Non-toxic alveolar oxygen concentration without hypoxemia during apnoeic oxygenation: an experimental study

Apnoeic oxygenation during neonatal intubation

'Apnoeic oxygenation' describes the diffusion of oxygen across the alveolar-capillary interface in the absence of tidal respiration. Apnoeic oxygenation requires a patent airway, the diffusion of oxygen to the alveoli, and cardiopulmonary circulation. Apnoeic oxygenation has varied applications in adult medicine including facilitating tubeless anaesthesia or improving oxygenation when a difficult airway is known or anticipated. In the paediatric population, apnoeic oxygenation prolongs the time to oxygen desaturation, facilitating intubation. This application has gained attention in neonatal intensive care where intubation remains a challenging procedure. Difficulties are related to the infant's size and decreased respiratory reserve. In addition, policy changes have led to limited opportunities for operators to gain proficiency. Until recently, evidence of benefit of apnoeic oxygenation in the neonatal population came from a small number of infants recruited to paediatric studies. Evidence specific to neonates is emerging and suggests apnoeic oxygenation may increase intubation success and limit physiological instability during the procedure. The best way to deliver oxygen to facilitate apnoeic oxygenation remains an important question.

The efficacy of apneic oxygenation to prevent hypoxemia during rapid sequence intubation in trauma patients

PURPOSE OF REVIEW

Rapid and effective airway management is priority for trauma patients. Trauma patients are often at an increased risk of experiencing hypoxia, and thus at increased risk of morbidity and mortality. Apneic oxygenation has been widely debated but has been reported to provide benefit in terms of increased peri-intubation oxygen saturation and decreased rates of desaturation. This review aims to evaluate the current literature on the efficacy of apneic oxygenation in the setting of rapid sequence intubation (RSI) in trauma patients.

RECENT FINDINGS

Two prospective studies published this year, demonstrated that apneic oxygenation was effective in reducing hypoxic events and hypoxic duration during RSI.

SUMMARY

The use of apneic oxygenation can play an important role in preventing hypoxic events in trauma patients undergoing RSI. The use of apneic oxygenation is cheap, and should be considered to reduce hypoxemic events. Additional studies are required to see the effects of apneic oxygenation on outcomes in trauma patients undergoing RSI, specifically desaturation and hypoxemic events and duration, and early onset mortality.

Apnoeic oxygenation in paediatric anaesthesia: a narrative review

Apnoeic oxygenation refers to oxygenation in the absence of any patient or ventilator effort to move the lungs. This phenomenon was first described in humans in the mid-20th century but has seen renewed interest in the last decade following the demonstration of apnoeic oxygenation with low-flow, and subsequently high-flow, nasal oxygen. This narrative review summarises our understanding of apnoeic oxygenation in the paediatric population. We examine the evidence supporting oxygenation via tracheal tube, modified laryngoscopes and nasal cannulae. The evidence for prolongation of safe apnoea time at induction of anaesthesia is also appraised. We explore the capacity for carbon dioxide clearance, flow rate selection with high-flow nasal oxygen and complications associated with the technique. It remains uncertain whether apnoeic oxygenation in paediatric patients results in a meaningful clinical benefit compared with standard care for outcomes such as the number of tracheal intubation attempts or the incidence of hypoxaemia. In particular, the role of apnoeic oxygenation in paediatric difficult airway management is unclear as this has not been the targeted focus of any published research to date.

Comparison of different methods for lung immobilization in an animal model

BACKGROUND AND PURPOSE

Respiratory-induced motion introduces uncertainties in the delivery of dose in radiotherapy treatments. Various methods are used clinically, e.g. breath-holding, while there is limited experience with other methods such as apneic oxygenation and high frequency jet ventilation (HFJV). This study aims to compare the latter approaches for lung immobilization and their clinical impact on gas exchange in an animal model.

MATERIALS AND METHODS

Two radiopaque tumor surrogate markers (TSM) were placed in the central (cTSM) and peripheral (dTSM) regions of the lungs in 9 anesthetized and muscle relaxed pigs undergoing 3 ventilatory interventions (1) HFJV at rates of 200 (JV200), 300 (JV300) and 400 (JV400) min^-1; (2) apnea at continuous positive airway pressure (CPAP) levels of 0, 8 and 16 cmH₂O; (3) conventional mechanical ventilation (CMV) as reference mode. cTSM and dTSM were visualized using fluoroscopy and their coordinates were computed. The ventilatory pattern was registered, and oxygen and carbon dioxide (pCO₂) partial pressures were measured.

RESULTS

The highest range of TSM motion, and ventilation was found during CMV, the lowest during apnea. During HFJV the amount of motion varied inversely with increasing frequency. The reduction of TSM motion at JV300, JV400 and all CPAP levels came at the cost of increased pCO₂, however the relatively low frequency of 200 min^-1 for HFJV was the only ventilatory setting that enabled adequate CO₂ removal.

CONCLUSION

In this model, HFJV at 200 min^-1 was the best compromise between immobilization and gas exchange for sessions of 10-min duration.

Influence of apneic oxygenation on cardiorespiratory system homeostasis

PURPOSE

The aim of this study was to elucidate the magnitude of variations in oxygenation indices and the pattern of hemodynamic changes in response to the net effect of tracheal apneic oxygenation (AO) with a view to define the safe time limit of its application.

METHODS

After obtaining Animal Research Ethics Committee approval, AO was applied in 12 piglets for 40 min. Arterial (a) and mixed venous (v) blood samples for oxygen (O2) and carbon dioxide (CO2) tension (PaO2/PvO2, PaCO2/PvCO2), O2 saturation (SaO2/SvO2), pHa, base excess (BEa), and bicarbonate (HCO3a) determination and for alveolar O2 tension (PAO2), PaO2/FiO2 and PaO2/PAO2 ratio, arterial-mixed venous O2 content (AVDO2), and O2 extraction ratio (O2ER) estimation were collected on anesthesia induction, 10, 20, 30, and 40 min during AO and 10 and 20 min after reconnection to the ventilator. Concomitant hemodynamic data were obtained.

RESULTS

Besides PvO2 and PAO2, AO adversely influenced PaO2 (248-113 mmHg), PaCO2 (35-145 mmHg), PvCO2, PaO2/FiO2, and PaO2/PAO2 in a time-depended fashion, whereas SvO2, AVDO2, and O2ER were minimally affected. P(a - v)CO2 was reversed throughout AO. Acid-base status derangement, consisting of HCO3a elevation, BEa widening, and acidemia (pH 6.9) maximized 40 min after AO. During AO, heart rate, systemic and pulmonary circulation pressures, and cardiac output were progressively elevated, whereas systemic vascular resistance was reduced. All the studied parameters reverted almost to baseline within the 20-min period of ventilator reconnection.

CONCLUSION

Tracheal AO for 40 min ensures acceptable blood oxygenation, promotes notable hypercapnic acidosis, and consequent transient hemodynamic alterations, which are almost completely reversible after reconnection to the ventilator.

Intensive buffering can keep pH above 7.2 for over 4 h during apnea: an experimental porcine study

BACKGROUND

Ventilation with low tidal volumes reduces mortality in acute respiratory distress syndrome. A further reduction of tidal volumes might be beneficial, and it is known that apneic oxygenation (no tidal volumes) with arteriovenous CO(2) removal can keep acid-base balance and oxygenation normal for at least 7 h in an acute lung injury model. We hypothesized that adequate buffering might be another approach and tested whether tris-hydroxymethyl aminomethane (THAM) alone could keep pH at a physiological level during apneic oxygenation for 4 h.

METHODS

Six pigs were anesthetized, muscle relaxed, and normoventilated. The lungs were recruited, and apneic oxygenation as well as administration of THAM, 20 mmol/kg/h, was initiated. The experiment ended after 270 min, except one that was studied for 6 h.

RESULTS

Two animals died before the end of the experiment. Arterial pH and arterial carbon dioxide tension (PaCO(2) ) changed from 7.5 (7.5, 7.5) to 7.3 (7.2, 7.3) kPa, P < 0.001 at 270 min, and from 4.5 (4.3, 4.7) to 25 (22, 28) kPa, P < 0.001, respectively. Base excess increased from 5 (3, 6) to 54 (51, 57) mM, P < 0.001. Cardiac output and arterial pressure were well maintained. The pig, which was studied for 6 h, had pH 7.27 and PaCO(2) 27 kPa at that time.

CONCLUSION

With intensive buffering using THAM, pH can be kept in a physiologically acceptable range for 4 h during apnea.