Navigating Ventilator Management


Navigating Ventilator Management

Page 16

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Navigating Ventilator Management

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Post-op Strategies Demand Precision and Practice

T-bone car crashes, motorcycle wrecks, bad falls, gunshot wounds and stabbings often occupy the time of RTs at Parkland Health Hospital in Dallas. When ADVANCE called, though, a patient with idiopathic pancreatitis, complicated by diabetes, was demanding much of their attention. Only in his 30s, the man was very sick indeed. Pancreatitis patients generally don’t die of ARDS, but they can descend into multiple organ failure, says Parkland therapist John Boynton, RRT.

“He started the day on room air. By day’s end he was fully ventilated,” Boynton recalls. “His abdomen was swollen so tightly with fluid we could barely ventilate his lungs.” The patient had high ventilator pressures due to abdominal compartment syndrome.

Surgeons opened the man’s abdomen and drained his fluids. As they did so, the pressure control needed to maintain his 700cc of tidal volume dropped from 50 cmH2O to 30 cmH2O. Once out of the OR, the man’s fate rested largely on Boynton, his fellow therapists and their skill at post-op ventilator management.

Ventilating acutely ill patients in the hours and days after surgery is an inexact and still-evolving science. Clinicians have developed modes more gentle and agreeable to patient needs, but they are still pursuing the Holy Grail–the optimal post-op ventilation strategy.

Weaning parameters are especially open to debate. Get patients off MV and do it ASAP is everyone’s battle cry. How best to liberate patients from their breathing machines remains the rub, however.

Inflection Curve
Immediately post-op, “we’re just trying to get them set up so they have a reasonable set of blood gases,” Boynton says, maybe a pH of 7.25 and a PaO2 of 60 or greater. “For someone as sick as we see, we’re not looking for optimal blood gases. We’re just making sure they are OK for the interim. If necessary, we practice permissive hypercapnea over the next 24 hours on the sickest of the sick.

“Once we get them settled in,” he continues, “if they’re very sick, we do a PEEP inflection curve on them at the bedside, once their hemodynamics can tolerate it. We use high PEEP on them if merited. With the curve information we try to use a lung protective strategy.”

Setting PEEP and tidal volume is like trying to navigate a fragile ship between two islands in rough seas. Set pressures too high and you could dash the patient against the rocks of volutrauma on port side. Set pressures too low and you veer close to jagged crags starboard: You risk exposing the patient’s lungs to “shear stress” as closed alveolar units are repetitively opened with each tidal volume.

Boynton’s team steered their pancreatitis patient between these two dangers by plotting his PEEP inflection curve.

“The upper and lower curve showed 20 of PEEP pressure for a lower inflection point and 50 of PEEP for an upper inflection point,” he explains. “We set him at 20 and 40. We’ve since come down to 15 after re-shooting his curve on 48 hours. He is doing pretty well on 40 percent oxygen, with a PaO2 of about 100. He’s not ready to wean yet, but he is resuscitated and better for sure… He is day-to-day instead of hour-to-hour like he was.”

With more reasonable ABGs and ventilator settings (PEEP less than 10, FiO2 of 50 or less), Boynton can change his patient from assist control to intermittent mandatory ventilation (IMV) pressure control mode. Then comes the task of weaning. The attending physician will write an order to initiate Parkland’s SICU weaning protocol to extubation, which can take hours or days.

Weaning, like setting pressures, demands thread-the-needle precision, too. Respiratory care pioneer Donald F. Egan calls separating patient from ventilator “a pure art.” But Martin Tobin, MD, Loyola University Medical Center, Maywood, Ill., believes weaning should be based on science.

By nature, doctors are conservative, preferring to move slowly so as not to do harm, Dr. Tobin told delegates at the ACCP’s annual conference in Chicago last year. Yet prolonged MV can cause numerous complications. Moving too cautiously or slowly on weaning “is not necessarily good for the patient,” he said.

On the other hand, give a patient a weaning trial too early and he may fail it, developing respiratory muscle fatigue as a result and adding injury to injury.

Judgments about whether to proceed with or stop a weaning trial are best made from objective data expressed in numbers, not from the skill, subjectivity and intuition of an individual physician, Dr. Tobin argued. “In medicine, we always try to decrease dependence on the skill of an individual physician, and we prefer to have objective measurements rather than an overall Gestalt opinion whenever that’s possible,” he said.

Liberating Patients
Following Dr. Tobin, Constantine Manthous, MD, took the podium to energetically espouse the view that mechanical ventilators are a “necessary evil” from which patients should be liberated at the earliest possible time. “We emphasize liberation as opposed to weaning,” he told ACCP delegates.

Dr. Manthous cited a “seminal” study of 300 patients randomized into one group weaned by RTs and nurses according to protocol and another group weaned under the guidance of physicians. The protocol group weaned in significantly less time, with fewer complications and at a lower cost than the physician-directed group.1

“I’m not going to suggest here that somehow nurses and respiratory therapists are superior to physicians in hastening weaning,” he said. “But they are better situated than physicians. They’re at the bedside throughout the day. They can take physiologic measurements and integrate them into the patient’s daily respiratory plan. And when armed with physiologic guidelines to initiate the weaning process, they can be highly effective in helping to expedite weaning outcomes.”

Dr. Manthous of Bridgeport Hospital, Bridgeport, Conn., went on to make these points:

  • Some historically used weaning parameters, while important as screening tools, “aren’t much better than the flip of a coin in determining whether a patient is ready to come off the ventilator,” he said. The rapid shallow breathing index, measured after a minute and again at 30 minutes of weaning, is better but not perfect. Therefore, the precondition for determining weaning appropriateness becomes the spontaneous breathing trial.
  • Studies suggest that trials using a T-piece and pressure support ventilation of less than or equal to 7 cmH2O are roughly equivalent approximations of spontaneous work of breathing to judge patient readiness to come off the ventilator.

    A major study that randomized 526 patients into receiving initial spontaneous T-piece breathing trials lasting either 30 or 120 minutes yielded no difference in extubation success rate.2 Thus, for the initial spontaneous breathing trial, “30 minutes is probably an adequate amount of time,” he said.

  • In patients who fail first-day spontaneous breathing trials, IMV weaning appears to prolong ventilator dependence.
  • Two-thirds of patients in all weaning studies are successfully extubated after their first two-hour T-piece trial. It follows that clinicians could have successfully extubated at least some of them by adopting more strenuous screening criteria to initiate breathing trials.

    “If two-thirds can be successfully extubated after a T-piece trial,” Dr. Manthous pointed out, finishing his talk emphatically, “weaning or gradual reduction is not necessary in the vast majority of patients.”

References

1. Ely W, et al. Effect on the duration of mechanical ventilation in identifying patients capable of breathing spontaneously. NEJM.1996;335(25):1864-9.

2. Esteban A, Alia I, Tobin MJ, Anselmo G, Gordo F, Vallverdu I, et al. Effect of spontaneous breathing trial duration on outcome of attempts to discontinue mechanical ventilation. Am J Respir Crit Care Med. 1999;159:512-8.

Michael Gibbons is senior associate editor of ADVANCE.

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