Legionella is an aerobic, gram-negative Bacillus. At present, there are 58 species and over 70 serotypes of Legionella identified, of which at least 24 species can cause lower respiratory tract infections in humans. Approximately, 90% of Legionella pneumonia is caused by Legionella pneumophila serogroup 1, which is widely distributed in warm, humid environments and can replicate in water at 25–42 °C. Humans usually become infected with Legionella through inhaling Legionella-containing aerosols from contaminated water sources (e.g., rain, pipes, air-conditioning systems) or inhaling directly contaminated water sources in specific conditions, such as water births. After entering the respiratory tract, Legionella can survive and replicate exponentially in human alveolar macrophages, releasing toxins and virulence factors, resulting in L. pneumophila (Bai et al. 2023).

The clinical manifestations of L. pneumophila infections are primarily respiratory. Two quite distinct kinds of respiratory illness may result from infection; the reasons for this dichotomy are not understood. The most common presentation is acute pneumonia, which varies in severity from mild illness that does not require hospitalization (walking pneumonia) to fatal multilobar pneumonia.

Typically, patients have high, unremitting fever and cough but do not produce much sputum.

The symptoms of Legionella infection undoubtedly result from a combination of physical interference with oxygenation of blood, ventilation-perfusion imbalance in the remaining lung tissue, and release of toxic products from bacteria and inflammatory cells. Bacterial factors include a protease that may be responsible for tissue damage. Cellular factors include interleukin-1, which produces fever after it is released from monocytes, and tumor necrosis factor, which may be responsible for some of the systemic symptoms; it is the causal agent of 5 to 12% of sporadic community-acquired pneumonia cases. Recent studies regarding severe community-acquired pneumonia have shown that Legionella pneumophila is the second most common cause of admission to ICU, not far behind pneumococcal pneumonia (Winn 1996).

Gastric stump carcinoma is a clinical entity that has been known in general surgery for decades. It has been calculated that 10% of patients undergoing distal gastric resection for benign disease will develop residual gastric cancer approximately 15–20 years after the first operation, and this is primarily due to gastroduodenal reflux (Degastrogastrectomy for cancer of the gastric stump 1999). The prognosis of gastric stump cancer is generally poor, especially due to the low resectability rates associated with intra-perioperative surgical complications (Sinning et al. 2007).

Anastomotic leakage (AL) after gastrectomy is one of the most severe postoperative complications and is related to increasing mortality. Gastric cancer remains one of the most common cancers worldwide, but the mortality shows a continuously decreasing trend on account of the developments in surgical technique and perioperative management. At present, radical gastrectomy is still the only probably curative therapy for resectable gastric cancer. Nevertheless, such surgical treatment includes the standard lymph node dissection and various reconstruction methods, and this high complexity of surgical procedure leads to a high risk of death and postoperative complications. AL is a destructive and potentially life-threatening postoperative complication, which is relevant to the increasing cost for treatment, the prolongation of hospitalization, and postoperative mortality. The incidence of AL has been reported to be 1 ~ 6% in gastric cancer patients after gastrectomy. In a study conducted on 3926 patients, AL after gastrectomy risk factors were analyzed. Univariate analyses indicated that in the elderly, the low concentrations of plasma hemoglobin, albumin, and cholesterol, diabetes, tumors located in the upper third stomach, the laparoscopic approach, proximal or total gastrectomy, esophagojejunostomy, and long operation time were hte indipendent risk facots facilitating AL development. Multivariate analysis revealed that albumin concentration, diabetes, the laparoscopic approach, and proximal or total gastrectomy were the independent risk factors facilitating AL development (He et al. 2023).

High-flow nasal cannula (HFNC) oxygen is a recently developed noninvasive oxygen therapy system. It can provide heated and moist oxygen through the nasal cannula, as well as offer a much higher and predictable gas flow rate (up 60 L/min) and FiO2 (up to 100%). Studies demonstrate that HFNC completely prevents hypoxia during sedated gastroscopy via two mechanisms. First, the high-flow produces positive pressure within the nasopharyngeal space and thoracic cavity, which reduces airway obstruction and increases the end-expiratory lung volume. Second, HFNC can produce positive pharyngeal pressure during expiration with a constant flow, with the pressure mainly determined by the volume of flow and expiratory flow of the patient. Because of its potential to improve oxygenation and ventilation, HFNC has been applied in many clinical situations to prevent hypoxemia, such as in awake fiber-optic intubation, conscious sedation during bronchoscopy, and some dental treatments under intravenous sedation. In addition, a few randomized controlled trials have shown that HFNC could also reduce the risk of hypoxemia during sedated digestive endoscopy, but some studies cannot draw the same conclusion (Zhang et al. 2022).

During gastroscopy, the patient’s mouth is kept open because of the gastroscopy tube. Therefore, it is reasonable to doubt the positive airway pressure mechanism. Maintaining a constant PEEP with HFNC is challenging because it can significantly decrease with open-mouth breathing. A recent systematic review demonstrated that when subjects ventilated with HFNC opened their mouth, hypopharyngeal pressure dropped from 5.2 (3.5, 7.0) cm H2O to 1.1 (− 0.9, 2.4) cm H2O with HFNC set at 50 L/min, and nasopharyngeal pressure dropped from 6.8 to 0.8 cm H2O with HFNC set at 60 L/min (Li et al. 2023). The goal of this conduct is to favor the washout of CO2 in the anatomical dead space, including more distal conducting airways, and to maximize the alveolar fraction of oxygen through the replacement of nitrogen to oxygen, stored in the lungs as functional residual capacity (FRC). It results in a reduction of rebreathing of CO2, decreases the available pressure gradient for oxygen transfer to the alveolus, and hastens the onset of hypoxemia (Bartlett et al. 1959).

We preferred to use propofol as a sleep inducer instead of a combination of midazolam and opioids due to its more predictable pharmacokinetic profile, rapid onset, and overlapping adverse effects for the dose that was used in our case.

In a recent retrospective study on procedure- and sedation-related adverse events in 73,029 endoscopies performed in the United States, Goudra et al. identified 44 patients who required endotracheal intubation and 14 deaths (Goudra et al. 2021). Therefore, reducing the incidence of hypoxia and severe hypoxia is always an important task during sedated endoscopy procedures. The optimal strategy for reducing the risks of adverse events caused by hypoxia is to prevent the development of hypoxia during the procedure.

A systematic review and meta-analysis on the effectiveness of high-flow nasal cannula during sedated digestive endoscopy demonstrated that compared to SNC (steady flow nasal cannula), HFNC not only reduce the incidence of hypoxemia but also reduce the requirements for airway interventions during sedated digestive endoscopy procedures, especially in patients at low risk for hypoxemia (Zhang et al. 2022).