The aims of mechanical ventilation are summarized in Table 1. Conventional mechanical ventilation is indicated where potentially reversible acute respiratory failure does not respond to the measures described above. A few patients with less severe lung injury respond to intermittent mandatory ventilation synchronized to their own spontaneous respiration. However, in practice, the majority of patients with established ARDS require continuous mandatory ventilation with varying amounts of positive end-expiratory pressure (PEEP). PEEP prevents repetitive opening and closing of recruitable alveoli and diminishes shear stress forces in the alveolar wall. It can be titrated against lung compliance in several ways to optimize alveolar recruitment ( Tuxen 1994). Ventilatory modes are usually volume or pressure preset
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Table 1 Objectives of mechanical ventilation
Traditional volume-preset modes of ventilation utilize constant inspiratory flow rates to produce supraphysiological tidal volumes. In theory, this diminishes dependent atelectasis and compensates for the increased dead-space asssociated with positive pressure ventilation. Mandatory respiratory rates are adjusted to maintain minute ventilation, and therefore PaCO2, within the normal range. At higher respiratory rates, incomplete exhalation can result in unintentional PEEP as a consequence of air trapping; this phenomenon is known as intrinsic or auto-PEEP. The obligatory nature of control-mode ventilation precludes any patient effort and usually requires neuromuscular blockade. The later development of assist modes initiated by the patient's own efforts allows more patient-ventilator interaction, but their utility in the critically ill is dependent on trigger sensitivity and type. Applying an external source of PEEP to the ventilator circuit may improve pressure triggering in patients with significant degrees of intrinsic PEEP. The volume-preset strategy has been employed as a mode of choice in patients with normal lungs for many years, but in the presence of poorly compliant lungs it may result in unacceptably high peak inspiratory airway pressures.
ARDS has heterogeneous effects on lung parenchyma. Poorly compliant areas of diseased lung are contiguous with relatively normal alveoli. Aerated pulmonary tissue is usually compliant and ventilates normally, unlike consolidated lung. Therefore patients with ARDS lungs can be thought of as having small-volume lungs. Conventional intermittent positive-pressure ventilation with 'standard' tidal volumes of 10 to 15 ml/kg causes hyperinflation or 'volotrauma' in less affected areas and can result in iatrogenic lung injury. Advances in ventilator design and the recognition that iatrogenic pulmonary damage results from volotrauma and/or barotrauma secondary to high airway pressures have led to the concepts of lung protection and pressure-limited ventilation. As a result, volume-cycled ventilation has been largely superseded by pressure-preset modes of ventilation in the management of acute lung injury and ARDS. These methods employ rapid inflows of inspiratory gas in conjunction with a predetermined airway pressure limit. This results in a square pressure wave and an exponential deceleration of flow during the inspiration ( Fig
2). Pressure-preset ventilation increases mean alveolar pressure but reduces peak inspiratory pressure. Tidal volume cannot be adjusted directly, but is determined by preset pressure limits and pulmonary compliance.
Fig. 2 Inverse ratio ventilation: airway pressure measured in (a) a volume-preset mode of ventilation (volume-controlled inverse ratio ventilation) and (b) with rapid inspiratory flow combined with a preset pressure limit (pressure-controlled inverse ratio ventilation). (Reproduced with permission from Keogh,,,§L§.L (1990))
The volume- and pressure-preset modes of ventilation can be manipulated further by adjusting cycling or 'duty' times. The inspiratory-expiratory time ratio (I:E ratio) of modern ventilators can be increased or reversed in order to sustain higher mean airway pressures throughout respiration and prevent alveolar collapse ( Fig..2). The prolonged inspiratory phase may also faciltate delivery of gases to alveoli with long time constants, but whether improvements in oxygenation are achieved without concomitant increases in auto-PEEP is a controversial issue. Volume-controlled inverse ratio ventilation delivers a preset tidal volume and provides predictable CO2 clearance, but may result in significant air trapping and hemodynamic compromise, secondary to 'stacking' of mandatory tidal breaths and high peak inspiratory airway pressures. Therefore volume-controlled inverse ratio ventilation has been largely superseded by pressure-controlled inverse ratio ventilation, which delivers a constant preset inspiratory pressure throughout the designated inspiratory time. Although this maneuver avoids high peak pressures, the mean airway pressure, which determines oxygenation, is increased. Inverse ratio ventilation is uncomfortable for the conscious patient and requires adequate sedation and usually neuromuscular blockade.
The potential for a reduction in the incidence of pulmonary barotrauma encouraged the development of high-frequency low-tidal-volume techniques as alternative modes of ventilation. Humidified ventilator gases can be driven into the airway with a preset inspiratory time at frequencies varying from 1 to 10 Hz and still permit passive expiration. Tidal volumes are smaller than dead-space volume and gas exchange is believed to depend on convective forces. Ventilating the lungs near their natural resonant frequency (approximately 5 Hz) increases mean airway pressures and alveolar recruitment, reduces peak airway inspiratory pressure, and permits adequate CO2 clearance (Swami—d Keogh 1992). Increasing the I:E time and driving pressure augments tidal volume and results in improved functional residual capacity.
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