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Intermittent mandatory ventilation (IMV) allows the patient to breathe spontaneously between machine-cycled or mandatory breaths. This concept originated in 1955 with an unnamed ventilator designed by Engstrom.1,2 In the early 1970s, Kirby et al3,4 introduced IMV as a means of ventilator support of infants with respiratory distress syndrome. In 1973, Downs et al5 were the first to propose IMV as a method to facilitate discontinuation from mechanical ventilation in adults. Those investigators6,7 also pioneered IMV use as a primary means of ventilator support during acute respiratory failure. Subsequently, breath-delivery design has been modified. Mandatory breaths initially delivered regardless of respiratory timing are synchronized with the patient’s inspiratory effort.8,9 This mode of ventilation has been termed intermittent demand ventilation, 8 intermittent assisted ventilation,9 and synchronous intermittent mandatory ventilation (SIMV). SIMV is an established partial mechanical ventilation mode in critically ill patients, both adult10 and neonate, worldwide.11 Currently, however, SIMV application in adults has declined except in North America12 and Australia–New Zealand,13 whereas in neonates, SIMV application remains prevalent.14 This chapter uses the terms IMV and SIMV interchangeably unless specifically indicated for clarification.


IMV is a means of ventilator support in which a preset number of positive-pressure (mandatory) breaths are delivered while the patient breathes spontaneously between the mandatory breaths. The mandatory breaths can be in the form of a preset volume (flow-limited, volume-cycled), pressure (pressure-limited, time-cycled),15 or a combination of pressure and volume (dual control).16 In principle, IMV is similar to controlled mechanical ventilation (CMV), in which the patient receives a predetermined number of mandatory machine-triggered breaths independent of spontaneous breathing effort. Likewise, SIMV is similar to assist-control ventilation (ACV), in which mandatory breaths are triggered by the patient. In contrast to CMV and ACV, however, in both IMV and SIMV the patient is allowed to breathe spontaneously between the mandatory breaths. In addition, with IMV and SIMV, the clinician can vary the ventilator support level according to the set IMV rate. At a high IMV rate, in which the patient’s spontaneous effort is suppressed, IMV provides full ventilator support. At a zero IMV rate, it provides no support, and all breaths are spontaneous. Between these extremes, IMV provides partial ventilator support.

System Design

Three types of IMV systems are described: continuous-flow IMV and pressure-triggered and flow-triggered SIMV systems.

Continuous-Flow IMV

The original IMV design uses a continuous-flow system.3 Two parallel circuits—one for the patient’s spontaneous breaths and the other for the mechanical breaths—are connected through a sidearm and a one-way valve, and share a common oxygen and air source. The continuous-flow IMV setup can be either an open or a closed system.17 The open system employs a reservoir tube that has a capacity of at least 1.5 times ...

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