Background

Aortic dissection (AD) is a lethal disease in which the inner layer of the aorta tears. The Stanford classification divides AD into two groups, namely Type A aortic dissection (TAAD) and Type B aortic dissection. TAAD results from pathological involvement of the ascending aorta and is associated with significant mortality and morbidity despite the numerous apparent improvements in diagnosis and management during the past six decades. Recent data from a study of 4428 patients between 1995 and 2013 show that the in-hospital and surgical mortality rates are still as high as 22% and 18% for patients with TAAD, even with modern medical and surgical/endovascular therapies [1]. Noticeably, for patients with acute [12,13,14,15,16]. However, the mechanism by which OSA affects the development of hypoxaemia and prolonged ICU stay in patients undergoing TAAD repair has not been fully elucidated. In this study, we sought to examine: 1) the prevalence of OSA in patients with TAAD, and 2) to evaluate the association of OSA with postoperative hypoxaemia and prolonged ICU stay following TAAD repair.

Methods

Study design and patient enrolment

This retrospective study was conducted in 252 consecutive patients with acute or chronic TAAD (either anterograde or retrograde subtype) who underwent total arch replacement between January 1 and December 31, 2018, at the Bei**g Anzhen Hospital. TAAD was diagnosed using computed tomographic angiography in all the patients.

Patients would be excluded from this study due to one of the following conditions:

  1. (1)

    diagnosed with OSA and treated by positive airway pressure before surgery,

  2. (2)

    unsuccessful sleep tests owing to clinical concerns, i.e., haemodynamic instability, unbearable pain, severe anxiety, etc.,

  3. (3)

    slept for less than 4 h in the evening of the sleep test owing to insomnia or unbearable pain, and

  4. (4)

    declined to take the sleep test.

A total of 124 patients completed the sleep test and 83 of them had complete sleep data and answered the STOP-BANG questionnaire [17]. These 83 patients were included in the final analysis (Fig. 1).

Fig. 1
figure 1

Flow chart of patient enrollment. TAAD type A aortic dissection, OSA obstructive sleep apnoea, PAP positive airway pressure

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Sleep study

OSA was diagnosed via completing sleep test within 90 days following surgical procedure. Nox T3 devices (Nox Medical, Reykjavík, Iceland) were used to perform the sleep tests, and an acceptable sleep test should contain a total sleep time > 5 h. Recorded electrodes included nasal pressure transducers, thoracic and abdominal plethysmography, cardiac pulse, snoring, body position, activity, and percutaneous oxygen saturation.

According to the detection of a nasal airflow transducer, apnoea was defined by breath cessation or ≥ 90% airflow drop that lasted longer than 10 s, while hypopnea was verified by at least 30% decline in airflow that lasted longer than 10 s and was accompanied by a 3% decrease in oxygen saturation. Apnoea hypopnea index (AHI) was defined as the sum of apnoea and hypopnea per hour. OSA was diagnosed when the AHI was 15/h or more, and severe OSA was defined by an AHI greater than 30/h. Hypoxaemia during sleep was determined by the average oxygen saturation, the nadir nocturnal oxygen saturation, the percentage of time with oxygen saturation of < 90%, and the oxygen desaturation index (i.e., oxygen saturation drop by ≥ 3% per hour). Daytime sleepiness was calculated using the Epworth Sleepiness Scale (ESS) [18].

Although not accepted as a diagnostic tool, the STOP-BANG score was completed [17] routinely on admission. Patients were assessed by answering “yes” or “no” to eight questions related to major subjective and objective manifestations of OSA. The sum of scores ranged from 0 to 8, and a modified STOP-BANG score of ≥ 4 was considered a high risk of OSA [3).

Table 3 Multivariate logistic analyses of risk factors for postoperative hypoxaemia and a prolonged intensive care unit stay
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Factors associated with prolonged ICU stay

With an overall area under the curve of 0.72 (95% CI 0.60–0.84; p = 0.002) (ROC analysis showed that a postoperative OI of 133.25 was the optimal cut-off value for predicting prolonged ICU stay (sensitivity: 63.6%; specificity: 75.4%; accuracy: 48.3%) (Fig. 3). When taken as a continuous variable, for every unit decrease in postoperative OI, the risk of prolonged ICU stay would be increased by 1% (OR 1.01; 95% CI 1.00–1.02, p = 0.008).

Fig. 3
figure 3

Receiver-operating characteristic curve of postoperative oxygenation index threshold for predicting prolonged intensive care unit stay. AUC area under the curve

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OSA was shown to be associated with prolonged ICU stay (unadjusted OR 3.84; 95% CI 1.32–11.17, p = 0.010; adjusted OR 4.05; 95% CI 1.27–12.90, p = 0.018) (Table 3). Furthermore, compared to an AHI of < 15/h, an AHI of > 30/h (severe OSA) was also identified to be associated with prolonged ICU stay (OR 5.46; 95% CI 1.66–19.49, p = 0.005), which persisted following adjustments for confounding variables (OR 5.60; 95% CI 1.59–21.75, p = 0.009) (Table 3).

Discussion

The results of this study show that OSA is prevalent in patients with TAAD, and severe OSA is predictive of postoperative hypoxaemia and prolonged ICU stay following TAAD repair. For patients with TAAD undergoing surgical repair, a sleep apnoea assessment with the STOP-BANG questionnaire prior to surgery could be helpful in recognising individuals who are at high risk of postoperative hypoxaemia and requiring perioperative intervention for OSA.

Here, the high prevalence of OSA in our cohort is in line with previous studies in patients with AD [9, 10]. OSA is considered to increase the risk of AD due to distinctive blood pressure surge and fluctuations, acceleration of atherosclerosis, and uniquely, exaggerated negative thoracic pressure, which induces strong shear forces onto the aorta [27,28,29]. According to the only literature on the prevalence of TAAD in patients with OSA available as of present [30], middle-aged men with features of being tall, fat and having comorbid hypertension are at high risk of TAAD.

As reported in our previous and present study, many patients with TAAD developed postoperative hypoxaemia, which was closely associated with poor operative outcomes [4,12,13,14], and this could prolong ICU stay and worsen surgical outcomes. The current study shows that severe OSA predicts postoperative hypoxaemia, which in turn is a strong predictor of prolonged ICU stay. Therefore, identification and treatment of preoperative OSA using non-invasive strategies, such as positive airway pressure could lead to a reduced postoperative hypoxaemia and a shorter ICU stay. Unfortunately, most patients with TAAD are in critical condition and need emergency surgery, which renders the evaluation by preoperative polysomnography, impractical. As intermittent desaturation is mainly caused by breathing events, preoperative oximetry is an alternative test that can be easily performed and used to identify patients with high odds of OSA. In addition, the STOP-BANG questionnaire is another approach to evaluate OSA, which could also serve as a valuable diagnostic clue considering the high consistency between the STOP-BANG score and the results of the sleep test, as shown in our study.

The major limitation of this study is inherent in the nature of TAAD, a clinical catastrophe that has to be managed by an emergency surgery, and this precludes the possibility of having a sleep test before surgical repair. Postoperative sleep assessment together with preoperative questionnaire could still generate incompetent data for determining a preoperative sleep status in individuals with significant changes in body weight and cardiopulmonary function, following an aortic repair. Second, many factors other than OSA can lead to postoperative hypoxaemia and prolonged ICU stay, such as comorbidities, complexity of the procedure and postoperative management. To avoid the risk of model overfitting in statistics, these data were excluded from the multivariate analysis; thus, our conclusion should be extrapolated with caution. This study only included cohorts that underwent a standardised Sun’s procedure for TAAD repair [20] and received a similar post operational care by the same professionals, ensuring the comparability among patients.

Conclusions

The results of this study demonstrate that OSA was highly prevalent in patients with TAAD, and this could predict postoperative hypoxaemia and prolonged ICU stay following surgical repair. Preoperative sleep assessment among patients with AD help identify OSA; further studies are warranted to investigate whether the treatment of OSA benefits the cohort.