Intermittent Mandatory Ventilation (IMV)

introduction

During IMV, breaths are delivered at a set rate, tidal volume, and flow (ie, mandatory breaths). In addition, the patient can breathe spontaneously through the ventilator circuit (ie, non-mandatory breaths). Many ventilators synchronize the mandatory breaths with inspiratory effort by the patient, which is called synchronized IMV (SIMV). Similar to ACV, the work of breathing can be excessive if the peak flow or trigger sensitivity of the mandatory breaths is insufficient.
Patient effort was previously believed to increase in proportion to the reduction of mandatory breaths only. More recent data suggest that this is not true. Instead, patient effort (measured as inspiratory work and the pressure-time product) increases in proportion to both mandatory and non-mandatory breaths. This is largely due to the inability of the respiratory center to rapidly adapt its output to intermittent support.

machine breaths can be patient or machine triggered
machine breaths can be volume or pressure triggered
spontaneous breathing allowed between mandatory machine breaths
often used in conjunction with Pressure Support Ventilation (PSV) to support spontaneous breaths

back up rate guarantees a minimum minute volume
less haemodynamic effects compared with Assist-Control Ventilation (ACV)
improved patient-ventilator synchrony
reduced muscle atrophy
reduced tendency to develop intrinsic PEEP
improved intrapulmonary gas distribution due to preservation of spontaeous breathing.

patient has an unstable respiratory drive or is unable to do all the work of breathing
it is desirable for the patient to breathe spontaneously (eg. minimise muscle atrophy)
the patientis able to assume a portion of the work of breathing