Critical Care Medicine |
|Chapter 72:||Respiratory Care|
Pressure-control and pressure-release modes of ventilation can provide any desired I:E ratio because the beginning and end of the inspiratory cycle occur at preset time intervals. The use of an inverse I:E ratio (inspiration longer than exhalation) was initially noted to improve oxygenation and ventilation in neonates with severe infant respiratory distress syndrome and was later reported to be useful in ARDS. 200 Figure 7227 illustrates that a 2-second inspiratory time is usually associated with a near no-flow state at end-inspiration. 201 A ventilator rate of 15/min results in a ventilatory cycle of 4 seconds, allowing for a 2-second inspiratory time while a 1:1 I:E ratio is maintained. As illustrated in Figure 7227, a rate greater than 15/min requires an inverse I:E ratio to achieve a 2-second inspiratory time.
|FIGURE 7227 Schematic illustration of a pressure-preset (40 cm H2O), time-initiated, time-cycled ventilator mode set to provide a 3-second cycle (rate of 20/min) with a 1:1 inspiration-to-expiration (I:E) ratio. Significantly diminished pulmonary compliance with relatively normal airway resistance is assumed. Proximal airway pressures are shown as a solid line and flow in airways as a dotted line. Inspiration (Inspir.): See Figure 7226. Expiration (Expir.): Flow has not reached zero at the end of the expiratory cycle (1 s); therefore, alveolar pressure is greater than baseline. The figure shows an airway pressure greater than baseline, but this may not be the case. To show an airway pressure equal to alveolar pressure may require a delay of next inspiratory cycle (expiratory occlusion pressure). Therefore, autopositive end-expiratory pressure (auto-PEEP) (intrinsic PEEP) is detected either by measuring airway pressure after an exhalation pause or noting that flow has not ceased at the end of the expiratory cycle. (From Shapiro and Peruzzi174 )|
Available data suggest that inverse-ratio ventilation serves to enhance alveolar recruitment primarily by creating auto-PEEP, 202 as shown in Figure 7227 . Those who prefer to avoid prescribing extrinsic PEEP levels in excess of 15 cm H2 O, but who also embrace the concept of inverse I:E ratios for support of oxygenation, must realize that the total PEEP level (auto-PEEP plus extrinsic PEEP) is the determinant of the extent of alveolar recruitment. When applying total ventilator support, there are no data to suggest that inverse-ratio ventilation has any advantage over a 1:1 I:E ratio (at a ventilator rate £15/min) as long as the appropriate extrinsic PEEP levels are prescribed. There is no evidence to suggest that high PaCO2 levels are significantly diminished by inverse ratios as compared with 1:1 ratios and appropriate extrinsic PEEP.
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