1) Ventilator modes have evolved significantly since the introduction of intermittent mandatory ventilation (IMV) in the 1970s. Early ventilators only provided continuous mandatory ventilation while IMV allowed for partial ventilator support. 2) While IMV was widely adopted based on early case studies, subsequent research questioned its physiological foundations and advantages over continuous mandatory ventilation. 3) A recent study found no difference in outcomes between synchronized IMV with pressure support compared to continuous mandatory ventilation, despite biases favoring IMV/pressure support. However, the study had limitations and more research is still needed.

1. Ventilator Modes : Where Have We Come
From and Where Are We Going?
Dean R. Hess
Chest 2010;137;1256-1258
DOI 10.1378/chest.10-0205
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2. Medicine (Dr Good), Department of Medicine, National Jewish support for IMV was based on a case series of six
Medical and Research Center.
Financial nonfinancial disclosures: The authors have reported patients. There is no control group, and not a single
to CHEST that no potential conflicts of interest exist with any P value is reported.
companies organizations whose products or services may be dis- So how is it, then, that IMV permeated the practice
cussed in this article.
Correspondence to: Michael D. Iseman, National Jewish Med- of mechanical ventilation? I think there are several
ical and Research Center, Department of Medicine, Division of reasons. First, it provided a potential solution to the
Mycobacterial and Respiratory Infections, 1400 Jackson St.
Denver, CO, 80206-1997; e-mail: isemanm@njc.org problem of poor patient-ventilator interaction of the
© 2010 American College of Chest Physicians. Reproduction ventilators in use at that time. Second, the authors
of this article is prohibited without written permission from the were enthusiastic about their discovery and widely
American College of Chest Physicians (www.chestpubs.org
site misc reprints.xhtml). promoted it. Third, respiratory therapists were
DOI: 10.1378 chest.10-0606 becoming more involved in the care of mechanically
ventilated patients. We (I was one of them) were an
adventurous group. The article by Downs et al4 not
Ventilator Modes only introduced the concept of IMV, but it provided
illustrations of how this could be done. Soon, an
Where Have We Come From and entire generation of respiratory therapists was adept
Where Are We Going? at jury-rigging H-valves and anesthesia bags into the
circuits of the Emerson and MA-1 to allow IMV.
Amodevolume, and flowthe ventilator controls pres-
sure,
describes how
within a breath, along with
The enthusiasm for IMV provided an opportunity
for manufacturers. Ventilators like the Emerson IMV
a description of how the breaths are sequenced.1 and BEAR 1 became available, which allowed IMV
Modern ventilators feature many modes, some without the need for any jury-rigging. In addition,
of which have been touted by manufacturers and the BEAR 1 had a demand valve, so IMV could be
zealous clinicians. Most new ventilator modes have accomplished without adding flow into the circuit.
become available with a paucity of evidence to guide This allowed the advantage of easier monitoring
their safe and effective implementation.2 Are these of exhaled tidal volume. The BEAR 1 also allowed
new modes an innovation or a solution in search of the mandatory breaths to be synchronized with the
a problem?3 patient’s spontaneous breathing efforts, hence syn-
When I was first introduced to ventilators, they con- chronized IMV (SIMV). The proposed advantage of
sisted of the Bird Mark 7, the Puritan-Bennett PR-2, SIMV was that it avoided breath stacking. Whether
and the Emerson Post-Op. I can recall our excitement this is important was debated at the time,5,6 but none-
when the first Puritan-Bennett MA-1 arrived! These theless all adult ventilators designed for acute care
ventilators were limited to a single breath type: pres- eventually offered SIMV, and SIMV became the sole
sure control (Mark 7 and PR-2) or volume control ventilator mode used in many ICUs.
(Emerson and MA-1). They only provided contin- Today, SIMV is quite different than the IMV first
uous mandatory ventilation (CMV), commonly called described in 1973. The mandatory breaths can be
assist-control (A C). The triggers on these early gen- either volume control, pressure control, or adaptive
eration ventilators were notoriously bad, promoting pressure control. The spontaneous breaths can be
poor patient-ventilator interaction. pressure support (PS) or adaptive pressure control,
It is against this background that intermittent and tube compensation can be used. The PS spon-
mandatory ventilation (IMV) was introduced in 1973. taneous breaths can be altered with rise time and
This allowed partial ventilatory support, in which flow cycle controls. It has always been the case
some of the minute ventilation is provided by the ven- that IMV and CMV are synonymous if the patient
tilator and the remainder is provided by the patient’s is making no spontaneous breathing efforts. But,
respiratory muscles. In their original report, Downs with the current generation of ventilators, the
et al4 promoted several advantages for IMV: reduced selection of breath parameters might also result in
weaning time, better control of Paco2, improved little difference between SIMV and CMV during
cardiac output, and better patient-ventilator inter- spontaneous breathing. Imagine, for example, if
action. Review of this paper illustrates how the the mandatory breaths are pressure control, the
scientific method has changed over the years. spontaneous breaths are PS, and the pressure con-
Numerous claims were made without any substan- trol and PS levels are identical.
tiation other than the authors’ experience. In fact, More than a decade after the introduction of IMV,
the reference for the statement, “Weaning time was studies began to appear that questioned the physio-
much less with IMV than with conventional tech- logic foundation of this mode. It was shown that Paco2
niques in infants who had RDS” is “personal com- was determined more by respiratory drive than the use
munication.” Little data are provided, and the entire of IMV.7 If some breath types are mandatory breaths
1256 Editorials
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© 2010 American College of Chest Physicians

3. provided by the ventilator and others are spontaneous received only SIMV/PS, 2% initially received SIMV/PS
breaths provided by the patient’s respiratory muscles, but were then switched to CMV, 11% initially received
how is it that the respiratory center and respiratory CMV but were switched to SIMV/PS, and 67% only
muscles can vary their output in anticipation of the received CMV. There was no advantage reported in
ventilator response on the next breath? Marini et al8 terms of clinical outcomes with SIMV PS compared
reported that inspiratory work increased progressively with CMV, despite treatment-allocation bias that
for both spontaneous and mandatory breaths as venti- favored SIMV PS.
lator support was withdrawn, suggesting little adapta- There are several limitations to the study by Ortiz
tion to the mandatory volume-control breaths during et al,15 which they acknowledge. Because only total
IMV. Imsand et al9 reported that electrical activations respiratory rate is known, we do not know the pro-
of the sternocleidomastoid and diaphragm were sim- portion of mandatory vs spontaneous breaths in the
ilar in successive spontaneous and mandatory breaths SIMV group, or the proportion of patient-triggered
during SIMV. They concluded that inspiratory motor vs ventilator-triggered breaths in the CMV group.
output is not regulated breath by breath but rather is The PS level in the SIMV PS group is also not known.
constant for a given level of ventilator support. Leung However, I doubt that these limitations had an impor-
et al10 reported that the addition of PS of 10 cm H2O tant impact on the important findings of this study;
with IMV resulted in respiratory muscle unloading there is no advantage in terms of clinical outcomes
for both the mandatory and spontaneous breaths. with SIMV PS compared with CMV.
The results of these studies suggest that, unlike what What additional lessons can be learned from this
has been commonly taught, SIMV does not allow study? Many new modes introduced in recent years,
partitioning the work of breathing between that done like SIMV, have strong support by some clinicians, but
by the ventilator and that done by the patient. have not been subjected to rigorous scientific study.
The most common use of IMV SIMV has been for None has been conclusively shown to improve patient
ventilator weaning. The intent is to gradually reduce outcomes. The Acute Respiratory Distress Syndrome
the mandatory breath rate until successful extuba- Network study,16 which reported a lower mortality
tion can occur. However, this notion was challenged with a tidal volume target of 6 mL kg in patients with
soon after the introduction of IMV by Schachter acute lung injury, is the only study of mechanical
et al.11 They reported that the length of hospitaliza- ventilation ever shown to improve patient outcome.
tion was 36 days for patients who received IMV and In that study, patients were ventilated with volume-
30 days for patients who received CMV, but this control CMV; neither SIMV nor any new ventilator
was not statistically significant because of the small mode was used. We should have sufficient evidence
sample size. Two studies of similar design reported of benefit for any new mode before it receives wide-
in the mid-1990s evaluated ventilator modes used spread adoption. For SIMV, it is unfortunate that it
for weaning.12,13 Both reported that the majority continues to have widespread use despite little evi-
of patients were successfully extubated on the first dence for benefit.
spontaneous breathing trial. More relevant to this
Dean R. Hess, PhD, RRT, FCCP
editorial, both studies reported the poorest weaning
Boston, MA
outcomes with SIMV. The available evidence does
not support weaning with the use of a gradual reduc- Affiliations: From the Department of Anesthesia, Harvard Med-
tion in ventilator support using SIMV.14 ical School; and the Department of Respiratory Care, Massachu-
Although SIMV has not been shown to be physi- setts General Hospital.
Financial nonfinancial disclosures: The author has reported
ologically sound or improves outcomes, it nonethe- to CHEST the following conflicts of interest: Dr Hess is a con-
less continues to be used frequently. It is against this sultant for Respironics, Novartis, and Pari. He receives royalties
from Impact.
background that the article by Ortiz et al15 is pub- Correspondence to: Dean R. Hess, PhD, RRT, FCCP, Respira-
lished in this issue of CHEST (see page 1265). They tory Care, Ellison 401, Massachusetts General Hospital, 55 Fruit
St, Boston, MA 02115; e-mail: dhess@partners.org
report that patients were more likely to receive SIMV © 2010 American College of Chest Physicians. Reproduction
with PS if they were from North America, had lower of this article is prohibited without written permission from the
severity of illness, or were ventilated postoperatively American College of Chest Physicians (www.chestpubs.org
site misc reprints.xhtml).
or for trauma. SIMV PS was less likely to be selected DOI: 10.1378 chest.10-0205
if patients were ventilated because of asthma or coma,
or if they developed complications such as sepsis or
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1258 Editorials
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5. Ventilator Modes : Where Have We Come From and Where Are We
Going?
Dean R. Hess
Chest 2010;137; 1256-1258
DOI 10.1378/chest.10-0205
This information is current as of November 27, 2011
Updated Information & Services
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