Tobin: “Minimal” PEEP and pressure support during SBT kills some patients (AJRCCM)

This entry was originally posted March 10, 2012. Dr. Tobin made further comments to the AJRCCM in July 2012, which are excerpted below.

Martin Tobin at Loyola University is worried about our standard practice in extubating patients. And since he wrote the book on mechanical ventilation and helped invent the Yang-Tobin index, if he’s worried, you should be, too.

After seeing a few patients who passed their spontaneous breathing trials (SBTs), but then rapidly entered respiratory failure soon after extubation and had difficult reintubations resulting in cardiopulmonary arrest and hypoxic brain injury, Tobin was troubled enough to write a commentary for the February 15 AJRCCM to share his concerns.

The problem, Tobin says, is our concept of  ”minimal ventilator settings.” In his view, there’s no such thing. He calls the idea of physiologic positive end-expiratory pressure (PEEP) ”a myth,” and argues that adding either 5 cm H2O as “physiologic” PEEP, or pressure support of  7 cm H2O to overcome the resistance in an endotracheal tube (or both, as is usually done) may actually reduce the “spontaneously” breathing patient’s workload by >40%. Success on these settings does predict extubation success in most patients, true; but a small minority will rapidly decompensate after extubation, with potentially catastrophic outcomes. Identifying and protecting these vulnerable patients should be our key task, Tobin argues:

But here’s the rub. The challenge of clinical medicine is not about taking care of the great majority of patients who do well irrespective of the methods employed by their physicians. Instead, the goal is to take feasible steps that have a high likelihood of circumventing a catastrophe in a small number of instances.

Why do patients in this subset decompensate so rapidly after extubation, after “passing” their SBT? Upper airway resistance (supraglottic edema), respiratory weakness masked by pressure support, and increased cardiac load induced by removal of CPAP, are the likely culprits, he believes.

Tobin argues for true T-piece spontaneous breathing trials on more patients being planned for extubation (and presumably for most or all of those considered to be at higher risk for extubation failure). In his opinion, this method will identify more patients unable to tolerate extubation, compared to SBTs on “minimal vent settings.” (He does acknowledge there’s a lack of evidence to prove this.)

Of course, T-piece trials are inconvenient, in that the ventilator circuit and sensors are not standing by to save the failing patient, who therefore requires near-continuous monitoring by a human being. (Tobin allows that an SBT with the vent set on “flow-by,” with pressure support and PEEP set at zero, is an acceptable though second-rate alternative.)  For those who would gripe about this, Tobin sends a zinger, one that can be applied well beyond this clinical scenario:

Taking simple steps to prevent infrequent occurrences that lead to a clinical catastrophe should dictate the practice of medicine, rather than employing approaches that are convenient to physicians and successful in most patients.

I have to trust Dr. Tobin’s gut and experience on this issue, but the validity of this last bit of professional (and borderline moral) criticism depends on there being some evidence that T-piece trials better predict extubation success than SBTs on “minimal vent settings.” He doesn’t reference any, and to my knowledge there is none. Would those unfortunate patients he helped care for have passed T-piece trials as well, and been extubated with the same sad outcomes?

With more and more COPD patients with ventilator-days in their future, it seems like an important randomized study for someone to do. To better identify an effect, enrollment could be restricted only to patients with severe heart or lung disease considered at high-risk for extubation failure, as were described in this paper. 

Dr. Tobin responded to the subsequent letters to the AJRCCM editor with what I found to be a trenchant and persuasive reply. He points out that the supposed absence of reported catastrophes in randomized trials on weaning does not mean they didn’t occur, but that the studies were underpowered to detect and quantitate them accurately. I recommend reading his complete text, but I’ll excerpt as much as I can:

That cardiopulmonary catastrophes are rare in weaning studies provides no solace to family members of a patient who breathed comfortably on a low level of pressure support or PEEP immediately before extubation but who was not able to cope with a 30–60% increase in respiratory load immediately after extubation, which led to a cardiac arrest and hypoxic brain injury. In the case of a patient who might experience cardiorespiratory difficulties after extubation, it is incumbent on physicians to ensure that the patient is able to breathe comfortably for about 30 minutes in the complete absence of pressure support or PEEP before removal of the endotracheal tube.

Tobin MJ. Extubation and the Myth of “Minimal Ventilator Settings”. Am J Respir Crit Care Med 2012; 185:349-350.

Dr. Tobin’s reply to letters-to-the-editor, AJRCCM July 15 2012.