We performed a single-center, retrospective, observational study in a university hospital. We collected data from patients admitted to the ICU between January 2011 and December 2017. The study design and protocol were approved by the Institutional Review Board (IRB) of Sapporo Medical University (IRB authorized number: 322-266). Owing to the observational nature of this study, the information was released on an opt-out basis.

We included patients (≥ 18 years) who had been intubated for > 72 h in the ICU of Sapporo Medical University Hospital and extubated after confirming successful SBT results defined as follows: respiratory rate < 30/min, SpO2 > 94%, heart rate < 140 bpm, no arrhythmia, no excessive increase in blood pressure and no effort breathing, under continuous positive airway pressure ≤ 5 cmH2O or pressure support ≤ 5 cmH2O or T-piece for ≥ 30 min with inspiratory oxygen concentration (FIO2) ≤ 0.4. SBT was performed when the patient fulfilled the condition described in Table 1, and the ultimate decision to extubate was made by the intensive-care clinician based on SBT data, as well as the patient’s hemodynamic stability, responsiveness, ability to follow commands, the strength of cough, and the ability to clear secretions. The exclusion criteria were patients after cardiovascular surgery or those with a tracheostomy. Extubation failure was defined as reintubation within 24 h after extubation.

Table 1 Criteria for initiating SBT

Information obtained from electronic medical records included age, sex, underlying disease, Charlson Comorbidity Index (CCI), patient category at ICU admission (postoperative or medical), ICU length of stay (LOS), Acute Physiology and Chronic Health Evaluation (APACHE) II score at ICU admission, Sequential Organ Failure Assessment score at ICU admission, ventilation days, 28-day mortality, delirium, RSBI, and frequency of tracheal suctioning. The primary outcome was the association between reintubation within 24 h after extubation and tracheal suctioning frequency (more than once every 2 h for up to 12 h) [5, 11] before extubation.

The definition of tracheal suctioning is as follows.

  1. 1)

    The patient’s effortful breathing is increased (increased respiratory workload findings).

  2. 2)

    Secretions are visible in the tracheal tube.

  3. 3)

    Coarse crackles that suggest the presence of secretions from the trachea to the right and left main bronchi are heard on chest auscultation, or there is a decrease in breath sounds on chest auscultation.

  4. 4)

    Palpation of the chest wall reveals vibrations associated with the movement of gas.

  5. 5)


  6. 6)

    Blood gases and blood oxygen saturation rate (SpO2) show hypoxemia.

  7. 7)

    An increase in airway pressure or a decrease in ventilation volumes.

Reintubation was performed when the patient had one or more of the following criteria: clinical signs of increased respiratory effort, upper airway obstruction, respiratory acidosis, hypoxemia (SpO2<90%), decreased consciousness with unprotected upper airway (GCS<8), severe tachycardia, and continued tachypnea.

Statistical analysis

Categorical variables were expressed as numbers and percentages. Continuous variables were expressed as means and standard deviations. Chi-square tests were used for the nominal variables. The Mann-Whitney U test was used for continuous variables. We hypothesized that the frequency of tracheal suctioning was an independent risk factor for reintubation. To evaluate this hypothesis, logistic regression analysis was performed to examine the odds ratio of the frequency of tracheal suctioning, adjusting for confounding factors that contribute to reintubation. We selected covariates based on the previous literature and clinical experience. Sex, CCI [7, 12], underlying pneumonia, APACHE II score at ICU admission [13], and ventilation days [12] were selected as confounding factors. The results of the multivariable analysis are shown with odds ratios (ORs), 95% confidence intervals (CIs), and p-values. Statistical significance was set at p <0.05. Statistical analyses were performed using SPSS Statistics version 27 (IBM Corp., Armonk, NY, USA).

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