Indications and options in mechanical ventilation

The decision to institute mechanical support should be made independently of that to perform tracheal intubation. Valid indications for tracheal intubation include: the need to assist ventilation or to deliver high levels of inspired oxygen; to protect the airway against aspiration; to clear secretions retained in the central airways; and to relieve upper airway obstruction. Noninvasive ventilation can often be used without the requirement of intubating the trachea, unless high pressure is needed or airway secretions need extraction. The primary indications from mechanical ventilation include: inadequacy of alveolar ventilation, inadequacy of arterial oxygenation, excessive respiratory workload, and cardiovascular insufficiency or congestion. Many physiological consequences of mechanical ventilation depend on the degree of effort made by the patient during the breathing pattern. Because positive pressure ventilation can influence intrapleural pressure as well as lung volume, mechanical ventilation can offer the loading conditions of both ventricles, either to the benefit or the detriment of the patient. Options for positive pressure mechanical ventilation include: pressure pre-set (pressure-targeted) ventilation and flow-controlled, volume-cycled ventilation. Because the product of pressure and flow determines the mechanical work performed on the lung and chest wall, different parameters need to be monitored when using one or the other of these cycling algorithms. The standard modes of positive pressure ventilation include: controlled mechanical ventilation, assist-controlled ventilation, synchronized intermittent mandatory ventilation, and pressure support ventilation. Dual control ventilatory modes combine the desirable characteristics of pressure-controlled breaths, and flow-controlled volume cycled breaths. Examples of these newer modes include: pressure-regulated volume control, volume support, and volume-assured pressure support. Airway pressure release and bi-level positive airway pressure are other intriguing options for partial ventilatory support. Adjuncts to mechanical ventilation that do not involve pressure to achieve gas exchange include permissive hypercapnia, inhaled nitric oxide, and tracheal gas insufflation. Noninvasive ventilation can prove useful when attempting to avoid intubation or shortly after extubation has been accomplished in a marginally compensated patient.