Biomedical subjects
R G Vanner
Publications and source records attributed to R G Vanner.
Ventilation of a model lung using various cricothyrotomy devices.
In this study we developed a model lung to compare the effectiveness of ventilation using four different cricothyrotomy devices. The Ravussin 13G cannula (VBM Medical), the Quicktrach cannula 4 mm ID (VBM Medical), the Melker cannula 6 mm ID (Cook) and a cuffed tracheal tube 6 mm ID were used in turn to ventilate the model lung through a cricothyrotomy over a range of upper airway resistances. The 6 mm cuffed tracheal tube provided consistently good ventilation independent of upper airway resistance. The 6 mm ID Melker device provided at least reasonable and at best very good ventilation, whatever the patency of the upper airway. The Ravussin cannula could ventilate well with the jet ventilator with low upper airway resistance but could not ventilate at all with complete upper airway obstruction. The Quicktrach performed poorly with low upper airway resistance but well with increased upper airway resistance. With its easier insertion, fewer complications compared to a surgical cricothyrotomy, and the ability to use it with a standard anaesthetic circuit, the authors feel that the 6 mm Melker canula is the technique of choice for emergency trans-tracheal ventilation.
Use of the harmonic scalpel in a patient with a permanent pacemaker.
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A novel method of measuring cricoid force.
The application of cricoid pressure is an effective means of preventing regurgitation of gastric contents when correctly applied. A force of 30 N (3 kg) is recommended for an unconscious patient. This study assesses the validity of using floor scales to measure cricoid force applied by anaesthetic assistants and compares their use to an established training technique. Forty subjects applied pressure to a cricoid model in a blinded manner, on three test occasions. For each test, cricoid pressure was maintained for 1 min and the highest and lowest forces recorded on the model were noted. The first test was before any instruction. The second test followed a period of practice on the cricoid model. For the final test, subjects stood upon a set of floor scales and noted their weight. Force was applied to the cricoid model until the weight on the floor scales reduced by 3 kg. Performance improved both following practice on the model and using the floor scales. Applying cricoid force while standing on floor scales and using the change in weight as a guide resulted in a predictable force on the cricoid model. The use of floor scales is a useful method of demonstrating the forces needed for effective cricoid pressure.
Airway obstruction with cricoid pressure.
Cricoid pressure may cause airway obstruction. We investigated whether this is related to the force applied and to the technique of application. We recorded expired tidal volumes and inflation pressures during ventilation via a face-mask and oral airway in 52 female patients who were anaesthetised and about to undergo elective surgery. An inspired tidal volume of 900 ml was delivered using a ventilator. Ventilation was assessed under five different conditions: no cricoid pressure, backwards cricoid pressure applied with a force of 30 N, cricoid pressure applied in an upward and backward direction with a force of 30 N, backwards cricoid pressure with a force of 44 N and through a tracheal tube. An expired tidal volume of < 200 ml was taken to indicate airway obstruction. Airway obstruction did not occur without cricoid pressure, but did occur in one patient (2%) with cricoid pressure at 30 N, in 29 patients (56%) with 30 N applied in an upward and backward direction and in 18 (35%) patients with cricoid pressure at 44 N. Cricoid pressure applied with a force of 44 N can cause airway obstruction but if cricoid pressure is applied with a force of 30 N, airway obstruction occurs less frequently (p = 0.0001) unless the force is applied in an upward and backward direction.
Cricoid pressure: which hand?
We studied 20 anaesthetic assistants applying simulated cricoid pressure with the left or right hand in random order. Simulated cricoid pressure was continued for up to 5 min with one hand and then, after resting, with the other hand. Applied pressure was measured at intervals and the subjects were blind to the results. Nineteen assistants were right-handed and all routinely applied cricoid pressure with their right hand. Mean (SD) force applied during simulated 'awake' cricoid pressure was 13.8 (5.7) N with either left or right hand, and during 'anaesthetised' cricoid pressure it was initially 25.1 (8.2) N and 24.7 (8.8) N with left or right hand, respectively. Mean force was maintained above 20 N and below 30 N throughout the study period with either hand. Force applied with the left hand was significantly lower than with the right hand but the difference was clinically insignificant (0.4 N). Inadequate or excessive force was more frequently associated with use of the left hand (p < 0.0001). Cricoid pressure was released before 5 min in three cases, two left-handed and one right-handed. Our results demonstrate that anaesthetic assistants apply a lower force than is classically taught and are able to maintain the force with either hand for a sustained period. Application with the left hand is justified where clinically indicated but may have a lower margin for error than when applied with the right hand.
Safe use of cricoid pressure.
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Gastric pressure during emergency caesarean section under general anaesthesia.
Gastric pressure and volume were measured in 20 pregnant women during emergency Caesarean section under general anaesthesia with neuromuscular block. Mean gastric pressure was 11 (range 4-19) mm Hg and we can predict that 99% of women undergoing emergency Caesarean section with neuromuscular block are likely to have gastric pressures of less than 25 mm Hg (mean + 3 SD). This has implications for the amount of cricoid pressure required during induction of anaesthesia. Gastric pressure increased during delivery to 19 mm Hg and fundal pressure caused a gastric pressure of 65 mm Hg in one woman. Gastric pressure decreased significantly after delivery (P < 0.001) to 8 mm Hg. Although we measured large gastric volumes (mean 112 (range 20-350) ml), there was no correlation between gastric volume and gastric pressure.
Caesarean section following a recent retinal haemorrhage.
We report a case of a 32-year-old woman who developed an antenatal Valsalva-induced retinal haemorrhage causing unilateral blindness 38 weeks into her pregnancy. Delivery was achieved by elective Caesarean section under epidural anaesthesia. The influence of anaesthetic technique on a recent retinal haemorrhage is discussed.
Cricoid pressure in chaos.
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The effect of cricoid pressure and neck support on the view at laryngoscopy.
Fifty female patients were studied to compare the view of the larynx at direct laryngoscopy under general anaesthesia with and without cricoid pressure applied. We also compared the view using the standard technique of cricoid pressure with that using cricoid pressure in an upward and backward direction and further investigated whether these views were improved with a firm foam rubber neck support. The order in which the types of cricoid pressure were applied was randomised and also blinded with a drape over the neck. Cricoid pressure was simulated on weighing scales after each case and a mean force of 3.2 kg was applied. The majority of views at laryngoscopy (95%) were grade 1, with too few grade 2 and 3 views for statistical comparison. Both types of cricoid pressure applied without neck support were more likely to give a better view than no cricoid pressure (p < 0.01) and cricoid pressure in an upward and backward direction was more likely to give a better view at laryngoscopy than the standard technique (p < 0.01). Neck support during the standard technique of cricoid pressure did not improve the view of the larynx at laryngoscopy. Cricoid pressure is likely to improve the view at laryngoscopy which may be further improved by applying it in an upward and backward direction.
Cricoid pressure--are two hand better than one?
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Regurgitation and aspiration of gastric contents in a child during general anaesthesia using the laryngeal mask airway.
We present a case report of a child who underwent general anaesthesia for elective surgery in whom regurgitation and aspiration of gastric contents were associated with the use of the laryngeal mask airway (LMA). Bronchospasm developed but mechanical ventilation was not required. The possible mechanisms causing regurgitation are discussed.
Laryngeal mask airway in the failed intubation drill.
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Mechanisms of regurgitation and its prevention with cricoid pressure.
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Regurgitation and the laryngeal mask airway.
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Gastro-oesophageal reflux and hiccup during anaesthesia.
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Can pre-emptive lumbar epidural blockade reduce postoperative pain following lower abdominal surgery?
In a double-blind study, 36 patients who received a standard general anaesthetic for abdominal hysterectomy or myomectomy, received either 15 ml of bupivacaine 0.5% with adrenaline by lumbar epidural injection 15 min before surgery (group A) or the same dose at the end of surgery but before waking (group B). Pain was assessed for 24 h by cumulative morphine dose (self-administered by patient-controlled analgesia), visual analogue scale and verbal rating score. Patients were included for analysis if they were pain free on waking and for at least 2 h after. There was no significant difference (p > 0.05) between the two groups in morphine dose, visual analogue scale or verbal rating score at 6 and 24 h after waking. As expected, there was a significant difference in the mean time of first use of patient-controlled analgesia (4.26 h in group A vs 5.06 h in group B, p < 0.05). Consequently, we compared the morphine dose, visual analogue scale and verbal rating score at 23 h in group A with those at 24 h in group B. Again there were no significant differences between the two groups. We were unable to demonstrate that epidural blockade had a significantly better effect on postoperative pain when administered before, rather than after, surgery.