CRICOID PRESSURE MAY PREVENT INSERTION OF THE LARYNGEAL MASK AIRWAY

We have studied 42 female patients undergoing elective day-casesurgery allocated randomly to two groups. After induction ofanaesthesia an attempt was made to insert a laryngeal mask airwayafter application of cricoid pressure in one group or with nocricoid pressure in the other. The anaesthetist was unawareof the application, or not, of cricoid pressure. Successfulinsertion was achieved at the first or second attempt in 19of the 22 patients in the non-cricoidpressure group, but inonly three of the 20 patients in the cricoid pressure group(X² 18.62, P <0.001). The laryngeal mask airway was theninserted successfully in all 17 patients after removal of cricoidpressure. The implications of having to remove cricoid pressureif a laryngeal mask airway is to be inserted are discussed.(Br. J. Anaesth. 1992; 69: 465–467)     

EFFECT OF THE LARYNGEAL MASK AIRWAY ON LOWER OESOPHAGEAL SPHINCTER PRESSURE IN PATIENTS DURING GENERAL ANAESTHESIA

We have studied the tone of the lower oesophageal sphincter(LOS) in 40 adults undergoing routine body surface surgery andallocated randomly to receive anaesthesia either by face maskand Guedel airway or by laryngeal mask airway. In the laryngealmask group there was a mean (SEM) decrease in barrier pressure(LOS minus gastric pressure) of 3.6 (1.4) cm H², compared witha mean increase of 2.2 (1.2) cm H2O in the face mask group (P< 0.005).     

PRESSURE EXERTED BY THE LARYNGEAL MASK AIRWAY CUFF UPON THE PHARYNGEAL MUCOSA

Ten patients were studied for each of the sizes 2, 3 and 4 laryngealmask airways (LMA) in order to calculate the pressure exertedby the cuff upon the pharyngeal mucosa. Using a non-invasivemethod of comparing intracuff pressures recorded both in vitroand in vivo, the transmitted pharyngeal mucosal pressures werecalculated over the clinical range of injection volumes. Cuffinflation with the "normal" injection volumes recommended resultedin the residual volumes of the cuffs being exceeded. The intracuffpressures recorded with the mask in situ at these normal injectionvolumes were in the range 103-251 mm Hg. The calculated transmittedmucosal pressures were substantial for all three sizes of cuffand potentially exceeded the capillary perfusion pressure ofthe adjacent pharyngeal mucosa, despite apparent pharyngealaccommodation to the mask. (Br. J. Anaesth. 1993; 70: 25–29)     

THE LARYNGEAL MASK AIRWAY FOR INTRAOCULAR SURGERY: EFFECTS ON INTRAOCULAR PRESSURE AND STRESS RESPONSES

Tracheal intubation, performed routinely during general anaesthesiain patients undergoing intraocular surgery, may have adverseeffects on cardiovascular function and intraocular pressure.This study assessed the suitability of the laryngeal mask airway(LMA) as a substitute for tracheal; intubation. Intraocularand systemic pressor effects, heart rate changes and catecholamineconcentrations were measured in two groups of 10 patients receivingstandardized anaesthesia with either a trachealtube (TT) ora LMA. There were significantly smaller changes in the pressorresponses to insertion and in concentrations of catecholaminesat critical times in the anaesthetic sequence in the LMA group.Mean (SEM) rate-pressure product was significantly smaller inthe LMA group compared with the TT group after both insertion(8276 (730) vs 13307 (1348), P<0.01) and removal (10 152(595) vs 14 137 (1044). P < 0.07; of the airway device. Thechange in intraocular pressure was significantly less in theLMA group at all time points after airway instrumentation thanthat in the TT group, with the greatest difference after extubation(–2.3 (2.4) mm Hg vs 74.5 (3.4) mm Hg. P < 0.071 ^*Present address: Department of Anaesthetics, Victoria Infirmary,Glasgow.     

COMPARISON OF THE EFFECTS OF THE LARYNGEAL MASK AIRWAY AND ENDOTRACHEAL INTUBATION ON VOCAL FUNCTION

We have compared changes in vocal function produced after insertionof a laryngeal mask airway (LMA) with those produced by trachealintubation in 20 patients. Using acoustic waveform analysis,we computed amplitude variability (AV), pitch variability (PV),harmonics-to-noise ratio (HNR) and additive noise level (ANL)before anaesthesia and at 1, 4, and 24 h after tracheal extubation.There were no significant changes in vocal function after extubationexcept for HNR ratio (P = 0.046) at 4 h in the LMA group. Therewere differences in all four variables at 1 h, 4 h, or both,after tracheal extubation compared with baseline in the trachealtube group. In both groups, all variables were the same as baselinevalues 24 h after extubation. We observed significant differencesin AV (4 h), PV and ANL (1, 4 h) values between the two groups.These observations suggest that the LMA causes less vocal changethan tracheal intubation (Br. J. Anaesth. 1993; 71: 648–650)     

COMPARISON OF THE REINFORCED LARYNGEAL MASK AIRWAY AND TRACHEAL INTUBATION FOR ADENOTONSILLECTOMY

One hundred and four patients were allocated randomly to receiveanaesthesia for adenotonsillectomy via either a reinforced laryngealmask airway or a tracheal tube. Airway maintenance and protectionwere assessed during and after operation. The reinforced laryngealmask did not interfere with surgical access; it resisted compressionand protected the lower airway from contamination with blood.Four patients were withdrawn from the laryngeal mask airwaygroup: two because of difficulty with placement, and two becausethe laryngeal mask was obstructed distally when the Boyle Davisgag was opened fully. In children, recovery was less eventfulin the laryngeal mask airway group, with less airway obstruction(P < 0.001) and better airway acceptance (P < 0.05). Thereinforced laryngeal mask airway provided a clear, secure airwayuntil recovery of protective reflexes. (Br. J. Anaesth. 1993;70: 30–33)     

REGURGITATION OF GASTRIC CONTENTS DURING GENERAL ANAESTHESIA USING THE LARYNGEAL MASK AIRWAY

We have investigated the incidence of regurgitation of gastriccontents during general anaesthesia administered via a laryngeaimask airway (LMA) or face mask and Guedel airway in 56 patientswith no risk factors for regurgitation. Patients swallowed agelatine capsule containing methylene blue 10 min before inductionof anaesthesia. Fibreoptic laryn–goscopy in the LMA groupor conventional laryn-goscopy in the face mask group was performedat the end of surgery. Dye was observed within the laryngeaimask in seven of 28 patients (25%). No patients in the facemask-Guedel airway group regurgitated dye (P = 0.005). Therewas no evidence of aspiration of dye.      

COMPARISON OF END-TIDAL AND ARTERIAL CARBON DIOXIDE MEASUREMENTS DURING ANAESTHESIA WITH THE LARYNGEAL MASK AIRWAY

We have confirmed the value of measurement of end-tidal carbondioxide concentration as an indicator of arterial carbon dioxidetension during the use of the laryngeal mask airway in healthypatients breathing spontaneously. The mean difference betweenarterial and end-tidal carbon dioxide tension was 0.52 kPa (range0–1.5 kPa), which is similar to the difference which hasbeen reported when a tracheal tube has been used. (Br. J. Anaesth.1993; 71: 734–735) ^*Present address, for correspondence: Department of Anaesthesia,St George's Hospital, Blackshaw Road, London SW17 OQT.     

COMPARISON OF THE USE OF THE LARYNGEAL MASK AND FACE MASK BY INEXPERIENCED PERSONNEL

Ten junior doctors with no postgraduate anaesthetic experienceattempted to ventilate the lungs of 50 anaesthetized patients,using either a laryngeal mask or a Guedel airway and face mask.Success was defined as the production of two successive tidalvolumes exceeding 800 ml within 40 s. The failure rate was significantlygreater using the laryngeal mask compared with the face mask(P < 0.05) and the average time was significantly longerwith the laryngeal mask than with the face mask (P < 0.01). The results from this investigation suggest the laryngealmask airway cannot be recommended as a resuscitation devicefor use by inexperienced operators.     

THE EFFECT OF PROPRANOLOL ON AIRWAY RESISTANCE

The effect of propranolol on the airway resistance (AWR) ofeighteen normal and seven asthmatic subjects has been studiedby whole body plethysmography. In normal subjects there wasa significant mean rise in AWR which was never accompanied byany respiratory upset. In asthmatic subjects there was a greatermean rise in AWR which usually resulted in dyspnoea and occasionallywheezing. In nine subjects, the studies were repeated afterthe prior administration of atropine which almost completelyabolished the rise in AWR after propranolol in normal subjectsand markedly reduced the rise in asthmatic subjects. The modeof action of propranolol in producing bronchoconstriction isdiscussed and the protective effect of atropine is stressed. ^* Present address: Department of Anaesthesia, University ofGlasgow, Western Infirmary, Glasgow, W.2.     

ARTERIAL OXYGEN SATURATION DURING INDUCTION OF ANAESTHESIA AND LARYNGEAL MASK INSERTION: PROSPECTIVE EVALUATION OF FOUR TECHNIQUES

In a prospective, randomized study of 87 patients, we have comparedthe incidence of hypoxaemia during induction of anaesthesiawith subsequent Laryngeal Mask Airway (LMA) insertion in healthyadults when four different techniques were used: one withoutsupplementary oxygen, and three with supplementary oxygen. Twelvepatients did not receive supplementary oxygen before LMA placement,25 underwent partial denitrogenation by breathing oxygen fromthe start of injection of the induction agent, 25 underwentformal denitrogenation by breathing oxygen for 3 min, and 25received five tidal volume breaths of oxygen by face mask usingpositive pressure immediately after induction of anaesthesia.Anaesthesia was induced with propofol 2.0 mg kg^–1 andfentanyl 1 µ kg^–1. Additional propofol was givenif required. Arterial oxygen saturation was measured by pulseoximetry. Desaturation occurred in 11 of 12 patients who didnot receive supplementary oxygen, and in 19 of 25 patients whoreceived manual ventilation with 100% oxygen after inductionof anaesthesia before LMA insertion. Full denitrogenation andpartial denitrogenation were equally successful in preventingdesaturation. Failure to position the LMA successfully occurredin 3% of patients, and some difficulty was encountered in another18%     

THE CRICOID YOKE--A DEVICE FOR PROVIDING CONSISTENT AND REPRODUCIBLE CRICOID PRESSURE

An instrument is described which, when used during the acceleratedinduction technique, ensures that consistent and adequate cricoidpressure can be applied. Mothers undergoing general anaesthesiafor elective Caesarean section were studied in order to illustratethe clinical application of the instrument. The consequencesto intubating conditions of applying adequate cricoid pressure,and an assessment of the instrument’s control over theincidence of regurgitation during operation were investigated.     

EFFECT OF AGE ON THE SENSITIVITY OF UPPER AIRWAY REFLEXES

We have recorded the threshold concentration of inhaled ammoniavapour required to elicit reflex glottic closure (NH3TR) in102 healthy, nonsmoking volunteers (39 female) aged 17-96 yrin order to assess the effect of age upon upper airway reflexsensitivity. A single measurement of sensitivity was made ineach subject using a system delivering small concentrationsof ammonia vapour for single intermittent breaths to the upperairway and recording glottic closure using an inspiratory pneumotachograph.We found a strong positive correlation between age and NH3TR,indicating a decrease in upper airway reflex sensitivity withincreasing age. (Br. J. Anaesth. 1993; 70: 574–575)     

EFFECT OF ORAL DIAZEPAM ON THE SENSITIVITY OF UPPER AIRWAY REFLEXES

In a double-blind, cross-over study, we have investigated theeffect of oral diazepam 20 mg and placebo on the sensitivityof upper airway reflexes in 10 male volunteers (aged 25–35yr). Upper airway reflex sensitivity (UARS) was assessed usingsmall concentrations of ammonia vapour as a stimulus to upperairway receptors. A threshold concentration of ammonia, at whichreflex glottic closure occurred in response to the ammonia stimulus,was used as a measure of UARS. With diazepam, there was significantdepression of UARS from 30 to 150 min after administration.(Br. J. Anaesth. 1993; 70: 131–134)      

CRICOID PRESSURE WITH OR WITHOUT THE "CRICOID YOKE"

The performance of cricoid pressure was studied in three groupsof medical personnel likely to be involved in its applicationusing the cricoid yoke and a test rig. The instrument enabledindividuals who had no previous experience in the applica-tionof cricoid pressure to achieve results as good as those obtainedby experienced anaesthetic staff. Furthermore, the instrumentimproved the consistency of the applied force in all groups,particularly if cricoid pressure was required for sustainedperiods of 30 s or more.     

RESISTANCE TO CONSTANT AIR FLOW IMPOSED BY THE STANDARD LARYNGEAL MASK, THE REINFORCED LARYNGEAL MASK AND RAE TRACHEAL TUBES

The reinforced laryngeal mask airway (RLMA) has been introducedrecently for head and neck surgery. Its resistance to constantair flow has been measured and compared with that of the standardlaryngeal mask airway (LMA). The RLMA resistance has also beencompared with that imposed by the standard oral Ring-Adair-Elwyn(RAE) tube of a corresponding size for a given patient. Theresistance to gas flow of the new RLMA was approximately threeto five times that of the LMA at two flow rates (0.5 and 7.0litre s^-1). The resistance of the RLMA 4 was intermediate betweenthat of RAE tubes sizes 8 and 9 mm, and that of RLMA 2 intermediatebetween that of RAE tubes sizes 5 and 6 mm. (Br. J. Anaesth.1993; 71: 594–596)