In recent decades, the prevalence of obesity (BMI >30–34 kg m−2), morbid obesity (BMI >35) and severe obesity (BMI >40) has dramatically increased worldwide.1 Adult obesity in the USA has increased from 14 to 31% in two decades (1978–2000) and is still increasing. In the UK, adult obesity increased from 6 to 21% in men and from 8 to 23.5% in women over a period of 20 years (1980–2001).2 In Sweden, the prevalence of obesity increased in adult (25–64 years) men from 10.4 to 19.1% and in women from 12.9 to 17.9% in two decades (1986–2004).3 As a consequence, many patients with morbid obesity are in need of surgery. Currently, bariatric surgery is one of the most important and fastest increasing surgical procedures.4
In adults, overweight has often been a long-lasting problem.3 Obese individuals frequently have multiple co-morbidities such as cardiovascular and respiratory problems. In fact, obesity is now considered the most common metabolic disease in mankind.5 Consequently, obesity is associated with an overall increase in mortality. Severe obesity has been shown to reduce life expectancy by 8–10 years.6
The excess body mass significantly stresses the physiological reserves of morbidly obese patients and also provides a challenge in operating rooms and ICUs.7,8 Obesity is considered an independent risk factor for ICU death.9 In anaesthesia, the risk of respiratory adverse events is approximately four times higher in obese patients than in lean patients.10 Morbid obesity has been associated with an increased risk of endotracheal intubation and mask ventilation difficulties, and may increase the likelihood of desaturation and aspiration compared to that in patients with normal BMI.11,12
Rapid sequence induction (RSI) is currently considered as the ‘golden standard’ for patients with increased risk of difficult mask ventilation (DMV) and aspiration.13 Consequently, during induction of anaesthesia, in patients with high BMI, maintaining spontaneous breathing can be the key element with regard to the avoidance of hypoventilation and desaturation, if fully awake endotracheal intubation techniques are not used. In this study, we evaluated the cardiovascular and respiratory consequences of volatile RSI in morbidly obese patients, with the aim of maintaining spontaneous breathing as long as possible.
The anaesthesia technique under investigation based on sevoflurane, propofol, suxamethonium and alfentanil was designed to maintain spontaneous breathing as long as possible and to keep the period of apnoea as short as possible before endotracheal intubation (Fig. 1).
The study included 34 patients with a mean BMI of 42.4 kg m−2 scheduled for bariatric surgery by laparoscopic Roux-en-Y gastric bypass (morbidly obese group) and 22 lean patients (mean BMI 25.6 kg m−2) scheduled for abdominal surgery with general anaesthesia (control group) (Table 1).
The study (DNR 09-042M) was approved by the Regional Ethical Review Board in Umeå, Sweden (Chairman A. Iacobaeus) on 30 October 2009. All included patients gave their consent after being informed about the study protocol orally and in written form, and were studied consecutively. Patients with unstable angina pectoris, systemic inflammatory response syndrome (SIRS)/sepsis or massive bleeding and those in need of awake intubation were excluded.
Data processing was done using Statistical Package for Social Sciences (SPSS) version 18.0. Comparison of the mean values between the different variables in the morbidly obese patients and the control group was performed by t-test for independent variables, supposing unequal variances. As the variables were not perfectly normally distributed and the number of cases was limited, a non-parametric test was also done by Mann–Whitney U-test. A P value less than 0.05 was considered as indicating statistical significance.
Information on diagnosed underlying preoperative co-morbidities, regular medication and patient characteristics was collected preoperatively. All patients fasted for more than 6 h and were premedicated with 2 g paracetamol orally. No sedatives were included in the premedication because of the potential risk of hypoventilation.7,8,14,15
In the preoperative room, an infusion of glucose 25 mg ml−1 at 1.5 ml per theoretical ideal body weight (IBW) per hour was started, and colloidal fluids were infused (approximately 500 ml) before induction of anaesthesia in both groups. IBW was calculated as the height in centimetre minus 100 in both sexes.7
A 20-gauge cannula (BD Arterial Cannula, REF 682245; Becton. Dickinson, Swindon, UK) was placed in the left radial artery. Invasive arterial blood pressure (IBP) was measured continuously with a standard transducer system (BD Arterial Cannula, REF 682245 and BD Transducer Blood Sampling Set, REF 685158) in the morbidly obese patients. An automatic non-invasive blood pressure (NIBP) measurement technique was used in the control group.
The patients were required to lie down in the supine position for 5 min without extra oxygen, before arterial blood samples were collected. An arterial blood sample was collected from the radial or femoral artery in the control group. Arterial blood samples were analysed immediately, using a blood gas analyser (Radiometer Copenhagen ABL 800 flex; Radiometer, Copenhagen, Denmark).
In the operating room, all patients were preoxygenated using the method described below (see anaesthetic technique). Peripheral oxygen saturation (SpO2) was registered just after the preoxygenation period and 1 min after endotracheal intubation. IBP in the morbidly obese group and NIBP in the control group were registered just before the preoxygenation period and 2 min after the endotracheal intubation in a 5–10° reverse Trendelenburg's position. A respirator-integrated timer was used to measure the three periods of interest: spontaneous breathing time (SBT), apnoea time and total time. The SBT was defined as the time (in seconds) from the beginning of the administration of sevoflurane to the disappearance of spontaneous breathing. This moment was detected using clinical signs and the point at which expiratory flow in the capnograph ceased. The apnoea time was defined as the time (in seconds) between disappearance of spontaneous breathing and successful endotracheal intubation (total time-SBT; Fig. 1). A successful endotracheal intubation was defined as the sensation and/or a clear visualisation of the endotracheal tube crossing the glottis,16 or as the first signs of expiratory flow in the capnograph (in patients with Cormack–Lehane 3–4). Intubation circumstances were evaluated using the Mallampati and Cormack–Lehane scales.12 All anaesthetic procedures were performed by the same anaesthetist.
We followed the common principles of RSI – no ventilation was performed before endotracheal intubation.17,18 Each patient climbed up onto the operating table and was carefully positioned to a semi-sitting 20–30° reverse Trendelenburg's position with large pillows under the shoulders and head. The head was placed in the ‘sniffing’ position to facilitate preoxygenation and endotracheal intubation.19 One minute before the preoxygenation period, atropine 0.5 mg and propofol 20 mg were administered intravenously, to facilitate breathing through the facemask system.
A scavenging facemask system (Medivent, Westfield, Massachusetts, USA) was used to avoid volatile gas spill in the operating room. Patients were preoxygenated for 2 min through a tight face mask with a continuous positive airway pressure (CPAP) of +8 cmH2O, FiO2 0.9 and a fresh gas flow of 10 l min−1. Each patient was asked to take two vital capacity breaths with a 3-s inspiratory pause (Fig. 1). Thereafter, the sevoflurane vapouriser was opened to 8%, keeping the face mask tight, and the timer was started. The patient was then asked to take another three vital capacity breaths, now with sevoflurane 8%, without an inspiratory pause.20,21 If the breaths were clinically evaluated as too small, a fourth breath was requested. In each case, the sevoflurane in the vapouriser was altered to 3% after 30 s. At this point, propofol 1.5 mg kg−1, alfentanil 20 μg kg−1 and suxamethonium 1 mg kg−1 were administered as a rapid sequence bolus (Fig. 1). Doses of propofol and alfentanil were based on the IBW. Doses of suxamethonium were based on total body weight (TBW).14,22 The clinically sufficient stage of anaesthesia was determined by absence of eyelash reflex and presence of miosis. In addition to depth of anaesthesia, an appropriate level of muscle relaxation for laryngoscopy was determined as when fasciculations in the face disappeared and the jaw felt relaxed. When an adequate depth of anaesthesia and muscle relaxation had been confirmed, the face mask was released and the patient was endotracheally intubated using a standard endotracheal tube with a stiff stylet, and a standard laryngoscope with a short handle and a Macintosh blade. Immediately after endotracheal intubation, the patient was repositioned to a 5–10° reverse Trendelenburg's position to ensure optimal venous return.7,19
After intubation, the patient was slightly hyperventilated using a pressure or volume controlled mode positive end-expiratory pressure (PEEP) +8 cmH2O, inspiration/expiration ratio to 1 : 1, tidal volume approximately 7 ml kg−1 IBW and minute ventilation approximately 95 ml kg−1 IBW. Ten minutes after intubation, a recruitment manoeuvre (holding +30 cmH2O pressure for 10 s) was performed in all patients. Anaesthesia was maintained by sevoflurane (0.8–0.9 minimum alveolar concentration, MAC), an infusion of remifentanil (0.25–0.35 μg kg−1 of IBW per minute) and vecuronium bolus 0.1 mg kg−1 of IBW.8,23 Mean arterial pressure was kept at at least 65 mmHg using ephedrine and phenylephrine and/or colloidal fluids if required during surgery.
The key observations in this study were a long period of spontaneous breathing during induction of anaesthesia resulting in a short period of apnoea before endotracheal intubation, and no desaturation or no systemic hypotension (Table 1, Figs 2 and 3). The methods used for preoxygenation and induction of sleep were successful. Slight injection pain was noticed with the bolus propofol given before preoxygenation [20 mg intravenously (i.v.)] in some patients. The patients were compliant in taking deep breaths with a tight face mask. The induction of sleep was smooth, without muscle twitches or undesirable movements in both groups. For the majority of patients, three to four deep inhalations of sevoflurane were enough to cause the eye flash reflex disappear. At 30 s from the beginning of the inhalation of sevoflurane, when propofol, alfentanil and suxamethonium were administered as a RSI bolus, every patient was breathing normally (Fig. 1, Table 1). Some of the patients could react to questions like ‘do you feel injection pain in your cannula’ with, for example shaking their heads, but always with closed eyes. All endotracheal intubations were successful at the first attempt. No cases of aspiration were observed. SpO2 was 100% before and after endotracheal intubation in all patients. No periods of desaturation or hypoxia were registered. In the morbidly obese group, the SBT was 67 (48–91) s, the apnoea time was 44.8 (21–65) s and the total time was 111.8 (82–155) s. In the control group, the SBT was 70.2 (50–88) s, apnoea time 45.7 (23–61) s and total time 116 s (100–141) s. No significant differences in measured time periods were found between the groups (Table 1, Fig. 2). After induction of anaesthesia, all patients were haemodynamically stable. Mean arterial pressure fell from baseline by a mean of 21.2% in morbidly obese group and 18.8% in the control group. There were no significant differences between the groups (Table 1, Fig. 3). End-tidal MAC value (et-MAC) registered immediately after endotracheal intubation was 0.8 in all the patients (Table 1).
According to a preoperative questionnaire, the majority of patients had been severely overweight for between 15 and 20 years (Table 2). The mean BMI was 42.4 (36–56.7) kg m−2 in the morbidly obese group and 25.6 (19.5–30.4) kg m−2 in the control group. The mean age was 43.5 (18–63) years in morbidly obese patients and 49.1 (20–68) years in controls. Most patients were women: 61.8 and 86.4% in the morbidly obese and control groups, respectively. Hypertension, snoring, obstructive sleep apnoea syndrome (OSAS) or obesity hypoventilation syndrome (OHS), dyspnoea [during exercise below a metabolic equivalent of task of 4],24 smoking, asthma and diabetes mellitus were common findings in the morbidly obese group. In contrast, no one in the control group had, for example, a diagnosis of OSAS/OHS or was suffering from dyspnoea (Tables 2 and 3). The preoperative arterial blood samples showed no significant differences between the groups except for higher haemoglobin and lower lactate values in obese patients (Table 4).
Limitations and strengths of the study
There are some limitations in this study. The study was a pilot setting and the limited power of this cohort can be a confounding factor for the results. Further, real endotracheal intubation difficulties, desaturation, instable haemodynamics, arrhythmias or other more rarely occurring problems such as compliance difficulties with inhalation anaesthetics may have been missed. However, all included patients were treated by the same standardised protocol and all anaesthetic procedures were performed by the same anaesthetist to minimise investigation bias.
To perform RSI in obese patients with low functional residual capacity and a propensity for atelectasis and hypoxia, while preventing circulatory adverse events, is challenging. Creating a safe time period for endotracheal intubation that is both long enough and has adequate oxygenation is crucial. Even if preoxygenation is adequate, the period of apnoea should be minimised as much as possible to reduce the risk of desaturation and hypoxia. We endeavoured to optimise the length of preoxygenation, spontaneous breathing and lung recruitment periods. The results of this study demonstrate that both morbidly obese and lean patients can be endotracheally intubated in a RSI manoeuvre that is safe and controlled, without desaturation or a significant fall in arterial blood pressure (Table 1, Fig. 3). The described protocol ensured a long spontaneous breathing period with CPAP in combination with rapid endotracheal intubation. The administration of sevoflurane, along with the timing and dosage of propofol, alfentanil and suxamethonium according to this protocol, proved to be a well functioning combination.
Sevoflurane was chosen for its low blood/gas partition coefficient, non-irritability and ability to maintain spontaneous breathing.25 Traditionally, common situations for using volatile anaesthetics (today mainly sevoflurane) for induction of anaesthesia are surgery in children, patients with upper or lower airway obstruction or for intra-thoracic processes: RSI for morbidly obese patients has not previously been included in this list. Different techniques for using sevoflurane as an induction anaesthetic agent have been described (e.g. single-breath, tidal volume, technique and vital capacity techniques).20,21 Despite the good qualities of sevoflurane, this volatile anaesthetic has not been utilised, as it could have been, in morbidly obese patients or in emergency situations, as an induction agent alone or in combination with other hypnotics. The latest guidelines for general anaesthesia for emergency situations do not suggest volatile agents at all as an alternative hypnotic agent; this means that i.v. induction is recommended for all situations when RSI is needed.13 However, this study clearly points to the possibility to use sevoflurane for anaesthesia induction in morbidly obese patients. The recruitment of the lungs in order to normalise ventilation/perfusion ratio may be a fundamental requirement for obtaining the desired rapid effect from volatile anaesthetics.
Propofol was chosen to provide larynx and pharynx relaxation, and alfentanil for its potency, rapidity of action and possibility to be administered as a rapid i.v. bolus.13,26 Additionally, alfentanil has no respiratory adverse effects.27 We utilised remifentanil as a perioperative drug after RSI to optimise the moment of extubation and a postoperative period. Suxamethonium was chosen because it is well documented in morbid obesity.14,22 Further, opioids and hypnotics have been shown to have fewer effects on intubation conditions when using suxamethonium compared with a non-depolarising muscle relaxation agent.13
Today, in clinical practice, most centres estimate lean body mass only approximately (e.g. with the formula height in centimetre minus 100).7 A more objective determination of the real lean body mass prior to anaesthesia, for example through body impedance measurements, would probably be preferable for optimising dosage and administration of narcotics to morbidly obese patients. The dosage of suxamethonium and estimation of lean body mass should perhaps be evaluated in more detail. In our cohort, one female patient developed very strong muscle fasciculation after a dose of succamethonium (1 mg per TBW). This was thought to be the result of sarcopenic obesity or low/normal pseudocholinesterase activity.14,22 It is possible that the dosage of suxamethonium should be reduced by 20–30% to approximately 0.7–0.8 mg kg−1 of TBW for obese patients with low physical activity.
No sedatives were given preoperatively because of an increased propensity for sleep apnoea and a possibly weakened response to CO2 in the brain respiratory centre in morbidly obese patients.8,15,28 The only sedative drug was 20 mg of propofol given as an i.v. injection 3 min before administration of sevoflurane. In our study protocol, relatively long-acting midazolam was replaced by ultra short-acting propofol to minimise potential respiratory and airway worries in the early postoperative period.29
In principle, there are three ways to obtain airway access in morbidly obese patients: RSI without mask ventilation, induction with mask ventilation or awake intubation. In the protocol outlined in this study and in our daily practice, we have used RSI for all patients undergoing bariatric surgery whenever possible. This is due to the high prevalence of diabetes mellitus (20.6% in our cohort) and possible related autonomic dysfunction (e.g. gastroparesis) and because of the obvious risk of DMV in morbidly obese patients.7,12,30 A BMI more than 26 kg m−2 has been considered as an independent risk factor for DMV.12 Alternatively, fully awake endotracheal intubation could be chosen as the most secure way.28 Nevertheless, a careful assessment of possible airway problems must be carried out preoperatively. Proper positioning in the operating room and a clear intubation algorithm is always crucial in these patients.7 The volatile part of the study protocol is required to reach an adequate level of anaesthesia for endotracheal intubation. The method described in this report can be suboptimal in patients with limited ability to ensure efficient spontaneous breathing and free airway as awake, for example in patients with huge face obesity. Consequently, those in need of awake intubation were excluded. However, these kinds of problems were not observed in this study cohort. It should be emphasised that although the method described in this paper could solve oxygenation and circulatory problems associated with RSI, it will not address occasional airway struggles. Furthermore, the protocol has been tested with BMIs up to approximately 57, but not with hyperobese patients (BMI >60). In patients with an extremely high BMI, fully awake intubation may be the safest method.8,28
In this study, we used traditional endotracheal intubation instruments (Macintosh). However, it may be more convenient and less traumatic if the newer video-optical endotracheal intubation techniques were used in these kinds of patients as a standard.31 Nonetheless, we only encountered a very few moderately difficult intubation circumstances (Table 1). The question whether the difficulties of endotracheal intubation in morbidly obese patients are due to obesity itself is discussed elsewhere.28,32 We did not routinely use the cricoid pressure manoeuvre during endotracheal intubation.13,33
Careful, thorough planning, using anaesthetist expertise, for all surgical procedures in morbidly obese patients is always essential.7,8,19 In principle, only an experienced anaesthetist should handle morbidly obese patients in daily practice. We recommend the routine assessment of procedural as well as core skills in anaesthesia before using this method in practice. Good skills and basic knowledge in physiology and pharmacology are essential when dealing with this method and these kinds of patients. A requirement for direct observation of a trainee in anaesthesia by an expert or the use of simulation models may be necessary to guarantee good clinical practice and quality.34 In the study, we demonstrated feasibility of the study protocol in both morbidly obese and normal weight patients. No patient developed cardiovascular or respiratory adverse effects. Our proposed method can be easily implemented in daily practice. The potential gas spill into an operating room environment during volatile induction can be solved by using a vacuum system. In conclusion, this method can be considered as an alternative when RSI is needed in both morbidly obese and lean individuals. The question of the most feasible way to perform RSI in morbidly obese and lean patients is interesting and merits further investigation.
The author T.P. made substantial contributions to the study concept and design and data collection. D.K. and O.W. contributed to the design, analysis and interpretation of data. S.A. contributed to the design, analysis and statistics of data. All authors helped in article preparation. This work is dedicated to the memory of colleague and friend of the authors, Dr Staffan Andersson, who died suddenly in December 2010. The authors are very grateful to him for his efforts and his enthusiasm for this study. This research was supported by the Norrbotten County Council. There are no conflicts of interest to disclose.
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Keywords:© 2011 European Society of Anaesthesiology
morbidly obese patients; rapid sequence induction; sevoflurane; spontaneous breathing; volatile rapid sequence induction