In patients who are mechanically ventilated for more than 72 hours weaning failure is a common issue. The Spontaneous breathing trial (SBT) is often done to assess if the patient can be extubated with a high chance of success. However, re-intubation rates are between 15 - 20 % after a successful SBT. The rapid shallow breathing index (RSBI) is an important parameter used in an SBT. Because the high incidence of extubation failure (re-intubation within 48 hours) a search for a better parameter than the RSBI is warranted. Using the measured end-tidal oxygen (etO2) of mechanically ventilated patients it is possible to calculate the VO2, which is a measure of patient effort. The VO2 is a parameter with the potential to predict weaning success or failure, together with other parameters of patient effort like the work of breathing (WOB), pressure time product (PTP) and esophageal pressure swings, reflecting muscle strength of the diaphragm. Therefore, the investigators want to investigate if these parameters are associated with an SBT success or failure.

Weaning failure is a common problem in patients who are mechanically ventilated for more than 72 hours. Prediction of successful extubation is crucial for mechanically ventilated (MV) intensive care patients. Prolonged intubation leads to prolonged mechanical ventilation, which is known to have undesirable and detrimental effects. Premature extubation, on the other hand, is associated with clinical deterioration, a second episode of mechanical ventilation, and prolonged mechanical ventilation, resulting in multiple adverse outcomes.

In addition, up to 20% of all Intensive Care Unit (ICU) patients fail to wean from mechanical ventilation. Prolonged mechanical ventilation is associated with half of the total time spent on the ICU. Improving the prediction of successful weaning provides physicians with a mean to optimize extubation timing, while reducing the incidence of premature extubation.

The rapid shallow breathing index (RSBI) is a parameter commonly used to predict extubation success, next to some other clinical parameters such as consciousness level, the ability to cough and muscle strength. The most commonly used RSBI cut-off value for predicting save extubation incorrectly classifies 15% to 20% of patients. Which means that those patients require another period of mechanical ventilation with possible adverse consequences. It is therefore necessary to develop new parameters or indices aiding in the guidance of extubation timing.

Various other parameters, such as work of breathing (WOB), transpulmonary pressure swings (PS) and the pressure time product (PTP), which are manifestations of patient effort, have been investigated. Although a difference in transpulmonary pressure swings during a spontaneous breathing trial (SBT) has been linked to weaning failure, it has yet to be determined whether transpulmonary pressure swings predict extubation success or failure. For all these measures of effort the placement of an esophageal catheter is obligatory.

Oxygen consumption (VO2) is a direct manifestation of patient effort. It is defined as the difference between the amount of oxygen inhaled and exhaled over a predetermined time interval. Using end tidal oxygen (etO2) measurements and volumetric calculations, it is feasible to track VO2 variations breath-by-breath. A change in oxygen consumption during an SBT could be an early indicator of patient effort and failure.

The investigators hypothesize that VO2 measurement, together with indices of patient effort, could be valuable additional parameters when estimating weaning failure or success, and therefore, aid in the prediction of extubation success or failure.

Inclusion Criteria:

• Age above 18 years
• > 24 hours mechanically ventilated
• Eligible for a spontaneous breathing trial according to the clinical team
• Regular SBT ("Regulier SBT") according to the Leiden University Medical Center (LUMC) protocol
• Hemodynamically stable
• Esophageal Catheter in situ

Exclusion Criteria:

• Severe chronic obstructive pulmonary disease (COPD) (Gold class IV) if it results in a non regular SBT according to the LUMC protocol.
• Heart failure (LVEF <30%) if it results in a non regular SBT according to the LUMC protocol
• Known pregnancy

The intended population consists of adults who are mechanically ventilated in the ICU and meet the criteria for a regular spontaneous breathing trial according to the ruling LUMC protocol.

Data will be shared upon request with researchers.