BJA Advance Access originally published online on December 2, 2006
British Journal of Anaesthesia 2007 98(1):136-140; doi:10.1093/bja/ael317
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Pressure support ventilation during fibreoptic intubation under propofol anaesthesia
1 Service d'Anesthésie, Institut Gustave Roussy 39 rue C Desmoulins, 94805 Villejuif, France
2 Département d'Anesthésie-Réanimation 4, Hôpital Pellegrin 33076 Bordeaux, France
*Corresponding author. E-mail: bourgain{at}igr.fr
Accepted for publication October 7, 2006.
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Abstract |
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Goal of the study. To assess the benefit of pressure support ventilation during fibreoptic intubation performed under propofol anaesthesia in patients having an anticipated difficult intubation.
Procedures. Thirty-two patients with ENT cancer, and having at least two criteria for anticipated difficult intubation were prospectively included. All patients received topical lidocaine 2% and propofol by plasma target control infusion (initial target concentration 3 µg ml–1, then adjusted to maintain loss of consciousness without apnoea). They were randomly assigned between two groups: spontaneous breathing (SB) or pressure support ventilation (with a support level set at 10 cm H2O) both using FIO2=1. Conditions for fibreoptic intubation, respiratory parameters (pulse oxymetry, ventilatory frequency, tidal volume and 
after intubation) and haemodynamic parameters were recorded.
Results. Patient characteristic data and intubation conditions were similar between both groups. All patients had a successful fibreoptic intubation and none needed a rescue procedure because of desaturation. In spite of a longer duration of intubation, after intubation was lower and tidal volume during intubation was higher with pressure support ventilation than in SB patients [38.1 (4.2) vs 42.3 (4.7) mm Hg and 371 (139) vs 165 (98) ml, respectively]. Desaturation episodes were observed in two SB patients conversely to no episode during pressure support ventilation, probably because of the higher minute ventilation.
Conclusion. Pressure support represents a useful method to improve ventilation during fibreoptic intubation under propofol anaesthesia in patients with an anticipated difficult intubation.
Keywords: airway, management; anaesthetics i.v., propofol; intubation, fibreoptic; pressure support
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Introduction |
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Fibreoptic tracheal intubation is a useful technique in the management of difficult airway. Academic societies recommend to perform fibreoptic intubation in an awake or lightly sedated patient.1 2 Fibreoptic intubation in an awake patient may increase the patient anxiety, induce a bad recall,3 or agitate and oppose the patient, hereby increasing the difficulties of the procedure. Light anaesthesia maintaining spontaneous breathing (SB), as provided by propofol, has been demonstrated to improve the patient comfort but may worsen the airway patency. During fibreoptic intubation in an anaesthetized patient, low oxygen saturation episodes induced by upper airway obstruction have been described.4 In several cases, these episodes led to stop the procedure and to switch to an alternative oxygenation technique (as the intubating laryngeal mask airway).5
The recent introduction of pressure support ventilation (PSV) in modern anaesthesia machines may be helpful to avoid inadequate ventilation in some situations. PSV is a form of partial ventilatory support in which each spontaneous breath is assisted to a constant and adjustable pressure applied during inspiration. If spontaneous ventilatory frequency decreases below a chosen threshold, the anaesthesia machine switches to a rescue pressure-controlled ventilation mode.
The aim of this prospective, randomized study was to compare SB and PSV during fibreoptic intubation in patients with anticipated difficult intubation anaesthetized by propofol.
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Patients and methods |
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After ethical committee approval and informed consent, all patients with upper airway cancer undergoing surgery and having at least two risk factors of difficult intubation1 were prospectively included. Patients with preoperative clinical symptoms or radiological evidence of upper airway obstruction were excluded because preoperative tracheotomy is often performed for these cases.
Before anaesthesia, patients were randomly assigned between two groups: SB or PSV. Patients did not receive any preoperative medication. In case of nasal intubation, lidocaine 5% with naphazoline was applied in the nasal cavities, 10 min before the induction of anaesthesia. The ‘spray as you go’ method was chosen to apply local anaesthetic (lidocaine 2%) on oral and pharyngeal mucosa. After 3 min of pre-oxygenation, induction was performed using target-controlled infusion of propofol (DiprifusorTM, Fresenius, Grenoble, France) with an initial plasma target concentration of 3 µg ml–1 without opioid or benzodiazepine. If the patient was still conscious after blood-effect site equilibration, the propofol target concentration was increased by 1 µg ml–1 step. If systolic arterial pressure decreased below 80 mm Hg, or in case of apnoea, propofol target concentration was decreased. Then, fibreoptic tracheal intubation started (LF3 Olympus with a video remote display). The operator (trainees or registered anaesthesiologists) was not blinded to the patient's group. Before tracheal passage of the fibreoptic bronchoscope, 3 ml of lidocaine 2% were injected above the vocal cords. In case of upper airway obstruction, which could not be managed with simple manoeuvres, transtracheal jet ventilation was prepared in the operating room and a surgeon was ready to perform a tracheotomy.
Three minutes before induction, patients started breathing spontaneously oxygen (fresh gas flow >6 litre min–1) delivered by the anaesthesia machine (Felix Taema France, Antony, France) in a circle system through a facial mask (Fig. 1) specially designed for fibreoptic intubation (Endoscopy mask, VBM-medical, Sulz, Germany). One of the authors who attempted to maintain a gas tight seal held the mask. In group SB, spontaneous ventilation was maintained until the trachea was intubated (Mallinckrodt safety-flex size 6.5). In the PSV group, PSV was started after loss of consciousness at a pressure level of 10 cm H2O. Minimal ventilatory frequency was set at 10 bpm: as a result, pressure support ventilation was triggered if spontaneous ventilatory frequency was higher than 10 bpm; otherwise pressure-controlled ventilation was delivered at a rate of 10 cycles per minute. If the tidal volume was lower than 200 ml, airway pressure level was increased by 2 cm H2O step. Ventilator parameters were standardized: 10 bpm, I:E ratio at 1:2, inspiratory trigger set at the minimal value and maximal inspiratory time at 1.3 s.
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Preoperative assessment included patient characteristic parameters (age, height, weight, sex) and predicted difficult intubation criteria: history of difficult intubation, systemic diseases usually associated with difficult intubation, inter-incisor gap <40 mm, modified Mallampati class including four categories, and neck mobility. Specific criteria related to ENT tumour were added: tongue mobility, cervical radiotherapy, cervical sclerosis and previous cancer ENT surgery.
Intubation conditions assessment included the following parameters: (i) duration of intubation defined as the time elapsed between starting fibrescopy (patient unconscious) and the first respiratory cycle observed on the capnograph after intubation; (ii) the predicted propofol effect-site concentration at the time of intubation; and (iii) clinical upper airway obstruction (partial or complete), hypoxaemia (
< 90%) and subjective difficulties were noted.
After intubation, the first value of end-tidal CO2 was also recorded. During fibreoptic intubation, the following variables were recorded every 2 min: expired tidal volume, ventilatory frequency, minute ventilation, mean airway pressure, and , as well as non-invasive blood pressure and heart rate.
Statistical analysis
The number of patients to include has been determined as follows: in our experience, propofol light anaesthesia, titrated as described, usually depresses minute ventilation by 50% (2.5 litre min–1 instead of 5 litre min–1) with a variability of 50–60%. Assuming that PSV would restore normal minute ventilation, a sample size of 15 patients in each group would have 95% power to detect a difference in means of –2.5 litre min–1 with a common standard deviation of 2 litre min–1, using a two group t-test with a 0.05 one-sided significance level.
Results are presented as mean (SD) or number of patients. Student's t-test was used to compare continuous variables. For others, exact Fisher's test was used. A value of P<0.05 was considered as a significant threshold.
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Results |
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A total of 32 patients [age: 59 (11) yr, height 164 (8) cm, weight 57 (10) kg] were included, 16 in each group. Preoperative variables were similar in both groups. Twenty-seven patients had been previously treated for a head and neck cancer either by surgery or by radiotherapy or both. Most of the patients were classified as class 4 Mallampati (n=28) and presented a severe reduction of inter-incisor gap [15(6) mm].
Predicted propofol effect-site concentration at tracheal tube insertion was similar between groups, with a large interindividual variability (Table 1).
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The number of patients having at least one episode of apnoea or upper airway obstruction was similar (Table 1). In both groups, decreasing the propofol target concentration by 0.5 µg ml–1 steps treated apnoea. In the PSV group, the anaesthesia machine easily ventilated all apnoeic patients whereas in two SB patients, fibreoptic intubation had to be stopped for face mask ventilation. In the SB group, upper airway obstruction (n=2) was partially controlled by adjusting the head position and by jaws thrust. In the PSV group, increasing the level of pressure support above 15 cm H2O was necessary in two patients (18 and 20 cm H2O).
The duration of the procedure was statistically longer in the PSV group with a very large range. In fact, randomization had assigned to that group the five most technically difficult intubation procedures: two oesophageal intubations were early recognized by capnography, two preoperative pharyngeal stenosis and one undiagnosed tracheal stenosis required the use of a size 5.0 tracheal tube. In all these cases, oxygenation was maintained and no rescue jet ventilation was necessary. Two transient episodes of <90% were observed in the SB group during apnoea.
The PSV group of patients had a significantly higher tidal volume and minute ventilation and a lower after intubation than the SB group (Table 2). during fibrescopy was greater, probably because the tidal volumes were sufficient to register a greater end-tidal value during mask breathing.
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Conversely, the ventilatory frequency and mean
were similar in both groups as well as blood pressure and heart rate. |
Discussion |
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TopAbstractIntroductionPatients and methodsResultsDiscussionReferences |
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Fibreoptic tracheal intubation is strongly recommended to manage predicted difficult airway. Historically, light sedation was first ensured by diazepam or midazolam. Patients were drowsy but still conscious and responsive. The rate of success was high, nevertheless 45–50% of the patients recalled the procedure and failure may be observed in restless patients or hyper-reactive upper airways.6 Hypoventilation was rare except when an opioid was given7 and desaturation could be prevented by preoxygenation8 and the use of pharyngeal oxygen supply. However, in patients having preoperative upper airway obstruction, complete loss of airway may occur at very light sedation level or even without any sedation.9 For this reason, those patients were excluded from our study, and had fully awake intubation or tracheotomy, as recommended by academic societies.1 2
General anaesthesia (defined by a loss of consciousness) with SB has then been proposed to improve the patient comfort (analgesia, anxiolysis and amnesia), to facilitate the procedure and to attenuate the response (respiratory or haemodynamic) to the tracheal tube insertion. It should be carefully titrated because an insufficient level of sedation may induce severe laryngospasm,10 whereas excessive sedation may impair airway patency or induce respiratory depression.6
However, in some cases, anatomical or technical reasons prolong the duration of difficult fibreoptic intubation (up to 28 min in the literature6 as in our results) and make desirable to maintain both stable spontaneous ventilation and sedation. Performing fibreoptic intubation in apnoeic patients requires a fast procedure, in order not to exceed the oxygen storages consumption (2–6 min).
As midazolam may be difficult to titrate to achieve deep sedation without hypoxaemia,6 general anaesthesia using sevoflurane or propofol has then been proposed. Sevoflurane anaesthesia has been successfully used in patients with predicted difficult intubation in several studies4 11 12 using various intubation techniques (gum elastic bougie, intubating laryngeal mask airway or rigid bronchoscope) without critical incident. It allowed satisfactory spontaneous ventilation in patients without difficult airways with only 16% of apnoea13 except when sufentanil was added (up to 28%).4 However, it requires a specially designed face mask and leaks make it difficult to maintain a stable end-tidal sevoflurane concentration and therefore a stable level of sedation.
Propofol is attractive because its i.v. delivery is independent from airway management. It provides unconsciousness, amnesia, good pharyngeal muscles and vocal cord relaxation. However, this relaxation may induce moderate hypoventilation and snoring14 with a reduction of tidal volume related to propofol sedative effects and reduction of cross-sectional area of the upper airways.
The mechanisms that lead to airway obstruction during anaesthesia demonstrated physiologic similarities with obstructive sleep apnoea as airway closure at the level of the soft palate.15 Positive pressure ventilation has been reported to improve upper airway obstruction in different clinical situations as sleep apnoea syndrome,16 post-extubation airway obstruction,17 but also during midazolam18 or propofol15 sedation. Positive airway pressure may counteract the anaesthesia-induced pharyngeal narrowing and inspiratory airway collapse as demonstrated by the application of nasal CPAP with magnetic resonance imaging.15 To facilitate fibreoptic intubation in anaesthetized patients, intermittent positive pressure ventilation through a face mask or a special canula sealed with an adhesive transparent dressing has been proposed.19 Nevertheless, clinical applications and limitations of these techniques have not been evaluated in a prospective study.
PSV, widely used in intensive care units, is now available in new anaesthesia machines. This method has been shown to provide better minute ventilation than SB in patients with a laryngeal mask20 and to improve gas exchange in severely hypoxaemic patients during broncho-alveolar lavage21 and during fibreoptic intubation.22
During anaesthesia, similar benefits may be expected by PSV: it was observed during sevoflurane induction in patients without expected difficult intubation,23 during isoflurane anaesthesia (increased tidal volume from 212 to 509 ml)24 and during anaesthesia using a laryngeal mask airway.20 In all these situations, clinical benefits were attributed to an increase in minute ventilation and a decrease in work of breathing.
The benefit of PSV during propofol anaesthesia and difficult airway management remained to be studied in other medical context as obesity or sleep apnoea syndrome.
Oxygen supply and a low number of patients included in this study do not allow us to make a conclusion about the preventive effect of PSV on hypoxaemic episodes. In SB patients, desaturation may be related either to upper airway obstruction or to more air mixture through any face mask leaks.
The level of inspiratory pressure necessary to achieve satisfactory tidal volume was low (always <20 cm H2O), and was unlikely to induce gastric gas insufflation although we did not evaluate this occurrence. Devitt and colleagues measured gastric insufflation at peak airway pressure of 15–30 cm H2O and found that gastric insufflation incidence rose from 2 to 35%.25
A reduction of the respiratory rate (RR) during PSV has been reported in intubated critically ill patients,26 in anaesthetized patients using isoflurane24 and in awake healthy subjects.27 We found no reduction in RR with PS; this may be related to the relatively low level of airway pressure which was required to maintain a satisfactory tidal volume in our patients or to the specific respiratory depressant effect of propofol.28
No difference in haemodynamic stability was observed between PSV and SB. This is not surprising because average intra-thoracic pressure differences were small and would not have induced detectable haemodynamic change as previously demonstrated when comparing positive pressure ventilation with SB during laryngeal mask anaesthesia.29
In conclusion, this study shows that PSV provides more effective ventilation than does SB during fibreoptic intubation in patients anaesthetized using a continuous propofol infusion.
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Acknowledgments |
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The anaesthesia machine was kindly supplied by Taema France (Antony, France).
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Footnotes |
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Presented in abstract at the ASA meeting 2003. 
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References |
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