Friday, February 11, 2011

CMS never events: Evidence of smoke in mirrors?

Let me tell you a fascinating story. In 1999, I was still fresh out of my Pulmonary and Critical Care Fellowship, struggling for breath in the vortex of private practice, when a cute little paper appeared in the Lancet from a great group of researchers in Spain, describing a study performed in one large academic urban medical center's two ICUs: one respiratory and one medical. Its modest aim was to see if semi-recumbent (partly sitting up) compared to supine (lying flat on the back) positioning could reduce the incidence of that bane of the ICU, ventilator-associated pneumonia (VAP). The study was a well done randomized controlled trial, and the investigators even went so far as to calculate the power (the number needed to enroll in order to detect a pre-determined magnitude of effect [in this case an ambitious 50% reduction in clinically suspected VAP]), and this number was 182 based on the assumption of a 40% VAP prevalence in the control (supine) group. The primary endpoint was the prevalence (percentage of all mechanically ventilated [MV] patients developing) and the secondary the incidence density (number of cases among all MV patients spread over all the cumulative days of MV [patient-days of MV]) of clinically suspected VAP, based on the CDC criteria, while microbiologically confirmed VAP (also rigorously defined) served as the secondary endpoint.

Here is what they found. The study was stopped early due to efficacy (this means that the intervention was so superior to the control in reaching the endpoint that it was deemed unethical after the interim look to continue the study), enrolling only 86 patients, 39 in the intervention and 47 in the control groups. And here are the results for the primary and secondary outcomes:

So, this is great! No matter how you slice it, VAP is reduced substantially; there is a microbiologically confirmed prevalence reduction of nearly 6-fold (this is unadjusted for potential differences between groups; and there were differences!). Well, you know what's coming next. That's right, the "not so fast" warning. Let's examine the numbers in context.

First of all, if we look at the evidence-based guideline on HCAP, HAP and VAP from the ATS and IDSA, the prevalence of VAP is generally between 5 and 15%; in the current study the control group exceeds 20%. Now, for the incidence density, for years now the CDC has been keeping and reporting these numbers in the US, and the rate in patients comparable to the ones in the study should be around 2-4 cases per 1,000 MV days. In this study, no matter how you slice it, clinically or microbiologically, the incidence density is exceedingly high, more in line with some of the ex-US numbers reported in other studies. So, they started high and ended high, albeit with a substantial reduction.

Second of all, there is a wonderful flow chart in the paper that shows the enrollment algorithm. One small detail has always been somewhat obscure to me: the 4 patients in the semi-recumbent group that were excluded from analysis due to reintubation (this means that they were taken off MV, but had to go back on it within a day or two), which was deemed a protocol violation. Now, you might think that 4 patients is a pretty small number to worry about. But look at the total number of patients in the group: 39. If the excluded 4 all had microbiologically confirmed VAP, that would bring our prevalence from 5% to 14% (6 out of 43). This would certainly be a less than 6-fold reduction in VAP.

Thirdly, and this I think is critical, the study was not blinded. In other words, the people who took care of the patients knew the group assignment. So what, you ask. Well remember that VAP is a pretty difficult, elusive and unclear diagnosis. So, let us pretend that I am a doc who is also an investigator on the study, and I am really invested in showing how marvelous semi-recumbent positioning is for VAP prevention. I am likely to have a much lower threshold for suspecting and then diagnosing VAP in the comparator group than in my pet intervention group. And this is not an indictment of anyone's judgment or integrity; it is just how our brains are wired.

Next, there were indeed important differences between groups in their baseline risk factors for VAP. For example, more patients in the control (38%) than in the intervention (26%) group were on MV for a week or longer, the single most important risk factor for developing VAP. Likewise, the baseline severity of illness was higher in the control than the intervention group. To be sure, the authors did statistical analyses to adjust these differences away, and still found an adjusted odds ratio of VAP among the supine group to be 6.8, with the 95% confidence interval between 1.7 and 26.7. This is generally taken to mean that, on average, the risk of VAP increases nearly 7-fold for supine position as opposed to semi-recumbent, and if the trial was repeated 100 times, 95 of those times this estimate would fall between a 1.7 and a 26.7-fold increase. OK, so we can accept this as a possible viable strategy, right?

But wait, there is more. Remember what we said about the odds ratio? When the event happens in more than 10% of the sample, the odds ratio vastly overestimates the risk of this event. 28.4% anyone?

Now, let's put it all together. A single center study from a Spanish academic hospital, among respiratory and medical ICU patients, with a minuscule sample size, yet halted early for efficacy, an exceedingly high baseline rate of VAP, a substantial number of patients excluded for a nebulous reason, unblinded and therefore prone to biased diagnosis, reporting an inflated reduction in VAP development in the intervention group. It would be very easy to write this off as a flawed study (like all studies tend to be in one way or another) in need of confirmatory evidence, if it were not so critical in the current punitive environment of quality improvement. (By the way, to the best of my knowledge, there is no study that replicates these results). The ATS/IDSA guideline includes semi-recumbent positioning as a level I (highest possible level of evidence) recommendation for VAP prevention, and it is one of the elements of the MV bundle, as promoted by the Institute for Healthcare Improvement, which demands 95% compliance with all 5 elements of the bundle in order to get the "compliant" designation. And even this is not the crux of the matter. The diabolical detail here is that CMS is creeping up on making VAP into one of their magical "never" events, and the efforts by hospitals will most assuredly be including this intervention. So, ICU nurses are already expected to fall in step with this deceptively simple yet not-so-easily executable practice.

And this is what is under the hood of just one simple level I recommendation by two reputable professional organizations in their evidence-based guidelines. One shudders to think...              

1 comment:

  1. So what happened to the incidence of VAP at the hospital after they stopped the study?

    That would be the real test. If they instituted semi-recumbent positioning and the incidence at their hospital stayed much lower than at hospitals where supine positioning was used then that would support the conclusion. If not then it was a chance difference.