What is the future of mechanical ventilation?
Jeffrey M. Feldman MD, MSE and Myron Yaster MD
The February 2024 issue of Anesthesia and Analgesia is devoted to reimagining how anesthesia will be delivered in the future. In today’s PAAD, Dr. Jeff Feldman of the Children’s Hospital of Philadelphia and I will discuss the vision of the future of mechanical ventilation in the OR inspired by Rubulotta et al.1 As many of you know, Jeff is a regular contributor to the PAAD and is one of the premier experts on intraoperative pulmonary physiology and mechanical ventilation.2-4 For purposes of brevity and my desire to keep PAADs to 5-6 minute reads, we will neither review the history of mechanical ventilation nor ventilation strategies in the ICUs. These are wonderfully described in the article, and I think well worth your time to read. Rather, we will concentrate on how mechanical ventilation may be managed in the OR of the future. Myron Yaster MD
Original article
Rubulotta F, Blanch Torra L, Naidoo KD, Aboumarie HS, Mathivha LR, Asiri AY, Sarlabous Uranga L, Soussi S. Mechanical Ventilation, Past, Present, and Future. Anesth Analg. 2024 Feb 1;138(2):308-325. doi: 10.1213/ANE.0000000000006701. Epub 2024 Jan 12. PMID: 38215710.
Mechanical ventilation is central to the practice of anesthesiology. In the OR, what began as a convenience to replace “squeezing the bag” has evolved in modern times to a sophisticated ventilator that not only mimics the capabilities of intensive care unit ventilators, but also supports efficient delivery of inhaled anesthetics. The majority of our patients present to the operating room with normal lung function, and the clinical challenge is to prevent atelectasis and avoid ventilator-induced lung injury (VILI). A smaller, but not insignificant, population of patients present to the OR with varying degrees of lung disease including frank ARDS. In those patients, mechanical ventilation becomes more challenging in that VILI is a greater risk and it is more difficult to achieve desirable targets of gas exchange.
Mechanical Power: A Unifying Concept of Ventilator-Induced Lung Injury (VILI)
The pathogenesis of VILI has become increasingly clear over the years, but the connection to pulmonary complications, and best practices for minimizing those complications, especially in the operating room, have to date eluded our understanding. Baro- volu-and atelecto-trauma have all been considered a root cause of VILI. Since the landmark paper demonstrating that mortality improved in adult ARDS patients by limiting tidal volume, lung protective ventilation using small tidal volumes has become a key strategy.5 Avoidance of atelectasis and repeated recruitment and derecruitment of the alveoli added the search for optimal PEEP as well as the use of recruitment maneuvers to the lung protective ventilation strategy. While lung protective ventilation has likely improved outcomes in the ICU and high-risk patients in the operating room, we still lack an evidence-based best practice.
In 2050, Rubulotta et al. predict that it is likely that we will have a much deeper understanding of VILI and therefore a more rational approach to lung protective ventilation. The concept of mechanical power is emerging as a unifying theory for our concepts of the potential for pressure, volume and PEEP to injure the lung. Power is the energy expended over time, usually expressed in joules per minute. With regard to mechanical ventilation, power is the product of pressure, tidal volume and respiratory rate. Pressure consists of three components static (PEEP), dynamic elastic (Pplat-PEEP) and dynamic resistive (Ppeak-Pplat).6 One early paper studying healthy piglets showed that despite a tidal volume of 38 mLs/kg, a respiratory rate sufficient to increase the power to 12 J/min or more was required to demonstrate lung injury.7 So much for low tidal volume ventilation! Subsequent studies have demonstrated an association between worsened outcomes in both ARDS patients and operative patients exposed to greater mechanical power during ventilation.8,9 Given this evolving theoretical foundation for VILI, it is not surprising that our existing strategies of limiting tidal volume and distending pressure while using recruitment maneuvers and PEEP to eliminate atelectasis have been effective. All of those strategies help to minimize the mechanical power required to achieve the gas exchange goals. Nonetheless, finding the optimal balance of pressure, volume and PEEP for the greatest benefit of each patient remains a clinical challenge.
Bedside Monitoring could use a touch of the Future
Bedside respiratory monitoring provides the tools for the clinician to link the science of respiratory physiology with the art of optimizing ventilation to achieve the desired goals of gas exchange while protecting the lung. The goals for mechanical ventilation are to achieve the best oxygenation at the lowest inspired oxygen concentration, the desired tidal volume using the least pressure and acceptable CO2 elimination. While modern monitoring tools for assessing oxygenation, lung compliance and ventilation are useful, hopefully the future will bring improvements.
· Oxygenation monitoring has been revolutionized by the pulse oximeter – non-invasive, easy to apply and provides an estimate of oxygen saturation in almost all patients. The greatest weakness of the pulse oximeter is that it is the measurement of saturation only. Consequently, the device is only sensitive for detecting problems with oxygenation if inspired oxygen concentration is minimized, ideally approaching 21%. Future enhancements should link the SpO2 measurement with inspired oxygen concentration and trend that relationship. Skin color, nail polish, low perfusion, light pollution are all well known to introduce bias into the pulse oximeter measurement. Enhancements of the basic design of that device will likely reduce bias and improve precision to provide more reliable oxygenation assessment.
· Lung Compliance assessment is seemingly a straightforward evaluation of the ratio of tidal volume to inspiratory pressure. Dynamic compliance (Volume/Peak Pressure) is relatively easy to assess but static compliance (Volume/Plateau Pressure) is a better reflection of intrinsic lung compliance. Given existing technology, while tidal volume assessment is generally reliable, determining plateau pressure at the bedside is not completely straightforward and could be improved. That said, Pressure-Volume loops have been incorporated into anesthesia workstations capable of measuring the breathing circuit compliance during the pre-use self-test. These loops provide breath to breath insight into the pressure-volume interaction and are useful for detecting decreases in lung compliance and the effect of interventions like recruitment maneuvers. Other technologies mentioned in the review with the potential to enhance compliance monitoring include electrical impedance tomography and esophageal manometry although the authors are correct to note that the additional equipment needed is an obstacle to routine use.
· Ventilation monitoring is currently based upon measurement of tidal volume, respiratory rate and estimation of PaCO2 by ETCO2. While the gradient between ETCO2 and PaCO2 is typically 5-10 mmHg, that difference is not reliable and is unknown in the absence of arterial blood gas measurements. How many of us have seen a reasonable ETCO2 value intraoperatively only to be surprised when a blood gas reveals more significant hypercarbia than expected. Tidal volume and apparatus dead space play an important role in determining the gradient. Furthermore, few if any workstations provide an easy view of the relationship between ventilator settings and ETCO2 over time. Future designs will hopefully make it easier for the clinician to manage the impact of mechanical ventilation on CO2 elimination.
Will AI Save Us?
Artificial intelligence is highlighted in the review article as a technology that can facilitate “personalized ventilation,” or optimized ventilation based upon the patient’s physiology and condition. Leaving the ICU out of the discussion, what does this mean for the operating room? When caring for a high acuity patient during a complex procedure, there are many priorities vying for the clinician’s attention. A significant opportunity for AI is to develop tools for continuously assessing monitored parameters and predicting untoward changes before they cause injury. An AI-based tool for hypotension prediction is already in use and there is no reason that similar tools cannot be developed to optimize mechanical ventilation.10 The computer is ideal for making continuous assessments and recommending, or even implementing, corrective actions. Imagine that the early onset of atelectasis is detected by continuous automatic assessment of lung compliance using feature extraction of plateau pressure from the pressure waveform. The inspired oxygen concentration can be reduced if needed to determine by pulse oximetry if there is an associated impact on oxygenation. Once the likelihood of atelectasis is established the clinician can be notified or a recruitment maneuver automatically begun to see if the problem can be improved.
One Last Comment on Split Ventilation
The authors of the review discuss the potential for split ventilation, using one ventilator to ventilate two or more patients, highlighting the success of that strategy in a few cases during the recent Covid pandemic. While the ability to make that strategy work during a crisis is laudable, it is our opinion that it is not a desirable strategy when preparing for future respiratory pandemics. Other work during the pandemic to develop simple, low-cost ventilators that could be built easily and cheaply, even in remote locations is much more promising. One such example is the Pufferfish ventilator designed at the University of Utah.
Send your thoughts and comments to Myron who will post in a Friday reader response.
References
1. Rubulotta F, Blanch Torra L, Naidoo KD, et al. Mechanical Ventilation, Past, Present, and Future. Anesthesia and analgesia 2024;138(2):308-325. (In eng). DOI: 10.1213/ane.0000000000006701.
2. King MR, Feldman JM. Optimal management of apparatus dead space in the anesthetized infant. Paediatric anaesthesia 2017;27(12):1185-1192. (In eng). DOI: 10.1111/pan.13254.
3. Feldman JM. Optimal ventilation of the anesthetized pediatric patient. Anesthesia and analgesia 2015;120(1):165-75. (In eng). DOI: 10.1213/ane.0000000000000472.
4. Feldman JM. Managing fresh gas flow to reduce environmental contamination. Anesthesia and analgesia 2012;114(5):1093-101. (In eng). DOI: 10.1213/ANE.0b013e31824eee0d.
5. Brower RG, Matthay MA, Morris A, Schoenfeld D, Thompson BT, Wheeler A. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The New England journal of medicine 2000;342(18):1301-8. (In eng). DOI: 10.1056/nejm200005043421801.
6. Gattinoni L, Tonetti T, Cressoni M, et al. Ventilator-related causes of lung injury: the mechanical power. Intensive care medicine 2016;42(10):1567-1575. (In eng). DOI: 10.1007/s00134-016-4505-2.
7. Cressoni M, Gotti M, Chiurazzi C, et al. Mechanical Power and Development of Ventilator-induced Lung Injury. Anesthesiology 2016;124(5):1100-8. (In eng). DOI: 10.1097/aln.0000000000001056.
8. Costa ELV, Slutsky AS, Brochard LJ, et al. Ventilatory Variables and Mechanical Power in Patients with Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med 2021;204(3):303-311. (In eng). DOI: 10.1164/rccm.202009-3467OC.
9. Santer P, Wachtendorf LJ, Suleiman A, et al. Mechanical Power during General Anesthesia and Postoperative Respiratory Failure: A Multicenter Retrospective Cohort Study. Anesthesiology 2022;137(1):41-54. (In eng). DOI: 10.1097/aln.0000000000004256.
10. Kouz K, Monge García MI, Cerutti E, et al. Intraoperative hypotension when using hypotension prediction index software during major noncardiac surgery: a European multicentre prospective observational registry (EU HYPROTECT). BJA Open 2023;6:100140. (In eng). DOI: 10.1016/j.bjao.2023.100140.