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Stanford University - School of Medicine Department of Anesthesiology, Perioperative and Pain Medicine, 300 Pasteur Drive, Room H3580, Stanford, CA 94305, USA
Although low tidal volume and reduced airway pressures are standard components of lung-protective ventilation, further individualization of ventilation may be necessary based on patient characteristics and surgical conditions.
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Noninvasive tools that provide real-time, continuous, dynamic lung imaging (lung ultrasound, electrical impedance tomography) or pressure measurements (esophageal manometry) may help to better identify different lung morphology including lung overdistension and atelectatic components as well as response to specific interventions.
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Using predictive risk scores to identify patients at risk for postoperative pulmonary complications in addition to recognition of specific patient populations (those with obesity, sepsis, or acute respiratory distress syndrome) or surgical procedures (laparoscopic, abdominal, or one-lung ventilation needs) that are at risk can help physicians implement ventilator management plans in the operating room to those that may benefit from lung-protective strategies.
Introduction
More than 2 decades ago, we learned that low tidal volumes (TVs; 6 mL/kg predicted body weight [PBW]) and lower plateau pressures (≤30 cmH2O) resulted in reduced patient mortality for patients suffering from acute respiratory distress syndrome (ARDS).
These strategies are now frequently referred to as lung-protective ventilation (LPV) and are distinctively different from the historical large TVs of 10 to 12 mL/kg used intraoperatively to promote lung recruitment and provide optimal oxygenation. With the growing evidence to support LPV in the management of ARDS in critically ill patients, anesthesiologists and intensivists have translated these strategies for the operating room (OR) in efforts to minimize postoperative pulmonary complications (PPCs) and prevent ventilator-induced lung injury (VILI). However, despite a solid trend in intraoperative ventilator strategies suggesting practice has changed, such as decreased TV
during the past decade, the evidence for specific intraoperative lung-protective ventilation (IOLPV) strategies that parallel those in the intensive care unit (ICU) remains incomplete.
As clinicians are faced with decisions regarding which strategies are beneficial in the OR versus the ICU, there are a growing number of critically ill patients that require surgical and procedural interventions during their hospital course. For simplicity of intraoperative management, there is an understandable desire to standardize mechanical ventilation for all patients undergoing surgery, yet there is evidence that this standardized approach may not be optimal or may even be detrimental depending on patient characteristics and surgical conditions.
Importantly, the stress and strain applied to the lung are not directly measured with standard LPV settings but are inferred from respiratory mechanics. Limitations of intraoperative mechanical ventilators, decreased access to ancillary staff and equipment, dynamic physiologic and positional changes occurring during surgery, in addition to the heterogeneity of the critically ill patient population, are all frequent barriers to implementing a single IOLPV framework for all ICU patients in the OR. Based on these challenges, an individualized approach to intraoperative mechanical ventilation that combines advanced monitoring tools and targets is increasingly becoming a point of focus for reducing PPCs.
Fogagnolo A, Montanaro F, Al-Husinat Li, et al. Management of Intraoperative Mechanical Ventilation to Prevent Postoperative Complications after General Anesthesia: A Narrative Review.
Deleterious outcomes of positive pressure ventilation (PPV) are not new concepts. Well before and directly contributing to the motivation for the landmark ARDSnet study,
Ventilator-induced lung injury (VILI) is described in reports of human disease following PPV as well as being studied in animal models preceding this and the term “respirator lung” was coined for patients exhibiting evidence of lung tissue disruption following mechanical ventilation.
Evidence of lung tissue damage, pulmonary edema, and development of hyaline membranes as precursors of pending severe and potentially life-threatening lung disease is present in the literature from the 1960s and 1970s.
Experimental pulmonary edema due to intermittent positive pressure ventilation with high inflation pressures. Protection by positive end-expiratory pressure.
Experimental pulmonary edema due to intermittent positive pressure ventilation with high inflation pressures. Protection by positive end-expiratory pressure.
For example, subjects with low airway but high transpulmonary pressure due to air hunger or significant spontaneous respiratory effort can generate significant gradients associated with lung injury.
Spontaneous breathing during lung-protective ventilation in an experimental acute lung injury model: high transpulmonary pressure associated with strong spontaneous breathing effort may worsen lung injury.
Similar to barotrauma, but more specifically related to damage resulting from overdistension of alveoli, high volumes during mechanical ventilation lead to alveolar rupture and potentially large air leak.
Just as overdistension of alveoli causes epithelial barrier disruption, repetitive opening and closing of the distal alveoli similarly results in the development of injury from pulmonary edema, epithelial cell dislodgement, and ultimately development of hyaline membranes.
Experimental pulmonary edema due to intermittent positive pressure ventilation with high inflation pressures. Protection by positive end-expiratory pressure.
Indeed, utilization of PEEP to maintain respiratory airway recruitment is now nearly ubiquitous following multiple publications and has been suggested as yet another basis for increased survival in patients with ARDS requiring mechanical ventilation.
In a fourth mechanism, stimulation of the inflammatory cytokines leads to an upregulation of hypertrophy and/or hyperplasia of airway epithelium and eventual pulmonary remodeling.
Notable substances include TNF-alpha, IL-8, and IL-6, and patients prescribed lung-protective ventilation seem to show reduced levels of these cytokines.
reducing airway pressure to the minimum required to provide adequate gas exchange, titration of ventilator settings to transpulmonary pressure as assessed through esophageal pressure monitoring,
Neuromuscular blockade is associated with the attenuation of biomarkers of epithelial and endothelial injury in patients with moderate-to-severe acute respiratory distress syndrome.
With studies providing evidence of lung injury through altered structure and ultimately function, the clinician is left without doubt that mechanical ventilation, although lifesaving and often necessary in the OR and the ICU, must be tailored to the individual patient just as any other therapy.
Discussion
The goal of intraoperative mechanical ventilation management is to optimize lung recruitment without overdistending compliant alveoli. In the critically ill patient, the low TV strategy has proven beneficial in preventing mechanisms of VILI that result in improved outcomes. Similarly, low TV ventilation (<8 mL/kg) versus high (>8 mL/kg) with general anesthesia (GA) has been associated with a decrease in PPCs.
Mechanical ventilation with lower tidal volumes and positive end-expiratory pressure prevents pulmonary inflammation in patients without preexisting lung injury.
A Meta-analysis of Intraoperative Ventilation Strategies to Prevent Pulmonary Complications: Is Low Tidal Volume Alone Sufficient to Protect Healthy Lungs?.
Going beyond low TV as the solo physiologic target for LPV and instead focusing on individualized targets for mechanical ventilation based on physiologic principles and dynamic monitoring may provide more effective intraoperative ventilation and reduce the occurrence of VILI.
Advanced physiologic targets
Alveolar Recruitment
Most patients develop atelectasis during anesthesia and surgery, the extent of which is determined by the type and duration of surgery, surgical technique, positioning, and underlying medical conditions.
PEEP can be used to prevent atelectrauma by alveolar recruitment. However, too much PEEP can have a hemodynamic effect, cause overdistention of aerated alveoli leading to increased dead space and shunt, and increased risk of cor pulmonale.
Association of positive end-expiratory pressure and lung recruitment selection strategies with mortality in acute respiratory distress syndrome: a systematic review and network meta-analysis.
Association of positive end-expiratory pressure and lung recruitment selection strategies with mortality in acute respiratory distress syndrome: a systematic review and network meta-analysis.
Ventilation strategy using low tidal volumes, recruitment maneuvers, and high positive end-expiratory pressure for acute lung injury and acute respiratory distress syndrome: a randomized controlled trial.
Association of positive end-expiratory pressure and lung recruitment selection strategies with mortality in acute respiratory distress syndrome: a systematic review and network meta-analysis.
Effect of Titrating Positive End-Expiratory Pressure (PEEP) With an Esophageal Pressure-Guided Strategy vs an Empirical High PEEP-Fio2 Strategy on Death and Days Free From Mechanical Ventilation Among Patients With Acute Respiratory Distress Syndrome: A Randomized Clinical Trial.
The use of PEEP alone cannot reopen all collapsed alveoli, so it is essential to use a recruitment maneuver (RM) to enable a complete reopening. Several methods for performing RMs have been described in the literature.
Most commonly, RMs are performed manually using the “bag squeezing” method of applying and holding a set inflation pressure for a specific duration of time using the airway pressure-limiting valve of the anesthesia machine.
Intraoperative protective mechanical ventilation for prevention of postoperative pulmonary complications: a comprehensive review of the role of tidal volume, positive end-expiratory pressure, and lung recruitment maneuvers.
However, ventilator-driven RMs are preferred to minimize the loss of PPV when switching back to the ventilator. Provided there are no contraindications, an inspiratory pressure of 40 cmH2O for 7 to 8 seconds is likely to result in full recruitment in most nonobese patients with healthy lungs.
Higher pressures and longer times may be needed for certain patients. For anesthesia ventilators that allow pressure-controlled ventilation, RMs can be performed by increasing PEEP up to 20 cmH2O in steps of 5 cmH2O (30–60 seconds per step) while maintaining a constant driving pressure of 15 to 20 cmH2O.
Intraoperative protective mechanical ventilation for prevention of postoperative pulmonary complications: a comprehensive review of the role of tidal volume, positive end-expiratory pressure, and lung recruitment maneuvers.
Although RMs have the potential to improve respiratory mechanics and gas exchange, they also have the potential to increase lung injury depending on the clinical circumstances.
Consequently, it is important to assess the extent of atelectasis and monitor the response when performing RMs. The recruitment-to-inflation ratio is a recently proposed approach to estimate recruitability at the bedside.
Retrospective studies have shown that DP is related to mortality in patients with ARDS even among those who received traditional protective ventilation.
The threshold value of DP for higher mortality in patients with ARDS is approximately 15 cmH2O, and each 1-cmH2O increase of DP was associated with a 5% increment in mortality. Higher DP values have also been associated with increased mortality in patients receiving pressure support ventilation.
In surgical patients, a meta-analysis of 17 randomized controlled trials found that intraoperative high driving pressure was associated with the development of PPCs, whereas no association was found with TV and PEEP.
Association between driving pressure and development of postoperative pulmonary complications in patients undergoing mechanical ventilation for general anaesthesia: a meta-analysis of individual patient data.
The deleterious effect of DP is believed to result from the concept of “functional lung size”—the volume of aerated lung available for ventilation. “Functional lung size” is reduced in patients with lung pathologic conditions such as atelectasis, consolidation, effusion, or fibrosis. Therefore, if lungs are either overdistended or underventilated in relation to their “functional lung size,” DP will increase. It has been suggested that DP may be used to set PEEP because the best compromise between overinflation and recruitment is determined at the lowest DP.
However, although studies clearly show that elevated DP is associated with increased complications, they do not confirm that active control of DP improves outcomes. Although a causal effect has not been demonstrated, it has been recommended that at least for patients with ARDS, DP should be targeted less than 13 cmH2O.
where the coefficient a represents the slope of the pressure time (PP-T) curve at a given time of measurement t, and c is the pressure at the initiation of inspiration. The constant SI, referred to as the “stress index,” is a number that describes the shape of PP-T curve.
Linearity of the PP-T curve (SI = 1) denotes compliance during TV insufflation, which is minimally injurious. Nonlinearity of the pressure–time curve denotes a nonconstant compliance during TV insufflation. If compliance increases during inspiration, as occurs with intratidal lung recruitment, the slope of the pressure–time curve decreases over time, resulting in a downward concavity (SI < 1). Implementation of increased PEEP to avoid the cyclic opening and closing of respiratory units, a mechanism of VILI, is beneficial in this instance. In contrast, if compliance decreases during inspiration, as suggested by an upward concavity of the pressure–time curve (SI > 1), this suggests that the lung is cyclically overdistended during inspiration. In this case, a reduction of PEEP and/or TV might be warranted. Use of the SI allows adjustment of the ventilatory settings to respiratory system characteristics of the patient, modifying VT and PEEP. Although measuring the SI requires complex calculations that are clinically onerous, it can be easily estimated by visualizing the PP-T curve and is demonstrated in Fig. 1.
Fig. 1A dynamic airway pressure during volume control ventilation with a constant inspiratory flow. The stress index (SI) is derived from changes in the slope of the pressure time during inspiration. Upward concavity of the pressure–time curve reflects decreasing compliance (overdistension, SI > 1) while downward concavity of the pressure–time curve indicates increasing compliance (atelectasis, SI < 1).
(From [Kallet RH. Should PEEP Titration Be Based on Chest Mechanics in Patients with ARDS? Respir Care. Jun 2016;61(6):876-90]; with permission.)
Mechanical power (MP) is a summary construct that attempts to relate the different ventilatory parameters set by the clinician at the bedside to the amount of energy transferred from the mechanical ventilator to the respiratory system.
where ΔV is tidal change in lung volume, RR is respiratory rate, Ers is respiratory system elastance, and Raw is airway resistance, and I:E is the inspiratory–expiratory ratio.
The MP equation illustrates the relative importance of each respiratory variable on the energy delivered to the lungs during mechanical ventilation. For example, the effect of TV, which is squared in the MP calculation, is predominant. The effect of PEEP is dichotomous: PEEP directly increases the MP but can also indirectly reduce it through a reduction in Ers. The MP equation also illustrates that the respiratory rate is linearly related to the amount of the energy delivered to the lungs. Studies have reported that increasing values of MP are associated with an increased risk of lung injury.
Intra-operative ventilator mechanical power as a predictor of postoperative pulmonary complications in surgical patients: A secondary analysis of a randomised clinical trial.
It is also important to recognize that despite low MP values, lung damage is still possible with inhomogeneous ventilation where atelectasis and hyperinflation simultaneously coexist.
Fogagnolo A, Montanaro F, Al-Husinat Li, et al. Management of Intraoperative Mechanical Ventilation to Prevent Postoperative Complications after General Anesthesia: A Narrative Review.
An important determinant of the appropriate size of the delivered TV during mechanical ventilation is the size of the dead space—the volume of inhaled gas that does not take part in gas exchange.
Dead space is divided into the anatomic dead space—which exists in the large and small airways that do not normally participate in gas exchange—and the alveolar dead space, which results when there is absence of blood flow to an area of lung that is still receiving ventilation. Physiologic dead space, the sum of anatomical and alveolar dead space, is a global measure of the efficiency of the lung function. It has been associated with outcome in patients with respiratory failure and may be helpful for selecting optimal PEEP.
However dead space is not routinely measured in anesthetic practice, due to difficulties in interpreting capnograms and in calculation methods. Use of minute ventilation is unable to adequately describe ventilation efficiency because it does not distinguish between alveolar and anatomic dead space ventilation. Volumetric capnography (VCap), which measures the volume of expired CO2 in one single breath, is a reliable real-time method for measuring dead space. In addition, VCap provides information about pulmonary perfusion, end-expiratory lung volume, and pulmonary ventilation inhomogeneities, which can be used for optimization of ventilation at the bedside.
Although further investigation is needed to establish the optimal use of VCap in the intraoperative setting, it is a potentially useful noninvasive tool to measure dead space and optimize intraoperative ventilation.
Advanced monitoring tools
Electrical Impedance Tomography
Electrical impedance tomography (EIT) is a noninvasive, radiation-free portable monitoring technique that provides images based on the electrical conductivity of tissue in the chest. Electrodes are placed on the chest wall, which record the surface voltage after the repeated application of a small amount of electrical current. The changes of electrical impedance over time are displayed dynamically in color-code images. By imaging breath-by-breath changes in ventilation distribution, EIT can be used intraoperatively to dynamically optimize ventilator settings.
EIT measurements of TV and ventilation distribution have been validated as accurate surrogates in comparison to CT scan and nitrogen washout, respectively.
EIT monitoring can identify regional ventilation heterogeneity, overdistention, and atelectasis, that are otherwise not identifiable by traditional protective ventilator-based metrics.
EIT helps inform the mechanical compromise between ventilation of nondependent and dependent lung regions, minimizing both overdistension and collapse. These beneficial effects are more pronounced in obese patients and those undergoing interventions that impair normal respiratory mechanics (eg, laparoscopy procedures or Trendelenburg positioning).
A variety of EIT-derived indices have been proposed to quantify temporal and spatial ventilation heterogeneity including the “global inhomogeneity index,” “regional ventilation delay,” and “dynamic relative regional strain.” Lung perfusion monitoring is another important feature of EIT that can be evaluated during general anesthesia. An IV injection of hypertonic saline during an expiratory breath-hold allows the calculation of pulmonary perfusion through impedance time-curves. Comparisons of impedance data for both ventilation and perfusion can then be analyzed and ventilator strategies can be altered based on knowledge of ventilation/perfusion mismatch.
Although the usefulness of EIT has been highlighted by a growing body of literature; limitations to its use include lower spatial resolution; lack of intrapatient and interpatient reproducibility secondary to variations in electrode placement; and deterrence due to setup time, equipment costs, and training of personnel.
Esophageal Manometry
Esophageal manometry is a clinical method used to separate the pressure applied to the respiratory system (Paw) into the component distending the chest wall (ie, pleural pressure, Ppl), and that distending the lung transpulmonary pressure (PL).
Transpulmonary pressure is defined as the difference between the airway pressure and the pleural pressure.
PL = Paw − Ppl
Esophageal manometry estimates the Ppl through an air-filled balloon catheter inserted in the esophagus. Although esophageal manometry is best known for its role in guiding mechanical ventilation in patients with ARDS,
it has also been used in the OR to optimize PEEP instead of relying on standard protective parameters that target Paw alone. To prevent atelectasis, it has been proposed to adjust PEEP such that expiratory transpulmonary pressure is slightly positive, so as to ensure that the lung (if recruitable) is maintained open.
To prevent injury from overdistension, attempts are also made to limit the inspiratory transpulmonary pressure.
Esophageal manometry can be used not only to monitor respiratory system mechanics during controlled ventilation but also to monitor patient’s respiratory muscle activity during spontaneous breathing. Although evidence-based guidelines recommend limiting inspiratory plateau pressure (Pplat) to 30 cmH2O especially in ARDS; however, Paw cannot be measured during normal spontaneous breathing. In contrast, PL can be measured during spontaneous breathing because it is calculated based on both Paw and Ppl. Studies in healthy individuals indicate that a PL of 25 cmH2O is the upper limit encountered by the lungs in normal life but it is unclear whether this limit is safe in injured lungs, and, a lower threshold than that experienced in healthy breathing may be required.
Despite these potential benefits of esophageal manometry, only limited studies have demonstrated a positive influence of esophageal manometry on patient outcomes.
In the OR setting, esophageal manometry may be especially helpful for selecting PEEP in patients with chest-wall abnormalities, such as those with obesity, or with surgical conditions such as increased intra-abdominal pressure due to laparoscopy or Trendelenburg positioning. Setting a desired target for transpulmonary pressures is another proposed method for determining appropriate TVs based on patient’s respiratory mechanics.
Fig. 2 demonstrates an example of using esophageal manometry and EIT together to optimize mechanical ventilation during pneumoperitoneum and Trendelenburg positioning in an obese patient.
Fig. 2The use of esophageal manometry and electrical impedance for intraoperative mechanical ventilation optimization in an obese patient.
Lung ultrasonography (LUS) is a noninvasive bedside tool that can quantify aerated lung mass and provide real-time information during mechanical ventilation.
In the perioperative setting, LUS has been used to dynamically detect the development of intraoperative atelectasis, alveolar consolidation, as well as diaphragmatic dysfunction and weakness.
LUS can be used to assess PEEP-induced lung recruitment, enabling optimization of the ventilatory settings. It can also help discriminate between a cardiac, lung parenchymal, or diaphragmatic cause of loss of lung aeration and, as a consequence, help to determine the most appropriate approach to management.
Although lung aeration reaeration can be evaluated using LUS, the ability to predict lung overinflation has been more challenging and the source of ongoing investigation.
Methods to increase adherence to intraoperative lung-protective ventilation
With multiple physiologic targets, equations, and studies to recall, creating an individualized approach to mechanical ventilation in the OR can be cumbersome and can be limited by the individual’s education and comfortability of IOLPV. IOLPV strategies that institute physiologic TVs (6–8 mL/kg cmH2O) and avoid zero PEEP settings can be facilitated by institutional quality improvement measures. Interventions that have shown improvement in the adherence to IOLPV include the removal of older anesthesia machines,
Replacement of anesthesia machines improves intraoperative ventilation parameters associated with the development of acute respiratory distress syndrome.
For the critically ill patient requiring mechanical ventilation in the OR, the use of a standardized hand-off tool to incorporate mechanical ventilation goals and targets between the ICU and OR team can be instrumental in continuing LPV in the OR setting.
Clinical relevance
Risk Evaluation and Prediction Scores
Selection of an individualized intraoperative ventilation strategy begins with an awareness of the factors that pose the greatest risk for PPCs. Acknowledging known risk factors and using risk prediction models can be useful, particularly in patients that seem to have healthy lungs despite being at risk for PPC or critical illness.
Epidemiology, practice of ventilation and outcome for patients at increased risk of postoperative pulmonary complications: LAS VEGAS - an observational study in 29 countries.
Incorporating these models into your intraoperative mechanical ventilation plan can help minimize “second-hit” exposures in conjunction with a LPV strategy.
Although 20% to 40% of ICU patients require mechanical ventilation, it is not uncommon to encounter spontaneously breathing critically ill patients that need a life-sustaining intervention in the operating suite to advance care. In these cases, it is vital to remember that protective ventilation begins with induction. Atelectasis occurs in 90% of patients undergoing GA, and it can persist for weeks postoperatively.
This is usually tolerated well by young, otherwise healthy patients. However, patients who are aged older than 50 years or who have a body mass index (BMI) greater than 40 kg/m2, an American Society of Anesthesiologists physical status greater than II, or obstructive sleep apnea may have less tolerance for the decreased functional residual capacity (FRC), shortening the time allowable for intubation. Supine positioning during anesthetic induction results in cephalad displacement of abdominal contents and compression of dependent lung regions. The 30-degree head-up and reverse Trendelenburg positions are associated with less reduction in FRC and should be used during anesthetic induction especially in obese individuals.
Critically ill patients are at a high risk for airway complications due to pathophysiologic alterations recently defined as the “physiologically difficult airway,” which can hinder optimal intubation times.
There is some literature to support the use of noninvasive positive pressure ventilation during induction to reduce atelectasis, minimize desaturations, and allow more time for intubation.
Sepsis remains one of the leading causes of ICU admission. Timely source control remains vital to standard treatment. Adequate source control within 6 hours of sepsis onset is associated with reduced risk-adjusted odds of 90-day mortality.
and although the use of minimally invasive procedures has increased, anesthetic consideration and ventilation strategies are necessary for many of these procedures. Sepsis and septic shock are known risk factors for ARDS.
The requirement for mechanical ventilation in the setting of sepsis can range from airway protection for procedures or altered mental status, to acute respiratory failure secondary to pneumonia, or to sepsis-induced ARDS. A multidisciplinary team coordination for induction and intubation is important given the possibility of hypoxemia, hypotension, and cardiac collapse that can result in the setting of a physiologically difficult airway. Given the high possibility of a metabolic acidemia at the time of induction and intubation, one must take caution of allowing permissive hypercapnia in the setting of low TVs in at risk patients, resulting in a worsening acidosis.
Many of the recommendations parallel to those already discussed, with the exception of a suggestion to use low TV compared with high TV ventilation for adults with sepsis-induced respiratory failure (without ARDS). While stating that the evidence is low and the recommendation is weak, given that sepsis is an independent risk factor for the development of ARDS, they recommend utilizing low TV strategies to avoid the underuse of or the delayed implementation of LPV in an at-risk population.
The clinical trials during the past 50 years exploring ways to prevent, mitigate, and treat ARDS are extensive and beyond the scope of this article. However, as this article does focus on the intraoperative strategies for critically ill patients, many of the parameters that were already explored are relevant to IOLPV in patients meeting criteria for the definition of ARDS. Institution of low TVs, high PEEP, and prone positioning for more than 12 hours have shown beneficial results in the outcomes of patients with ARDS and should be incorporated into IOLPV protocols to minimize VILI.
Although proning patients in the OR may not be practical, considerations of performing bedside interventions in the ICU to limit interruptions in proning protocols could be considered. Anesthesiologists frequently use intraoperative neuromuscular blocking agents (NMBA) to facilitate intraoperative ventilator synchrony and optimize surgical conditions. However, the most recent studies suggest that prolonged NMBA are no longer beneficial for ARDS and new recommendations regard limiting their use or implementing intermittent boluses over continuous infusions.
The OR and ICU team should coordinate goals for NMBA before transport and incorporate these new guidelines in the OR when feasible.
Special Surgical Cases and Patient Populations
Patient positioning and surgical conditions are important to consider in determining an LPV strategy. Laparoscopic surgery can have negative effects on respiratory mechanics resulting from the pneumoperitoneum, which reduces FRC and promotes atelectasis formation. Pneumoperitoneum also results in reduced compliance of the respiratory system such that a greater amount of pressure will be required to expand the chest wall to achieve a given TV. Many laparoscopic procedures also use the Trendelenburg position that further increases the transmission of intra-abdominal pressure to the chest. Other unique considerations include lateral positioning and single lung ventilation, which also result in changes in lung compliance, resistance, and in TV distribution. The PROTHOR study found a decrease in PPCs in one-lung ventilation with the use of low TV ventilation (6 mL/kg PBW), RMs, and high PEEP.
Protective ventilation with high versus low positive end-expiratory pressure during one-lung ventilation for thoracic surgery (PROTHOR): study protocol for a randomized controlled trial.
In addition, compliance of the ventilated lung was improved and as a result, driving pressure was decreased. Other trials, such as the iPROVE-OLV are ongoing.
Patients with a BMI greater than 35 can develop rapid atelectasis, a decrease in FRC, and are susceptible to positioning changes in the OR. A recent study in JAMA compared the use of high PEEP (12 cmH2O) with RMs with low PEEP (4 cmH2O) with no RMs. Both groups received low TV ventilation of 7 mL/kg PBW in obese (BMI >35) patients undergoing noncardiac surgery. No differences in PPCs were found. The higher PEEP group had lower driving pressures but they were limited by hemodynamic instability.
Effect of intraoperative high positive end-expiratory pressure (PEEP) with recruitment maneuvers vs low PEEP on postoperative pulmonary complications in obese patients: a randomized clinical trial.
The individualized PEEP was superior to either fixed low levels or hither level PEEP with regards to oxygenation, driving pressures, and indices of regional ventilation but did not show a difference in PPCs. Ventilator settings often need to be readjusted in response to surgical conditions, adding complexity to the ventilatory management of patients in the OR, and further studies may be needed to elucidate optimization of intraoperative mechanical ventilation in special patient populations.
Recommendations for intraoperative lung protective ventilation
Recent guidelines and several reviews focusing on intraoperative ventilation for patients with and without ARDS have recently been published.
Based on these publications and the prior discussion in this article, some recommendations for an individualized approach to IOLPV are provided.
Ventilatory parameters should be selected and titrated based on close monitoring of targeted physiologic variables and individualized goals. Although low TV and reduced airway pressures are standard components of LPV, further individualization of ventilation may be necessary based on patient characteristics and surgical conditions. Lung recruitability is essential to identify to determine the potential benefit of RMs and PEEP titration. DPs provide a target to adjust TV and possibly to optimize PEEP. For most patients, DP should be maintained less than 13 cmH2O. Advanced monitoring tools including EIT, esophageal manometry, and LUS require additional effort and skill for interpretation but should be considered for patients and during surgical procedures that compromise respiratory mechanics and make standard interpretation difficult. Measures including mechanical power, stress index, and dead space provide additional insight into physiology of mechanical ventilation and may provide targets for further optimization.
Summary
Based on the past 20 years of evidence for LPV in ARDS, anesthesiologists have an opportunity to advance intraoperative ventilatory management of critically ill patients in the OR. Delineating which parameters and strategies are best used in the OR versus the ICU will still require more investigation; however, the trend for low TVs, adequate PEEP, and driving pressure targets have gained traction as strategies to prevent PPCs and ARDS. Implementing educational strategies using the EMR and feedback dashboards and resetting default ventilator settings are 2 methods that can be used to change practice. Incorporating more advanced monitoring tools and assessments of respiratory mechanics may help anesthesiologists improve respiratory care provided at the individual level.
Clinics care points
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When implementing a LPV strategy in the OR include both low TVs and the use of PEEP in ventilator settings and avoid the use of low TVs without PEEP given the risk of atelectrauma.
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When monitoring driving pressure in the OR, target driving pressures that increase compliance or to a goal of less than 13.
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When performing a recruitment maneuver in the OR, look for evidence of atelectasis and recruitability by using the stress index and avoid using recruitment maneuvers in patients with overdistension.
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When performing recruitment maneuvers in the OR, consider setting the peak inspirator pressure at 40 cmH2O for 7 to 8 seconds or an upward titration of PEEP up to 20 cmH2O in a pressure-controlled mode but avoid the “bag squeezing” technique to minimize the loss of positive pressure ventilation.
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When inducing anesthesia and providing mechanical ventilation for patients with obesity, limiting atelectasis starts with induction and patients should be intubated in the 30-degree head-up and reverse Trendelenburg position.
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When intubating critically ill patients in the OR at risk for a physiological difficult airway, use noninvasive mechanical ventilation during induction.
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If your patient presents in sepsis or with septic shock, use low TV strategies in the OR given the high risk of ARDS in patients with sepsis.
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When caring for patients with sepsis or septic shock who require mechanical ventilation in the OR, avoid worsening acidemia with permissive hypercapnia.
•
If a patient undergoing laparoscopic surgery develops worsening gas exchange with insufflation, increase PEEP and try recruitment maneuvers and consider a change in position or surgical technique if unsuccessful.
References
Brower R.G.
Matthay M.A.
Morris A.
et al.
Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome.
Fogagnolo A, Montanaro F, Al-Husinat Li, et al. Management of Intraoperative Mechanical Ventilation to Prevent Postoperative Complications after General Anesthesia: A Narrative Review.
Experimental pulmonary edema due to intermittent positive pressure ventilation with high inflation pressures. Protection by positive end-expiratory pressure.
Spontaneous breathing during lung-protective ventilation in an experimental acute lung injury model: high transpulmonary pressure associated with strong spontaneous breathing effort may worsen lung injury.
Neuromuscular blockade is associated with the attenuation of biomarkers of epithelial and endothelial injury in patients with moderate-to-severe acute respiratory distress syndrome.
Mechanical ventilation with lower tidal volumes and positive end-expiratory pressure prevents pulmonary inflammation in patients without preexisting lung injury.
A Meta-analysis of Intraoperative Ventilation Strategies to Prevent Pulmonary Complications: Is Low Tidal Volume Alone Sufficient to Protect Healthy Lungs?.
Association of positive end-expiratory pressure and lung recruitment selection strategies with mortality in acute respiratory distress syndrome: a systematic review and network meta-analysis.
Ventilation strategy using low tidal volumes, recruitment maneuvers, and high positive end-expiratory pressure for acute lung injury and acute respiratory distress syndrome: a randomized controlled trial.
Effect of Titrating Positive End-Expiratory Pressure (PEEP) With an Esophageal Pressure-Guided Strategy vs an Empirical High PEEP-Fio2 Strategy on Death and Days Free From Mechanical Ventilation Among Patients With Acute Respiratory Distress Syndrome: A Randomized Clinical Trial.
Intraoperative protective mechanical ventilation for prevention of postoperative pulmonary complications: a comprehensive review of the role of tidal volume, positive end-expiratory pressure, and lung recruitment maneuvers.
Association between driving pressure and development of postoperative pulmonary complications in patients undergoing mechanical ventilation for general anaesthesia: a meta-analysis of individual patient data.
Intra-operative ventilator mechanical power as a predictor of postoperative pulmonary complications in surgical patients: A secondary analysis of a randomised clinical trial.
Replacement of anesthesia machines improves intraoperative ventilation parameters associated with the development of acute respiratory distress syndrome.
Epidemiology, practice of ventilation and outcome for patients at increased risk of postoperative pulmonary complications: LAS VEGAS - an observational study in 29 countries.
Protective ventilation with high versus low positive end-expiratory pressure during one-lung ventilation for thoracic surgery (PROTHOR): study protocol for a randomized controlled trial.
Effect of intraoperative high positive end-expiratory pressure (PEEP) with recruitment maneuvers vs low PEEP on postoperative pulmonary complications in obese patients: a randomized clinical trial.
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