The Daily Dose • Sunday, April 16, 2023

Coming to the Rescue When Ventilators are not Enough

Christian S. Guay, MD

Anesthesiology and critical care interweave across multiple domains. Especially notable is the use of mechanical ventilation to maintain oxygenation and acid-base homeostasis. Although most patients undergoing surgery have normal lung function, there are instances when advanced knowledge of mechanical ventilation and rescue therapies can be life-saving, for example patients with acute respiratory distress syndrome (ARDS) requiring emergent surgical procedures.

In the session, co-sponsored by the Society of Critical Care Anesthesiologists (SOCCA), “When the Anesthesia Machine Ventilator Is Not Enough,” on Saturday, April 15, at the IARS 2023 Annual Meeting, attendees learned about ventilator mechanics, advanced forms of mechanical ventilation titration, ARDS and rescue therapies for when ventilators are not enough.

Ross Blank, MD, an Assistant Professor of Anesthesiology at Michigan Medicine, kicked off the session by discussing the similarities and differences between ventilators integrated into anesthesia machines versus ICU ventilators. Most of the early differences between analog ventilators in these settings have been phased out over time, and modern anesthesia machines have almost all of the same ventilation modes and settings as ICU ventilators. Modes exclusive to ICU ventilators primarily rely on proprietary approaches to incorporating spontaneous breathing and weaning from mechanical ventilation. Key differences to note include that modern anesthesia machine ventilators integrate partial rebreathing of exhaled gas scrubbed of CO2, adjustable fresh gas flow, anesthetic vaporizers and volatile anesthetic scavenging systems. 

When an anesthesiologist is preparing to care for a patient admitted to the ICU, they should carefully note all of the ventilator settings in the ICU and pre-set them on the anesthesia machine ventilator to ensure a smooth transition. Most patients will tolerate ventilation with a self-inflating bag and PEEP valve, but patients with severely compromised lung function may benefit from a transport ventilator that can replicate the ICU ventilator settings. Finally, clamping the endotracheal tube during ventilator changes can help prevent de-recruitment and resulting hypoxia.

Oliver Panzer, MD, an Associate Professor at the Hospital for Special Surgery, followed with a presentation on ARDS and advanced ventilation techniques. After discussing ARDS definitions and standards of care, Dr. Panzer dove into evidence that has emerged since the landmark ARDSNet article published in NEJM (2000). Driving pressure (plateau pressure – PEEP) in particular has emerged as an important target when managing mechanical ventilation and should be kept below 15 cmH2O whenever possible. Dr. Panzer also described the stress index method of determining when patients are on the steep ascending curve of pressure-volume loops, and therefore have optimal compliance. Different phenotypes of ARDS are also emerging (e.g. focal vs. non-focal), which may benefit from different ventilation strategies.

Because sometimes a ventilator simply is not enough to maintain oxygenation, Bhoumesh Patel, MD, an Assistant Professor of Anesthesiology at Yale School of Medicine, closed the session with a discussion of venovenous extracorporeal membrane oxygenation (VV ECMO). The process starts with the insertion of large-bore venous cannulas to drain and re-infuse a large portion of the patient’s cardiac output after passing it through a membrane that allows for gas exchange (notably O2 and CO2). This allows the machine to replace some or all of the gas exchange functions of the lungs, either as a bridge to lung transplantation or recovery of native lung function. Perioperative indications include inadequacy of mechanical ventilation, critical airway compromise, advanced tracheal or carinal surgery, critically low preoperative pulmonary or right heart reserve, and cases when lung isolation is not feasible. In general, VV ECMO flow should be maintained at 50-80 ml/kg/min or > 60-70% of the patient’s native cardiac output. Flow determinants include the pump speed (rotations per minute; RPM), volume status, intrathoracic and abdominal pressures, and recirculation between cannulas. Dr. Patel concluded his presentation with a discussion of intensivist-lead ECMO cannulation programs and a call to action for the education and training of intensivists in this exciting and growing field.