Mechanical Ventilation: Which Counts More, Volume or Pressure?

NASHVILLE—At the Society of Critical Care Medicine’s 2009 Critical Care Congress, experts debated whether targeting volume or pressure is the most important consideration in the management of critically ill patients with regard to ventilator-induced injury.

OVERDISTENTION INJURY

Neil R. MacIntyre, Jr, MD, Professor of Pulmonary and Critical Care Medicine at Duke University School of Medicine in Durham, North Carolina, was asked to argue in favor of volume over pressure.

“It is the physical overdistention of the lung that actually causes lung injury,” Dr. MacIntyre said. “As patients start to reach the higher parts of vital capacity, stretching occurs in the lipid monolayer. Near total lung capacity, there are probably some breaks in this membrane,” he explained. “Liposomes fill these holes much like the Dutch boy and the dike. When the lipid monolayer is overstretched repeatedly, you get breaks that are not patched up quite so quickly and you get a stress failure.” The issue, he said, is not just the alveolar pressure, but also the transpulmonary pressure—the pressure across the alveolar epithelial cells—that reflects lung volume.

He referred to an animal study during which the lungs were constrained and not allowed to expand. Even though the pressures inside the alveoli were high, both the transalveolar pressures and the volumes were near normal. As a consequence, not much injury occurred.

Clinically speaking, perhaps the more important issue is mortality. In the ARDSNet (ARDS Network) trial, patients with early ARDS randomly assigned to conventional mechanical ventilation (maintenance of the lowest positive end-expiratory pressure [PEEP] with a tidal volume of 12 mL/kg) or protective mechanical ventilation (high PEEP and a tidal volume of less than 6 mL/kg). “Reducing the tidal volumes from 12 mL/kg to 6 mL/kg ideal body weight produced a very impressive 10% absolute risk reduction in mortality and time off the ventilator,” Dr. MacIntyre noted. However, plateau pressures were different between the groups, Dr. MacIntyre pointed out.

“Was it the reduction in tidal volumes that reduced the mortality and improved outcome—what I call tidal stretch—or was it the reduction in plateau pressures—or maximal stretch—from the low 30s [cm of water above the PEEP value] in the high tidal volume group to the mid 20s in the low tidal volume group, that caused the improvement in outcome?” If it was the maximal stretch, it would not matter what the tidal volume was as long as the plateau pressure was in a “safe range”—about 30 cm of water, he contended. However, if there is tidal injury as well, then there may not be a “safe” plateau pressure, and the goal would be to reduce both plateau pressure and the tidal volume.

Mayo Clinic researchers looked at ICU patients who were on the ventilator for more than a year for reasons other than ARDS. They found that among the patients who developed lung injury, the strongest predictor by far was the size of the tidal volume. Compliance in the high tidal volume group was the best with so-called “safe” plateau pressures—in the mid 20s. After the study, the Mayo Clinic implemented a protocol to reduce tidal volumes in their ICU. Over the course of several years, the average tidal volume in the Mayo Clinic dropped from about 10 mL/kg to around 7 mL/kg, with an “impressive” reduction in the development of acute lung injury on the ventilator.

“I will always submit we try to keep the plateau pressures as low as we can because we are trying to protect healthier regions of the lung, yet I also believe that tidal stretch can be just as important as maximal stretch. I think our goal in respiratory care is to keep the lungs as near normal as we possibly can, and I’m not really sure there is a safe plateau pressure.”

A DIFFERENT APPROACH

“Dr. MacIntyre and I will probably end up at the same point, although I think it is critical how you approach the problem,” said Robert Kacmarek, PhD, RRT, Director of the Respiratory Care Department at Massachusetts General Hospital in Boston. “My approach is that pressure is more important than tidal volume.”

One of the goals of the ARDSNet protocol was to maintain a plateau pressure of less than 30 cm of water. Plateau pressure, Dr. Kacmarek said, is important because to date it is the best, easiest-to-obtain indication of the peak alveolar pressure. “Regardless of the tidal volume that we deliver, stretch will be measured by how much pressure gradient exists across the alveoli, not by how much pressure is actually applied.”

To illustrate this point, he compared healthy persons to ARDS patients. “Healthy individuals with a transpulmonary pressure of 10 cm of water can produce a tidal volume of 2 L easily, which is translated into 28.5 mL/kg of predicted body weight in a person who weighs 70 kg. That is not a problem because their total respiratory system compliance is good.” But while healthy individuals do not develop lung injury, ARDS patients are a different story.

“For example, we ventilate a patient with high plateau pressures—35 cm of water associated with a tidal volume of 800 mL, 11 mL/kg predicted body weight. When that patient’s compliance is 23 mL/cm of water, many would agree that this is creating the potential for ventilator-induced lung injury. However, it is not the tidal volume but rather the actual pressure that is distending the alveoli and is the critical factor in determining whether or not injury exists. Data from the ARDSNet clearly supports that conclusion,” Dr. Kacmarek said.

When looking at the relationship between plateau pressure and mortality in the original ARDSNet trial, he said, even though the tidal volume was the same between the 6 mL/kg group and the 12 mL/kg group, there was a big difference in mortality depending on plateau pressure. When looking at the relationship between plateau pressure and mortality in the original ARDSNet trial, he said, mortality was based on plateau pressure, not with tidal volume. “Regardless of which tidal volume group there were randomized to, if patients had a high plateau pressure, there was high mortality. If patients a low plateau pressure, there was low mortality,” he said. Other studies have found the same trend.

With regard to the Mayo Clinic data that Dr. MacIntyre cited, Dr. Kacmarek noted that the researchers neglected to report plateau pressures. “I hope nobody goes away thinking I believe we should be using large tidal volumes, because that is not what I’m saying. What I’m saying is that we should focus on the plateau pressure because that clearly is more critical in determining the level of injury.”

Dr. Kacmarek noted that 6 mL/kg might not be the correct tidal volume if a patient’s plateau pressure is higher than 30. “I would argue that in some patients, perhaps plateau pressure should be less than 4 mL/kg to avoid the injury associated with that high plateau pressure. If we are causing a significant level of alveolar stretch, we are going to create injury and we should be decreasing that tidal volume to get the plateau pressure to around 25 cm of water.”

In patients with very low tidal volumes, Dr. Kacmarek advised physicians to consider what is in patients’ best interest at that time. “If we want patients to have a small tidal volume, you have to sedate them and take over,” he said.

“So what is better for that patient’s general course at that point in time? Allowing them a little larger tidal volume than you would normally consider or sedating them to accept a much smaller tidal volume? This is difficult to answer but this is a decision we all have to make on a daily basis as we ventilate patients. As we get to those higher plateau pressures, in some cases we need to consider tidal volumes smaller than 6 mL/kg.”

—Glenn S. Williams

Suggested Reading
Amato MB, Barbas CVS, Medeiros DM, et al. Effect of protective-ventilation strategy on mortality in the acute respiratory distress syndrome. N Engl J Med. 1998;338(6):347-354.

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