Carbon dioxide dynamics during apneic oxygenation: the effects of preceding hypocapnia

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 Effect of High-Flow Nasal Oxygen on Carbon Dioxide Accumulation in Apneic or Spontaneously Breathing Adults During Airway Surgery: A Randomized-Controlled Trial

BACKGROUND

High-flow nasal oxygen (HFNO) is an emerging technology that has generated interest in tubeless anesthesia for airway surgery. HFNO has been shown to maintain oxygenation and CO2 clearance in spontaneously breathing patients and is an effective approach to apneic oxygenation. Although it has been suggested that HFNO can enhance CO2 clearance during apnea, this has not been established. The true extent of CO2 accumulation and resulting acidosis using HFNO during prolonged tubeless anesthesia remains undefined.

METHODS

In a single-center trial, we randomly assigned 20 adults undergoing microlaryngoscopy to apnea or spontaneous ventilation (SV) using HFNO during 30 minutes of tubeless anesthesia. Serial arterial blood gas analysis was performed during preoxygenation and general anesthesia. The primary outcome was the partial pressure of CO2 (Paco2) after 30 minutes of general anesthesia, with each group compared using a Student t test.

RESULTS

Nineteen patients completed the study protocol (9 in the SV group and 10 in the apnea group). The mean (standard deviation [SD]) Paco2 was 89.0 mm Hg (16.5 mm Hg) in the apnea group and 55.2 mm Hg (7.2 mm Hg) in the SV group (difference in means, 33.8; 95% confidence interval [CI], 20.6-47.0) after 30 minutes of general anesthesia (P < .001). The average rate of Paco2 rise during 30 minutes of general anesthesia was 1.8 mm Hg/min (SD = 0.5 mm Hg/min) in the apnea group and 0.8 mm Hg/min (SD = 0.3 mm Hg/min) in the SV group. The mean (SD) pH was 7.11 (0.04) in the apnea group and 7.29 (0.06) in the SV group (P < .001) at 30 minutes. Five (55%) of the apneic patients had a pH <7.10, of which the lowest measurement was 7.057. No significant difference in partial pressure of arterial O2 (Pao2) was observed after 30 minutes of general anesthesia.

CONCLUSIONS

CO2 accumulation during apnea was more than double that of SV after 30 minutes of tubeless anesthesia using HFNO. The use of robust measurement confirms that apnea with HFNO is limited by CO2 accumulation and the concomitant severe respiratory acidosis, in contrast to SV. This extends previous knowledge and has implications for the safe application of HFNO during prolonged procedures.

Does apneic oxygenation prevent desaturation during emergency airway management? A systematic review and meta-analysis

RéSUMé: OBJECTIF: L'oxygénation apnéique (OA) par lunettes nasales est une méthode de prévention de la désaturation en oxygène au cours des intubations en urgence. L'objectif de cette revue systématique était de déterminer l'efficacité de l'OA sur la prévention de la désaturation en oxygène au cours des intubations en urgence.

SOURCE

Des recherches systématiques ont été effectuées dans trois bases de données électroniques (MEDLINE, EMBASE et CINAHL) pour identifier les études portant sur la prévention de la désaturation en oxygène au moyen de l'OA par lunettes nasales. Notre critère d'évaluation principal était l'incidence des désaturations telle que définie dans chaque étude; nous avons ensuite évalué l'incidence de la désaturation sévère en oxygène (SpO₂ < 80%). Une méta-analyse a été effectuée sur les études présentant des données sur la désaturation en oxygène telle que définie par chaque étude et chez des patients ayant une désaturation sévère pour générer une estimation groupée de l'effet.

CONSTATATIONS PRINCIPALES

Au total, 544 études ont été examinées, parmi lesquelles dix (2 322 patients) satisfaisaient tous les critères d'éligibilité. Comparativement à l'absence d'OA, l'utilisation de cette méthode a été associée à une réduction de la désaturation en oxygène (risque relatif [RR] : 0,76; intervalle de confiance à 95% [IC] : 0,61 à 0,95; P = 0,02), mais n'a pas été associée à une réduction de la désaturation sévère (RR, 0,65; IC à 95% : 0,38 à 1,11; P = 0,12). Néanmoins, il y avait une hétérogénéité significative des facteurs liés aux patients, des interventions et des définitions de la désaturation en oxygène entre les études.

CONCLUSION

Nos constatations suggèrent que l'OA par lunettes nasales est associée à un moindre risque de désaturation en oxygène au cours des intubations en urgence. Cependant, compte de tenu de l'hétérogénéité des études, d'autres essais de grande qualité sont nécessaires pour déterminer quels patients pourraient bénéficier de l'OA au cours des intubations d'urgence.

Apneic Oxygenation Versus Low-Tidal-Volume Ventilation in Anesthetized Cardiac Surgical Patients: A Prospective, Single-Center, Randomized Controlled Trial

OBJECTIVES

To compare the physiology of apneic oxygenation with low-tidal-volume (VT) ventilation during harvesting of the left internal mammary artery.

DESIGN

Prospective, single-center, randomized trial.

SETTING

Single-center teaching hospital.

PARTICIPANTS

The study comprised 24 patients who underwent elective coronary artery bypass grafting surgery.

INTERVENTIONS

Apneic oxygenation (apneic group: 12 participants) and low-VT ventilation (low-VT group: 12 participants) (2.5 mL/kg ideal body weight) for 15 minutes during harvesting of the left internal mammary artery.

MEASUREMENT AND MAIN RESULTS

The primary endpoint was an absolute change in partial pressure of arterial carbon dioxide (PaCO₂). Secondary endpoints were changes in arterial pH, pulmonary artery pressures (PAP), cardiac index, and pulmonary artery acceleration time and ease of surgical access. The mean (standard deviation) absolute increase in PaCO₂ was 31.8 mmHg (7.6) in the apneic group and 17.6 mmHg (8.2) in the low-VT group (baseline-adjusted difference 14.2 mmHg [95% confidence interval 21.0-7.3], p<0.001). The mean (standard deviation) absolute decrease in pH was 0.15 (0.03) in the apneic group and 0.09 (0.03) in the low-VT group baseline-adjusted difference 0.06 [95% confidence interval 0.03-0.09], p<0.001. Differences in the rate of change over time between groups (time-by-treatment interaction) were observed for PaCO₂ (p<0.001), pH (p<0.001), systolic PAP (p = 0.002), diastolic PAP (p = 0.023), and mean PAP (p = 0.034). Both techniques provided adequate ease of surgical access; however, apneic oxygenation was preferred predominantly.

CONCLUSIONS

Apneic oxygenation caused a greater degree of hypercarbia and respiratory acidemia compared with low-VT ventilation. Neither technique had deleterious effects on PAP or cardiac function. Both techniques provided adequate ease of surgical access.

Jet ventilation for surgical interventions in the upper airway

The clinical applications of jet ventilation (JV) in ear, nose, and throat surgery can be best understood by the characteristics that distinguish this form of ventilation from conventional positive pressure ventilation. By definition, JV is based on the application of gas portions under high pressure through an unblocked catheter into the airway, which is open to the ambient air. Beneficial opportunities arise in JV, which otherwise are not available in regular ventilation.

Voluntary hyperventilation before a rapid-sequence induction of anesthesia does not decrease postintubation PaCO2

To prevent hypercapnia, voluntary hyperventilation is recommended for patients with increased intracranial pressure before the induction of general anesthesia. We sought to determine whether this maneuver results in a lower PaCO2 than breathing 3 min of oxygen 100% by face mask (preoxygenation) after intubation. Thirty patients requiring general anesthesia were randomly assigned to breathe either 3 min of oxygen 100% by face mask (Group P) or 1 min of oxygen 100% followed by 2 min of voluntary hyperventilation with oxygen 100% (Group H). All patients received a standard rapid-sequence induction of anesthesia followed by a 90-s period of apnea. Patients were then tracheally intubated and mechanically ventilated. Five arterial blood gas samples were taken: with room air, after preoxygenation or hyperventilation, after 60 and 90 s of apnea, and after tracheal intubation. Voluntary hyperventilation decreased PaCO2 before rapid-sequence induction (hyperventilation, 30.0 +/- 3.5 mm Hg versus preoxygenation, 37.9 +/- 5.2 mm Hg; P < 0.0001), but after 60 s of apnea, both groups had similar PaCO2 (hyperventilation, 36.1 +/- 3.3 mm Hg versus preoxygenation, 35.6 +/- 3.4 mm Hg; P = 0.673), and no benefit was found after intubation (hyperventilation, 40.5 +/- 3.9 mm Hg versus preoxygenation, 41.4 +/- 2.7 mm Hg; P = 0.603). We conclude that voluntary hyperventilation before rapid-sequence induction does not provide protection against potential hypercapnia during intubation.

IMPLICATIONS

Voluntary hyperventilation before anesthesia induction is recommended for patients with increased intracranial pressure to prevent hypercapnia. This randomized, prospective study demonstrated that this maneuver does not result in a lower postintubation PaCO2 than standard preoxygenation.