SUCCESSFUL techniques for mechanically supporting ventilation have been available since the 1920s. Positive-pressure ventilation was initially utilized in conjunction with anesthesia and surgery, while negative-pressure ventilators (iron lungs) were most commonly employed outside the operating room. The efficacy of negative-pressure techniques for supporting acutely ill patients was seriously questioned during World War II, and positive-pressure ventilation was firmly established as the superior technique by 1955.1,2 The advantages of positive-pressure ventilation are the ability to ventilate adequately despite increased airway resistance and/or decreased lung compliance, patient accessibility since the ventilator is not "encasing" the patient, and provision of adequate bronchial hygiene via endotracheal tube suctioning
The disadvantages of positive-pressure ventilation are directly related to pulmonary physiology. Gas moves into the lungs in response to transairway (tracheal to alveolar) pressure gradients. Spontaneous ventilation produces a transairway pressure gradient by decreasing pleural pressures, thereby creating transpulmonary pressure (tracheal to pleural pressure) gradients.