Evidence based mechanical ventilation

For years technological advances in respiratory care have outpaced clinical evidence, resulting in a confusing array of opinions about the best way to deliver and “wean” from mechanical ventilation (MV). But evidence based medicine is here, enabling a systematic approach. This approach is nicely summarized in a review by Neil MacIntyre in the November issue of Chest. The article is worth reading in the original. A few summary points are made below.

The review pertains primarily to acute respiratory distress syndrome (ARDS) and acute lung injury (ALI) but many of the principles are applicable to other forms of respiratory failure. MV itself can injure the lungs, a concept known as ventilator induced lung injury (VILI). There are two well known mechanisms of this injury: volutrauma (over distension of alveoli) and atelectrauma (alveolar shearing caused by repetitive opening and closing of marginal or “recruitable” alveoli). These mechanisms of injury suggest a rationale for the prevention of both forms of VILI by maintaining tidal ventilation between the lower and upper inflection points of the pressure volume curve. Theoretically this means utilization of low tidal volumes combined with high levels of peep designed to recruit alveoli. However, somewhat surprisingly, only one of these strategies—low tidal volumes—is supported by high level clinical evidence. As for the use of peep, the best approach may be that outlined by the ARDS net peep scale.

The current synthesis of evidence also enables a systematic, almost algorhythmic, approach to discontinuation of MV. The process of discontinuation has traditionally been termed “weaning”, a misconception, since it implies that liberation from mechanical ventilation is done gradually. This concept was the basis for the now outmoded approaches of gradual reduction in levels of pressure support or frequency of machine delivered breaths in the SIMV mode. (Patients with poor cardiovascular reserve may occasionally constitute an exception to the rule that true weaning is not necessary. Because resumption of spontaneous breathing is a cardiovascular stress, patients with marginal reserve may experience ischemia and pulmonary edema upon abrupt withdrawal of ventilatory support. That issue is addressed briefly in another review in the same issue of Chest).

The process has two phases, the first of which consists of clinical assessment for readiness, and which should be done daily. The criteria that must be met are 1) evidence of stabilization and/or improvement in the original disease process; 2) blood gas parameters reflecting PO2/FiO2 > 150, peep level of less than 8, FiO2 less than 0.5, and ph greater than 7.25; 3) hemodynamic stability; and 4) ability to make an inspiratory effort. This assessment must also take into account the patient's general condition and neurologic status.

Once those criteria are met the patient may be considered for advancement to the second phase, the spontaneous breathing trial, which can be done with the patient on a T-tube, C-pap, or pressure support. If the patient maintains a frequency/tidal volume ratio under105 over the first several minutes, the trial is continued for 30 minutes to 2 hours. Tolerance of the period of spontaneous breathing without adverse clinical signs generally indicates readiness for withdrawal from MV, tempered by clinical judgment. Patients who fail the trial should have their disease process, nutrition, pain control, fluid balance and overall management reviewed and breathing trials repeated as appropriate.