Effect of cricoid pressure on the view at laryngoscopy.

The impact of cricoid pressure on laryngoscopy is unknown. We have developed a quantitative method of recording the laryngoscopic view using a rigid, zero-degree endoscope. We found that an image matching the laryngoscopist's view could be obtained by positioning the endoscope along the laryngoscopist's 'line of sight'. Photographing this image allowed us to measure laryngeal exposure. We set out to define the effect of cricoid pressure on laryngoscopy using this method. In 40 patients undergoing elective surgery, laryngoscopy was performed with cricoid pressures of 0-60 N, increasing by increments of 10 N. We photographed the laryngoscopic view at each force and recorded dynamic images as cricoid pressure was released. The change in laryngoscopic view with increasing cricoid pressure fell into one of four broad patterns: little change (11 subjects); gradual deterioration (10 subjects); improvement at low force (< 20 N) followed by deterioration (9 subjects); improvement at high force (> 30 N) (10 subjects). We identified five subjects with a good initial view (anteroposterior length of the rima glottidis > 5 mm) who showed a marked deterioration in laryngoscopic view as cricoid pressure increased; in three of these subjects this progressed to obscure the larynx completely at a force of 30 N, 40 N and 60 N, respectively. We conclude that the effect of cricoid pressure on laryngoscopy is complex. However, in some individuals, a force close to that currently recommended (30 N) may cause a complete loss of the glottic view.

Intragastric pressure and its relevance to protective cricoid force.

Cricoid pressure is frequently used to protect the anaesthetised and paralysed patient from passive regurgitation. Although intragastric pressure (Pga) drives regurgitation, its relevance in the setting of protective cricoid force has been largely ignored. We sought to define the likely range of Pga encountered in the population at risk. We studied 100 consecutive patients presenting for surgery requiring mechanical ventilation. We measured respiratory swings in Pga during mechanical ventilation in the paralysed state following rapid sequence induction (n = 24) and routine induction of anaesthesia (n = 76). Pga (mmHg) in the whole group recorded at end-inspiration (Pga-In) and end-expiration (Pga-Ex) was [mean (SD)]: Pga-In 6.48 (2.60) mmHg and Pga-Ex 3.23 (2.24) mmHg. We found no correlation between Pga and body mass index (r2 = 0.018). These findings have implications for the level of cricoid force required to protect a patient during the induction of anaesthesia.

Cricoid pressure: knowledge and performance amongst anaesthetic assistants.

We investigated the cricoid pressure technique of 135 anaesthetic assistants attending the annual conference of the British Association of Operating Department Assistants in May 1997. Their knowledge and training were assessed using a structured interview. Technique was assessed using a simulator measuring applied force during sham cricoid pressure. Our additional aims were to see whether a knowledge of the required force and practical training in the application of a target force would affect performance. Our results highlight a lack of knowledge relating to the manoeuvre. Only about one-third of subjects could quote an appropriate force and fewer than half could give a single contraindication to its use. Very few subjects had been trained on a model before practising the technique on a patient. Technique was poor and we observed a large variation in the force actually applied. Performance, as assessed by the variability of forces applied and proportion of subjects applying force within our target range (30-44 N), was improved markedly by providing simple instruction about the required force in an understandable form. Performance was further improved by practical training in the application of target force on a simulator.

Cricoid pressure: can protective force be sustained?

We studied six operating department assistants performing simulated cricoid pressure on a model of the larynx with the arm either flexed to 90 degrees (flexed position) or fully extended with the elbow locked (extended position). Subjects were asked to maintain forces of 20, 30 and 40 Newtons (N) for a target time of 20 min. Subjects rated pain during each assessment on a four-point verbal rating scale (VRS): 1 = uncomfortable; 2 = hurting; 3 = hurting a lot; and 4 = agony. Times to onset of pain were short and mean times to VRS 3 at each force studied were: 40 N, flexed position 2.3 min, extended position 5.4 min; 30 N, flexed position 4.0 min, extended position 7.5 min; and 20 N, flexed position 9.6 min, extended position 12.5 min. None of our subjects was able to sustain 40 N for the target time. Mean times to release at 40 N were: flexed position 3.7 min, extended position 7.6 min. Only one subject was able to sustain 30 N and then only using the extended arm. Mean times to release at 30 N were: flexed position 6.4 min, extended position (five subjects) 10.8 min. Two subjects with the arm flexed and five with the arm extended achieved the target time at 20 N. Mean times to release at 20 N were: flexed position (four subjects) 13.2 min, extended position (one subject) 14.6 min. Use of the extended arm consistently prolonged times to pain and fatigue. These findings are relevant to the management of cricoid pressure during failed intubation.

Abdominal muscle activity and intraabdominal pressure after upper abdominal surgery.

To examine the influence of abdominal muscle activity on intraabdominal pressure, the integrated surface EMG from upper abdominal muscle (EAB) was related to changes in intragastric pressure (PGA) in six patients after upper abdominal surgery. A similar respiratory pattern of EMG activity was observed in all subjects. EAB increased rapidly at the onset of expiration, and thereafter more slowly throughout expiration. At the onset of inspiration EAB decreased abruptly, and was small during inspiration. Changes in intragastric pressures were closely related to changes in EAB. In five patients PGA and EAB waveforms were almost identical over the respiratory cycle. In one subject a biphasic change in PGA during inspiration was observed, suggesting the influence of other respiratory muscles. Abdominal muscle action results in changes in intraabdominal pressure previously attributed to diaphragmatic impairment.

Single-shot i.v. anaesthesia for outpatient dental surgery. Comparison of 2,6 di-isopropyl phenol and methohexitone.

A single i.v. bolus dose of propofol 3 mg kg-1 was compared with methohexitone 2 mg kg-1 as the sole anaesthetic agent for simple dental extraction in outpatients. Induction of anaesthesia was smoother with propofol, with a lower incidence of excitatory phenomena. Pain on injection was a common complication of both drugs and related to the site of injection. The Leeds Psychomotor Tester was used to assess psychomotor performance during the recovery. Rate of recovery from anaesthesia was similar with both agents, and there was little residual impairment of psychomotor function 40 min after induction.