2.2. Anaesthetic methods for ambulatory knee surgery
Outpatient knee surgery requires substantial modifications of the traditional inpatient anaesthetic practices. Anaesthesia should be specifically tailored for ambulatory surgery, and the anaesthetic drugs must have consistent onset and offset times, permitting rapid changes in the levels of drug effect (Philip 1997). Side effects that are tolerated in an inpatient context, such as nausea, vomiting, and pain, are unacceptable in an outpatient setting, where these complications may delay discharge or even cause unanticipated overnight admissions (Meridy 1982, Gold et al. 1989). Vomiting and pain have been documented as the major anaesthetic causes of overnight admissions in all of the large series in both America and Europe (Green & Jonsson 1993). These side effects are considered by both patients and anaesthesiologists alike as the ones most desirable to avoid (Marcario et al. 1999). Prevention of postoperative pain, nausea and vomiting is critical to successful implementation of a fast-tracking program in an ambulatory setting (White & Song 1999).
Patients’ cooperation is essential at all stages of the ambulatory surgical process, from preparation to recovery at home. Patients’ expectations about what will happen during their ambulatory surgical experience must be appropriate, to make them satisfied with their care (Philip 1992). Recent changes in fasting policies (ASA 1999) have allowed patients to continue chronic medications and to avoid the uncomfortable symptoms of dehydration. The use of small doses of sedative-anxiolytic drugs as premedication has been shown to improve the perioperative experience of patients without any adverse effects on the recovery process (Van Vlymen et al. 1999).
Anaesthetic techniques that optimize the intraoperative surgical conditions while providing for rapid, early recovery have assumed increased importance. The introduction of more rapid and shorter-acting volatile anaesthetics (desflurane and sevoflurane), intravenous anaesthetics (propofol), opioid analgesics (remifentanil) and muscle relaxants (mivacuronium, rapacuronium) has allowed practitioners to achieve more consistently a recovery profile that facilitates fast-tracking after the administration of general anaesthesia (Savarese et al. 1988, Song et al. 1998, 1999). The use of nonsteroidal anti-inflammatory drugs (NSAID) (Souter et al. 1994) and local anaesthetics has become increasingly important in controlling pain during and after ambulatory surgery (White 2000).
2.2.1. Local anaesthesia
Lower extremity surgery can be performed with peripheral nerve blocade. When a thigh tourniquet is not used, the procedure can be safely done under infiltrative local anaesthesia of the knee cavity and through the ports used to introduce the instruments (Kelly et al. 1999). Patel et al. (1986) used femoral nerve block for knee arthroscopy, which allowed earlier discharge and improved postoperative analgesia. The choice of anaesthesia in routine knee arthroscopy varies considerably. The concerns about local anaesthesia include the fear that it will take longer to perform surgery and that the anaesthesia will be inadequate, leading to patient discomfort. Forssblad and Weidenhielm (1999) showed data from patients (n = 6519) who had undergone knee arthroscopy under local anaesthesia (n = 4101) and general anaesthesia (n = 2418). Only 0.9% of the arthroscopies performed under local anaesthesia could not be performed safely due to patient discomfort.
There are many studies to support the conclusion that knee arthroscopy under local anaesthesia can be considered a reliable, well tolerated and safe alternative to conventional procedures (Butterworth et al. 1990, Iossifidis 1996, Lorentsen et al. 1997, Ramanathan 1998).
Lintner et al. studied retrospectively 256 outpatient knee arthroscopies (local and general anaesthesias) and prospectively 100 knee arthroscopies performed using local anaesthesia. They compared the local and general anesthesias in terms of efficacy, cost-effectiveness and safety. The data showed that the use of local anaesthesia for outpatient knee arthroscopy is safe, effective, well accepted and cost-effective compared to general anaesthesia. (Lintner et al. 1996). When a combined sciatic-femoral nerve block with 25 ml of 2% mepivacaine was used, a slightly longer preoperative time was needed, but similarly effective anaesthesias with no differences in home discharge times were seen when comparing this block to spinal anaesthesia with 8 mg of hyperbaric bupivacaine (Casati et al. 2000).
2.2.2. Spinal anaesthesia
The first operation under spinal anaesthesia was performed in 1898, when the German surgeon August Bier administered cocaine intrathecally for surgical anaesthesia (Bier 1899). At that time, large-diameter spinal needles were commonly used, which frequently resulted in postdural puncture headache (PDPH) (Gielen 1989). In the years to follow, the technique declined in popularity, but resurged with the introduction of small-diameter spinal needles. The relatively low risk of PDPH with these needles (Pittoni et al. 1995, Corbey et al. 1997, Lambert et al. 1997, Despond et al. 1998, Spencer 1998, Flaatten et al. 2000) has contributed to the increasing popularity of spinal anaesthesia in the ambulatory setting (Halpern & Preston 1994, Kuusniemi 2001).
The advantages of spinal anaesthesia for ambulatory surgery include ease of administration, rapid onset and high reliability (Standl et al. 1996, Alon et al. 2000). The anaesthetised area can be limited to the surgical site (Kuusniemi et al. 2000), the common side effects of general anaesthesia (nausea, vomiting, drowsiness) are reduced, the risks of general anaesthesia (difficult intubation, pulmonary aspiration, malignant hyperthermia) are minimised, and improved analgesia is provided in the postoperative period (Allen et al. 1993). The benefits of spinal anaesthesia are most evident in the postoperative phase. The residual block protects the patient from initial pain after the block has worn out (Dahl et al. 1997, Raeder 1999). Dahl et al. explained that alleviation of the initial, severe postoperative pain results in lesser activation of the pain-enhancing mechanisms in the medullary cord, thus preventing the amplification of pain usually seen when pain is inappropriately treated (Dahl et al. 1997). Spinal anaesthesia is associated with a lower incidence of postoperative nausea and vomiting (PONV), drowsiness and postoperative pain compared to general anaesthesia (Dahl et al. 1990, Mulroy & Willis 1995, Standl et al. 1996). These symptoms are the most frequently reported causes for delays in discharge time among ambulatory patients (Pavlin et al. 1998).
Although spinal anaesthesia is considered a simple procedure with a high margin of safety, it is not entirely free from risks. The severe neurological complications associated with spinal anaesthesia and other central blocks may be due to the neurotoxic effects of local anaesthetics, direct neural tissue injury caused by a needle or catheter and spinal cord compression by an epidural haematoma or abscess (Alahuhta 2001). In a retrospective review by Horlocker et al. (1997), one disc space infection and one paraspinal abscess were found, but complete neurologic recovery was demonstrated in both patients. In a follow-up of 18,000 consecutive central blocks, 20 neurological complications related to regional anaesthesia were found (Dahlgren & Törnebrandt 1995). In France, 103,730 regional anaesthesias were studied (Auroy et al. 1997). The incidence of severe, anaesthesia-related complications was found to be very rare, less than 0.1%. There were 24 neurological deficits among the approximately 40,000 spinal anaesthesias, 12 of which were associated with trauma evidenced by either paresthesia or pain on injection. In Finland, the incidence of serious complications following spinal anaesthesia was 0.45:10,000 (Aromaa et al. 1997). Although major complications are rare, they can be devastating to the patient and the anaesthesiologist. For this reason, the patients must be postoperatively followed closely to detect potentially treatable sources of neurologic injury (Horlocker & Wedel 2000).
In recent years, the popularity of spinal anaesthesia has been growing among the outpatient population. There have been attempts to find targeted spinal anaesthesias for outpatients (Kuusniemi 2001, Valanne et al. 2001), where the side effects are minimal and the components of ideal spinal anaesthesia maximal. These goals can be approached with a right choice of local anaesthetic and the use of adjuncts to augment spinal anaesthesia (Liu 1997).
2.2.2.1. Lidocaine
Spinal lidocaine has been a popular choice for ambulatory spinal anaesthesia since its introduction in 1945. After that, more than 100 million patients have been operated under lidocaine spinal anaesthesia (Van Zundert 1999). Lidocaine has been popular because of the rapid repression of the sensory and motor blockade (Atanassoff 2001). Though lidocaine has enjoyed a long history of safety and popularity, it has recently come under scrutiny because of transient neurologic symptoms (TNS), which were first described by Schneider et al.1993. They reported four patients who, after uneventful spinal anaesthesia with hyperbaric 5% lidocaine, developed a triad of symptoms including low back pain and dysaesthesia with radiation to the buttocks, thighs and lower limbs 1–20 hours after recovery from spinal anaesthesia. The pain was described as dull and aching, and it occasionally decreased when the patient stood up and walked around. It responded well to NSAIDs and resolved spontaneously within two to five days. There were no sensory, motor or reflex disturbances, nor bladder or bowel dysfunctions.
In the recent years, TNS has been shown to occur after all spinal anaesthetisias, but the incidence seems to be significantly higher after lidocaine (Hampl et al. 1999). The incidence of TNS after lidocaine spinal anaesthesia has been reported to range from 0% to 40% (van Zundert 1999). A reduction in lidocaine concentration does not seem to decrease the risk (Pollock et al. 1999), a case report of TNS even after spinal anaesthesia with 1% plain lidocaine has been described (Henderson et al. 1998). The other factors that have been suggested to increase the risk for TNS include the addition of adrenaline and phenylephrine, the lithotomy position and outpatient status (Hampl et al. 1999). On the contrary, Lindh et al. did not identify early ambulation after spinal anaesthesia with 2% hyperbaric lidocaine as a risk factor (Lindh et al. 2001).
The aetiology of TNS is still speculative. Hampl and co-workers described several cases similar to those reported by Schneider et al. in various clinical studies (Hampl et al. 1995, 1996), but actual neurologic symptoms were never described. Wong and Slavenas found no cases of TNS among 67 obstetric patients after spinal anaesthesia with 5% lidocaine (Wong & Slavenas 1999). Hiller and Rosenberg reported a 30% incidence of TNS after spinal anaesthesia with 4% mepivacaine (Hiller & Rosenberg 1997), while Liguori et al. found no cases of TNS after spinal anaesthesia with 1.5% mepivacaine for knee arthroscopy, but a 22% incidence after 2% lidocaine (Liguori et al. 1998). Drasner pointed out that 1.5 and 5% lidocaine produce equally effective spinal anaesthesias, and the risk of neurotoxic injury can be minimised by reducing dose and concentration, although such modifications do not seem to affect the incidence of TNS (Drasner 1998).
The recommendations to reduce the risk of neurologic symptoms after spinal lidocaine include (Alahuhta 2001):
Use of the lowest effective concentration and dose. For short-acting spinal lidocaine, the maximum dose is 60 mg at a concentration not higher than 2%.
Adrenaline should be avoided as an adjuvant.
Avoidance of spinal lidocaine among patients positioned with the knees or hips flexed.
2.2.2.2. Bupivacaine
Bupivacaine has been in clinical use since 1963 (Savarese & Covino 1986). It has been classified as an agent of high anaesthetic potency and long duration of action. Interest in small doses of subarachnoid bupivacaine for spinal anaesthesia in ambulatory patients has arisen due to complaints of radiating backache after spinal lidocaine. Many studies indicate that bupivacaine 0.5% also causes TNS, but less often than lidocaine (Hampl et al. 1995, Pollock et al. 1996, Freedman et al. 1998, Kokki et al. 1998a, Kokki et al. 2000a, Kuusniemi 2001). Few findings exist about the minimal effective doses of bupivacaine for ambulatory anaesthesia. The majority of studies have used relatively large doses (7.5–20 mg) and have not examined the anaesthetic recovery profiles quantitatively. Recent dose-response data on the clinical anaesthetic characteristics of spinal bupivacaine indicate that small doses can be used for ambulatory anaesthesia (Ben-David et al. 1996a, Liu et al. 1996, Tarkkila et al. 1997, Kuusniemi 2001, Valanne et al. 2001). Small doses of bupivacaine (< 10 mg) should be used in ambulatory anaesthesia to avoid prolonged detrusor block, inability to void and excessively prolonged time until discharge (Kamphuis et al. 1998). Hyperbaric bupivacaine in doses of 6–8 mg has also been found to be a suitable alternative to spinal lidocaine for surgical procedures with a mean duration of about one hour (Gentili et al. 1997).
2.2.2.3. Mepivacaine
The clinical anaesthetic characteristics of mepivacaine are similar to those of lidocaine (Zayas et al. 1999). In an investigation by Liguori et al. 1998, 60 patients undergoing knee arthroscopy received either 45 mg of 1.5% mepivacaine or 60 mg of 2% lidocaine through a 27 G Whitacre needle. There was no difference between the two local anaesthetics with respect to recovery from the sensory or motor block or the discharge criteria. There was a difference in the incidence of TNS with no spinal headache in the patients receiving mepivacaine in contrast to 22% of those with lidocaine anaesthesia.
2.2.2.4. Ropivacaine
Ropivacaine is a new amide local anaesthetic, which was approved in Europe about 15 years ago. It is less lipid-soluble than bupivacaine and is reported to be 20% less potent than bupivacaine at equal doses (Polley et al. 1998). Ropivacaine produces less motor blockade and is of shorter duration than bupivacaine (Scott et al. 1995, Markham & Faulds 1996, Zaric et al. 1996). The decreased potency of ropivacaine offers a potential for more rapid recovery and is better suited to be used as an outpatient spinal anaesthetic. However, dose-response data have indicated that equipotent doses of ropivacaine will have similar recovery times as bupivacaine (McDonald et al. 1999, Gautier et al. 1999) with no signs of TNS (Gautier et al. 1999). Ropivacaine in equipotent doses has been shown to be virtually indistinguishable from bupivacaine for clinical anaesthesia without any obvious advantages (Atanassoff 2001).
2.2.3. Epidural anaesthesia
Epidural anaesthesia is not so frequently used for day-case anaesthesia as spinal anaesthesia (Raeder & Korttila 1996). Some patients may fear postspinal headache after a negative previous experience with spinal anaesthesia. Some female patients may have good experiences of epidural anaesthesia as a method of labour pain relief and request the same method again. Sometimes surgical procedures may have an unpredictably long duration and an extra dose of local anaesthetic is needed through an epidural catheter (Knize & Fishell 1997). Drugs of long duration are not appropriate, because ambulatory surgical procedures usually take less than 1–2 hours. Prolonged epidural anaesthesia may result in prolonged postoperative bed occupancy, delayed discharge and an increased risk of urinary retention (Raeder & Korttila 1996). In adults, lidocaine seems to be the drug of choice for epidural anaesthesia in day cases (Kopacz & Mulroy 1990, Seeberger et al. 1994). Due to the need of prolonged postoperative surveillance, the use of opioids in the epidural mixture is usually not recommended in day-surgery, but some authors have recently advocated the use of opioids other than morphine for this indication (eg. alfentanil, fentanyl, sufentanil) in order to speed up onset, to improve the quality of the block and to improve postoperative analgesia (Kwa et al. 1995). There are recommendations to use epidural anaesthesia for day-case surgery in the lower half of the body if the surgery is expected to be last for more than 2 hours, if the extent of surgery is very unpredictable or if there is a special request from the patient (Raeder 1999).
2.2.4. General anaesthesia
Since the first application of diethyl ether in 1846 (Kennedy & Longnecker 1990), numerous agents have been investigated as potential clinical inhalable general anaesthetics and abandoned for diverse reasons, including adverse effects and high flammability (Robbins 1946, Vitcha 1971, Wallin et al. 1972, Calverly 1986). The availability and clinical introduction in 1956 of the first nonflammable agent, halothane (Bryce-Smith & O"Brien 1956, Johnstone 1956), revolutionised inhalation anaesthesia. Further work, modelled on halothane, led to the development of a new generation of inhalation anaesthetic agents (enflurane, isoflurane, desflurane and sevoflurane) in the quest for an ideal agent conferring the following key properties (Jones 1990, Marshall & Longnecker 1990):
Rapid and tolerable induction
Rapid recovery
Rapid adjustment of the depth of anaesthesia
Adequate skeletal muscle relaxation
Wide margin between the concentrations producing the desired pharmacological effect and those producing toxicity
Absence of toxic effects or other adverse events at normal doses
The use of drugs intravenously to facilitate the production of general anaesthesia started during the late nineteenth century, when morphine was sometimes used to supplement inhaled anaesthetics (Way & Trevor 1986). In the early years of the twentieth century, barbiturates were discovered (Weese 1933). Thiopental was first used in anaesthesia almost 70 years ago by Waters and colleagues (Pratt et al. 1936) and soon after that by Lundy (Lundy 1935). Since that time, thiopental has been established as an intravenous anaesthetic drug against which all the more recently introduced drugs (e.g. propofol) are compared.
Concerns regarding the side effects of succinylcholine (Smith et al. 1993) and the neuromuscular reversal drugs (Ding et al. 1994, Watcha et al. 1995) have increased interest in the use of more rapid and shorter-acting nondepolarizing neuromuscular blocking drugs in the ambulatory setting. The availability of mivacurium and rocuronium provide anaesthesiologists with alternatives to succinylcholine for outpatient anaesthesia (Tang et al. 1996).
Although various forms of anaesthesia are used for ambulatory knee surgery, general anaesthesia has remained as a popular method for many of these operations. Both surgeons and patients prefer general anaesthesia (Fairclough et al. 1990), and the recent advances in inhalational and intravenous methods of anaesthesia induction have made general anaesthesia safer and more predictable. Duncan and colleagues (Duncan et al. 1992) evaluated 6914 adult ambulatory surgery patients and reported that only 8% of all outpatients experienced a postanaesthesia care unit (PACU) complication. Of the complications that were reported, respiratory and circulatory complications accounted for only 0.4% and 0.3%, respectively. In this outcome study, the presence of preexisting underlying disease was the most important factor in determining which outpatients were at risk of developing a postoperative complication.
2.2.4.1. Isoflurane
Isoflurane was the most widely used potent inhaled anaesthetic before the introduction of desflurane and sevoflurane (Eger 1993). It is still widely used for the maintenance of anaesthesia in outpatients (Herregods et al. 1988, Ghouri et al. 1991a, Eriksson et al. 1995, O’Hara et al. 1996, Philip et al. 1996), especially combined with propofol induction of anaesthesia (Gupta et al. 1992). Isoflurane has a slightly pungent odour and irritates the airways, and it is thus less readily accepted by patients for mask induction of the anaesthesia than sevoflurane (Sloan et al. 1996).
2.2.4.2. Desflurane
Desflurane was registered in Finland in 1994. It differs from its predecessors in having lower solubility in blood and tissues (Eger 1993). The lower solubility imparts greater control over the maintenance of anesthesia and more rapid elimination and recovery from anaesthesia. In other respects, the pharmacological properties of desflurane resemble those of isoflurane (Eger 1993). Transient airway irritant effects are the most common adverse effects during the induction of anaesthesia with desflurane, and this agent is hence not recommended for induction (Patel & Goa 1995). A rapid concentration increase has also been shown to provoke autonomic nervous system hyperactivity and haemodynamic instability (Ebert & Muzi 1993).
2.2.4.3. Sevoflurane
Sevoflurane is one of the new generation of inhalational general anaesthetic agents, and it was synthesised in 1971 (Wallin et al. 1972, Frink & Brown 1993). It has been in clinical practice in Japan since 1990 and in Finland since 1995. Sevoflurane is a colourless, nonflammable liquid of mild ethereal odour with lower solubility in lipids (Malviya & Lerman 1972) and blood (Yasuda et al. 1991) than halothane or isoflurane but not desflurane (Patel & Goa 1996). The solubility of sevoflurane in blood does not change significantly with age (Malviya & Lerman 1972), unlike that of isoflurane or other inhalable agents (Eger et al. 1971, Lerman et al. 1984). The anaesthetic potency of sevoflurane, quantified as the minimum alveolar concentration (MAC) that, at a steady state, produces immobility in 50% of individuals exposed to a noxious stimulus (Eger et al. 1965), is almost 50% lower than that of isoflurane, but almost 30% more higher than that of desflurane (Patel & Goa 1996). The plastic/gas and rubber/gas partition coefficients (i.e. solubility in the rubber and plastic components of an anaesthesia breathing circuit) consistently result in the following order: halothane > isoflurane > sevoflurane > desflurane (Targ et al. 1989). Sevoflurane is the volatile anaesthetic agent least irritant to the airways (Van Hemelrijck et al. 1991, Doi & Ikeda 1993). These properties allow both rapid induction and recovery and fast changes in the administration (Eger 1994). The partial pressure of the gas in the brain increases more rapidly than it does with the older inhaled anaesthetics (Yurino & Kimura 1993), and when the administration of sevoflurane is discontinued, the fall in the partial pressure of the gas in the brain is rapid, resulting in rapid recovery.
The structural formulae of halogenated inhaled general anaesthetic agents are shown in Fig. 1, and the most common physical characteristics of these anaesthetics are shown in Table 2.
Figure 1. Structural formulae of halogenated inhalable general anaesthetic agents (Patel & Goa 1996).
Table 2. Physical characteristics of inhalable anaesthetic agents (Rosenberg 1999).
| Parameter | Halothane | Enflurane | Isoflurane | Desflurane | Sevoflurane |
|---|---|---|---|---|---|
| Molecular weight | 197.4 | 184.5 | 184.5 | 168 | 200 |
| Specific gravity (20°C) | 1.86 | 1.52 | 1.50 | 1.48 | 1.52 |
| Boiling point (°C) | 50.2 | 56.5 | 48.5 | 22.8 | 58.6 |
| Vapour pressure at 20°C (mmHg) | 244 | 172 | 240 | 669 | 157 |
| MAC with 100% O2 (%) | 0.75 | 1.68 | 1.15 | 6–7 | 2.0 |
| MAC with 70% N2O (%) | 0.29 | 0.57 | 0.50 | ~3 | 0.66 |
| blood/gas partition coefficient (37° C) | 2.3 | 1.9 | 1.4 | 0.42 | 0.65 |
| preservative | thymol | – | – | – | – |
| stability in soda-lime | unstable | stable | stable | stable | unstable |
| metabolism (%) | 20 | 2.4 | 0.17 | 0.02 | 2.5 |
2.2.4.4. Propofol
In adult ambulatory practice, anaesthesia is usually induced with a short-acting intravenous anaesthetic. Propofol was introduced into clinical practice in 1984, and its advantages as an induction agent and also as an agent for maintaining anaesthesia were soon noted: rapid, smooth induction of anaesthesia, fast recovery and a low incidence of postoperative nausea and vomiting (Langley & Heel 1988, Boysen et al. 1989, Korttila et al. 1992). When used in combination with fentanyl or alfentanil, propofol is suitable for the provision of total intravenous anaesthesia (TIVA) (Langley & Heel 1988). With target-controlled infusion (TCI) of propofol, where a computer-controlled pump delivers a specific targeted plasma concentration, the use of TIVA may increase (Coetzee et al. 1995). Infusions of subanaesthetic doses of propofol have been used to sedate patients for surgery under regional anaesthesia (Korttila 1999). The most common physical and clinical characteristics of propofol compared to the other intravenous anaesthetics are shown in Table 3.
Table 3. Physical and clinical characteristics of intravenous anaesthetics (Scheinin 1999).
| Anaesthetic | Clearance (ml/min/kg) | VDss (l/kg) | T (elimination) (h) | Induction dose (mg/kg) |
|---|---|---|---|---|
| thiopentone | 3–4 | 1.5–3.0 | 7–15 | 3–5 |
| metohaxital | 10 | 2.2 | 2–6 | 1–1.5 |
| propofol | 22–30 | 1.5–3.0 | 4–24 | 2–2.5 |
| diazepam | 0.3–0.5 | 1.0–1.5 | 30–50 | 0.3–0.5 |
| midazolam | 6–11 | 1.0–1.5 | 1.7–2.6 | 0.1–0.3 |
| ketamine | 11–20 | 3.0 | 2–3 | 1.5–2 |
| etomidate | 18–25 | 1.5–3.5 | 3–5 | 0.2–0.3 |
2.2.4.5. Mivacurium
Mivacurium is a benzylisoquinolinium muscle relaxant, which is rapidly hydrolysed by plasma cholinesterases (Bevan 1995). Although the onset time of an intubating dose of mivacurium (0.15–0.2 mg/kg) is quite long (2.5–3 min), recovery begins within 15 min and is virtually complete after 30 min (Smith 1994). In patients carrying atypical forms of plasma cholinesterase or having renal or hepatic dysfunction, the action of mivacurium is prolonged (Ostergaard et al. 1993, Savarese et al.1995), but the residual mivacurium-induced neuromuscular block can be antagonised with neostigmine (Lessard et al. 1997). Mivacurium has a potential for histamine release, and although this is seldom a problem in normal practice, some difficulty may arise if large doses are administered rapidly (Smith 1994). Mivacurium is a non-cumulative agent, and it is suitable for short-term ambulatory anaesthesia, but may be also used for longer operations as continuous infusion (Diefenbach et al. 1995).
2.2.4.6. Rocuronium
Rocuronium is a steroid muscle relaxant structurally related to vecuronium (Smith 1994). Because of its low potency, it has a very rapid onset of action (Kopman 1993). A dose of 0.6 mg/kg produces complete twitch depression within 75–150 seconds and acceptable intubation conditions after 60 seconds (Kopman 1993). The clinical duration of action of an intubating dose of rocuronium is very similar to that of vecuronium. Rocuronium was used widely during the late 1990"s, but since that, there have been many reports of anaphylactic reactions during the induction of anaesthesia using rocuronium for muscle relaxation (Matthey et al. 2000, Heier & Guttormsen 2000).
The pharmacodynamics of the most common muscle relaxants are shown in Table 4.
Table 4. Pharmacodynamics of muscle relaxants (Erkola 1999).
| Relaxant | ED95 dose (mg/kg) | Intubation dose (mg/kg) | Time to maximal effect (min) | Time to intubation (min) | Clinical duration (min) | Recovery index (min) | Maintaining dose (mg/kg) |
|---|---|---|---|---|---|---|---|
| mivacurium | 0.08 | 0.25 | 2.3 | 1.5–2.0 | 18–22 | 6.5 | 0.05–0.2 |
| rapacurium | 1.30 | 1.0–2.5 | 1.5–2.5 | 1–1.5 | 15–45 | 7–12 | 0.5 |
| atracurium | 0.25 | 0.5 | 3.0 | 2–2.5 | 35–45 | 11–16 | 0.1–0.2 |
| cisatracurium | 0.05 | 0.15 | 3.5 | 2–3 | 55 | 10 | 0.03 |
| rocuronium | 0.3 | 0.6–1.0 | 1.5–2.5 | 1 | 30–60 | 7–5 | 0.1 |
| vecuronium | 0.045 | 0.1 | 3.0 | 2–2.5 | 30–45 | 10–20 | 0.015–0.03 |
| pancuronium | 0.06 | 0.1 | 3.5–4 | 3–4 | 75–115 | 30–50 | 0.015 |
2.2.4.7. Short-acting opioids
Opioids are frequently administered in the immediate preinduction period to suppress autonomic responses to endotracheal intubation and during the maintenance of general anaesthesia to prevent autonomic responses to painful stimuli. Although morphine and pethidine can be used in outpatient anaesthesia, they are not so popular as the more potent, rapid and shorter-acting opioid analgesics fentanyl, sufentanil, alfentanil and remifentanil (Van Vlymen et al. 2000). The basic pharmacokinetic data for opioids are shown in Table 5.
A study comparing morphine and fentanyl in ambulatory surgical patients reported higher pain scores and more analgesic use in the fentanyl group. Morphine produced better-quality analgesia, but was associated with increased nausea and vomiting, especially after discharge. There were no differences in recovery times or discharge times despite the shorter duration of fentanyl action (Claxton et al. 1997). Compared with a standard inhaled anaesthetic, most investigators have demonstrated improved intraoperative conditions and more rapid emergence from anaesthesia when fentanyl or one of its newer analogues was administered as a part of a balanced anaesthetic technique (White et al. 1986, Ghouri & White 1991). When sufentanil infusion was compared with fentanyl for the maintenance of general anaesthesia with nitrous oxide, its use was associated with less nausea and postoperative pain (Phitayakorn et al. 1987).
Alfentanil has a rapid onset and a short duration of action (Van Vlymen & White 2000). These characteristics make it particularly useful in the outpatient setting. Most investigators have reported faster emergence and recovery of psychomotor function after alfentanil-based anaesthesia compared with fentanyl (Van Vlymen & White 2000). During the last decade, alfentanil has been the most widely used opioid in ambulatory TIVA regimens (Philip et al. 1997b).
Remifentanil is an ultra-short-acting opioid analgesic with an analgesic potency similar to that of fentanyl (Van Vlymen & White 2000). It is metabolised by nonspecific esterases via a process that allows rapid systemic elimination (Glass 1995, Michelsen & Hug 1996). In consequence of that, remifentanil has short-lived opioid side effects, but also results in short-lived analgesia (Smith 1999). When remifentanil was compared with alfentanil as part of a total intravenous anaesthesia technique, remifentanil provided more effective suppression of intraoperative responses, but prolonged awakening and recovery room stay (Philip et al. 1997b). There was an earlier need for analgesics postoperatively with remifentanil, and both groups had similar discharge times. The later studies have shown that the adjunctive use of remifentanil infusion during desflurane-N2O anaesthesia facilitated early recovery without increasing PONV, pain or need for rescue medication after ambulatory laparoscopic surgery (Song & White 1999). Studies involving the use of remifentanil in combination with less soluble anesthetics suggest that low-dose infusion (0.05–0.2 µg/kg/min) may produce a significant anaesthetic-sparing effect (Song et al. 1998b).
Table 5. Pharmacokinetic data for opioids (Laitinen & Salomäki 1999).
| Parameter | Morphine | Oxycodone | Pethidine | Fentanyl | Alfentanil | Sufentanil | Remifentanil |
|---|---|---|---|---|---|---|---|
| pKa | 8 | 8.9 | 8.5 | 8.4 | 6.5 | 8 | 7.1 |
| % un-ionized at pH 7.4 | 23 | – | < 10 | < 10 | 90 | 20 | 67 |
| Octanil-water partition coefficient | 1.4 | 1.6 | 39 | 813 | 145 | 1778 | 18 |
| Percentage bound to plasma proteins | 20–40 | – | 70 | 84 | 92 | 93 | 70 |
| T (h) | 2.9 | 2–4 | 3–5 | 3.7 | 1–2 | 2–3 | 0.1–0.2 |
| VDcc (l/kg) | 0.1–0.4 | – | 1–2 | 0.5–1 | 0.1–0.3 | 0.2 | 0.2 |
| VDss (l/kg) | 3–5 | 3–6 | 3–5 | 3–5 | 0.4–1 | 2.5–3 | 0.3–0.4 |
| Clearance (ml/kg/min) | 15–30 | 6–19 | 8–18 | 10–20 | 4–9 | 10–15 | 42 |
2.2.4.8. Laryngeal mask airway
The laryngeal mask (LMA) airway was designed by Brain in 1981 as a new concept in airway management (Brain 1991). The beauty of the LMA is that it forms an airtight seal by enclosing the larynx rather than plugging the pharynx and hence avoids airway obstruction in the oropharynx. The LMA appears to be a safe and acceptable technique for day-case anaesthesia (Coyne 1990, Smith & White 1992, Goodwin et al. 1992). The placement of the LMA is easy to learn (Davies et al. 1990). Muscle relaxants and laryngoscopy are not necessary, and the emergence and recovery times are shorter when LMA is used and similar to those in patients on whom a face mask is used (Smith & White 1992). Compared with endotracheal intubation, the insertion of a LMA causes a minimal cardiovascular response and is better tolerated at lighter levels of anaesthesia (Pennant & White 1993). Postoperative side effects, such as the incidence of sore throat, are markedly reduced when a LMA is used. In a large survey, 47% of intubated patients complained of sore throat postoperatively versus only 7% of the patients with a LMA (Alexander & Leach 1989, Joshi et al. 1997). When compared with anaesthesia with a face mask and an oral airway, patients with a LMA had fewer desaturation episodes, fewer intraoperative airway manipulations and fewer difficulties in maintaining an airway (Smith 1992).