Synchronized intermittent mandatory ventilation: time to send this workhorse out to pasture.

After its development as a weaning modality in the early 1970s,1 intermittent mandatory ventilation (IMV) quickly became a very popular mode of ventilation. Continuous mandatory ventilation, the prevailing mode in use prior to the introduction of IMV, permitted the patient to determine the number of ventilator breaths by sensing patient inspiratory effort. In contrast, IMV mode delivered a set number of controlled mandatory breaths while allowing the ventilated patient to breathe spontaneously (and unassisted) between mandatory breaths. Weaning was to occur by slowly reducing the set ventilator rate, allowing the patient to gradually take over the work of breathing.1 To overcome inherent asynchrony with IMV, a synchronized mode known as synchronized intermittent mandatory ventilation (SIMV) was developed, which senses patient inspiratory effort and enables the patient to receive “synchronized” patient-triggered mandatory breaths up to the set rate. SIMV quickly became the preferred mode in many ICUs, for both ventilation and weaning, in the overwhelming majority of institutions.2 However, enthusiasm for SIMV subsequently waned as physiologic studies examining its effects on ventilated patients questioned its theoretical benefits. Hudson et al3 showed that IMV may be helpful in correcting abnormally high pH in patients with respiratory alkalosis, but at the cost of increased work of breathing. Subsequently, both Marini et al4 and Imsand et al5 showed that work of breathing during SIMV was determined by the work of the spontaneous breaths, and that the mandatory delivered breaths did not effectively unload the respiratory muscles in critically ill patients with respiratory failure. Those authors concluded that neurologic control of the respiratory muscles, and hence respiratory muscle work, was not adaptable on a breath-by-breath basis, as is purported to occur by initial supporters of SIMV. The addition of pressure support for the spontaneous patient breaths to the mandatory patient-triggered breaths was subsequently found to decrease the work of breathing during SIMV,6 and use of SIMV with pressure support (SIMVPS) ventilation became more widespread. Henceforth, SIMV developed both ardent supporters and detractors. While experts debated the physiologic advantages and disadvantages of SIMV, randomized controlled trials were initiated to compare clinical outcomes of the various ventilator modes. Esteban et al7 and Brochard et al8 both showed that SIMV, as a weaning strategy, was inferior to either pressure support or T-piece trials, with SIMV on average slowing the weaning process from mechanical ventilation by 2–4 days. Thus, over time, the percentage of patients being weaned via SIMV or SIMV-PS declined from over 90% in the 1980s to just under 18% in 2004.9 More recently, in a large multinational observational study examining thousands of patients, Ortiz et al10 found that, compared to continuous mandatory ventilation, SIMV-PS was less likely to be used in Latin America or Europe, and more likely to be used in North America, and in patients with lower severity of illness scores, as well as those ventilated postoperatively or for trauma. While use of the SIMV mode for weaning declined, it also became clear that patient-ventilator asynchrony is associated with adverse outcomes, including a longer duration of mechanical ventilation. Studies by Chao et al11 and Thille et al12 clearly established that link, and suggested that elimination of asynchrony can also facilitate the process of liberation from mechanical ventilation. However, while the trials examining patient-ventilator asynchrony have examined mainly patients in medical ICUs, where the continuous mandatory ventilation mode predominates, patients in surgical ICUs are more likely to be ventilated via SIMV or SIMV-PS. Considering that patient populations differ substantially between the different types of ICUs, studies examining the occurrence and clinical impact of asynchrony in non-medical ICUs are of interest.