Apnoeic oxygenation in pregnancy: a modelling investigation

Subglottic tracheal stenosis complicating pregnancy: A case report

The case presented details an uncommon case of subglottic tracheal stenosis exacerbated by pregnancy. We outine the multidisciplinary management involved and the outcomes for the pregnancy. The case serves as a reminder that shortness of breath in pregnancy has a broad differential diagnosis, and stridor is always abnormal.

Peri-intubation oxygenation for Caesarean delivery: is there an optimal technique?

Peri-intubation oxygen administration to the pregnant patient during induction of general anaesthesia is critical to avoiding hypoxaemia and harm to the mother and fetus. Recent modelling comparing low-flow with high-flow nasal oxygen in simulated term pregnant women of varying body habitus, taken together with previous work, suggests that face mask preoxygenation with the use of low-flow or high-flow nasal oxygen during the period of apnoea prolongs the safe apnoea period, with the benefit varying by body habitus. Low-flow compared with high-flow nasal oxygen may be easier to combine with face mask preoxygenation and is readily available in all operating theatres, although future improvements in high-flow nasal oxygen delivery systems may improve ease of use for this indication.

Neuraxial and general anaesthesia for caesarean section

Caesarean section (CS) is one of the most performed operations worldwide. In many parts of the world, there has been a reduction in anaesthetic associated obstetric mortality, and this has been attributed to the increased use of neuraxial anaesthesia and improved safety of general anaesthesia, alongside improved training and organisational changes. In resource-limited countries, anaesthesia contributes disproportionately to maternal mortality, with one in seven deaths being due to anaesthesia. A major contributory factor to this is the severe shortage of trained anaesthetic providers. Goals for anaesthesia for CS include the woman's comfort and foetal well-being, focusing on strategies to minimise morbidity and mortality for both. Anaesthetic options for CS include neuraxial techniques (spinal or combined-spinal epidural or epidural extension of labour analgesia) and general anaesthesia. There is increasing evidence of the benefit of neuraxial techniques over general anaesthesia in terms of maternal and foetal outcomes. For elective CS, spinal and combined-spinal anaesthesia predominate. General anaesthesia is mainly reserved for Category 1 CS where there is an immediate threat to the life of the mother or the baby. This review discusses the practical aspects of neuraxial and general anaesthesia for CS.

Ventilation strategies for front of neck airway rescue: an in silico study

BACKGROUND

During induction of general anaesthesia a 'cannot intubate, cannot oxygenate' (CICO) situation can arise, leading to severe hypoxaemia. Evidence is scarce to guide ventilation strategies for small-bore emergency front of neck airways that ensure effective oxygenation without risking lung damage and cardiovascular depression.

METHODS

Fifty virtual subjects were configured using a high-fidelity computational model of the cardiovascular and pulmonary systems. Each subject breathed 100% oxygen for 3 min and then became apnoeic, with an obstructed upper airway. When arterial haemoglobin oxygen saturation reached 40%, front of neck airway access was simulated with various configurations. We examined the effect of several ventilation strategies on re-oxygenation, pulmonary pressures, cardiovascular function, and oxygen delivery.

RESULTS

Re-oxygenation was achieved in all ventilation strategies. Smaller airway configurations led to dynamic hyperinflation for a wide range of ventilation strategies. This effect was absent in airways with larger internal diameter (≥3 mm). Intrapulmonary pressures increased quickly to supra-physiological values with the smallest airways, resulting in pronounced cardio-circulatory depression (cardiac output <3 L min^-1 and mean arterial pressure <60 mm Hg), impeding oxygen delivery (<600 ml min^-1). Limiting tidal volume (≤200 ml) and ventilatory frequency (≤8 bpm) for smaller diameter cannulas reduced dynamic hyperinflation and gas trapping, preventing cardiovascular depression.

CONCLUSIONS

Dynamic hyperinflation can be demonstrated for a wide range of front of neck airway cannulae when the upper airway is obstructed. When using small-bore cannulae in a CICO situation, ventilation strategies should be chosen that prevent gas trapping to prevent severe adverse events including cardio-circulatory depression.

Effect of variable pre-oxygenation endpoints on safe apnoea time using high flow nasal oxygen for women in labour: a modelling investigation

BACKGROUND

Studies of pulmonary denitrogenation (pre-oxygenation) in obstetric populations have shown high flow nasal oxygen therapy (HFNO) is inferior to facemask techniques. HFNO achieves median end-tidal oxygen fraction (FE'O₂) of 0.87 after 3 min. As HFNO prolongs safe apnoea times through apnoeic oxygenation, we postulated that HFNO would still extend safe apnoeic times despite the lower FE'O₂ after pre-oxygenation.

METHODS

The Interdisciplinary Collaboration in Systems Medicine simulation suite, a highly integrated, high-fidelity model of the human respiratory and cardiovascular systems, was used to study the effect of varying FE'O₂ (60%, 70%, 80%, and 90%) on the duration of safe apnoea times using HFNO and facemask techniques (with the airway open and obstructed). The study population consisted of validated models of pregnant women in active labour and not in labour with BMI of 24, 35, 40, 45, and 50 kg m^-2.

RESULTS

HFNO provided longer safe apnoeic times in all models, with all FE'O₂ values. Labour and increased BMI reduced this effect, in particular a BMI of 50 kg m^-2 reduced the improvement in apnoea time to 1.8-8.5 min (depending on the FE'O₂), compared with an improvement of more than 60 min in the subject with BMI 24 kg m^-2.

CONCLUSIONS

Despite generating lower FE'O₂, HFNO provides longer safe apnoea times in pregnant subjects in labour. Care should be taken when used in patients with BMI ≥50 kg m^-2 as the extension of the safe apnoea time is limited.

A prospective, up-down sequential allocation study investigating the effectiveness of vital capacity breaths using high-flow nasal oxygenation versus a tight-fitting face mask to pre-oxygenate term pregnant women

BACKGROUND

The role of high flow nasal oxygenation (HFNO) for pre-oxygenation before obstetric general anaesthesia remains unclear. This study aimed to determine the number of vital capacity breaths using HFNO required to pre-oxygenate 90% of parturients to an end-tidal oxygen concentration fraction (F_ETO₂) of ≥0.90 (termed EN90).

METHODS

Using up-down, sequential allocation trial design, volunteer term parturients undergoing caesarean delivery were investigated with HFNO with their mouth closed, followed by mouth open, and if F_ETO₂ ≥0.90 was not achieved after a maximum of 20 vital capacity breaths, pre-oxygenation was attempted with a face mask. The primary outcome was the number of vital capacity breaths required using HFNO (mouth open and closed) to achieve EN90. Secondary outcomes included assessment of EN90 using mouth open versus mouth closed and face mask pre-oxygenation, maternal satisfaction and evaluation of fetal cardiotocography.

RESULTS

Twenty women at term were recruited. Successful pre-oxygenation occurred in 4 (20%), 3 (15%) and 14 (70%) women with HFNO mouth closed, HFNO mouth open, and via face mask respectively. At up to 20 vital capacity breaths, face mask pre-oxygenation was more successful at achieving EN90 compared with both HFNO with a closed (P=0.006) or open (P=0.001) mouth. Closed mouth HFNO did not outperform open mouth pre-oxygenation.

CONCLUSION

Face mask pre-oxygenation is more effective at achieving EN90 compared with to HFNO within a clinically acceptable number of vital capacity breaths. Further studies are needed to determine the role of HFNO in optimising the time before desaturation and for apnoeic oxygenation in term parturients.

Transnasal Humidified Rapid-Insufflation Ventilatory Exchange for Elective Laryngeal Surgery During Pregnancy: A Case Report

Transnasal humidified rapid-insufflation ventilatory exchange (THRIVE) maintains oxygenation and blunts the partial pressure of carbon dioxide (PaCO2) rise in nonpregnant subjects during apnea. Physiologic changes of pregnancy may attenuate the utility of THRIVE. We present a nulliparous patient at 31 weeks' gestation undergoing tracheal dilation requiring general anesthesia without intubation utilizing THRIVE. Our data confirms prior reports in nonpregnant patients showing markedly extended time to desaturation. However, PaCO2 rise more closely mirrors classic apneic oxygenation in nonobstetric patients. The PaCO2 elevation and subsequent acidosis may limit the utility of THRIVE for prolonged apnea in pregnant surgical patients.

Effect of oxygen fraction on airway rescue: a computational modelling study

BACKGROUND

During induction of general anaesthesia, patients frequently experience apnoea, which can lead to dangerous hypoxaemia. An obstructed upper airway can impede attempts to provide ventilation. Although unrelieved apnoea is rare, it continues to cause deaths. Clinical investigation of management strategies for such scenarios is effectively impossible because of ethical and practical considerations.

METHODS

A population-representative cohort of 100 virtual (in silico) subjects was configured using a high-fidelity computational model of the pulmonary and cardiovascular systems. Each subject breathed 100% oxygen for 3 min and then became apnoeic, with an obstructed upper airway, during induction of general anaesthesia. Apnoea continued throughout the protocol. When arterial oxygen saturation (Sao₂) reached 20%, 40%, or 60%, airway obstruction was relieved. We examined the effect of varying supraglottic oxygen fraction (Fo₂) on the degree of passive re-oxygenation occurring without tidal ventilation.

RESULTS

Relief of airway obstruction during apnoea produced a single, passive inhalation (caused by intrathoracic hypobaric pressure) in all cases. The degree of re-oxygenation after airway opening was markedly influenced by the supraglottic Fo₂, with a supraglottic Fo₂ of 100% providing significant and sustained re-oxygenation (post-rescue Pao₂ 42.3 [4.4] kPa, when the airway rescue occurred after desaturation to Sao₂ 60%).

CONCLUSIONS

Supraglottic oxygen supplementation before relieving upper airway obstruction improves the effectiveness of simulated airway rescue. Management strategies should be implemented to assure a substantially increased pharyngeal Fo₂ during difficult airway management.

Use of transnasal humidified rapid-insufflation ventilatory exchange (THRIVE) to facilitate the management of subglottic stenosis in pregnancy

Subglottic stenosis is a rare condition. Diagnosis is often delayed as symptoms are attributed to other causes, such as asthma. This problem may be compounded in pregnancy when dyspnoea may be attributed to normal physiological changes. In respiratory compromise, surgical intervention may be required and airway management is challenging as endotracheal intubation may be traumatic or, in severe cases, impossible. Transnasal humidified rapid-insufflation ventilatory exchange (THRIVE) is a novel open-airway apnoeic technique using high-flow nasal oxygen. It does not expose the patient to the risks of jet ventilation, nor does it require the placement of an airway device to effectively oxygenate the patient. In pregnancy, elevation of maternal carbon dioxide is of particular concern as it may result in a worsening fetal acidosis. While THRIVE has been shown to provide some clearance of carbon dioxide, a patent airway is required for it to function effectively. In this case report we describe the management of a pregnant patient who underwent balloon dilatation for severe subglottic stenosis at 23 weeks' gestation using THRIVE and we summarise the evidence supporting the use of this new technique in pregnant women.

High-flow nasal oxygenation for anesthetic management

High-flow nasal oxygenation (HFNO) is a promising new technique for anesthesiologists. The use of HFNO during the induction of anesthesia and during upper airway surgeries has been initiated, and its applications have been rapidly growing ever since. The advantages of this technique include its easy set-up, high tolerability, and its abilities to produce positive airway pressure and a high fraction of inspired oxygen and to influence the clearance of carbon dioxide to some extent. HFNO, via a nasal cannula, can provide oxygen both to patients who can breathe spontaneously and to those who are apneic; further, this technique does not interfere with bag-mask ventilation, attempts at laryngoscopy for tracheal intubation, and surgical procedures conducted in the airway. In this review, we describe the techniques associated with HFNO and the advantages and disadvantages of HFNO based on the current state of knowledge.

Transnasal Humidified Rapid-Insufflation Ventilatory Exchange for Preoxygenation Before Cesarean Delivery Under General Anesthesia: A Case Report

Transnasal humidified rapid-insufflation ventilatory exchange has been shown to improve oxygenation and increase apnea time in difficult airway cases. It may also be beneficial in patients vulnerable to rapid desaturation due to limited pulmonary reserve. We report the use of transnasal humidified rapid-insufflation ventilatory exchange for preoxygenation before a cesarean delivery under general anesthesia in a patient with respiratory distress because of pneumonia and heart failure from severe mitral stenosis. To our knowledge, the use of this technique has not been previously reported in pregnant patients.

High Flow Humidified Nasal Oxygen in Pregnant Women

Failed airway management in the obstetric patient undergoing general anaesthesia is associated with major sequelae for the mother and/or fetus. Effective and adequate pre-oxygenation is an important safety strategy and a recommendation in all current major airway guidelines. Pre-oxygenation practice in the obstetric population may be suboptimal based on current literature. Recently, clinical applications for high flow nasal oxygen, also known as transnasal humidified rapid insufflation ventilatory exchange or THRIVE, are expanding in the non-obstetric population and may have theoretical benefits if used for pre-oxygenation and apnoeic oxygenation in the obstetric population. We review the current literature surrounding high flow nasal oxygen relevant to the pregnant woman. We also propose a basis for potential advantages and complications for its use in this context.

Efficacy and Safety of Using High-Flow Nasal Oxygenation in Patients Undergoing Rapid Sequence Intubation

Objective

To assess the efficacy and safety of high-flow nasal oxygen (HFNO) therapy in patients undergoing rapid sequence intubation (RSI) for emergency abdominal surgery.

Methods

HFNO of 60 L.min^-1 at an inspiratory oxygen fraction of 1 was delivered 4 min before laryngoscopy and maintained until the patient was intubated, and correct intubation was verified by the appearance of the end-tidal CO₂ (EtCO₂) waveform. Transcutaneous oxygenation (SpO₂), heart rate and non-invasive mean arterial pressure were monitored at baseline (T0), after 4 min on HFNO (T1) and at the time of laryngoscopy (T2) and endotracheal intubation (ETI) (T3). An SpO₂ of <3% from baseline was recorded at any sampled time. The value of EtCO₂ at T3 was registered after two mechanical breaths. The apnoea time was defined as the time from the end of propofol injection to ETI. RSI was performed with propofol, fentanyl and rocuronium.

Results

Forty-five patients were enrolled. SpO₂ levels showed a statistically significant increase at T1, T2 and T3 compared with those at T0 (p<0.05); median SpO₂% (interquartile range) was 97% (range, 96%-99%) at T0, 99% (range, 99%-100%) at T1, 99% (range, 99%-100%) at T2 and 99% (range, 99%-100%) at T3. Minimal SpO₂ was 96%; no patient showed an SpO₂ of <3% from baseline; mean EtCO₂ at the time of ETI was 36±4 mmHg. Maximum apnoea time was 12 min.

Conclusion

HFNO is an effective and safe technique for pre-oxygenation in patients undergoing rapid sequence induction of general anaesthesia for emergency surgery.