Initiation of respiration may be ventilator triggered ('control breaths') or patient triggered ('assisted breaths'). The inspiratory time and I:E ratio remain constant during ventilator-triggered ventilation but in an assist mode the inspiratory time usually remains fixed, resulting in a variable I:E ratio. Some ventilators allow only ventilator-initiated pressure-controlled breaths, while others provide an assist-control mode and even pressure-controlled synchronized intermittent mandatory ventilation. Owing to the dependence of tidal volume on the I:E ratio, controlled ventilation is usually performed. While sedation and even paralysis are often required, particularly with a prolonged inspiratory time, some patients find this ventilatory mode comfortable. The appropriate inspiratory time may be determined by slowly increasing it while monitoring the increasing tidal volume until no further tidal volume increase occurs ( Blanch ef ai 1993b). Further prolongation of the inspiratory time may be necessary to improve oxygenation (see discussion of iny.e.^S.® Ia.ti.9 y®ntiia!i0.0 below).

Each breath is delivered as a constant pressure maintained for a defined inspiratory time, with flow decelerating exponentially at a rate determined by the patient's thoracic compliance and airways resistance (Fig 3). This square-wave pressure delivery with decelerating flow appears to have certain physiological advantages, allowing maximal mean airway pressure with minimal peak airway pressure. This may improve oxygenation and dead-space while minimizing lung injury. Unlike volume-preset ventilation, the maximal flow rate is not preset but is a function of the preset pressure and airways resistance. However, the actual maximal flow rate delivered in pressure-control ventilation is limited by the flow capabilities and internal resistance of the ventilator. Pressure and flow patterns similar to those characterizing pressure-controlled ventilation may be emulated to some degree with volume-controlled ventilation when a decelerating flow rather than the usual constant or sinusoidal flow pattern is used. Pressure-controlled ventilation is time cycled, differing from volume-controlled ventilation which is volume cycled and pressure support ventilation which is flow cycled. Exhalation occurs passively with opening of the exhalation valve at the end of the inspiratory time. PEEP may be used to prevent derecruitment of alveoli or, alternatively, prolongation of the inspiratory time (at the expense of expiratory time) may produce intrinsic PEEP with similar physiological effects.

Fig. 3 Schematic diagram of airway and alveolar pressure (upper trace) and flow (lower trace) during pressure-controlled inverse ratio ventilation. Note the square-wave pressure curve (black line) with shaded alveolar pressure. Flow decelerates exponentially during inspiration. Note that at end-expiration there is still expiratory flow and positive alveolar pressure (auto-PEEP). (Reproduced with permission from Marcy andm.Marinii (199il}.)

Inverse ratio ventilation

Each respiratory cycle in conventional ventilation is characterized by a short inspiratory phase and longer expiratory phase (e.g. an I:E ratio of 1:2), usually allowing for complete exhalation. Extending the inspiratory time to a ratio greater than unity improves oxygenation, lung compliance, and dead-space ratio in some patients with acute lung injury. Histopathological evidence of reduced lung injury may also be demonstrated. While this benefit of inverse ratio ventilation has been shown in animal studies and small clinical series, no randomized prospective trial has been performed; whether there is benefit remains unclear. Ventilation at I:E ratios ranging from 1:1 to 4:1 is used, but adverse hemodynamic effects and the potential for barotrauma are more common at very high I:E ratios.

A prolonged inspiratory time has the beneficial effect of increasing mean airway pressure and allowing adequate time for ventilation of alveoli with long inspiratory time constants. However, from a theoretical point of view, auto-PEEP generated during inverse ratio ventilation will produce a relatively smaller increase in the regional functional residual capacity of lung units with short expiratory time constants, and hence is potentially not as effective as applied PEEP. The short expiratory time can produce dynamic hyperinflation or intrinsic PEEP. This may be responsible in part for improved oxygenation. Pressure-controlled inverse ratio ventilation may have advantages over similar effects produced by volume-controlled ventilation with an inspiratory pause ( BJ,a.n..c.h ei.,.a( 1993b). The maintenance of pressure during the prolonged inspiratory phase overcomes loss of airway pressure which may occur in the volume-controlled situation owing to leaks and alveolar recruitment.

Close monitoring is necessary after initiation of pressure-controlled inverse ratio ventilation to ensure adequate oxygenation, minute ventilation, and hemodynamic stability. The development of excessive intrinsic PEEP may cause hemodynamic compromise and reduce tidal pressure and, therefore, minute ventilation. Deep sedation or paralysis may be required to prevent dyssynchronous breathing. Inverse ratio ventilation should not be used in the presence of airflow obstruction where exhalation time needs to be maximized.

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