have basically been replaced by these other two classes of drugs because they are not true analgesics or amnestics; they produce these effects only in doses sufficient to produce unconsciousness. At lower doses, they are actually antianalgesic and may produce exaggerated responses to pain and decreased cooperation.

E. Dexmedetomidine is an α₂ agonist that has been used for sedation in the intensive care unit and for surgical procedures (5). It has the advantage of avoiding respiratory depression, but may produce hemodynamic side effects, particularly bradycardia and hypotension. It appears to potentiate analgesia and reduce inhalation anesthetic requirements. It is administered as an intravenous bolus (1 μg/kg over 10 minutes) followed by an infusion (0.3–0.7 μg/kg/h) because of its short half-life (2 hours). It may be useful as intraoperative sedation with peripheral nerve blocks (infusion of 0.7 μg/kg/h equivalent to propofol 35 μg/kg/min), but its hypotensive effect may make it undesirable in conjunction with neuraxial blockade. It has no amnestic effects.

F. Ketamine has been used in low doses (20–30 mg intravenously) as a sedative during performance of regional blockade because of its analgesic properties in this dose range. Larger doses are associated with hallucinations on emergence. This drug has the advantage of maintaining cardiovascular stability and producing less respiratory depression and obtundation of airway reflexes. It is most useful for sedation during spinal anesthesia for fractured hip repair in the elderly, where the analgesia and cardiovascular support are beneficial.

G. Oxygen. Although not a sedative drug, oxygen is appropriate as a supplemental drug when most of these sedatives and analgesics are used. Opioids particularly produce respiratory depression, and this is potentiated by the addition of benzodiazepines. Oxygen desaturation is frequent, and nasal prongs or a face mask are useful, especially in the elderly (6).

H. Other adjuvants

1. The preoperative visit has been shown to be extremely effective in reducing anxiety in patients.

2. Kind attention to the patient’s concern and situation will reduce the need for any of the preceding medications. Small gestures, such as comfortable positioning of the table and the offer of a warm blanket, are greatly appreciated.

3. Music also has sedative properties. Selected tapes can be provided through a portable cassette player or a piped-in music system. This will not only distract and pacify most patients, but the headphones will also eliminate many of the anxiety-provoking sounds and conversations of the induction room and the operating room.

I. General anesthetic agents are sometimes helpful and occasionally necessary. When upper abdominal surgery is being performed under intercostal, paravertebral, spinal, or epidural block, supplemental general anesthesia with an endotracheal tube is advisable to obtund diaphragmatic sensation, protect the airway, and provide controlled ventilation. The presence of a regional block reduces the minimum alveolar concentration (MAC) of inhalational anesthesia, and when the endotracheal tube is the most significant stimulus the patient may perceive, a fraction of MAC is usually sufficient (7). As mentioned elsewhere, performance of regional blocks in an unconscious adult may increase the risk of nerve injury.

III. Monitoring

A. Intraoperatively, patients undergoing regional anesthesia require the same standards of monitoring as those receiving general anesthesia, including electrocardiogram (ECG), blood pressure device, and a pulse oximeter, as per standard American Society of Anesthesiologists (ASA) guidelines.

B. The patient undergoing regional anesthesia must also be monitored closely for the expected hemodynamic changes of blocks, and especially for the signs of potential systemic toxicity.

1. Specific monitoring to detect increasing blood levels of local anesthetic must focus on the patient’s mental status and therefore requires constant verbal contact. The anesthetist or assistant should engage in conversation with the patient and be alert for the first signs of a change in mental concentration or slurring of speech, especially in the first 20 minutes following injection of a large quantity of local anesthetic.

2. A pulse oximeter is the most frequently applied monitor in an induction area. Providing information about heart rate as well as oxygen saturation, it is useful during regional anesthesia, particularly when sedation may produce respiratory depression. It is also an effective pulse counter for monitoring heart rate changes with an epinephrine-containing test-dose solution.

3. Blood pressure monitoring is essential following spinal, epidural, or sympathetic blocks. An automatic noninvasive device with a short cycling time is ideal because it leaves the anesthesiologist’s hands free to make interventions during the early stages of hypotension. A baseline blood pressure value should be established before performing any blocks that will produce sympathetic blockade.

4. The block level should also be monitored, especially when a sympathectomy is produced. Block level and blood pressure should be measured every 3 to 5 minutes for the first 15 minutes following the block to warn of unexpected high levels. Block level should be monitored during the course of epidural or spinal anesthesia, because both of these techniques may demonstrate a change of level over the first hour.

5. A final note. All blocks involving significant quantities of local anesthetic must be administered in a location that provides immediate resuscitation equipment.