High-Flow Heated and Humidified Oxygen Therapy and Gastric Distension

High Flow Heated and Humidified Nasal Oxygen therapy (HFHHNO) is being increasingly used in the emergency medicine and intensive care settings to manage patients with acute hypoxemic respiratory failure, and to optimize pre-oxygenation prior to intubation in patients with mild-to-moderate hypoxemia.

More recently, there have been reports of applications in the anesthesia perioperative setting. In fact, it is possible that the single greatest potential advantages of HFHHNO for anesthesia practice is that (unlike face mask and CPAP devices) oxygen administration can be maintained during periods of apnea with the potential to significantly prolong the apnea time available for safe and effective airway management.

However, the low levels of positive airway pressure associated with HFHHNO has raised the question of whether prolonged use could result in gastric insufflation thus increasing the risk of regurgitation and aspiration with an unprotected airway. Gastric distension is a concern for anesthesiologists because it leads to the activation of a parasympathetically-mediated reflex through the vagus nerve that leads to secretion of acetylcholine by enteric neurons. In turn, increased acetylcholine activates M3 receptors on parietal cells resulting in increased secretion of gastric acid. The combination of an increased volume of gastric secretions and high intraluminal pressure may place patients at risk of pulmonary aspiration.

The primary objective will be any change in gastric fluid volume from the baseline to each scan following HFHHNO therapy. Participants will undergo an abdominal ultrasound scan prior to any oxygen therapy to provide a baseline. One 30-minute session of oxygen therapy will follow(up to 60-70 L/min). After session, another ultrasound scan will take place, identical to the baseline to obtain the same measurements. From the images taken by the ultrasound, a mathematical model can be applied to calculate the gastric volume (and any change).

The secondary outcome will be the incidence of "gastric air distension" defined by qualitative ultrasound as a distended antrum with air content that blurs the posterior gastric wall.

Inclusion criteria

• Healthy volunteers aged 18 to 70 years
• Male or female
• American Society of Anesthesia physical status classification I and II
• Height greater than 145 cm
• Ability to understand the study protocol and provide informed consent. Communication difficulties will not be an impediment to participate. In case of a language barrier, translation services will be sought as per usual institutional practice.

Exclusion criteria

• Subjects predisposed to have an increased residual gastric volume at baseline (e.g. diabetes or known gastric dysmotility)
• History of major upper gastrointestinal disease (including hiatus hernia or prior gastroduodenal surgery)
• Morbid obesity (Body Mass Index > 40 Kg/m2)
• Chronic Obstructive Pulmonary Disease (Emphysema or Chronic Bronchitis)
• Subjects diagnosed with type I and II Diabetes