Body position and the effectiveness of mask ventilation in anaesthetised paralysed obese patients: A randomised cross-over study : European Journal of Anaesthesiology | EJA

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Airway-ventilation

Body position and the effectiveness of mask ventilation in anaesthetised paralysed obese patients

A randomised cross-over study

Chang, Jee-Eun; Seol, Taikyung; Hwang, Jin-Young

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European Journal of Anaesthesiology 38(8):p 825-830, August 2021. | DOI: 10.1097/EJA.0000000000001473
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Abstract

Introduction

Mask ventilation is an integral part of airway management during general anaesthesia and critical care. Maintaining upper airway patency is important for successful mask ventilation, and difficult and impossible mask ventilation can lead to serious complications such as hypoxia or cardiac arrest. Difficult mask ventilation can also lead to gastric insufflation during induction of anaesthesia, increasing the likelihood of regurgitation with risk of pulmonary aspiration.

Airway management is more challenging in obese patients.1 Not only do they have impaired respiratory mechanics, including a decreased functional residual capacity (FRC) and vital capacity, increased airway resistance, and reduced oxygen reserve, but they also have an increased metabolic rate and oxygen consumption.2 Maintaining airway patency may be difficult in obese patients due to the high collapsibility of upper airway structures and technical difficulty in applying the mask. When obese patients move from the sitting to the supine position, there is a further deterioration in respiratory mechanics,3 putting them at higher risk of rapid desaturation during apnoea.4 The supine position worsens upper airway obstruction due to gravitational effects on the upper airway structures,5 and airway manoeuvres for achieving upper airway patency may be less effective in obese patients, particularly in the supine position. A BMI of 30 kg m−2 or greater has been reported to be an independent predictor of difficult mask ventilation.6 According to a previous study,7 the incidence of difficult mask ventilation was 8.8% in obese patients,7 which is significantly higher than that reported in the general population (2.2%).8

The sitting position provides better respiratory mechanics and decreases collapsibility of the passive pharynx in anaesthetised patients with obstructive sleep apnoea.9 The head-up position also improves preoxygenation4 and the laryngoscopic view.10 However, its effect on the effectiveness of mask ventilation has not been evaluated. We postulated that mask ventilation would be improved in the 25° semisitting position compared with that in the supine position. We compared the effectiveness of mask ventilation in the supine and the 25° semisitting position in anaesthetised paralysed obese patients.

Methods

The current study was approved by the Institutional Review Board of SMG-SNU Boramae Medical Center in Seoul, Republic of Korea (20190712/20-2019-49/082) on 4 March 2020, and registered at ClinicalTrials.gov (NCT03996161). Written informed consent was obtained from each participant in the study.

Adults with a BMI at least 30 kg m−2 undergoing general anaesthesia for elective surgery were enrolled. The exclusion criteria included structural abnormalities or diseases of the upper airway, risk of aspiration, cardiovascular or respiratory diseases, or pregnancy. Mallampati classification, thyromental distance, cervical extension and mandibular protrusion were assessed preoperatively. Mandible protrusion was graded in the following manner: grade A, the lower teeth can protrude further than the upper teeth; grade B, the lower teeth meet with the upper teeth in the midline; and grade C, the lower teeth do not reach the upper teeth.

No premedication was given. Patients were monitored with pulse oximetry, electrocardiography, noninvasive arterial pressure and capnography during their procedure. After preoxygenation with 100% oxygen, anaesthesia was induced with intravenous propofol 1 to 2 mg kg−1 lean body weight (LBW). LBW was calculated as follows: [9270 × total body weight (kg)/(6680 + 216) × BMI (kg m−2)] in men, and [9270 × total body weight (kg)/(8780 + 244) × BMI (kg m−2)] in women.11 After checking for loss of consciousness, ventilation with sevoflurane (2 vol.%) in 100% oxygen was manually assisted by a board-certified staff anaesthesiologist using a well fitted face mask. Neuromuscular blockade was induced using rocuronium 0.8 mg kg−1 of LBW, and monitored by train-of-four stimulation using an acceleromyograph (TOF-Watch SX, Organon Ireland Ltd., Dublin, Ireland). At a train-of-four count of zero, two-handed mask ventilation was started using an anaesthesia machine (Datex Ohmeda Aestiva 5; General Electric company, Chicago, Illinois, USA) in the following alternating patient positions: supine and 25° semisitting positions by an assigned random order from a computer-generated program. The 25° semisitting position was achieved by "breaking" the operating table and adjusting the chest and head sections until they were 25° head up, as measured by an inclinometer. During two-handed mask ventilation, the thumb and thenar eminence of each hand held each side of the facial mask, and the second to fifth digits of each hand grasped and lifted the mandible with the mouth opened and the head tilted. If required, a step stool was used during mask ventilation in the semisitting position. Mechanical ventilation in a pressure-controlled mode was established with a peak inspiratory pressure of 15 cmH2O, a respiratory rate of 15 bpm, an inspiratory-to-expiratory ratio of 1 : 1, and no positive end-expiratory pressure. According to the product manual of the ventilator, the accuracy of a delivered-to-monitored volume is better than 10 ml at volumes less than 60 ml, better than 15 ml at volumes less than 210 ml and better than 7% at volumes more than 210 m. The investigator who performed mask ventilation was unaware of the ventilatory outcomes displayed on the anaesthesia monitor screen. Exhaled tidal volume and peak inspiratory pressure were recorded by another investigator during 10 consecutive breaths in each position. Relocation of the mask was not permitted during the recording. The last five breaths in each position were included in the data analysis. Insufficient mask ventilation was defined as an average exhaled tidal volume of less than 4 ml kg−1 of LBW. Dead space ventilation was defined as an average exhaled tidal volume of less than 2.2 ml kg−1 of LBW12 together with no clinical manifestations of ventilation such as absence of end-tidal CO2 tracing. If hypoxaemia occurred (pulse oxygen saturation <90%), the study was terminated, and subsequent management was at the discretion of the attending anaesthesiologists. After completion of the study protocol, tracheal intubation was performed in the position chosen by the anaesthesiologist.

The presence of predictors for difficult mask ventilation, other than BMI at least 30 kg m−2, were recorded: age more than 55 years, high Mallampati classification (3 or 4), thyromental distance less than 6.0 cm, limited cervical extension, limited mandibular protrusion (grade B or C), obstructive sleep apnoea, habitual snoring and edentulism.6,13

The primary outcome was the average exhaled tidal volume (ml kg−1 LBW) during the latter five respirations in each patient position. Secondary outcomes were respiratory minute volume (ml kg−1 min−1 LBW), counted as the total sum of exhaled tidal volumes during the five respirations in each patient position multiplied by three, peak inspiratory pressure, and the occurrence of insufficient ventilation or dead space ventilation.

The sample size was determined based on a preliminary study that measured the average exhaled tidal volume in the supine position (7.5 ml kg−1) in 15 patients. Assuming a difference of 10% of the mean in the average exhaled tidal volume between the two different positions and a SD of 1.5 ml kg−1, 34 patients were required with 80% power and 95% significance level. Consequently, 38 were enrolled to take account of possible dropouts.

Statistical analyses were performed using SPSS Statistics 26 (IBM Corp., Armonk, New York, USA). Normality of data distribution was tested using the Shapiro–Wilk test. Data are expressed as the number of patients (%) and mean ± SD. A paired t test or Wilcoxon's signed rank test was conducted to analyse the exhaled tidal volume, respiratory minute volume, and peak inspiratory pressure between the two patient positions. The χ2 test or McNemar test was performed to compare the occurrence of insufficient or dead space ventilation between the two patient positions. The presence of a carry-over effect was checked using a linear mixed-effect model. A P value of less than 0.05 was considered statistically significant.

Results

Forty-five patients were recruited from March to June 2020. Seven did not meet the inclusion criteria, leaving 38. Of these, two were excluded due to protocol violations, and the remaining 36 were included in the final analysis, 19 women and 17 men. Their mean age was 46 ± 16 years, with height 165 ± 11 cm, weight, 97 ± 14 kg and BMI 36 ± 3 kg m−2 (Fig. 1).

F1
Fig. 1:
Flow diagram.

Predictors for difficult mask ventilation are shown in Table 1. Ventilatory outcomes according to the position and the period are presented in Table 2. The linear mixed-effect model revealed no significant carry-over effect of the first position on the outcomes in the second position, the exhaled tidal volume (P = 0.492), minute volume (P = 0.492) and peak inspiratory pressure (P = 0.339). Ventilatory outcomes in the two patient positions are shown in Table 3. Exhaled tidal volume was significantly improved in the semisitting position compared with that in the supine position 9.3 ± 2.7 vs. 7.6 ± 2.4 ml kg−1 (P < 0.001). Respiratory minute volume was higher in the semisitting position compared with that in the supine position 139.6 ± 40.7 vs. 113.4 ± 35.7 ml kg−1 min−1 (P < 0.001). Peak inspiratory pressure during mask ventilation was higher in the semisitting position than in the supine position 15.5 ± 0.5 vs. 15.3 ± 0.4 cmH2O (P = 0.008). Insufficient ventilation occurred in one patient during mask ventilation in the supine position, and dead space ventilation was not observed in any patient. No difference was observed in the occurrence of insufficient or dead space ventilation between the two patient positions during mask ventilation 0/36 vs. 1/36, semisitting vs. supine (P = 0.314). Ventilatory outcomes are presented according to the sequence in the supine and 25° semisitting positions in Table 3. Exhaled tidal volume and respiratory minute volume were significantly higher in the semisitting position than in the supine position irrespective of the sequence of patient position.

Table 1 - Patients with predictors for difficult mask ventilation
Older than 55 years 11 (31)
High Mallampati classification (3 or 4) 6 (17)
Thyromental distance <6.0 cm 1 (3)
BMI ≥ 30 kg m−2 36 (100)
Limited cervical extension 2 (6)
Limited mandibular protrusion (grade B or C) 19 (53)
Obstructive sleep apnoea 8 (22)
Habitual snoring 20 (56)
Edentulous 4 (11)
Values are number of patients (%). Mandibular protrusion: grade B, the lower teeth meet with the upper teeth in the midline; grade C, the lower teeth do not reach the upper teeth.

Table 2 - Ventilatory outcomes stratified by the supine and 25° semi-sitting positions and cross-over sequence
First Second
Supine, n=18 25° semisitting, n=18 Supine, n=18 25° semisitting, n=18 P value (carry-over effect)
Exhaled tidal volume (ml kg−1) LBW 7.8 ± 2.4 9.0 ± 3.0 7.3 ± 2.4 9.6 ± 2.5 0.492
Minute volume (ml kg−1 min−1) LBW 116.8 ± 35.5 134.5 ± 44.4 109.9 ± 36.6 144.7 ± 37.2 0.492
Peak inspiratory pressure (cmH2O) 15.2 ± 0.3 15.5 ± 0.5 15.3 ± 0.4 15.4 ± 0.4 0.339
Insufficient or dead space ventilation 1 (6) 0 (0) 0 (0) 0 (0)
Values are mean ± SD or n (%). Insufficient ventilation was described as an average exhaled tidal volume of less than 4 ml kg−1 of LBW, and dead space ventilation was defined as an exhaled tidal volume of less than 2.2 ml kg−1 of LBW with no clinical manifestation of ventilation. LBW, lean body weight.

Table 3 - Ventilatory outcomes in the supine and 25° semi-sitting positions
Supine, n=36 25° semisitting, n=36 P value
Exhaled tidal volume (ml kg−1) LBW 7.6 ± 2.4 9.3 ± 2.7 <0.001
Minute volume (ml kg−1 min−1) LBW 113.4 ± 35.7 139.6 ± 40.7 <0.001
Peak inspiratory pressure (cmH2O) 15.3 ± 0.4 15.5 ± 0.5 0.008
Insufficient or dead space ventilation 1 (3) 0 (0) 0.314
Values are mean ± SD or n (%). Insufficient ventilation was described as an average exhaled tidal volume of less than 4 ml kg−1 of LBW, and dead space ventilation was defined as an exhaled tidal volume of less than 2.2 ml kg−1 of LBW with no clinical manifestation of ventilation.

No patients experienced hypoxaemia during the study period. After completion of the study protocol, tracheal intubation was performed in all patients.

Discussion

The current study showed that mask ventilation is more effective in the 25° semisitting position than in the supine position in anaesthetised paralysed obese patients. Exhaled tidal volume and minute volume were improved in the semisitting position compared with the supine position.

Our finding that the 25° semisitting position improved mask ventilation is in line with previous studies showing that patient positioning affects upper airway collapsibility.9,14 Tagaito et al.9 reported that postural change from the supine to sitting position in anaesthetised patients with obstructive sleep apnoea increased cross-sectional area of the pharynx and decreased closing pressures of the airway. They explained that postural changes influence the gravitational direction exerted on the upper airway structures. Ikeda et al.14 indicated that 30° head elevation reduced collapsibility of the upper airway during midazolam sedation. Airway manoeuvres, such as jaw thrust, can be more effective in the semisitting position because the perpendicular component of gravity on the upper airway structures decreases. However, it should be noted that the 25° semisitting position may have a small influence on airway patency because approximately 90% of the weight is still pushing onto the upper airway according to vector mathematics (cosine of 25°) in the 25° semisitting position.

Atelectasis frequently occurs during induction of general anaesthesia and paralysis.15 Changing from the upright to the supine position leads to a decrease in FRC and an increase in airway resistance, and the effects of the supine position on atelectasis are greater in obese individuals due to the greater abdominal mass.5 Therefore, obese patients are more susceptible to atelectasis than normal weight patients during general anaesthesia and paralysis.16 Furthermore, the amount of atelectasis formation is more severe, and atelectasis is sustained over a longer period postoperatively in obese patients.17,18 Atelectasis may lead to peri-operative hypoxaemia,18,19 and contribute to localised infection and pulmonary complications.20 Various strategies such as an alveolar recruitment manoeuvre, and adjustment of inspired concentration of oxygen have been suggested for the prevention of atelectasis in the peri-operative period.16 Difficult and impossible mask ventilation can also lead to atelectasis during the induction of anaesthesia. Mask ventilation in the semisitting position might alleviate the development of atelectasis following improved respiratory mechanics compared with that in the supine position, although this was not evaluated in our study.

Preoxygenation in the 25° head-up semisitting position in the morbidly obese has been reported to be more effective at increasing the desaturation safety period than the supine position.4 Preoxygenation in this head-up position may provide more oxygen storage in a larger lung volume and reduce the risk of developing atelectasis and shunting, resulting in more efficient oxygenation. Ensuring a greater margin of safety for oxygenation during pre-oxygenation would be clinically useful in cases of difficult and impossible mask ventilation.

The semisitting position may be also advantageous during tracheal intubation. According to Lee et al.10 the laryngeal view is significantly improved in the 25° back-up position by relieving the force of gravity during direct laryngoscopy. They suggested that gravity moves oropharyngeal and laryngeal structures slightly more caudally in the 25° back-up position compared with the supine position, and the decreased angle between the laryngeal angle and line of vision facilitates a better laryngeal view.

The semisitting position, by reducing upper airway collapsibility and improving respiratory mechanics, would be clinically beneficial for effective preoxygenation, mask ventilation and tracheal intubation during induction of anaesthesia. The more upright position, however, against a background of general anaesthesia-induced cardiovascular depression, may lead to hypotension. Therefore, use of the semisitting position should be carefully considered and managed in patients with cardiovascular and neurologic comorbidities.

Although our study included patients with and without predictors for difficult mask ventilation other than obesity, the effect of other predictors for difficult mask ventilation on ventilatory outcomes was not evaluated because the study was not powered to do so. Another point to note is that for mask ventilation we used the V–E technique as opposed to the C–E technique. The V–E technique employs the thumb and thenar eminence of each hand to press the mask onto the face, and the second to fifth digits of both hands to grasp and lift the mandible forming an ‘E’ shape during two-handed mask ventilation.21 Based on a previous study in anaesthetised obese patients, the V–E technique was found to be more effective for two-handed ventilation than the C–E technique, where the mask is held with each thumb and index finger over each side of the mask in a ‘C’ shape, and the third to fifth digits of each hand lift the mandible in an ‘E’ shape.

It has been suggested that the application of positive end-expiratory pressure during induction of anaesthesia in obese patients will increase the duration of nonhypoxic apnoea22 and prevent atelectasis formation.23 However, a comparison of 10 cmH2O positive end-expiratory pressure with zero end-expiratory pressure at a peak inspiratory pressure at least 20 cmH2O24 found a significantly higher incidence of gastric insufflation with the former, indicating caution when positive end-expiratory pressure was used during mask ventilation. Another study25 showed that pressure-controlled ventilation with a peak inspiratory pressure of 15 cmH2O and zero end-expiratory pressure achieved a lower occurrence of gastric insufflation with sufficient pulmonary ventilation during induction of anaesthesia. These studies were performed in nonobese patients,24,25 and it is unclear which combination of peak inspiratory pressure and positive end-expiratory pressure would provide the best balance between the probability of sufficient ventilation and the probability of absence of gastric insufflations in the obese. In our study, the effect of position on mask ventilation was evaluated after achieving maximal neuromuscular blockade, extending the period of mask ventilation more than is usually required in clinical practice. Because the obese may be at risk of aspiration due to increased intra-abdominal pressure,26 in the current study, a peak inspiratory pressure of 15 cmH2O and no positive end-expiratory pressure were applied during mask ventilation to minimise the possibility of gastric insufflation.

This study is a 2 × 2 cross-over design. The main disadvantage of a cross-over design is a possible carry-over effect of the first treatment on the second treatment. In the current study, there was no significant carry-over effect of the first position during the second period.

The current study had several limitations. First, the investigator who performed mask ventilation was not blinded to the patient position, but complied with the rules of the study protocol and was unaware of the ventilatory data. Second, the study was performed in anaesthetised, paralysed patients. The use of a neuromuscular blocking agent may influence the efficiency of mask ventilation although it is a controversial issue.27,28 Thus, our findings may not be generalisable to nonparalysed or spontaneously breathing patients. Third, an oral airway was not used in this study because it was intended to evaluate the effect of two different patient positions on the efficiency of mask ventilation irrespective of the use of an oral airway. Fourth, this study was conducted in obese patients, and obesity is a predictor for difficult mask ventilation.6 However, mask ventilation can be difficult or impossible due to mechanisms other than obesity, and the effect of the semisitting position on mask ventilation might differ in these situations. Fifth, we did not directly evaluate how the semisitting position affects respiratory mechanics during mask ventilation. Further studies are required to evaluate the respiratory mechanics in the semisitting position. Lastly, this study was conducted in obese patients with a mean BMI of 36 kg m−2. The effectiveness of mask ventilation in the 25° semisitting position may be different between obese and morbidly obese patients (BMI ≥ 40 kg m−2). Therefore, our finding may not be applicable to morbidly obese patients.

In conclusion, in the semisitting position for a given peak inspiratory pressure, the tidal volume is increased compared with in the supine position. There was no significant difference in adequacy of ventilation or difficulty in its performance.

Acknowledgements relating to this article

Assistance with the study: none.

Financial support and sponsorship: none.

Conflicts of interest: none.

Presentation: none.

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