The choice of ventilator settings should be guided by clearly defined therapeutic end points. In most instances, the primary goal of mechanical ventilation is to correct abnormalities in arterial blood-gas tensions. In most patients, this is accomplished easily by adjusting the minute volume to correct hypercapnia and by treating hypoxemia with oxygen (O2) supplementation. Because the volume, frequency, and timing of gas delivered to the lungs have important disease-specific effects on cardiovascular and respiratory systems functions, the physician must avoid simply managing the blood-gas tensions of the ventilator-dependent patient. After a brief review of the capabilities of modern ventilators, this chapter discusses the mechanical determinants of patient–ventilator interactions and defines therapeutic end points in common respiratory failure syndromes. These sections provide background for the major thrust of the chapter, which is to detail the physiologic consequences of positive-pressure ventilation and to develop recommendations for ventilator settings in various disease states based on this knowledge.
The incorporation of microprocessors into ventilator technology has made it possible to program ventilators to deliver gas with virtually any pressure or flow profile. Significant advances have been made in producing machines that are more responsive to changes in patient ventilatory demands, and most full-service mechanical ventilators display diagnostic information contained in airway pressure (Paw), volume (V), and flow (
) waveforms. Because of these added capabilities, the practitioner is being challenged with a staggering array of descriptive acronyms for so-called new modes of ventilation. To avoid unnecessary confusion, it is useful not to focus on specific modes for the moment but rather to consider three general aspects of ventilator management: (a) the choice of inspired-gas composition, (b) the means to ensure the machine’s sensing of the patient’s demand, and (c) the definition of the machine’s mechanical output.
Choice of Inspired-Gas Composition
Practically speaking, decisions regarding the composition of inspired gas concern only the O2 concentration (see “Acute Lung Injury and Hypoxic Respiratory Failure” below). Although there may be occasions when the care provider considers supplementing the inspired gas with nitric oxide, the efficacy of nitric oxide therapy for most forms of hypoxic respiratory failure remains to be established.1 There has been growing interest in the biologic effects of hypercapnia on gas exchange, vascular barrier properties, and innate immunity.2–7 Therapeutic hypercapnia, that is, the deliberate supplementation of inspired gas with carbon dioxide (CO2), however, cannot be recommended at this point in time. On extremely rare occasions, it may be appropriate to use a helium-oxygen mixture in an attempt to lower the flow resistance across a lesion in the distal trachea or mainstem bronchi, and there has been some interest in the use of helium in asthma.8 Currently, these approaches must be considered experimental.
Machine’s Sensing of Patient’s Demand (Ventilator Triggering)
Ideally, a mechanical ventilator should adjust not only its rate but ...