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successfully extubated (p ¼ .004). This supported previous studies in concluding that the variation in brain natriuretic peptide level before and after SBT could provide a noninvasive manner in which to augment the value of a SBT in predicting weaning outcome.14 At this time, this is not routine or widespread practice. Transthoracic echocardiography is widely used in ICUs to assist in the assessment of haemodynamic status, and certain aspects of haemodynamic status may impact weaning. The E/A ratio is the ratio of the early (E: ventricular filling during ventricular relaxation) to late (A: ventricular filling from atrial contraction) ventricular filling velocities. In a healthy heart, the E velocity is greater than the A velocity. However, in diastolic dysfunction, due to impairment of ventricular relaxation, a greater portion of the end-diastolic volume results from late filling rather than early filling and the normal E/A ratio (i.e., ,1) is reversed. The E/Ea ratio is a Doppler-derived index that may be more sensitive than the E/A ratio in identifying left venticular diastolic dysfunction in patients with preserved left ventricular systolic function. Lamia et al reported that in a group of patients who had already failed an SBT, the combination of E/A . 0.95 and E/Ea . 8.5 measured with transthoracic echocardiography at the end of an SBT allowed an accurate noninvasive detection of weaning-induced pulmonary artery occlusion pressure elevation.15 WHAT IS THE ROLE OF ULTRASONOGRAPHY IN WEANING? In addition to echocardiography, ultrasonography of the lungs and airways has potential use as a bedside procedure in predicting some aspects of weaning failure. B-lines are vertical reverberation artefacts (as opposed to horizontal A-lines, which are present in normal lungs) that arise from the pleural line and move in synchrony with the lung. They are usually considered to be a sign of increasing lung density and decreasing air content. The presence of B-lines on lung ultrasonography may give an early indication of heart failure or consolidation. Using M-mode ultrasonography, diaphragmatic dysfunction (vertical excursion of ,10 mm and paradoxical movement) was found in 29% of medical critical care unit patients without any history of diaphragmatic disease. A statistically significant higher rate of early and delayed weaning failure occurred in these patients with diaphragmatic dysfunction.16 A cuff-leak test can be used prior to extubation if there is concern about postextubation stridor due to airway oedema or swelling. It has been suggested that a measured difference of at least 110 mL or 10% of the inspiratory tidal volume, measured by a ventilator before and after cuff-down during an assist-control mode of ventilation, essentially rules out postextubation stridor. However, a leak volume of less than this is often associated with successful extubation.17 One study showed that Bmode laryngeal ultrasonography could be used to look at the air-column width on endotracheal tube cuff deflation and hence evaluate the air leak. A narrower air column on cuff deflation was found to correlate with a lower risk of postextubation stridor.18 It should be emphasised that this aids in the prediction of extubation failure rather than weaning failure per se. LONG-TERM AND PROLONGED MECHANICAL VENTILATION Some patients may simply never wean from mechanical ventilation. Up to 50% of difficult-to-wean patients require prolonged ventilation. According to a National Association for Medical Direction of Respiratory Care Consensus Conference, prolonged mechanical ventilation should be defined as the need for 21 consecutive days of mechanical ventilation for 6h/d.19 Successful weaning in prolonged mechanical ventilation constitutes complete liberation from mechanical ventilation (or a requirement for only nocturnal NIV) for 7 consecutive days. The previously described clinical review also highlights that there is no evidence-based time limit for when further attempts at weaning can be declared futile. Similarly, there are also no evidence-based guidelines to inform decisions as to whether withdrawal of or continued use of potentially lifelong ventilatory support is the correct course of action. Decisions such as
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