Intubation, with subsequent mechanical ventilation, is a common life-saving intervention in emergency departement. Many different strategies of positive-pressure ventilation are available. These are based on various permutations of triggered volume-cycled and pressure-cycled ventilations and are delivered at a range of rates, volume and pressures. Poor ventilatory management can inflict serious pulmonary and extrapulmonary damage that may not be immediately apparent,
Because many of the effects of ventilator-induced lung injury are delayed and not seen while patients are in the emergency fepartement, much of our understanding of the adverse consequences of volume trauma, air-trapping, baro trauma and oxygen toxicity has come from the critical care literature. While the fundamental principles underlying mechanical ventilatory support have changed little over the decades, much progress has been made in our understanding of the secondary pathophysiologic changes associated with positive-pressure ventilation.
Ventilatory strategies have been devised for different disease processes to protect pulmonary parenchyma while maintaining adequate gas exchange, and they may be responsible for the increased rates of survival for pathologies such as acute respiratory distress syndrome.
MODES OF MECHANICAL VENTILATION
- Volume-cycled mode
Inhalation proceeds until a set tidal volume is delivered and is followed by passive exhalation. A feature of this mode is that gas is delivered with constant inspiratory flow pattern, resulting in peak pressure applied to the airways higher than that required for lung distention (plateau pressure). Since the volume delivered is constant, applied airway pressures vary with changing pulmonary compliance (plateau pressure) and airway resistance (peak pressure)
Because the volume-cycled mode ensures a constant minute ventilation despite potentially abnormal lung compliance, it is a common choice as an initial ventilatory mode in the emergency departement. A major disadventage is that high airway pressures may be generated, potentially resulting in barotrauma. Close monitoring and use of pressure limits are helpful in avoiding this problem. Note that ventilators set to volume-cycled mode function well as monitors of patients pulmonary compliance, which will be decreased in physiological states such as worsening acute respiratory distress syndrome, pneumothorax, right mainstem intubation, chest wall rigidity, increased intra-abdominal pressure and psychomotor agitation. These pathophysiological states increase peak pressure and should be considered whenever pressure alarms are sounded.
- Pressure-cycled mode
A set peak inspiratory pressure is applied, and the pressure difference between the ventilator and the lungs results in inflation until the peak pressure is attained and passive exhalation follows. The delivered volume which each respiration is dependent on the pulmonary and thoracic compliance.
A theorical adventage of pressured-cycled modes is a decelerating inspiratory flow pattern, in which inspiratory flow tapers off as the lung inflates. This usually results in a more homogeneous gas distribution throughout the lungs. However, no definite evidence exists that this results in a reduction of the rate of ventilator-induced lung injury or overall mortality. Nevertheless, pressured-cycled ventilation has achieved considerable popularity in the intensive care setting for management of patients with acute respiratory distress syndomes, whose lungs are most likely to be characterized by a broad range of alveolar dysfuction and are also most vulnerable to the effects of barotrauma and volutrauma.
A major disadvantage is that dynamic changes in pulmonary mechanics may result in varying tidal volumes. This necessitates close monitoring of minute ventilation and limits the usefulness of this mode in many emergency departement patients. However, newer ventilators can provide volume-assured pressure-cycled ventilation, which incresae peak pressures as needed to deliver a preset minimum tidal volume.