Pre-oxygenation in pregnancy by rizmaadlia

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Pre-oxygenation in pregnancy: an investigation using physiological modelling

Dr. Rudi MP
www.themegallery.com

Background
 Despite the increased use of regional anaesthetic techniques for Caesarean section, general anaesthesia is still required under certain circumstances.

 Rapid sequence induction has been recommended during pregnancy due to the increased risk of regurgitation and aspiration.  Hypoxaemia during the inevitable period of apnoea presents a hazard to mother and baby.  The period of apnoea may become prolonged if difficulties are experienced in managing the airway.
 The Confidential Enquiry into Maternal and Child Health (CEMACH) report for 2000–02 highlighted seven deaths directly due to anaesthesia, an increase from the previous triennium.

 Three of these resulted from oesophageal intubation and subsequent apnoea.
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Background
 The time to the onset of life-threatening oxygen desaturation during apnoea can be extended by preoxygenation (denitrogenation) of the lungs, and

 It is universal practice to employ some variant of this technique when inducing general anaesthesia for Caesarean section.  Pregnancy is associated with considerable changes to the cardiovascular and respiratory systems.  Reduced functional residual capacity, coupled with increased minute ventilation, predicts more rapid preoxygenation.  There is a modest body of research in small numbers of subjects to support this.
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 However, research in pregnant patients suffers from ethical and practical constraints that have limited the work in this field.  Studies have found conflicting results and recommended different techniques of pre-oxygenation.

 Computer simulation can be used to undertake precisely controlled investigations difficult or impossible to achieve in vivo.  We aimed to use the Nottingham Physiology Simulator to establish the differences between pregnant and nonpregnant women during pre-oxygenation, and the optimal method of pre-oxygenation in pregnancy. An accompanying paper studies apnoea in pregnancy.
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Methods
 The Nottingham Physiology Simulator (NPS) is an integrated computational model of the respiratory and cardiovascular systems that has been described previously.

 It has been validated for the investigation of preoxygenation in healthy adults.  We present further validation of the NPS for use in pregnancy. Version NPS270307 was used for this study and is available to download free via the corresponding author.  The NPS was re-initiated prior to each simulation run.
 After configuration of the virtual subject under study, full equilibration within the model’s compartments was ensured, initially breathing room air.
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 To investigate the variation in response that might be found in a population due to physiological heterogeneity, we modelled multiple pregnant and non-pregnant women.

 Firstly, an ‘average’ pregnant and an ‘average’ non-pregnant subject were generated from published mean physiological values. An additional four subjects, predicted to undergo slow or rapid equilibration during pre-oxygenation were then created (Table 1), with baseline physiological parameters from within the ranges found in published studies.  We ensured that in any individual subject, the overall physiological configuration produced baseline arterial blood gas values within an acceptable range.
 Pregnant subjects were configured to represent a nonlabouring, full-term normal pregnancy, with the subject lying supine with left lateral tilt.
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 Non-pregnant subjects were configured as healthy individuals lying supine.

 All six virtual subjects underwent tidal breathing of 100% oxygen for 10 min.
 For vital capacity pre-oxygenation, the subjects were initially configured as tidal breathing air, and then the slow, average and rapid subjects took 1500, 2000 and 2500 ml breaths, respectively, of 100% oxygen at a rate of 10 breaths.min)1 for 5 min.

 During pre-oxygenation, PE´ O2 was recorded at 1-s intervals.
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Results
 Pre-oxygenation during tidal breathing proceeded more rapidly in pregnant than in non-pregnant subjects (Fig.1).  The difference was marked, such that the slowest pregnant subject equilibrated more rapidly than the fastest nonpregnant subject.

 In pregnancy, the median [range] time to undergo 95% of the maximum change in PE´ O2 (i.e. 95% complete preoxygenation) was 1 min 37 s [1:23–1:52], compared to 2 min 51 s [2:28–3:15] in the non-pregnant subjects.
 The median [range] time to achieve 99% of the maximum change in PE´ O2 was 3 min 14 s [3:13–3:24] in pregnant subjects, compared to 5 min 16 s [5:01–5:37] in nonpregnant subjects.

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Vital capacity pre-oxygenation proceeded marginally more rapidly in pregnancy (Fig. 2). However, when the number of breaths required to achieve the same PE´ O2 as after 95% complete tidal pre-oxygenation was analysed, seven breaths were required [range 5–10] in pregnant subjects, compared to six breaths [range 4–9] in nonpregnant subjects. Vital capacity breathing accelerated preoxygenation to produce a similar rate in pregnant and non-pregnant women.
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Discussion
 We have demonstrated that pre-oxygenation during tidal breathing proceeds more rapidly in pregnant than in nonpregnant women.

 This difference is predicted by increased ventilation of a relatively small functional residual capacity in pregnancy.  This finding is in keeping with those of previous researchers.  We found that 2 min tidal breathing of 100% oxygen ensured > 95% complete pre-oxygenation in full-term pregnancy.  During vital capacity pre-oxygenation, equilibration also proceeds slightly more rapidly in pregnancy, as shown in Fig. 2.

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 The difference between pregnant and non-pregnant women is small compared to that seen during tidal breathing.  Previous authors have also reported more rapid vital capacity pre-oxygenation in pregnancy.

 An unexpected finding in our study is that pregnant women require slightly more breaths than non-pregnant women to achieve the same PE´ O2 as after 95% complete tidal pre-oxygenation.
 This can be explained by the differing target PE´ O2 levels for preoxygenation, a higher PE´ O2 being achievable in pregnancy, due to reduced PACO2.
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 For this study, we used submaximal vital capacity breaths. This was based on the maximum tidal volume possible with a fresh gas flow of 10 l.min)1 and a 2-l bag, as these would be most commonly available. We have found no published evidence that pregnant and nonpregnant women differ in their ability to perform such submaximal vital capacity breathing.  Four vital capacity breaths have been recommended for preoxygenation in pregnancy. However, other authors have reported this to be inadequate, and we found that four breaths are not sufficient to achieve optimal pre-oxygenation. Chiron et al. recommended eight breaths, but we found that up to 10 breaths were required to achieve the same PE´ O2 as after 95% complete tidal pre-oxygenation.

 There is a striking difference between the graph for tidal breathing and that for vital capacity breathing.
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 During tidal pre-oxygenation, there is no overlap of the pregnant and non-pregnant populations, but during vital capacity breathing, they overlap extensively. Vital capacity breathing accelerates preoxygenation in pregnant and non-pregnant women, but the benefit is greater in nonpregnant women

 Pregnant women have higher minute ventilation at rest and therefore have relatively less to gain by ‘rapid’ vital capacity pre-oxygenation. We would therefore suggest that this technique is less useful in pregnancy.  Although there is a time benefit, this is moderate, and will be offset by the time required to explain the procedure and difficulty in ensuring adequate technique.
 We chose to study subjects configured as in a supine position. In nonpregnant women, pre-oxygenation in a head-up position has been found to delay the onset of desaturation during apnoea [23, 24].

 However, Baraka et al. [23] failed to demonstrate any benefit in pregnancy, and the (non-significant) trend was towards detriment.
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 We must acknowledge some limits of our investigation. Applying the results of theoretical modelling to real patients needs to be done with care. The most important question is whether the model gives a true representation of in vivo physiology.  The Nottingham Physiology Simulator has been extensively validated against clinical studies [15–18]. In addition, we have undertaken validation of the model for use in pregnancy as part of this investigation  In a novel approach for computer simulation, we modelled a population of virtual subjects, rather than a single idealised subject, to allow us to demonstrate the range of responses that might be seen in a real population.  This aids both the clinical extrapolation of our findings and comparison with previous in vivo work. There may be patients who lie outside the range of our virtual population.

 However, we should expect the majority of patients to lie within the limits of our findings.
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 In this study, we have developed a model of preoxygenation in nonlabouring, full-term normal pregnancy.  The clinical applicability of our findings may therefore be limited to elective Caesarean sections under general anaesthesia, an uncommon event in current UK practice.

 However, most in vivo research of physiological changes and preoxygenation during pregnancy is also in non-labouring, normal volunteers for obvious practical reasons.
 It was necessary in the first instance to create a model of such subjects, to allow validation of the model in this study, we have developed a model of preoxygenation in non-labouring, full-term normal pregnancy.  The clinical applicability of our findings may therefore be limited to elective Caesarean sections under general anaesthesia, an uncommon event in current UK practice.
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 However, most in vivo research of physiological changes and preoxygenation during pregnancy is also in non-labouring, normal volunteers for obvious practical reasons.

 It was necessary in the first instance to create a model of such subjects, to allow validation of the model against this work.
 We plan subsequently to use the Nottingham Physiology Simulator to study the effects of labour and of obstetric morbidity on preoxygenation in pregnancy.

 In summary, we would recommend that in pregnancy, 2 min of tidal breathing of 100% oxygen is necessary to ensure at least 95% complete pre-oxygenation.
 The importance of a tight-fitting facemask cannot be overstressed [25].  Vital capacity pre-oxygenation offers considerably less benefit in pregnant than in non-pregnant patients.
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