Is difficult or failed intubation a confounder or an effect modifier for hypoxaemia? Comment on Br J Anaesth; 125: e81-7

Abstract

Editor—Bonnet and colleagues1Bonnet M.P. Mercier F.J. Vicaut E. et al.Incidence and risk factors for maternal hypoxaemia during induction of general anaesthesia for non-elective Caesarean section: a prospective multicentre study.Br J Anaesth. 2020; 25: e81-e87Abstract Full Text Full Text PDF Scopus (8) Google Scholar reported the results of an observational study that showed that parturients who encountered difficult or failed intubation were at increased risk for hypoxaemia after intubation (adjusted odds ratio =19.1 [8.6–42.7]). Baseline predictors for difficult intubation were collected: it is possible that these identified risk factors2Quinn A.C. Milne D. Columb M. Gorton H. Knight M. Failed tracheal intubation in obstetric anaesthesia: 2 yr national case-control study in the UK.Br J Anaesth. 2013; 110: 74-80Abstract Full Text Full Text PDF PubMed Scopus (117) Google Scholar attributed to difficult or failed intubation prolonged the intubation time leading to hypoxaemia. It is important to know whether difficult or failed intubation is a potential confounder or an effect modifier (in other words, the interaction effect). A variable is considered a confounding variable if it is associated with both the exposure and outcome variables, but is not associated with the causal pathway between the exposure and the outcome (Fig. 1a). As shown in Figure 1b, it is likely that the occurrence of difficult or failed intubation might be the mediator between predictor variables of difficult intubation and hypoxaemia. If so, then, it would be appropriate to test the interaction effect of the covariate (i.e. the presence or absence of difficult/failed intubation in the regression model). The authors mentioned that 9.6% of the parturients needing non-elective Caesarean section had severe pregnancy-induced hypertension. It is understood that the authors used rapid sequence induction of anaesthesia and tracheal intubation to secure the airway. However, in this group of patients, the transient, but severe hypertension that accompanies tracheal intubation can result in fatal neurological complications. As a result, guidelines recommend administration of agents such as short-acting beta blockers, opioids, or vasodilators to blunt the intubation stress response. The authors need to justify not using the above medications. Further, if the authors had used these agents, it would have been interesting to see the effect on hypoxaemia, especially with the short-acting opioids. The authors cited an article suggesting that head-up positioning does not prolong the safe apnoea time in the obstetric population.3Baraka A.S. Hanna M.T. Jabbour S.I. et al.Preoxygenation of pregnant and nonpregnant women in the head-up versus supine position.Anesth Analg. 1992; 75: 757-759Crossref PubMed Scopus (59) Google Scholar This unexpected result may be attributed to several factors: small sample size (10 parturients in supine vs 10 parturients in head-up position), lack of a reliable tool to assess lung denitrogenation (i.e. monitoring of end-tidal concentration of expired oxygen), and no information on parturient BMI. On the contrary, a study by Hignett and colleagues4Hignett R. Fernando R. McGlennan A. et al.A randomized crossover study to determine the effect of a 30° head-up versus a supine position on the functional residual capacity of term parturients.Anesth Analg. 2011; 113: 1098-1102Crossref PubMed Scopus (31) Google Scholar on healthy term parturients showed a significant increase in functional residual capacity with the 30° head-up position in comparison with the supine position. In addition, the head-up position improves the glottic view at laryngoscopy.5Lee B.J. Kang J.M. Kim D.O. Laryngeal exposure during laryngoscopy is better in the 25 degrees back-up position than in the supine position.Br J Anaesth. 2007; 99: 581-586Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar In one survey, the majority of respondents preferred head-up or ramped positioning before induction of anaesthesia in obstetric patients.6Desai N. Wicker J. Sajayan A. Mendonca C. A survey of practice of rapid sequence induction for caesarean section in England.Int J Obstet Anesth. 2018; 36: 3-10Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar It is necessary to conduct a multicentre randomised clinical trial to determine whether head-up position in comparison with supine position prolongs the time to desaturation during the apnoea phase in rapid sequence induction for Caesarean section. The authors declare that they have no conflicts of interest. Incidence and risk factors for maternal hypoxaemia during induction of general anaesthesia for non-elective Caesarean section: a prospective multicentre studyBritish Journal of AnaesthesiaVol. 125Issue 1PreviewPregnant women are at increased risk of hypoxaemia during general anaesthesia. Our aim was to determine the incidence and the risk factors that contribute to hypoxaemia in this setting. Full-Text PDF Open Archive