Pre-oxygenation is a relevant element of induction of general anaesthesia affecting the safety of patients. Detailed guidelines for proper and effective pre-oxygenation are available [1]. According to the current recommendations, the procedure consists of 3-minute passive oxygenation, which in the majority of patients ensures appropriate oxygen concentration in the lungs (ETO2 >90%). In morbidly obese patients, due to numerous severe changes in the respiratory and cardiovascular system, standard pre-oxygenation may prove ineffective. In such patients, the functional residual capacity (FRC), considered the main store of oxygen, is limited. Additionally, the positioning of the patient on the operating table further reduces the FRC. Abnormalities in the pulmonary circulation and in the perfusion-ventilation ratio result in a substantial shunt of non-oxygenated blood in the lungs, even up to 10-20% [2].

The aim of the present study was to determine the time needed to achieve ETO2 >90% in morbidly obese patients during standard pre-oxygenation before induction of general anaesthesia.

METHODS

The study design was approved by the Ethics Committee. Patients qualified for scheduled procedures to treat obesity were enrolled. The inclusion criteria were consent for involvement in the study and BMI >40 kg m-2. The exclusion criteria were as follows: lack of consent, concomitant respiratory diseases or severe cardiovascular diseases.

Patients were positioned on the operating table in the head-elevated laryngoscopy position (HELP), i.e. with the bedhead elevated by 25o. Pre-oxygenation was administered using a tightly adherent facial mask through a respiratory system of the anaesthesia machine with the oxygen flow 8 L min-1 and the relief valve open. Patients were asked to breathe regularly with normal tidal volume. The time from mask placement to attainment of ETO2 > 90% was determined. ETO2 was measured using a gas analyzer of the anaesthesia machine.

The results were analysed statistically to find possible correlations between body weight, BMI and age vs. time taken to achieve the required level of ETO2.

RESULTS

Forty-three patients were enrolled to the study: 23 female and 20 male patients aged 40.9±10.7, body weight 144.26±27.7 kg, and height 173.5±9.8 cm. The body mass index was 47.9 ±6.4 kg m-2 (range 40-68 kg m-2).

The time needed to achieve the expired oxygen concentration of 90% was 295±124.5 sec. In only 11 patients (25%), 3 min (210 sec) were enough to achieve the required ETO2 (Fig. 1).

The distribution of the parameter analysed – the time needed to achieve ETO2 = 90%, demonstrates that 95% of obese patients should achieve ETO2 > 90% after 450 sec. No correlation was found between BMI, body weight and age vs the time needed to achieve ETO2 = 90% during pre-oxygenation (Fig. 2, 3, 4).

DISCUSSION

The FRC decreases with increasing BMI. It is estimated that in morbidly obese patients (BMI >40 kg m-2), the FRC may be reduced by 2/3 compared to height-matched non-obese patients [3]. During spontaneous breathing, obese patients use almost entire expiratory residual volume having no respiratory reserves. This is caused by elevation of the diaphragm induced by increased intra-abdominal pressure and accumulation of fatty tissue in the thorax. In the recumbent decubitus or sitting position, the FRC further decreases compared to the upright position. The anti-Trendelenburg position does not bring beneficial outcomes [4] – the FRC is still markedly reduced.

It was found that after standard 3-minute pre-oxygenation, the safe apnoea period (SAP) in obese patients is almost 2.5 times shorter compared to normal-weight patients (196 sec vs 595 sec) [5]. Various methods of pre-oxygenation were studies to improve its effectiveness and lengthen the SAP. Passive oxygenation for 5 min was compared with 4 deep breaths within 30 sec; the SAPs were similar [6]. It was demonstrated that the HELP position of the obese patient during passive oxygenation lengthened the SAP in comparison with oxygen therapy in the recumbent position [7]. After 3 min of respiration with 100% oxygen, PaO2 in the HELP group was higher. The difference in SAP was 50 sec, on average (in HELP group – 201 sec, in controls – 155 sec). Moreover, the effects of sitting position on the efficacy of pre-oxygenation in obese patients were analysed. After 8 deep breaths with 100% oxygen through the facial mask and the anaesthetic circuit at the oxygen flow of 10 L min-1, patients in sitting position had longer SAP (214 sec) compared to patients in recumbent position (162 sec) [8]. The comparison of the HELP, recumbent and anti-Trendelenburg positions during 3-minute standard pre-oxygenation in obese patients showed the longest SAP in the anti-Trendelenburg group  (153 sec vs 123 sec vs 178 sec, respectively) [9].

The use of CPAP 7.5 cm H2O (0.74 kPa) during pre-oxygenation in obese patients to increase its effectiveness lengthened the SAP to 240 sec compared to standard pre-oxygenation for 3 min (203 sec) [10]. Similar results were obtained using pressure support (PS) during which the target value of VT  was 8-10 mL kg-1 and  PEEP 5 cm H2O (0.49 kPa) [11]. After 3 min of PS, values of PaO2 were higher in comparison with standard pre-oxygenation [11].

Moreover, the use of BiPAP for pre-oxygenation of obese patients was very effective [12]. The 5-minute standard pre-oxygenation was compared with NIV BiPAP with the inspiratory pressure 17 cm H2O (1.67 kPa), expiratory pressure 7 cm H2O (0.69 kPa) and VT about 8 mL kg-1. In the BiPAP group, PaO2 reached 95 mm Hg (12.67 kPa) whereas in the standard pre-oxygenation group 60 mm Hg (8 kPa). Another study demonstrated that standard passive oxygenation for 3 min (oxygen flow – 5 L min-1) through the catheter inserted in the naso-oral cavity lengthened the SAP from 145 sec to 240 sec [13]. After 3-minute standard pre-oxygenation, ETO2  reached the value of 89%.

The available literature lacks publications concerning the analysis of the time needed to achieve 90% concentration of oxygen in the expired air during pre-oxygenation of obese patients. Most studies were focused on SAP measurements. Moreover, the influence of body weight on the time required to achieve the suitable ETO2 has not been analysed.

The literature reports and our observations suggest that proper management of pre-oxygenation in morbidly obese patients (BMI >40 kg m-2) should involve head-elevated positioning of the patient [14] and passive oxygenation through a facial mask and anaesthetic circuit at the oxygen flow of 8-10 L min-1 and PEEP of 5 cm H2O (0.49 kPa) for 8 min or until ETO2 >90% is achieved.

CONCLUSIONS

1. Obese patients require longer passive oxygenation during induction of general anaesthesia.

2. No correlation was found between body weight, BMI and age of patients vs. time needed to achieve effective pre-oxygenation.

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REFERENCES

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3.     Jones R, Nzekwu MM: The effects of Body Mass Index on lung volumes. Chest 2006; 130: 827-833.

4.     Benedyk PS, Baun MM, Keus L, Jimenez C, Morice R, Bidani A, Meininger JC: Effects of body position on resting lung volume in overweight and midly obese to moderately obese subjects. Respiratory Care 2009; 54: 334-339.

5.     Berthoud MC, Anaes BM, Peacock JE, Anaes FC, Reilly CS: Effectiveness of preoxygenation in morbidly obese patients. Br J Anaesth 1991; 67: 464-466.

6.     Goldhnberg ME, Norris MC, Larijani G, Marr A, Seltzer JL: Preoxygenation in the morbidly obese: a comparison of two techniques. Anesth Analg 1989; 68: 520-522.

7.     Dixon BJ, Dixon JB, Carden JR: Preoxygenation is more effective in the 25 degrees head-up position than in the supine position in severely obese patients: a randomized controlled study. Anesthesiology 2005; 102: 1110-1115.

8.     Altermatt FR, Munoz HR, Delfino AE, Cortinez LI: Pre-oxygenation in the obese patient: effects of position on tolerance of apnoea. Br J Anaesth 2005; 95: 705-709.

9.     Boyse JR, Ness T, Castroman P, Gleysteen JJ: A prelimination study of the optimal anaesthesia positioning for the morbidly obese patient. Obes Surg 2003; 13: 4-9.

10.    Cressey DM, Berthoud MC, Reilly CS: Effectiveness of continuous positive airway pressure to enhance pre-oxygenation in morbidly obese women. Anaesthesia 2001; 56: 670-689.

11.    Baillard C, Fosse JP, Sebbane M, Chanques G, Vincent F, Courouble P, Cohen Y, Eledjam JJ, Adnet F, Jaber S: Noninvasive ventilation improves preoxygenation before intubation of hypoxic patients. Am J Resp Crit Care Med 2006; 174: 171-177.

12.    El-Khatib M, Kanazi G, Baraka A: Noninvasive bilevel positive airway pressure for preoxygenation of the critically ill morbidly obese patient. Can J Anesth 2007; 54: 744-747.

13.    Baraka AS, Taha SK, Siddik-Sayyid SM, Kanazi GE, El-Khatib MF, Dagher CM, Chehade JMA, Abdallah FW, Hajj RE: Supplementation of pre-oxygenation in morbidly obese patients using nasopharyngeal oxygen insuflation. Anaesthesia 2009; 62: 769-773.

14.    Tanoubi I, Drolet P, Donati F: Optimizing preoxygenation in adults. Can J Anaesth 2009; 56: 449-66.

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Address:

*Tomasz Gaszyński
Klinika Anestezjologii i Intensywnej Terapii
Uniwersytet Medyczny w Łodzi
ul. Kopcińakiego 22, 90-153 Łódź
e-mail: tomgaszyn@poczta.onet,pl

Received: 01.04.2010
Accepted: 29.07.2010