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Apnoeic oxygenation in pregnancy: a modelling investigation
Abstract Number: S-05
Abstract Type: Original Research
General anaesthesia in the parturient carries increased risk of failed intubation and a faster time to critical desaturation during apnoea, when compared to the non-pregnant population. Oxygenation can occur during apnoea due to continued uptake of oxygen at the alveolar level, provided that there is an open airway. Supplemental oxygen via nasal cannula during apnoea can increase the time to critical desaturation in obese patients. (1) This study looks at the effect of apnoeic oxygenation in a virtual parturient.
The Nottingham Physiology Simulator is a computational model of the respiratory and cardiovascular system, validated for the investigation of apnoea in pregnancy. (2) The virtual subject was created using published physiological data and, where that was not available, values were inferred from studies in non-pregnant subjects or from physiological theory. (2)
The virtual subject was pre-oxygenated for 3 min of tidal breathing using 100% oxygen. Apnoea was then initiated and continued until desaturation occurred. During this study, data were collected at 0.025s intervals on SaO₂, PaCO₂ and pH. The simulation was repeated introducing oxygen at increasing concentrations at the open glottis.
After pre-oxygenation, increasing FiO₂ at the open glottis increased the time taken to desaturate during apnoea. (Figure 1) The increase in apnoea time is greater at higher oxygen concentrations. PaCO₂ increased and pH decreased in a linear fashion during apnoea with pH 7.0 and PaCO₂ 19.8kPa at 11min of apnoea.
Delivering supplemental oxygen at the open glottis in this simulation prolongs the time taken to desaturate during apnoea. The time to desaturate at high oxygen concentrations exceeds that needed to create clinically significant respiratory acidosis. Increases in concentration of glottic oxygen may be seen via the delivery of oxygen via nasal cannula. During quiet respiration 10-15l/min O₂ via nasal cannula can deliver FiO₂ 0.35-0.4. (3) A recent study in patients with difficult airways showed that nasal OptiFlow™ prolonged the time for SpO₂ >90% during apnoea with less significant respiratory acidosis. (4) Our study suggests a benefit in delivering supplemental oxygen to the apnoeic parturient during the induction of anaesthesia. Further work needs to determine the glottic FiO₂ provided by different nasal oxygen delivery devices, at varying flow rates, during apnoea.
1. J Clin Anesth 2010; 22:164-168
2. Anaesthesia 2008; 63(3):264-9
3. Anaesthesiology 1976; 44:71-74
4. Anaesthesia 2014 doi:10.111/anae.12923