Application of basic physiology principles at bedside has changed the approach to the treatment of patients with acute respiratory distress syndrome (ARDS) and refractory hypoxemia. Current standard of care for patients with ARDS includes a low tidal volume ventilation strategy (6 mL/kg of ideal body weight), keeping plateau pressures below 30 cm H2O (Brower RG, et al.), driving pressures below 15 cm H2O and adequate positive end-expiratory pressures (PEEP) to keep the alveoli open without overdistension (Villar J, et al. ). However, at this time, despite the awareness of the importance of this intervention, there is no consensus regarding the best method to determine ideal PEEP at the individual patient level.
A thorough understanding of the basic physiologic concepts regarding respiratory pressures is of paramount importance to be able to formulate an opinion. The transpulmonary pressure (or lung distending pressure) is the gradient caused by the difference between alveolar (PA) and pleural pressure (PPL). In order to prevent lung collapse at end-expiration, PA must remain higher than PPL such that the gradient remains outward, preventing end-expiratory collapse and atelectotrauma. To accomplish that, it is necessary to know the end-expiratory PA and PPL. Esophageal balloon pressures (PES) represent central thoracic pressures, but, despite positional and regional variations, they are a good surrogate for average “effective” PPL (Baedorf KE, et al.).
Understanding that the value of the PES represents a practical PPL makes it easier to appreciate the potential usefulness of an esophageal balloon to titrate PEEP. The objective of PEEP titration is to prevent de-recruitment, maintain alveolar aeration, and improve the functional size of aerated alveoli. If the applied PEEP is lower than the PPL, the dependent lung regions will collapse. On the other hand, if PEEP is higher than the PPL, the lung would be overdistended, causing barotrauma and hemodynamic compromise.
The question is: Should we use esophageal balloons?Yes, we should.
A single center randomized control trial (EPVent) compared PEEP titration to achieve a positive PL vs standard of care lung protective ventilation (Talmor D, et al.). The PEEP titration group used significantly higher levels of PEEP, with improved oxygenation and lung compliance. However, there was no significant difference in ventilator-free days or mortality between the groups.
Obese patients are also likely to benefit from PEEP titration guided by an esophageal balloon, as they often have higher levels of intrinsic PEEP. Therefore, the application of higher levels of PEEP to compensate for the higher levels of intrinsic PEEP may help reduce work of breathing and prevent tidal recruitment-de-recruitment and atelectasis. Additionally, low to negative transpulmonary pressures measured using the actual values of PES in obese patients and obese animal models predicted lung collapse and tidal opening and closing (Fumagalli J, et al.). It is useful to remember that the compliance of the respiratory system (Crs) is the total of the sum of the compliance of the chest wall (Ccw) and the lung compliance (CL). In obese patients, Ccw has a much more significant contribution to the total Crs, and the clinician should be really interested in the CL. At the bedside, esophageal manometry can be very useful to distinguish the contribution of CL and Ccw to the total Crs.
No, we shouldn’t.
Another randomized controlled trial (EPVent-2), by the same group, compared PEEP titration guided by esophageal pressure with empirical PEEP titration, in patients with moderate to severe ARDS (Beitler JR, et al.). The primary outcomes of interest, death, and mechanical ventilator-free days through day 28 were not different between the groups.
Additionally, placement of an esophageal balloon is challenging and operator-dependent. The balloon portion of the esophageal catheter should be positioned in the lower third of the esophagus, behind the heart. Catheter placement is typically performed by inserting it into the stomach to a depth of about 60 cm, and gently pressing on the abdomen and observing a sudden increase in pressure on the ventilator screen. It is then withdrawn to about 40 cm, while looking for cardiac oscillations and pressure change (Talmor D, et al.). One can see how easily it would be to insert the esophageal balloon incorrectly. A misplaced balloon won’t provide accurate PES and can potentially cause harm.
Final answer: It depends on each individual patient.
Arguments for and against using an esophageal balloon to titrate PEEP in patients with ARDS and refractory hypoxemia are ongoing. Even the two most cited and applied trials on the matter (EPVent and EPVent-2) reported contradictory results. However, when analyzed in depth, both showed better oxygenation with the use of esophageal balloon. EPVent had improvement in oxygenation as its primary endpoint, and it was significant in the esophageal balloon group. EPVent-2 had oxygenation goals, in the form of need for rescue therapies for refractory hypoxemia, as secondary endpoints. Nonetheless, the patients in the esophageal balloon group in EPVent-2 required prone positioning less frequently, had lower use of pulmonary vasodilators, and a lower rate of ECMO consultations. Even though those trials did not show a mortality benefit, both showed an oxygenation benefit.
The ideal single tool that would indicate the “perfect “PEEP for each patient remains to be described. Until then, PEEP titration guided by a combination of ARDSnet PEEP tables, while maintaining a plateau pressure below 30 cm H2O and considering a driving pressure below 15 cm H2O should be a clinician’s goal. In patients in the extremes of height and body weight, and/or with conditions that would increase intra-abdominal pressure, such as ascites, a well-placed esophageal balloon while patient is supine might be beneficial.
The truth of the matter is, PEEP should be titrated by a trained intensivist in conjunction with the multidisciplinary ICU team, at patients’ bedside taking into consideration each individual’s unique physiologic and pathophysiologic characteristics at that moment.
Dr. Gallo de Moraes is Assistant Professor of Medicine, and Dr Oeckler is Assistant Professor of Medicine, Division of Pulmonary and Critical Care, Mayo Clinic, Rochester, Minnesota.