Author: Naveed Saleh MD MS
In two studies presented at Euroanaesthesia 2019, researchers showed that flow-controlled ventilation (FCV) improved gas exchange, and that the linearized expiratory airflow during FCV may be particularly lung-protective in obese patients.
FCV is a novel type of ventilation, which delivers a constant positive flow during inspiration and maintains a constant negative flow during expiration. With FCV, pressure increases linearly during inspiration—similar to volume-controlled ventilation (VCV)—and decreases linearly during expiration.
“To be [cautious], we started in lung-healthy patients, and even in those patients with close-to-optimal conditions we could find improvements. I think this has high potential to change ventilation strategies,” said Stefan Schumann, PhD, of the Department of Anesthesiology and Critical Care, Medical Center, University of Freiburg, in Germany.
In the first controlled crossover trial, 21 consecutive healthy patients who were scheduled for elective abdominal surgery were randomly assigned to receive one of two ventilation sequences—one patient was later excluded. Both sequences began with seven minutes of VCV, with one group receiving a sequence of VCV followed by FCV, and the other group receiving the reverse. The primary outcome was the arterial oxygen pressure.
In the study, FCV enhanced arterial oxygenation and carbon dioxide elimination without interfering with respiratory mechanics or hemodynamic variables. Furthermore, with similar tidal volumes and plateau pressures, the mean airway pressure was elevated in FCV versus baseline ventilation.
“This study is the first in humans to compare the effects of FCV and VCV on arterial blood gases. A linearized expiratory flow linearizes the expiratory pressure drop and thereby improves pulmonary gas exchange, without affecting VT [ventricular tachycardia], VF [ventricular fibrillation], Pplat [plateau pressure] and PEEP [positive end-expiratory pressure]. This may potentially be an approach for lung-protective mechanical ventilation. It follows that the potential beneficial effects found in this interventional crossover trial have to be validated in long-term studies,” the authors noted.
In the second study, the team assessed respiratory system mechanics and hemodynamic variables in 23 consecutive obese patients scheduled for bariatric surgery during VCV and FCV in a crossover setting. Of note, three patients were eventually excluded. Just as with the first study, after seven minutes of VCV, either a sequence of VCV/FCV or FCV/VCV was administered. The primary end point was the difference in end-expiratory lung volume (EELV) between VCV/FCV and baseline ventilation.
According to the results, compared with VCV, expiratory flow during FCV offered enhanced maintenance of lung aeration with the same VT, Pplat and PEEP. The difference in EELV between baseline ventilation and FCV (–126±207 mL) was less than between baseline and VCV (–316±254 mL; P<0.001). The research was presented during the 2019 annual meeting of the European Society of Anaesthesiology.
Protecting Lungs With Linear Flow
In an interview with Anesthesiology News, Timothy Gaulton, MD, MSc, an assistant professor of anesthesiology and critical care at the Hospital of the University of Pennsylvania, in Philadelphia, helped place findings into context from the second study involving FCV in obese patients.
“Keeping the lung open and full of air is important to prevent perioperative complications like low levels of oxygen in the blood and atelectasis or lung collapse,” he said. “Previous experimental data have also suggested that this type of ventilation may also reduce the amount of injury to the lung caused by mechanical ventilation.”
He added the need for protecting the lungs against low levels of oxygen and atelectasis are even more important for obese patients.
“Patients who are obese are particularly susceptible to these complications due to the larger size of their chest and abdomen compared to patients who are not obese,” Dr. Gaulton said. “They have almost double the risk of pulmonary complications after surgery compared to nonobese patients. It is therefore critical that new strategies be developed—such as this study proposes—to increase the safety of mechanical ventilation in [those who are obese].”
Dr. Gaulton was encouraged by the results of the study and the potential for this technique to improve ventilation in obese patients. He also noted the added benefit of using electrical impedance tomography (EIT) as a noninvasive tool to measure aeration in the lungs for the study.
“The authors report that, in a population of obese patients having bariatric surgery, the use of flow-controlled ventilation as compared to conventional ventilation increases the volume of air in the lungs at the end of expiration, as measured by EIT,” he said. “This form of ventilation when utilized during the procedure can reduce the amount of atelectasis that develops when obese patients are under anesthesia.”
Dr. Gaulton compared the study with others in the field considering the use of this novel method of ventilation for improving lung mechanics in obese surgical patients. He specifically pointed to recent work that has also focused on these topics. One such study published in JAMA in June compared different levels of PEEP and use of recruitment maneuvers on pulmonary complications after noncardiac, nonneurologic surgery in obese adults (2019;321:2292-2305). The study found that higher levels of PEEP did not reduce pulmonary complications despite it being well known that this approach to ventilation prevents atelectasis and improves respiratory mechanics, according to Dr. Gaulton.
“This should introduce some caution to the interpretation of the present study and warrants future studies to correlate their findings with clinical outcomes before this approach can be adopted into clinical practice,” Dr. Gaulton said. “This mode of ventilation also requires a specific ventilator that would need to replace the ventilators currently used at most institutions; this may be a substantial cost against a clinical benefit that is unclear.”