Cuff volume and size selection with the laryngeal mask

By the year 2000, the laryngeal mask will have been used in approximately 100 million patients worldwide. Currently, at least 30% of anaesthetised patients in the UK [1] and 20% of patients in the USA [2] are managed using the laryngeal mask. The popularity of the device stems from its ease of use, its role in the difficult airway and the advantages it offers over the face mask and tracheal tube [3, 4]. The advent of the laryngeal mask has certainly forced anaesthetists to re-consider the practice of routine airway management as a whole. In many circumstances in which intubation and ventilation was once considered mandatory, the laryngeal mask has been used safely, and may even offer advantages. For example, the laryngeal mask can be used during adenotonsillectomy and is associated with fewer complications, such as aspiration of blood, than tracheal intubation [5]. Although the laryngeal mask has a low failure and complication rate, considerable efforts have been made to improve its clinical success and to make its use even safer. One recent advance is in the selection of appropriate size and cuff volume of the laryngeal mask. We felt it appropriate to summarise current knowledge of this issue and to make evidence-based recommendations for clinical practice. The instruction manual of the laryngeal mask originally indicated that the size 3 should be used in children or small adults of > 25 kg and the size 4 in adults of either average or large size. After introduction of the size 5, the inventor recommended using as large a size of mask as possible [6]. Despite these instructions, the size 3 laryngeal mask seems to be used frequently in females and the size 4 in males. The reasons for this practice are not clear, but it is possible that clinicians may have considered that larger masks would be more difficult to insert and more likely to produce pharyngolaryngeal injury. Several researchers have attempted to find a method for selecting an appropriate size of the laryngeal mask [7-11]. Voyagis and colleagues found that the mask provided a better airtight seal during controlled ventilation using the sex-related selection method (size 4 in females and size 5 in males) than the weight-related method (size 3 in patients weighing < 70 kg, size 4 for patients weighing 70–90 kg and size 5 for patients weiging > 90 kg) [7]. Berry and colleagues sought the optimal size by ease of insertion, sealing effect, gastric insufflation and anatomical position of the laryngeal mask, and found that either size 4 or 5, but never size 3, was optimal. Selection of the mask based on sex was better than selection based on weight, and the larger mask (size 4 in females and size 5 in males) was a better choice than the smaller (size 3 in females and size 4 in males) [8]. Asai and colleagues have shown that a larger size (size 4 in females and size 5 in males) provided an airtight seal more frequently than a smaller size (size 3 in females and size 4 in males). There were no significant differences in the ease of insertion or calculated pressure exerted on the pharynx between the two sizes of mask in either sex [10]. Brimacombe and Keller reported that in males, size 5 provided a better airtight seal than size 4, whereas in females both size 5 and 4 provided adequate seals with similar pressures on the pharynx and similar position of the mask in the two sizes [9]. In summary, all the above studies indicate that selection of the size of the mask based on sex is appropriate, and that size 5 is more appropriate than size 4 in males, whereas either size is appropriate in females, and that the use of size 3 in adults is not desirable. Some may feel that size 5 may often be too large to insert, particularly in females. However, studies have shown that this is not the case, and even size 5 can be inserted easily in females [8-10]. There are, however, limitations to size selection based solely on the patient's sex. One possible problem is that, although a larger mask provides a better airtight seal than a smaller one, the larger mask may sometimes protrude into the mouth of smaller patients. Because the laryngeal mask is designed to conform to the hypopharynx (laryngeal part of the pharynx) and the proximal part of the cuff should be positioned caudal to the level of rami of the jaw and tonsils, the cuff should not be seen on opening of the patient's mouth [12, 13]. If the cuff is in the oral cavity, it may interfere with tonsillectomy and may increase the theoretical risk of sore throat or ischaemic change to lingual nerves. One study showed that when a larger laryngeal mask was used, the cuff was often seen in the back of the mouth in small patients (< 165–170 cm in height in males and 160–165 cm in females) [11]. This was consistent with another study, which concluded that, in patients who were < 165 cm in height, the size 4 would be more appropriate than the size 5, in terms of oropharyngeal sealing pressure and the fibreoptic position of the mask [8]. It seems reasonable to conclude that a larger mask be placed first, and if the cuff is visible in the back of the mouth, the mask should be replaced with a mask one size smaller. For example, it may be preferable to use a small mask when adenotonsillectomy is planned. The manufacturer recommends that the cuff is inflated with the minimum volume of air required to provide an effective seal (minimum effective volume) and that this volume is maintained by intermittent withdrawal of gas during anaesthesia using nitrous oxide. However, it is common practice to inflate the cuff with the maximum recommended volume (size 3, 20 ml; size 4, 30 ml; size 5, 40 ml), as illustrated in several studies [14-16], and to regard these as the ‘recommended volumes’. Recent studies indicate that a cuff that is inflated with the maximum volume of air may cause clinical problems. One problem is that inflation of the cuff with the recommended maximal volume may not produce the best airtight seal [17, 18]. For example, when a size 4 or 5 mask was used, the airtight seal was highest at approximately one-third to two-thirds the maximum recommended cuff volume and decreased thereafter [18, 19]. Similarly, the airtight seal was significantly better when the intracuff pressure was adjusted to 60 cmH2O than when it was 180 cmH2O [17]. It is considered that when the mask is inflated maximally, the more rigid cuff is less able to adapt to the variable contours of the pharynx [18]. If the maximum recommended volume is exceeded, and occasionally in patients with an unexpectedly small pharynx, it is possible that the cuff may even be displaced from the pharynx with a sudden loss of seal. Because of less effective sealing, the incidence of gastric insufflation may also be increased if the cuff is inflated with a high volume [19]. Another potential problem associated with the maximally inflated cuff is ischaemia of the oropharyngeal mucosa. When the pressure exerted by the mask on the pharynx is greater than the mean mucosal capillary perfusion pressure, there is a risk of mucosal ischaemia. Capillary perfusion in the posterior pharynx is reduced progressively when mucosal pressure is increased from 24 to 57 mmHg [20], which is similar to or slightly higher than the tracheal mucosal pressure [21]. When the cuff is inflated with the maximum volume of air, the intracuff pressure is initially > 100 mmHg, it increases steadily when nitrous oxide is used, and may exceed 200 mmHg during a prolonged procedure [22-24]. In contrast, when the cuff is inflated to the minimum effective volume, the intracuff pressure is much lower than the pressure of the maximally inflated cuff [10]. A high intracuff pressure in itself does not necessarily mean that the pressure on the pharynx is excessively high, because there is no clear relationship between intracuff pressure and the pressure on the pharynx [10, 22, 25]. In fact, the pressure on the pharynx may be unchanged or even decrease at some locations as the intracuff pressure increases [25]. Therefore, optimal cuff volume should be sought based on the pressure exerted on the pharynx, rather than the intracuff pressure. In early studies, the exerted pressure on the pharynx was obtained by calculating the difference between intracuff pressures measured with the mask in place in the patient (P in vivo) and held in air (P ex vivo), with the cuff inflated with the same volume of gas (P = P in vivo − P ex vivo) [10, 22, 23]. These studies suggested that exerted pressures were usually higher than the capillary perfusion pressure when the cuff was inflated with the recommended maximum volume of air [10, 22, 23]. Calculation of the exerted pressure on the pharynx, however, has two limitations. First, because the calculated pressure is the average of pressures exerted by the cuff of the laryngeal mask on the contacted surface, it includes the pressure exerted on surfaces other than the oropharynx (e.g. the pressure exerted by the cuff on the tube of the laryngeal mask near the junction of the mask and tube). Second, it is not possible to obtain the pressure exerted against the pharynx by the back plate or by the tube of the laryngeal mask. By directly measuring the exerted pressures at several sites, Brimacombe and colleagues found that the measured pressures were generally much lower than the calculated pressures [18, 25]. They also found that the exerted pressure did not, in fact, distribute evenly on the pharynx, and the pressure exerted by the proximal part of the cuff on the tube was considerably higher [26]. This partly explains why the calculated pressure is greater than the measured pressure. Nevertheless, direct measurement indicates that the exerted pressure generally increases with increasing cuff volume [18, 26], and when the cuff is inflated with the recommended maximal volume or close to it, the exerted pressure can exceed the capillary pressure [18]. These high pressures are exerted on the posterior pharyngeal wall by the posterior surface of the tube, and on the base of the tongue by the anterior surface of the cuff [18]. Therefore, these oropharyngeal sites may be the most vulnerable sites for possible mucosal ischaemia. This theoretical risk of mucosal ischaemia can be reduced by adjusting the cuff volume and by avoiding inflating the cuff with the maximum volume of air. It has been reported that, by inflating the cuff of either a size 3 or 4 mask with a fixed volume of 10 ml, it is possible to reduce the calculated pressure exerted on the pharynx to less than the capillary perfusion pressure [23]. However, this may reduce the efficacy of preventing gas leakage and soiling from liquid that may pool in the oropharynx. A better method is to inflate the cuff with the minimum effective volume. Compared with the cuff inflated with the maximum volume of air, the cuff inflated with the minimum effective volume exerts significantly lower pressure on the pharynx [10]. Direct measurement of the mucosal pressure suggests that the pressure is generally lower than the capillary pressure when a size 4 or 5 cuff is inflated with a volume of air up to ≈ 30 ml [9]. One important factor to remember is that, when the cuff is inflated maximally, the pressure on the pharynx is greater with a larger mask than a smaller one [9, 10, 26]. This indicates that the theoretical risk of ischaemia is greater with a larger laryngeal mask if the mask is inflated maximally. In contrast, when the cuff is inflated with the minimum effective volume, there is no significant difference in the exerted pressure between the two sizes of mask [9, 10, 26]. Therefore, the cuff volume should be adjusted to the minimum, particularly when a large size mask is used. The minimum volume for obtaining a clinically acceptable seal is generally 10–15 ml for sizes 3, 4 and 5 [8]. Most patients can be ventilated adequately with peak airway pressures < 15 cmH2O [27], but an airtight seal should be > 15 cmH2O if there is a maximum risk of aspiration from above the cuff [28]. With this range of cuff volumes, studies using cadavers suggest that the mask can attenuate the flow of liquid from the oesophagus to pharynx [29] and can protect the airway from soiling from above the cuff [28]. In summary, a reasonable method is to initially inflate the cuff with ≈ 10–15 ml of air and, if there is a gas leak around the mask at an airway pressure of < 15cmH2O, 5–10 ml of air is added. When nitrous oxide is used, it may be preferable to adjust occasionally the cuff to contain the minimum effective volume, by monitoring the intracuff pressure using a manometer or by palpation of the pilot balloon [30]. It appears to be generally appropriate to choose a relatively large laryngeal mask and to inflate the cuff with the minimum effective volume, having regard to the following two major factors: the pharyngeal pressure and the incidence of air leak around the mask during intermittent positive pressure ventilation. The presence of an air leak may be clinically irrelevant when spontaneous breathing is maintained during use of the laryngeal mask. The possibility of pharyngeal ischaemia could also be clinically unimportant, because the duration of use of the laryngeal mask is usually short. What matters then is whether selection of a mask size or cuff volume alters peri-operative morbidity. When the cuff is overinflated, it may increase the incidence of complications, such as coughing or increased heart rate, during anaesthesia and during emergence from it, because the overinflated cuff may stimulate the pharyngolarynx. However, in one report, there were no clinically significant differences in complications or cardiorespiratory parameters during emergence between cuffs inflated with high and low cuff volumes [31]. Adjusting the cuff volume appears to alter the incidence of postoperative sore throat. Several studies showed that, compared with a maximally inflated cuff, one which was adjusted to half the maximal volume or to the ‘just-seal’ condition was associated with a significantly lower incidence of sore throat [31-34]. Therefore, these results indicate that lowering the pressure exerted on the pharynx by adjusting the cuff volume does reduce postoperative sore throat. One study, however, showed that, when the intracuff pressure was adjusted continuously to 30 mmHg, the incidence of postoperative sore throat was similar to that in another group in which the intracuff pressure was adjusted to 180 mmHg [35]. It is less clear whether the difference in the mask size alters the incidence of postoperative sore throat. One researcher has claimed that a size 3 mask (among sizes 2.5, 3 and 4) in females or size 4 (among sizes 3, 4 and 5) in males was associated with the lowest incidence [36]. However, this claim is based on one report in which no data were provided and historical controls were used. A large mask may not exert a greater pressure on the pharynx than a smaller mask, but a large mask could produce abrasion of a wider area during insertion. One serious complication associated with the use of the laryngeal mask is neuroparalysis in the oropharynx. Although rare, there have been 12 reports of temporary or long-term paralysis of the lingual, hypoglossal or recurrent laryngeal nerves [37, 38]. There has also been one report of transient tongue cyanosis [39]. Some considered that too high a pressure exerted by the device on the oropharynx may have caused these complications [40, 41]. Interestingly, most cases involved the use of a size 3 mask in adults. It is possible that this may have encouraged overinflation of the cuff in an attempt to obtain an effective seal. Brain argues that these complications were induced due to malpositioned laryngeal masks, because a correctly inserted mask should not be in contact with the area in which these nerves and vessels run [42]. Nevertheless, adjustment of the cuff volume might have avoided the complication, even if the mask was malpositioned and compresses the nerve. Based on the current state of knowledge, it appears that selection of the size of the laryngeal mask should be based primarily on sex (size 5 in males and size 4 or 5 in females). Previoulsy, the instruction manual for the device indicated that size selection should be based on weight [43], but the latest manual [12] concedes that size selection based on weight is a rough estimate and acknowledges the sex-based selection method. Nevertheless, it seems unreasonable to argue which single factor should be used exclusively for size selection, as there is no clear-cut relationship between sex, weight, height, body mass index and the size or shape of the oropharynx. When an appropriate size has been chosen, the cuff volume should be adjusted to the minimum, particularly in cases in which a large mask is used. If a satisfactory airtight seal cannot be obtained, a larger size may be selected; a size 6 is now available. Flexion of the head–neck or gentle application of pressure to the front of the neck may also improve seal [8, 44]. There have been more than 400 studies and over 2000 reports on the device. The laryngeal mask is now in routine use worldwide. One might well wonder therefore why there is further need to publish or read new articles about it. We are still far from achieving a thorough understanding of the device and there are many questions to be answered. For example, there are considerable variations between studies of the incidence of gastro-oesophageal reflux (0–80%) [45-47]. Some also claim that the laryngeal mask is less effective in children than in adults, whereas others have not found such differences. Such a high incidence of a complication is a concern, but in fact no convincing methods to reduce the complications have been offered. Whereas an inadequate depth of anaesthesia and users' skill might have caused discrepancies, selection of a suboptimal size and cuff volume might well also have produced such high incidence of complications. No formal studies are available either for the appropriate size in children or the effect of the different size and cuff volume on gastro-oesophageal reflux. The inventor [48] and others have frequently stressed that failure to adhere to the manufacturer's instructions will result in an increased incidence of complications and a lack of success with the device. Clinicians' practice should be based on a systemic appraisal of the best evidence available. The inventor [48] and others [49, 50] have frequently pointed out that the incidence of complications associated with the use of the laryngeal mask can be reduced by following the manufacturer's instruction manual, which is now based mainly on randomised controlled studies. There are some circumstances in which clinicians may be reluctant to follow these instructions. For example, although the instruction manual indicates an optimal insertion method, one report indicates that < 35% of anaesthetists use this method [51]. To some extent this may reflect the fact that a few studies show that alternative insertion methods are better than the recommended technique [52-54]. Neither the manufacturer nor even the inventor is always correct. Therefore, the safest and most efficacious method should be sought and the instructions or guidelines should be amended. Studies into appropriate size selection and cuff inflation volume have provided reasonably consistent results, allowing rational anaesthetic management of the individual. Whether such a selection influences the clinical outcome is yet to be determined, it is at least no longer acceptable to choose leisurely the size of the laryngeal mask and to inflate the cuff with the maximal volume of air.