Abstract
In Reply.— Dr Johnson's statement that negative-pressure and positive-pressure ventilators are identical in physical and physiologic principles is not correct. Studies and clinical experience have shown that positive-pressure ventilation results in an increase in mean intrathoracic pressure.1This increase in mean intrathoracic pressure may result in diminution of cardiac output2and disruption of ventilation-perfusion relationships.3,4The same is not true of negative-pressure ventilators. We do not disagree with Dr Johnson about the advantages of volume ventilators over pressure ventilators. However, as stated in our article, we limited our discussion to volume ventilators since it is currently believed that pressure-cycled machines are not desirable. Our article discusses the principles of ventilatory support of acutely ill patients. We did not intend any injustice to patients requiring long-term ventilatory support. The support of patients with chronic irreverisble ventilatory insufficiency requires consideration of factors not necessarily pertinent to acute short-term ventilation
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