In this multicenter retrospective study, a novel objective measure for complications of mechanical ventilation predicted patients' ventilator, intensive care, and hospital days as well as the traditional CDC surveillance definition for VAP. The novel definition for ventilator-associated complications, however, was a superior predictor of hospital mortality. Surveillance using the novel definition was faster than conventional surveillance, requiring a mean of 1.8 minutes per patient versus 39 minutes per patient for VAP. Qualitative analysis of both VAC and VAP events suggested that they were both predominantly attributable to similar frequencies of pneumonia, pulmonary edema, acute respiratory distress syndrome, and atelectasis.
The superior association of VAC with mortality compared to VAP might be due to the threshold effect inherent in the VAC definition. Only patients whose complications were severe enough to merit an increase in ventilator support met criteria for VAC whereas patients with stable ventilator support could still be labeled with VAP on the basis of more subjective criteria such as rales, delirium, and changes in the quality and quantity of pulmonary secretions. Indeed, about half of patients labeled with VAP did not meet criteria for VAC. By definition, these patients had stable ventilator settings despite their purported pneumonias. Mixing these stable patients with patients who do have pronounced evidence of impaired oxygenation may be the reason that VAP was not associated with increased mortality: patients with benign disease may be “diluting” the mortality signal of patients with more severe disease. As such, the relative insensitivity of VAC relative to VAP may in fact be a strength of VAC surveillance since it selects for patients with more severe and hence meaningful complications.
The association of ventilator setting increases with mortality is analogous to the partial pressure of arterial oxygen to fraction of inspired oxygen ratio (PaO2/FiO2). A sustained decrease in the PaO2/FiO2 ratio is also an independent marker for mortality in ventilated patients. Changes in ventilator settings are more suitable for continuous population surveillance than changes in PaO2/FiO2 ratios, however, since ventilator settings are available on every patient for every day of mechanical ventilation whereas PaO2/FiO2 ratios are only available when clinicians choose to obtain an arterial blood gas, typically an intermittent event.
Concern that VAC is merely a marker for the manner in which patients die on mechanical ventilation (i.e. progressive increases in ventilator support for refractory hypoxemia) is allayed by the consistent correlation between VAC and lengths of stay in the analysis of survivors alone. Likewise, VAC is unlikely to be simply a marker for severity of illness since by definition it is only triggered by a deleterious change in patients' respiratory status after a period of stable or improving respiratory status. Worsening oxygenation after a sustained period of stability or improvement is more likely to indicate a complication than progression of underlying disease.
The VAC definition can be rapidly applied either electronically or manually. The dramatically lower time required for VAC surveillance presumes the raw data is pre-organized into a linelist with each patient's daily minimum ventilator settings. Hospitals without information systems to automatically generate linelists of patients' daily minimum PEEPs and FiO2s can have nurses or respiratory therapists record these two values every 24 hours on a dedicated spreadsheet by the bedside. Spreadsheets of this nature enable infection preventionists to rapidly complete VAC surveillance by consolidating and simplifying patients' ventilator data for rapid review. Hospitals can also consider providing visual plots or statistical process control charts of daily minimum PEEPs and FiO2s for clinicians at the bedside to rapidly alert them to evolving VACs.
Many observers have questioned the validity of comparing VAP rates between hospitals as well as the clinical significance of reports of “zero” VAP rates in some hospitals. 
The distribution in VAC rates between the hospitals in this study compared with the spread in VAP rates is informative. Amongst patients ventilated for 7 days or less, the observed VAP rates varied from 0 to 4% but when VAC rates were calculated, the range was only 7 to 9%, suggesting a measure that is both more uniform and able to detect complications in populations with ostensibly zero VAPs. A similar narrowing of the distribution between hospitals was observed for patients ventilated >7 days.
A potential criticism of VAC relative to VAP is that it does not indicate specific etiologies for patients' decompensations that can be used to inform future care improvement efforts. The ostensible specificity of a VAP diagnosis, however, is illusory. In this study, qualitative analysis by a critical care physician confirmed only one third of VAPs and identified many additional pneumonias missed by VAP criteria. Poor correlation between VAP clinical criteria and patients' true underlying disorders is consistent with prior investigations.
Indeed, it is striking that similar proportions of VAC events and VAP events were attributed to same array of significant complications including pulmonary edema, acute respiratory distress syndrome, and atelectasis in addition to pneumonia. This implies that VAP surveillance both misses and mislabels many important complications. Lumping many complications together as pneumonia risks missing important alternative domains for care improvement initiatives. Labeling patients' adverse events as VACs rather than pneumonias is a more frank and therefore useful description of what can and cannot confidently be discerned by surveillance.
In addition, shifting the focus of surveillance from pneumonia alone to complications in general emphasizes the importance of preventing all complications of mechanical ventilation, not just pneumonia. Hospitals should consider treating VACs as sentinel events that catalyze a multidisciplinary, open-minded evaluation of what might have precipitated the patient's deterioration. Shifting focus from pneumonia alone to complications in general sidesteps arguments about whether or not implicated patients truly had pneumonia (a distraction that sometimes overshadows critical analyses of VAPs at present) and instead invites caregivers to try to work out what did go wrong. A sentinel analysis might conclude that the patient's deterioration was due to VAP but could just as well attribute decompensation to poor fluid management, barotrauma, thromboembolic disease, or lobar collapse secondary to mucous plugging. Ideally, open-minded analyses of complications will generate broader and more nuanced views as to what practices can be improved. Grouping VACs by suspected etiology might reveal patterns of potentially modifiable precipitants. Ultimately, this process should lead to a broader “ventilator bundle” with added measures to promote early extubation, encourage protective lung ventilation, prevent pulmonary edema, minimize blood transfusions, and better manage secretions. The Institute for Healthcare Improvement's bundle anticipates this direction: it includes thromboembolism and stress ulcer prophylaxis in addition to pneumonia specific measures such as elevating the head of the bed.
There are important limitations to this work. It is a purely observational, retrospective study limited to three medical centers. VAP assignments were made by infection preventionists from chart reviews – different infection preventionists conducting prospective surveillance might have made different determinations. VAP surveillance time estimates should be treated as approximations since they measure time for retrospective chart review rather than daily, prospective, bedside assessments. The results of this investigation need to be reproduced prospectively in different settings to assure validity and generalizability.
The thresholds for stability prior to VAC eligibility, minimum changes in ventilator settings, and duration of changes merit further evaluation. In particular, the 2 day window of stable or decreasing ventilator settings prior to VAC eligibility might need to be lengthened to avoid mislabeling patients who require staggered increases in ventilator support when intubated for respiratory failure from a disease that continues to progress after intubation. Increasing the minimum thresholds for rises in ventilator settings and duration of increased ventilator support might further improve correlation between VAC and adverse outcomes.
Future changes in ventilator management strategies or the introduction of novel modes of ventilation might alter the performance or feasibility of VAC criteria. There is also some risk that clinicians may be loathe to increase patient's ventilator support, even when clinically indicated, to prevent their patient from being labeled with VAC. However, we believe the risk of this happening is low since failure to maintain patients' oxygenation in a safe zone is an egregious clinical error.
In recent years, hospitals have made admirable progress in reducing their VAP rates.
The median VAP rate in hospitals reporting to the National Safety Healthcare Network has decreased from 4.6 per 1000 ventilator-days from 1992–2004 to 2.0 per 1000 ventilator-days in 2006–2008.
In addition, multiple hospitals have reported extended periods without any VAPs.
While these decreases may partly be due to the subjectivity permitted by the current VAP definition, it is clear that VAP is becoming a vanishing target upon which to focus surveillance and prevention efforts. Surveillance for VAC identifies more patients who might have suffered complications of care (almost three times as many patients met criteria for VAC compared to VAP) and therefore constitutes a broader group upon which to focus quality improvement efforts.
Most public health departments and funding agencies have shied away from compelling hospitals to report VAP rates and from making VAP a non-reimbursable event in light of the complexity and subjectivity of VAP surveillance.
An alternative measure is needed to promote quality assessment, benchmarking, and care improvements for ventilated patients. VAC has many features that make it a promising alternative: the definition's simplicity minimizes the extra burden upon hospital personnel to complete surveillance, its objectivity makes it less susceptible to gaming, and the close association between VAC and adverse outcomes make it a meaningful target for prevention. VAC's emphasis on complications in general rather than pneumonia per se sidesteps the inherent limitations of VAP diagnosis. This has the additional advantage of inviting thoughtful case-by-case analyses of affected patients to identify broad areas for improvements in care beyond just pneumonia prevention alone. Further study is now needed on the extent to which VAC rates can be lowered through meaningful improvements in care.