Tracheal Gas Insufflation Helps Improve Gas Exchange in Primary Severe ARDS

Key Point

In early-onset, primary severe ARDS, short-term high-frequency oscillation plus tracheal gas insufflation improves gas exchange.

Some promising laboratory experiments have more than a few intensivists thinking that adding tracheal gas insufflation to short-term high-frequency oscillation may be the best way yet to improve gas exchange in severe ARDS, and a study by Spyros D. Mentzelopoulos, MD, PhD, has shown support for this belief. In the study, short-term high-frequency oscillation plus tracheal gas insufflation (HFO-TGI) improved the Pao2/Fio2 (fraction of inspired oxygen) ratio and other gas exchange measures in patients with severe ARDS better than did HFO alone or conventional mechanical ventilation.

In addition, HFO-TGI produced both larger reductions in shunt fraction and an improved oxygenation index relative to standard HFO. Dr. Mentzelopoulos and colleagues suggested that “the improved Pao2/Fio2 during HFO-TGI relative to HFO could be explained mainly by the TGI-induced PEEP effect. This effect could also have enhanced molecular diffusion. Indeed, at a fixed oscillatory pressure amplitude, increases in lung volume rapidly decrease the resistance to diffusion, thus improving gas exchange.”

Compared with standard HFO and conventional mechanical ventilation, HFO-TGI may be less traumatic to patients, Dr. Mentzelopoulos told Pulmonary Reviews. “A lower product of mean airway pressure and Pa o2/Fio2 means better oxygenation with the use of lower pressures,” explained Dr. Mentzelopoulos, a faculty member in the First Department of Critical Care at the University of Athens Medical School, Greece.

The authors acknowledged the small size of the study population—14 patients diagnosed with primary ARDS—but noted that physiologic studies are often small. Patients met the following inclusion criteria: age between 18 and 75, body weight greater than 40 kg, ARDS diagnosis within the past 72 hours, severe oxygen disturbances, and a need for endotracheal intubation and mechanical ventilation. “PEEP, Fio2, tidal volume, and breathing rate were set in accordance with the ARDS Network protocol,” noted the authors.

The patients received 60 minutes each of HFO alone and HFO-TGI in random order, with an interval of no more than 24 hours between treatments; the average TGI volume was 6.1 L/min. The HFO and HFO-TGI sessions were preceded and followed by conventional mechanical ventilation as recommended by the ARDS Network. 

“During HFO sessions, mean airway pressure was set at 1 cm H2O above the point of maximal curvature of the respiratory system expiratory pressure-volume curve,” the authors related. “Gas exchange and hemodynamics were determined before, during, and after HFO sessions.” Neither HFO nor HFO-TGI was associated with adverse effects such as HFO-associated excessive hypercapnia, hemodynamic perturbations, or post-HFO bronchoscopic evidence of tracheal mucosal injury.

Pao2/Fio2 was significantly higher during HFO-TGI than it was during HFO or conventional mechanical ventilation (174.5 vs 136.0 and 105.0 mm Hg, respectively). Pao2/Fio2 returned to baseline values within three hours after HFO alone, the authors observed.

Compared with HFO and conventional mechanical ventilation, HFO-TGI was associated with significant improvement in shunt fraction (0.36 vs 0.42 and 0.45, respectively) and mixed venous oxygen saturation (77.8% vs 75.9% and 71.8%, respectively). The oxygenation index was significantly improved during HFO-TGI relative to HFO but did not differ significantly between conventional mechanical ventilation and either HFO or HFO-TGI.

Relative to conventional mechanical ventilation, neither HFO nor HFO-TGI significantly affected Paco2, arterial pH, or hemodynamics, nor did they appreciably alter respiratory mechanics or pressure-volume curves.

“The unchanged Paco2 during HFO-TGI is surprising,” the authors noted, “because TGI and endotracheal tube cuff leak independently reduce Paco2 during HFO.” Furthermore, TGI at flow rates of 15 L/min has been shown to reduce Paco2 by nearly 30% in dogs with lung injury.

“However, we employed 2.5-times lower TGI flows, the effects of which were not previously evaluated,” reasoned the authors. “Also, the present study’s baseline Paco2 was 34% lower. In dogs, Paco2 results of TGI may be enhanced by anatomical dead-space size (6 vs 2 mL/kg in humans) and improved collateral ventilation.”

In addition to its small sample size, the study was limited by its lack of diverse causes of ARDS. “Pneumonia-induced ARDS usually results in increased recruitment potential,” the authors explained. “This may have limited the variability of gas exchange responses to the present study’s interventions, thus facilitating detection of physiologically important differences in a small sample.”            

—Timothy Begany

Reference
Mentzelopoulos SD, Roussos C, Koutsoukou A, et al. Acute effects of combined high-frequency oscillation and tracheal gas insufflation in severe acute respiratory distress syndrome. Crit Care Med. 2007;35(6):1500-1508.

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