Lung-Protective Ventilation Varies With Mechanical Type

Author: Thomas Rosenthal

Anesthesiology News

Delivery of lung-protective ventilation (LPV) varies significantly according to the mode of mechanical ventilation (MV) that ICU patients receive. While some systems can be chosen to enhance ventilation for patients with low compliance or those with lung conditions, specific methods can be harnessed to protect lung health for the majority of patients, a new study concluded.

“Certain modes of ventilation were more clearly associated with the delivery of tidal volume and drive pressures that we would associate with being lung protective,” said Craig Jabaley, MD, the medical director of the cardiothoracic ICU at Emory University Hospital and an assistant professor of anesthesiology at Emory University School of Medicine, in Atlanta.

Ideally, Dr. Jabaley said, best practices for LPV include a tidal volume setting of no more than 6 mL/kg of predicted body weight (PBW) and limitation of driving pressure of no more than 14 cm H₂O. The study also identified incongruity between tidal volume and driving pressure, which may separate out some methods as better suited for patients with poor compliance.

The researchers explored these ventilation methods because of their variety, which is often left up to the physician’s discretion. “We know from large epidemiological studies that the bedside clinician’s approach to selection of mode of ventilation is pretty heterogeneous,” Dr. Jabaley said. “That is really in stark contrast to the selection of a lot of other ventilator settings. The reason is probably we have a lot of good evidence to suggest what sort of tidal volume people should be getting and what level of PEEP [positive end-expiratory pressure] people should be getting, but we don’t have a lot of really great evidence to guide us as to the mode of mechanical ventilation.”

Dr. Jabaley said when it comes to a selection between volume control and pressure control, there is a lack of research and guidelines to help clinicians make decisions. “Clinicians are left to their own devices,” he said.

Because of this, the authors wanted to look at what they said were the modern concepts of LPV, which include limitation of tidal volume and driving pressure, as well as titration of PEEP to optimize compliance. These methods are not just for patients with lung conditions, such as acute respiratory distress syndrome (ARDS), but can be applied routinely to prevent lung injury, the authors noted.

The study, presented at the 2018 annual meeting of the Society of Critical Care Medicine (abstract 1016), recorded the MV parameters for 39,115 patients in the 20 ICUs of Emory Healthcare’s four hospitals, from Oct. 4, 2011, to Dec. 31, 2016.

Volume-targeted pressure-controlled modes (7.31mL/kg of PBW) and assist-control pressure-controlled modes (7.38mL/kg of PBW) produced the lowest mean tidal volume in the study. Adaptive support ventilation (8.19mL/kg of PBW; P<0.001 for both comparisons) delivered the highest mean tidal volumes. It was also found that the volume-targeted pressure-controlled and assist-control pressure-controlled modes produced higher mean driving pressure compared with adaptive support ventilation (18.7 and 19.0 compared with 14.14; P<0.001 for both comparisons). The lowest mean driving pressures were delivered via assist-control, and the highest by synchronized intermittent mandatory ventilation (14.3 vs. 20.7; P<0.001). Across the board, mean PEEP was 6.37 (interquartile range, 5.00-8.00).

Past studies did not show as definitive a difference for ventilation methods for certain patients, said Daniel Talmor, MD, MPH, the chief of the Department of Anesthesia, Pain Medicine and Critical Care at Beth Israel Deaconess Medical Center, and a professor of anesthesiology at Harvard Medical School, in Boston, who was not connected to the study but asked to comment. “In studies in patients with ARDS, no mode of ventilation has been shown to be superior to another,” he said. “This study suggests that there may indeed be subtle differences in the way each mode of ventilation delivers LPV.”

Different Means to Same End

Dr. Jabaley noted that even though there are various MV modes, each can provide consistent volume as per best practices.

For MV there are three major modes, he said. In the first, clinicians set the amount of airway pressure to deliver, which becomes the independent variable, and tidal volume becomes the dependent variable. The second category is the opposite: Clinicians select the tidal volume to be delivered, which becomes the independent variable, and airway pressure becomes the dependent variable, he said.

The third category was what he termed a bucket of ventilator modes for people who are breathing spontaneously and initiating meaningful respiratory effort on their own. For these patients, Dr. Jabaley said, clinicians deploy pressure support ventilation or bilevel ventilation, which is also called airway pressure release ventilation.

Of the three, the first category piqued the researchers’ curiosity. “What we had wondered was, for patients on a volume-control mode of ventilation, would their tidal volume be more in line with those recommended in terms of best practices than those who are receiving pressure-control ventilation?” Dr. Jabaley said.

Pressure control can mean more maintenance by the physician, but it also can give patients variability. “If your patient is on volume-control ventilation, you walk over to the ventilator and you select the tidal volume based on the patient’s weight,” he said. “Whereas if you walk over to the ventilator and you select the pressure-control mode of ventilation, you’d select a certain amount of airway pressure to deliver to the patient, so it requires a lot more vigilance to maintain a tidal volume. As the compliance of the respiratory system gets worse or gets better, the tidal volume goes up and down.”

Dr. Jabaley said for the same amount of airway pressure, as the patient’s lungs get sicker they get stiffer, and clinicians deliver lower tidal volume. But, he said, if someone improves and the patient’s lungs become more elastic and flexible, clinicians could deliver high tidal volume if they are not paying close attention.

Although pressure control requires much attention, the researchers did not find worse outcomes. “Surprisingly, what we found in the ICU [was that] clinicians actually did a good job delivering a very consistent tidal volume between both of those groups,” Dr. Jabaley said.