The Anesthesia Intensive Care Unit (AICU), managed by anesthesiologists, provides intensive care, monitoring and treatment for critically ill patients after surgery until they are recovered and transferred to a normal ward. Some of patients, unfortunately, experience a delayed discharge from the AICU for more than 24 h due to various causes, leading to the increase in medical cost and patient’s dissatisfaction. This single-center retrospective study investigated the incidence of delayed discharge from the AICU in a tertiary hospital and analyzed its risk factors. It is found that a younger or older age, higher ASA physical status classification, BMI outside of the normal range, preoperative complications, emergency surgeries and intraoperative massive hemorrhage were significantly correlated with delayed discharge from AICU.

We consistently observed a large proportion of patients at the age of 65 years and above in the AICU (43.09%), accounting for 47.15% of cases remaining in AICU for greater than 24 h [9]. The number of elderly surgical patients is annually on the rise, consisting of a high-risk group with preoperative frailty, perioperative complications and impaired organ functions. Cao et al. [10] suggested that the advanced age is mainly responsible for delayed postoperative recovery and transfer to the ward. In the present study, children and teenagers younger than 18 years were also the high-risk group of delayed discharge from AICU. A retrospective analysis further revealed that most of them were scoliosis patients, accompanying tissue and organ dysplasia at varying degrees. The prolonged duration of scoliosis surgery, coupled with significant intraoperative blood loss, substantially increased the perioperative risk, ultimately resulting in delayed discharge from the AICU [11].

The ASA physical status classification is a vital indicator in preoperative assessments, and a poor physical status is a risk factor for prolongation of postoperative recovery and unexpected transfer to ICU [12, 13]. We consistently showed that ASA Class III-IV was significantly correlated with delayed discharge from AICU. BMI is also a determinant of delayed discharge from AICU due to the negative influence of obesity on sleep apnea, delayed extubation and perioperative hypoxemia, especially in patients treated with opioids. Most studies only used dichotomies for BMI. Therefore, low body weight had been ignored. Studies have found a significant correlation between low body weight (BMI < 21 kg/m2) and delayed resuscitation, in addition to morbid obesity [14]. Further analysis reveals that the majority of low body weight patients are scoliosis patients. These patients generally have some degree of developmental and nutritional problems. And their surgery time is long, the degree of surgery is complex, and there is a lot of intraoperative bleeding, which can lead to a delay in transferring out of AICU after surgery. The present study revealed that both BMI < 18.5 kg/m2 and > 25 kg/m2 were risk factors for delayed discharge from AICU.

Patient’s physical condition, together with surgery-associated factors both influence the perioperative rehabilitation of AICU patients. For surgery-associated factors, surgical specialties, type of surgery, duration of surgery and intraoperative conditions are closely linked with postoperative recovery [15,16,17]. The majority of emergency surgeries are performed in critically ill patients usually with respiratory and circulatory dysfunction. The severe illnesses largely increase the incidence of delayed discharge from AICU or even the possibility of being transferred to ICU for more intensive care. In addition, a linear correlation exists between intraoperative blood loss and incidence of delayed discharge from AICU. Massive hemorrhage causes a perioperative hypoperfusion. Moreover, blood product transfusion in turn imbalances the physiological homeostasis, prolongs the pharmacokinetics and pharmacodynamics of anesthetic drugs, and influences the nervous, muscular and functional recovery, eventually prolonging AICU stay [18]. However, we did not identify a significant correlation between the duration of surgery and delayed discharge from AICU, and it may be attributed to the treatment of critical illnesses in the tertiary hospital, types of surgical specialties and high-quality postoperative care.

Complex surgeries, such as spine surgeries (7.74%), orthopedic surgeries (2.58%) and vascular surgeries (2.37%), generally need longer duration of surgery and anesthesia maintenance, more complicated methods of anesthesia, greater influences on physiological functions and higher incidence of complications, resulting in a high incidence of delayed discharge from AICU [13]. Postoperative hemorrhage (22.17%), septic shock (18.55%), hypoperfusion (18.10%) and pulmonary insufficiency (17.19%) were the most common causes of delayed discharge from AICU. The above patients usually experienced a longer admission in intensive care units because of the extensive damages to important organs and systems [19, 20]. Most postoperative recovery events are related to the cardiopulmonary system, and hypoperfusion, hypoxia and infections serve as huge challenges in perioperative management [21]. A large-scale retrospective study reported that the incidence of postoperative cardiovascular and respiratory complications in patients suffering from a delayed transfer to the ward is inflated to 40.33% [22].

A nomogram for predicting the risk of delayed discharge in AICU patients was developed and validated with excellent discrimination and calibration in the current study. We visualized these data using nomograms, which is more conducive to clinicians’ judgment and targeted treatment. The result of nomogram showed that age of younger than 18 years, ASA physical status of class IV and V, and intraoperative massive hemorrhage of 2000–3000 ml were the highest scoring factors. We retrospectively analyzed these factors and found that most of them were concentrated in adolescent spinal correction patients, who were also the group with the highest proportion of AICU delayed discharge. Because such patients have multiple preoperative complications, complex surgeries, and rapid changes in their condition, comprehensive perioperative evaluation and comprehensive management were necessary. By combining with the nomogram, personalized prevention and intervention based on risk factors can be achieved, thereby improving the treatment rate of critically ill patients and providing them with comfortable perioperative care.

Outperformed in the research of delayed discharge from AICU, our study for the first time assessed its risk factors. In addition, we introduced the binary logistic regression model and established a nomogram model. Indeed, there are some limitations in our research. First of all, it was a single-center retrospective study that lacked an external validation. Second, nomogram is based on retrospective studies, requiring further validation in prospective cohort and clinical trials. In addition, this study only analyzed all patients who entered AICU. In fact, there are certain differences in postoperative management among patients undergoing different surgeries, which is also the content that needs further analysis and research in our subsequent experiments. In future research, we will use a multicenter prospective study model to further explore the treatment and prognosis of patients with different diseases in AICU, in order to further expand our findings.