A highly illustrative example of how different factors can lead to imbalance between ventilatory needs and respiratory capability is provided by acutely hyperinflated patients. In these patients, the load of the inspiratory muscles is increased for a variety of reasons. First, airway obstruction and/or decreased elastic recoil lead to prolongation of expiration that cannot be completed before the ensuing inspiration. It implies that at the end of an expiration there is still a positive pressure at the alveolar level. Consequently, during the next inspiration the inspiratory muscles have to develop an equal amount of pressure before airflow begins. Second, because of hyperinflation tidal breathing occurs at a steeper portion of the pressure-volume curve of the lung, further increasing the load. At the same time that the load is severely increased, the neuromuscular competence is decreased due to muscular weakness. Hyperinflation forces the inspiratory muscles to operate at an unfavourable position in their length-tension curve. In a state of hyperinflation the costal and crural fibres of the diaphragm are arranged in series, rather than in parallel, and this diminishes the force that can be generated. The resultant flattening of the diaphragm increases its radius of curvature and, according to Laplace's law (Pdi = 2Tdi/Rdi; where Pdi is the pressure-generating capacity, Tdi is the tension and Rdi is the radius of curvature), diminishes its pressure-generating capacity for a given tension developed.
The daily screening of patients who are on mechanical ventilation with the aim of identifying those able to breathe spontaneously is, possibly, the best approach to reduce the duration of ventilatory support. Standard weaning criteria were used in all of the aforementioned studies to identify patients who were able to resume spontaneous breathing, and patients who did not meet such criteria remained on mechanical ventilation. The ability to breathe spontaneously is adequately tested by performing a trial with either T-tube or pressure support of 7 cmH2O. A duration of 2 h has been extensively evaluated, but weaning outcome is the same when the duration is reduced to 30 min. Patients failing the initial spontaneous breathing trial need a gradual withdrawal of ventilatory support. It is known that SIMV is the most ineffective method of weaning those patients. With respect to the use of pressure support or T-tube, clinicians should choose the method they feel most comfortable with and individualize the strategy to meet the patient's needs. We recommend the use of a once daily trial of spontaneous breathing in difficult-to-wean patients for three main reasons: it leads to extubation twice as quickly as PSV; it simplifies management, because the patient's ability to breathe without ventilatory support needs to be assessed only once a day; and it allows a prolonged period of rest, which may be the most effective method to permit adequate time for muscle recovery. 2b1af7f3a8