Both premature and delayed extubation prolong the duration of mechanical ventilation and the intensive care unit (ICU) length of stay and increase morbidity and mortality. Therefore, accurate prediction of postextubation distress and the early diagnosis of the causes responsible for failure of a trial of pressure support ventilation are of paramount importance to improve the outcome of mechanically ventilated patients in the ICU.

This observational study is designed to test the ability of cardiac and diaphragm function assessed by bedside ultrasound to predict extubation failure within 48 h and re-intubation within 1 week after extubation.

Weaning from mechanical ventilation is the process of gradual ventilatory support reduction to the patient subjected to mechanical ventilation for more than 24 hours. Determining the correct time to extubate mechanically ventilated patients is a crucial issue in the critical care practice.

Spontaneous breathing trial (SBT) is recommended to predict weaning outcome. However, 13% to 26% of patients who are extubated following a successful SBT need to be reintubated within 48 hours.

Traditional indicators, such as respiratory rate (RR), minute ventilation, tidal volume (VT), and the rapid shallow breathing index (RSBI), can reflect patients' integral conditions, but none has shown great prognostic accuracy for weaning failure.

Although there are several causes of weaning failure, cardiac function deterioration during the weaning process combined with acute pulmonary edema is considered the leading cause of weaning failure. The transition from positive to negative thoracic pressure increases venous return and left ventricular afterload, decreases left ventricular compliance and may induce cardiac ischemia. All of these factors tend to increase ventricular filling pressures and may consequently lead to weaning-induced pulmonary edema. The efficacy of echocardiography for predicting weaning failure has been reported, however there is debate over the predictive value of echocardiography in this setting continues due to differences in weaning technique and outcome evaluation.

The diaphragm is the principal respiratory muscle. With an excursion of 1 to 2 cm, it provides nearly 75% of the resting pulmonary ventilation, while during the forced breathing, its amplitude is up to 7 to 11 cm. However, the diaphragm is vulnerable to damage from hypotension, hypoxia, and sepsis, which are very common in critically ill patients. While in surgical patients, diaphragm dysfunction is often caused by acute insults such as trauma or surgical procedures. In addition, mechanical ventilation itself can decrease the force of the diaphragm and induce diaphragmatic dysfunction, named as ventilator-induced diaphragmatic dysfunction. Many studies have shown that Diaphragm dysfunction is responsible for a number of pulmonary complications, including atelectasis and pneumonia, which are risk factors for extubation failure. and might lead to weaning failure and long-term mechanical ventilation.

Some studies have reported that diaphragmatic excursion (DE) or and diaphragmatic thickening fraction (DTF) could predict extubation failure.

Although transthoracic echocardiography and diaphragm ultrasound have been confirmed in independently assessing extubation outcomes, few studies have shown their different roles in the weaning process until now.

• Inclusion criteria: All patients aged more than 40 years who are intubated and mechanically ventilated for more than 48 hours, with improvement of underlying cause of acute respiratory failure and considered ready to undergo a spontaneous breathing trial (SBT) by the attending physician on the basis of readiness-to-wean criteria used in our surgical ICUn Criteria

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Exclusion Criteria:

1. Planning for tracheostomy after the SBT.

2. Severe mitral stenosis, severe regurgitation, or prosthetic mitral valve.

3. History of diaphragmatic dysfunction as diaphragmatic palsy, cervical spine injury, or neuromuscular disease.

4. Pneumothorax or pneumo-mediastinum.

5. Use of muscle-paralyzing agents within 48 h before the study.

6. Poor echocardiographic windows or difficult windows of diaphragmatic movement as women in late pregnancy or obese patients.

7. Extubation failure definitely caused by upper airway obstruction.

8. Planned prophylactic noninvasive ventilation (NIV) after extubation.

9. Atrial fibrillation or atrioventricular conduction abnormality.

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All patients aged more than 40 years who are intubated and mechanically ventilated for more than 48 hours, with improvement of underlying cause of acute respiratory failure and considered ready to undergo a spontaneous breathing trial (SBT) by the attending physician on the basis of readiness-to-wean criteria used in our surgical ICUn Criteria